Published in the Encyclopedia of Life Support Systems, 2001 · Metric system multiples are used to describe large amounts of activity so that a kBq is 1000 Bq, an MBq is 106 Bq, a

Published in the Encyclopedia of Life Support Systems, 2001

Nuclear Energy P. Andrew Karam Keywords: nuclear energy, reactors, radioactive waste, naturally-occurring radioactive materials, nuclear fuel cycle, uranium ore, uranium enrichment, desalinization, fossil fuels, radioactive emissions, radiation health risks, cancer, gaseous diffusion, AVLIS, Oklo reactor, Chernobyl, high-level radioactive waste, low-level radioactive waste, WIPP, Windscale, IAEA, pebble-bed nuclear reactors Contents 1) Introduction 2) The Nuclear Fuel Cycle 3) Kinds of Nuclear Reactors 4) Environmental Issues and Sustainability 5) Conclusions Glossary of Terms Activation products – Atoms that become radioactive after being bombarded with neutron radiation (see neutron activation) AVLIS – atomic vapor laser isotopic separation; a method of using carefully tuned laser beams to increase the amount of 235U to make reactor fuel Becquerel (Bq) – a unit of radioactivity equal to one radioactive disintegration per second. Metric system multiples are used to describe large amounts of activity so that a kBq is 1000 Bq, an MBq is 106 Bq, a GBq is 109 Bq, and a TBq is 1012 Bq Boiling water reactor – a type of nuclear reactor in which water in the reactor core boils, generating steam to produce power Control rods – neutron-absorbing rods (often made of cadmium, hafnium, silver, indium, or some other neutron-absorbing metal) that are used to control reactor power; inserting control rods into the reactor core brings the criticality to a halt Depleted uranium – uranium from which 235U has been removed (in order to make enriched uranium); depleted uranium has less than 0.72% 235U Desalinization – the process of removing salt from sea water to make it drinkable or useable in agriculture Emergency cooling system – a system designed to remove waste heat from a reactor’s core in the event an emergency renders the reactor coolant pumps inoperable Enriched uranium – uranium in which the amount of 235U has been increased to a level greater than 0.72% in order to make reactor fuel Fission products (also called fission fragments) – radioactive atoms formed from an atom that fissions; fission of 235U leads to the formation of fission fragments with atomic masses around 100 and 135 atomic mass units. Fuel – the fissionable material (usually enriched uranium) that makes it possible to sustain a critical chain reaction; the uranium fuel is loaded into fuel rods which, in turn, are assembled into fuel assemblies


Gas centrifuge – a method of uranium enrichment in which centrifugal force is used to help separate molecules containing the lighter atoms of 235U from the uranium feed Gas-cooled reactor – a type of nuclear reactor in which gas (usually helium) is used to transfer the heat of nuclear fission from the reactor core to a steam generator or turbine Gaseous diffusion – a method of enriching uranium in which the slightly lighter molecules gas molecules containing 235U are more likely to penetrate (or diffuse through) a permeable barrier Graphite-moderated reactor – a type of nuclear reactor in which graphite is used to moderate (or slow down) neutrons, making fission possible Mill and mine tailings – the waste materials left over after digging uranium ore from the ground and removing the uranium from it for further processing Moderation – the process of slowing down fast neutrons released from nuclear fission by causing them to collide, and exchange energy with atoms of the moderator Neutron activation – bombarding stable atoms with neutron radiation will result in some atoms absorbing neutrons; this process makes the atoms radioactive Nuclear criticality – the process by which a nuclear reactor produces power; in a critical reactor, the number of neutrons (a measure of reactor power production) remains constant over time – note that all nuclear reactors are critical when operating Nuclear fission – when some atoms absorb neutrons they will split into two or more parts and will emit neutrons; this process is called nuclear fission Nuclear reactor – a device that is designed to produce power by allowing nuclear fission to proceed in a controlled manner for prolonged periods of time Ore – a geologic body in which it is economically feasible to recover minerals or metals for industrial or commercial use Pebble bed modular reactor – a type of nuclear reactor in which small spheres of moderator and fuel are loosely stacked; helium is circulated through the spheres as a coolant Person-Sv (or person-rem) – radiation exposure to a group of people, defined as the summation of dose to every person measured; for example, a dose of 0.1 Sv to a group of 10,000 people will give a collective dose of 1000 person-Sv Pressurized water reactor – a type of nuclear reactor in which the coolant (water) is pressurized to keep it from boiling as it transfers heat from the core to a steam generator Primary plant (systems) – the parts of a nuclear reactor that come in direct contact with coolant that has passed through the reactor core Radiation – the transfer of energy from one place to another via an intermediary; in particular, ionizing radiation uses alpha, beta, or gamma radiation to transfer excitation energy from an unstable atomic nucleus to an absorber Radiation dose – a measure of energy deposited in an absorber by ionizing radiation; 1 Gray (Gy) is the dose resulting from the absorption of 1 Joule of energy per kg of absorber. The Sievert (Sv) is a measure of dose equivalence and accounts for the fact that some kinds of radiation are more damaging than others Reactor coolant pumps – pumps used to circulate reactor coolant (usually water) through the reactor core SCRAM – an emergency reactor shutdown; scrams can be initiated automatically or manually Secondary plant (systems) – in a pressurized water reactor, the systems that do not have direct contact with water that has passed through the reactor core Sievert – exposure to ionizing radiation that produces the biological damage equivalent to depositing 1 Joule of gamma radiation per kilogram of body tissue – the US unit is the rem


Steam generator – a piece of equipment in which hot gas or water passes through a heat exchanger and is used to boil water to produce steam; the steam is then used to turn a turbine to produce electrical power TRU – an acronym for trans-uranic; any element heavier than uranium Turbine – a piece of equipment in which a hot gas such as steam causes the turbine to turn; the other end of the turbine is connected to an electrical generator to produce electricity Uranium enrichment – the process of increasing the amount of 235U from 0.72% (which is found in natural uranium) to a higher percentage needed for most nuclear reactors to operate

1. Summary A significant fraction of the global electricity supply is produced by the world’s 400+ (as of June, 2001) nuclear power plants. Other nuclear reactors produce radioactive isotopes for research and medical treatment or generate radiations used in other scientific research. Nuclear energy also promises to help reduce the emissions of greenhouse gases, and some have noted that nuclear energy may be the best way to supply the growing demand for electrical energy without further contributions to global warming. However, nuclear energy is not an unmixed blessing. Nuclear power plants have also been decried as being costly, unsafe, and environmentally unfriendly. Several accidents have tarnished nuclear power’s image, and a few of these have resulted in fatalities. Nuclear reactors generate both high-level and low-level radioactive waste, and the ultimate disposition of these wastes is subject to a great deal of public scrutiny and generates considerable concern. The fears of the public and the environmentalists are further heightened by widespread fear of radiation and its potential long-term effects on the public health. Nuclear power cannot be ignored as a potential source of global energy because of its undeniable benefits; neither can it be embraced unquestionably, for the reasons noted above. In this article, we will discuss many of the issues surrounding nuclear energy. This discussion will include a brief description of the manner in which nuclear reactors are currently used, a more detailed description of the nuclear fuel cycle and the theory underlying nuclear reactor operations and design, and a discussion of the scientific basis for many of the public health concerns raised by nuclear power. Finally, we will discuss some of the political and environmental issues surrounding nuclear energy, comparing nuclear reactors to other energy sources in an effort to provide an unbiased comparison of the benefits and drawbacks they provide.

2. Introduction a) A brief history of nuclear reactors and their uses

The first man-made nuclear reactor was constructed at the University of Chicago by a team led by Enrico Fermi in 1943. Built as part of the Manhattan Project, this nuclear reactor was designed and constructed explicitly for research leading to the eventual construction of the world’s first nuclear weapons. Although much scientific research suggested that uranium could be used to generate a self-sustaining nuclear chain reaction, until Fermi’s reactor achieved criticality, this had not been demonstrated in practice. Using knowledge from this reactor, other scientists were able to develop not only nuclear weapons, but also built the first plutonium production reactors, used to create fuel for other atomic bombs. Thus, from the very start, nuclear reactors became intimately associated with nuclear weapons; an association that has


since haunted all discussion of nuclear energy. However, it is noteworthy that Germany, the United States, and Japan all understood the value of nuclear-powered submarines even before the Manhattan Project, and all three nations, however fleetingly, investigated this possibility at one time. Following the end of World War II, the United States, the Soviet Union, Canada, and Great Britain were the first nations to continue exploring the potential of nuclear energy for both civilian and military purposes. In particular, the United States and the Soviet Union, spurred on by the Cold War, led the way in designing and building increasingly sophisticated designs for both nuclear reactors and nuclear weapons, as well as devising an increasing number of uses for nuclear power in other settings. This led to development and construction of small reactors for use in research, somewhat larger reactors used to produce radio-labeled compounds for research and medical purposes, and the investigation of “portable” nuclear reactors for generating power in near-combat or remote locations. In addition, nuclear reactors were utilized by several navies, where they revolutionized submarine warfare. In 1954, the first commercial nuclear power plant went on-line in the Russian city of Obninsk, near Moscow. This was followed in 1956 by the British plant in Calder Hill, and in 1957, the first American commercial nuclear power plant went on-line in Shippingport, Pennsylvania. The 1950s and, to a lesser extent, the 1960s were an age of nuclear optimism, particularly in the US and the Soviet Union. The widespread use of nuclear power was seen as a nearly unlimited source of inexpensive, reliable power that would help to lift much of the world out of poverty while simultaneously providing fresh water, new drugs, and reduced environmental costs. However, the continued development of nuclear weapons, their testing in the atmosphere, and the growing awareness of the potential for radiation injury began to concern some. In the 1960s, with the growing strength of the global environmental movement, these concerns began to be voiced to governments with increasing volume. One milestone along this path was Linus Pauling’s successful campaign to halt atmospheric nuclear weapons testing, which was given enhanced visibility by his subsequent Nobel Peace Prize for his efforts. However, it was not until 1979, with the accident at the US nuclear power plant at Three Mile Island (TMI) that the anti-nuclear power movement really took off. Although the TMI accident resulted in exceedingly low radiation exposure to the general public, the reactor core was destroyed, and the perception was that it represented a narrowly-averted disaster. Coming on the heels of the successful (although technically inaccurate) movie “The China Syndrome”, the accident was an unmitigated disaster for the US nuclear power industry. Seven years later, much more serious accident at the Soviet (now Ukranian) Chernobyl nuclear reactor gained global notoriety, the political results of which are still felt today. As of this writing (June, 2001), the global outlook for nuclear energy is mixed. Japan, France, and to a lesser extent Russia, Canada, and Great Britain seem to have mature and relatively politically secure nuclear power capabilities. Several European nations, however (including Sweden and Germany) have announced plans to eliminate nuclear power plants, although the source of alternate energy has not yet been announced. Still other nations (particularly China and, to a lesser extent, Iran) are embarking on large programs to increase their dependence on nuclear energy, and the US stance remains mixed and undecided.


b) Nuclear reactor theory and operations – a general description

Nuclear reactors generate energy by fissioning (splitting) atoms of uranium. This simple statement hides a great deal of physics and engineering. The physics describes why splitting atoms produces energy and how this fissioning can be maintained for prolonged periods of time, and the engineering is necessary if this energy is to serve any useful purpose. It is not immediately obvious that simply splitting a uranium atom should release energy. After all, splitting a log, or a stone, or any other object we are familiar with requires energy – swinging an ax is hard work. Similarly, there is no obvious reason that fissioning one atom should result in a second atom splitting, just as it is not readily apparent why uranium must be used instead of, say, lead or iron. A brief foray into nuclear structure is necessary to understand this, but the description is neither mathematical nor abstract. All atoms are composed of a central nucleus surrounded by a cloud of electrons. The electrons do not concern us for the purposes of this discussion. The nucleus, in turn, is made up of protons with a positive electrical charge and neutrons with no charge, all confined to a very small space. Similar electrical charges repel one another, and the protons in the nucleus are subject to strong forces that try to force the nucleus apart. What holds atoms together is a force, called the strong nuclear force, and this force is carried by the neutrons. The neutrons are the duct tape that helps to hold the protons together. However the strong force only works over very short distances, so as atomic nuclei become larger, the strong force loses its ability to hold onto all of the protons. This means that, in general, large, heavy atoms are inherently less stable than small, light atoms. Another way to look at it, using the duct tape analogy, is that a piece of tape has a finite length. If we use a 30 cm piece of tape to hold together a few sticks, it will serve quite well. However, as the group of sticks grows, the tape is less able to wrap around to hold them all, and the entire bundle becomes less stable and easier to tear or fall apart. Uranium is the largest atom that exists in abundance on Earth. There are small amounts of plutonium that are present naturally, and large amounts of plutonium and even heavier elements are formed in stellar explosions, but they are not long-lived and are uncommon on Earth. This means that uranium is also the atom most likely to fall apart on its own (called spontaneous fission) or to be forced apart (induced fission) by adding a neutron to the atomic nucleus. Uranium also comes in several “flavors”, or isotopes. The chemical properties of an atom are determined by the number of protons in the nucleus. Every atom with 82 protons (lead) is chemically identical, as is every atom with 92 protons (uranium). However, atoms of the same element can have different numbers of neutrons present, giving them a variety of atomic weights and different atomic properties. In the case of uranium, 99.2% of the uranium in the world has 92 protons and 146 neutrons, giving it an atomic weight of 238 (written as 238U, or U-238). About 0.72% has three fewer neutrons; 235U. In spite of having the same chemical properties, U-235 and U-238 have different nuclear properties, and U-235 is more likely to absorb passing neutrons than is U-238. When this happens the strong nuclear force, already stretched thin by the sheer size of the nucleus, can no longer hold the atom together and it falls apart. As the atom fissions, it produces two fission fragments (which are radioactive), 2 – 3 neutrons, gamma rays,


and energy. The energy released is what we harness to make electricity, the neutrons go on to cause other fissions, and the fission fragments become radioactive waste. As noted above, between 2 and 3 neutrons are released from each fission. Some of these neutrons go on to be absorbed by the water, steel, and lead of the reactor plant and are lost to the plant. Others are absorbed by uranium atoms, but do not cause fission, and still others escape the reactor altogether. The entire secret of nuclear reactor design is to arrange the fuel in such a way that, for each atom of uranium that fissions, exactly one neutron is produced that goes on to create another fission. When this happens, the total number of fissions in the reactor at any time is constant, so the production of energy is constant. Making this happen requires a certain mass of uranium arranged in a certain configuration, called the “critical mass” and “critical geometry”. When you achieve such conditions, the nuclear reactor is said to be “critical”. Put another way, all nuclear reactors are critical when they are operating, and nuclear criticality in an operating nuclear reactor is hardly an emergency (as a corollary, those who understand this fact are usually amused by television shows or movies in which someone announces in a panic-stricken voice that “the reactor is critical” – this just indicates that the writer is not terribly knowledgeable about nuclear reactors, and suggests that their writing should be viewed with some degree of skepticism). On an atom-by-atom basis, nuclear fission releases a tremendous amount of energy. Splitting one uranium atom produces about 100 times as much energy as burning one molecule of gasoline, so nuclear energy can produce much higher energy densities than can plants that rely on chemical reactions (such as combustion). However, this energy is useless unless it can be harnessed in a usable form; we cannot simply pump through wires. In the case of nuclear reactors, the energy generated by fission turns into heat, which heats the reactor fuel. The fuel is surrounded by a coolant, usually water, and the heat energy is transferred into the water. The hot water, in turn, is used to produce steam, which turns turbines, which generate electricity. Although the process sounds somewhat laborious, it is no more so than many other forms of electricity generation, and the efficiency of most nuclear power plants (i.e. the ratio of electrical energy to thermal energy) is higher than many competing forms of energy generation. Technical note – in reality, energy cannot be created, it can only be changed from one form to another. Nuclear reactors release energy already present in an atomic nucleus, turn it into heat energy, and the heat energy is transformed into electrical energy. The total amount of energy contained in the power lines coming out of a nuclear reactor plant is the same as the total amount of energy originally present in the uranium atoms that were fissioned.

c) Uses of nuclear reactors Although the most visible and obvious uses to which nuclear reactors have been put are the generation of electrical energy and the production of materials for nuclear weapons, they have found many other uses in the half-century or so they have been in use. In particular, this section will these two uses, the use of nuclear reactors in military, non-weapons programs, the production of isotopes for medical and research purposes, and desalinization plants. Generating power has already been discussed above, and nuclear weapons are beyond the scope of this chapter, so the next few paragraphs will discuss the use of nuclear reactors for military (non weapons-related), research, and medical purposes.


i) Military, non-weapons use The first non-weapons use to which nuclear energy was applied was in nuclear submarines. Every major naval power immediately understood that nuclear energy offered the promise of creating a submarine force that could operate submerged for prolonged periods of time, virtually undetectable. Earlier submarines were hybrid machines – they ran on diesel engines on the surface and on batteries while submerged. Since their batteries could only give a limited amount of service before recharging, diesel submarines were designed to operate on the surface of the ocean, submerging only when necessary to attack or to hide. Nuclear reactors, unlike diesel engines, do not need air or oxygen to produce power. A nuclear submarine could operate at full power, completely submerged, almost indefinitely. The reactors actually produce more than enough energy to meet the ship’s needs, leaving additional energy for distilling fresh water, purifying the atmosphere, and more. In addition, freed from the constraints of a large battery for underwater attacks or evasions (and the diesels to recharge it), much more of the volume could be devoted to carrying weapons, electronics, and crew. The development of nuclear submarines was the most significant revolution in the history of submarines and was one of the biggest innovations in the history of modern naval warfare. Although nuclear power has also been put in use on surface combatants by the US and the Soviet Union (now Russia), its advantages on the ocean’s surface are not nearly as pronounced as they are beneath the waves. In addition to the naval use of nuclear reactors, some nations experimented with “portable” nuclear power plants that could be used to supply energy to military headquarters in remote locations. Another proposed use was in a nuclear airplane, a project begun but abandoned by the US in the 1960s, and others have used nuclear reactors in space. This latter use should not be confused with radio-isotopic thermal generators (or RTGs) which are used on most deep-space missions to the outer solar system. RTGs make use of heat released by radioactive decay, but they do not use nuclear fission for this purpose, they do not generate radioactive fission products or high levels of neutrons.

ii) Medical and research uses Although only a few isotopes will fission, most can be induced to capture neutrons, protons, or other atomic particles under the appropriate conditions. When this happens, the resulting atom will become radioactive. One example of this is the formation of radioactive carbon in the atmosphere. In this reaction, a cosmic ray neutron will strike a nitrogen atom, ejecting a proton from the nucleus and turning it into a carbon atom. This reaction is written 14 14N n C p+ = + This reaction is the one that creates the carbon-14 used to date archeological artifacts, tree rings, and many other objects. Similar reactions can be used to create many other isotopes that are widely used in research, to diagnose medical conditions, or to treat cancer and some other diseases. Research performed with the aid of nuclear reactor-generated isotopes includes genetic


sequencing, investigation of basic biological functions, the development and understanding of new drugs, and better understanding of brain functions. Nuclear reactors are also used as a source of neutrons for other research purposes. Neutrons can be used to probe the structure of matter and to investigate the chemical composition of geologic specimens (also called rocks). Cell cultures can be exposed to radiation from nuclear reactors to learn more about how DNA is damaged and repaired, nuclear reactor-produced neutrons have also been used to help treat some forms of cancer, and chemical compounds containing radioactive atoms created in nuclear reactors are used in research and in the diagnosis or treatment of disease.

3. The Nuclear Fuel Cycle As mentioned above, natural uranium cannot sustain a nuclear chain reaction; for this to occur, the fraction of U-235 present in the uranium must be increased from 0.72% to at least 1% of the total number of uranium atoms present. In reality, the uranium must be enriched further yet, because commercial nuclear reactors use fuel containing from 3-6% U-235. Some nuclear reactors, primarily those used for research, make use of fuel enriched to 20% U-235, and some military nuclear reactors use fuel that is nearly pure U-235; similar to the concentrations used in some older nuclear weapons. As an aside, this means that commercial and research nuclear reactors cannot explode like nuclear bombs – it is physically impossible for them to do so because of the relatively low concentrations of fissionable uranium. The process of making fuel for nuclear reactors begins when the uranium ore is mined and processed, continues through the process of uranium enrichment, and culminates with fabrication of the nuclear reactor fuel. Eventually, the U-235 in the reactor fuel is fissioned to the point at which nuclear reactions no longer occur, and the reactor is refueled so it may continue operating. The spent fuel is then either stored on site, sent for disposal, or recycled. This whole process is called the nuclear fuel cycle and is the subject of this section.

d) Uranium mining The first step in the nuclear fuel cycle is locating a uranium ore deposit and bringing the ore to the earth’s surface. Virtually all rocks, soils, and most waters contain trace amounts of uranium. However, the uranium is present in low quantities, and it is not possible to recover the uranium at a reasonable cost. The term “ore” is an economic term, not a scientific one; ore is the presence of a material (usually metal) in a place and chemical concentration that makes it possible to mine at a profit. For example, deep-sea manganese nodules contain large amounts of very high-purity manganese, but their location on the seafloor makes the metal horrendously expensive to recover. These nodules of nearly pure metal are not considered ore because of this. On the other hand, large mining operations in the America West make a lot of money mining ore that contains small amounts of manganese, simply because it is found near the earth’s surface and the mining is not expensive. In the case of uranium, a large number of ore deposits have been found on virtually every continent. Among the most famous are those in the American west, Australia, Canada, and several places in Africa. Some of these ore deposits are near the earth’s surface, and strip mining


operations are sufficient to recover the ore, while others are deeply buried, requiring mine shafts and tunnels. Regardless of how the ore is mined, it is brought to the surface along with tens of thousands (or even millions) of cubic meters of other rock. This other rock (mine tailings) is often mildly radioactive (because of elevated, but not economic levels of uranium); it is generated in the process of digging to the ore body. Most mines generate large piles of tailings, but in the case of uranium mining, the tailings are often regulated and must be treated as radioactive waste. This can create problems, because the piles can leach uranium into ground or surface waters, they emit radioactive radon gas, and many nations’ laws will not permit their disposal into landfills or even into the mine shafts from which they were excavated. This forces companies to find some way to contain the tailings, with varying degrees of expense and success.

e) Processing the ore After removal from the ground, the uranium ore is shipped to a processing facility. Here, the rock is crushed and chemically processed to remove as much of the uranium as possible. The uranium may then be chemically treated to turn it into uranium hexafluoride (UF6), making it suitable for enrichment via gaseous diffusion (described in the following section). If another enrichment process is used, the uranium may be placed in another chemical form more conducive to that particular process. Following removal of uranium from the rock, there is, again, a great deal of waste material. Some uranium ores contain less than 1% uranium by weight, and far less by volume. This means that virtually all of the ore brought to the uranium mill is discarded as waste, generating huge piles of mill tailings. As with the mine tailings, this waste is mildly radioactive and emits radon. This, too, must be properly handled to adhere to regulatory requirements and to minimize environmental and health effects. It must be pointed out that, in the case of both mine and mill tailings, the risk from radioactivity present in the waste materials is exceptionally low, and there have been no documented human health effects from this radioactivity. It must also be noted that the radiation levels from the ore and the UF6 containers are also too low to create a health risk to the public.

f) Uranium enrichment As of this writing, there are three primary methods of uranium enrichment used by most nations with domestic uranium processing. These are gaseous diffusion, gas centrifuge, and atomic vapor laser isotopic separation (AVLIS). The first two of these make use of the very slight difference in weight between 235UF6 and 238UF6. Each will be described briefly.

i) Gaseous diffusion Uranium hexafluoride is a very volatile compound, meaning it becomes a gas at a relatively low temperature. When cylinders of UF6 arrive at a gaseous diffusion plant, pipes are connected to the container and the cylinder (which may be 3 meters long and over a meter in diameter, containing 14 tonnes of UF6) are heated with steam until the UF6 vaporizes. This vapor is introduced into the gaseous diffusion plant through the pipes. A uranium hexafluoride molecule that contains an atom of U-235 is about 0.85% lighter in weight than molecules containing a U-238 atom. Since the temperature of a gas is a measure of


the speed (and kinetic energy) of the molecules in that gas, this means that molecules of 235UF6 are moving very slightly faster than molecules of 238UF6 with the same temperature. This, in turn, means that the 235UF6 molecules are able to strike a barrier marginally faster than their heavier counterparts, and if the barrier is properly constructed, when the UF6 gas diffuses through, it contains slightly more U-235 than does the original gas. The increase in U-235 is very small, so it takes hundreds or thousands of stages to increase the uranium enrichment to the point at which it is possible to make nuclear reactor fuel. In fact, the gaseous diffusion plants in the US each contain three buildings with a total area of over 40 hectares, all devoted exclusively to enriching uranium to from 3-20% U-235. The chief advantage of the gaseous diffusion process is that it requires relatively few people to maintain the machinery. In addition, it does not require a high level of technology, and the process is relatively simple conceptually. However, the process demands an extraordinarily high amount of electrical energy because of the large number of units in the uranium enrichment “cascade”, and much of the machinery now in use in this process is relatively old. Finally, UF6 is toxic and corrosive, and the large amounts of fluorine (usually in the form of hydrogen fluoride, or HF) is also a serious safety concern. Gaseous diffusion was the mainstay of US uranium enrichment efforts for over 60 years. However, as of this writing, this form of uranium enrichment is being phased out in favor of more efficient and less expensive methods, such as those described in the following sections.

ii) Gas centrifuge Gas centrifuges take advantage of the different molecular weights of UF6 in another manner. Just as a layer of oil will sit atop a layer of water, a layer of 235UF6 will lay on top of a layer of 238UF6 because of the slight difference in molecular weight. In this enrichment process, however, the “layers” of gas are formed by introducing it into a series of centrifuges spinning at a very high rate of speed. The UF6 is flung to the outside of centrifuge with the lighter molecules rising to the “top”, marginally closer to the center of the spinning cylinder. The engineers who design the gas centrifuges can calculate the thickness of each layer of UF6 because they can control the speed at which the gas enters the centrifuge, the size of the centrifuge, and the rate at which it spins. Pipes are connected to the centrifuge in the location where the lighter (235UF6) is concentrated, and the enriched uranium is led away to the next stage of the enrichment process. This is just like pouring oil and water into a jar together and placing a pipe to remove the oil alone – if the pipe is placed so that it only protrudes into the layer of oil, then only oil will be removed. The chief advantage of this method of uranium enrichment is that it requires far less energy than does gaseous diffusion. It is also more efficient in terms of space, because the centrifuges are smaller than the gaseous diffusion converters, and each individual stage is more efficient at enriching uranium. However, the centrifuges operate at high speeds and with very precisely machined pieces; they are more likely to break and they require much more maintenance to keep running. For this reason, gas centrifuge uranium enrichment facilities require more workers, which somewhat offsets the lower energy costs to operate. Finally, both this method and


gaseous diffusion produce enormous volumes of waste, in the form of depleted uranium (which is uranium containing less than 0.72% U-235). This depleted uranium (DU) is only mildly radioactive and poses little to no health risk from radioactivity (in spite of some claims to the contrary), but it is chemically toxic and must be handled with care because of its chemical risks.

iii) Atomic Vapor Laser Isotopic Separation (AVLIS) The final form of uranium enrichment is AVLIS, so far used only in demonstration projects, but not in full production mode. AVLIS is perhaps the most elegant system of uranium enrichment, and it likely represents the future for this industry. In AVLIS, a gas of uranium atoms is illuminated with a precisely tuned laser beam. This laser beam imparts more energy to U-235 atoms than to U-238 atoms, and the U-235 atoms are “kicked” a bit to the side away from the laser beam. As with the gas centrifuge process, pipes are set up to capture the stream of U-235 atoms and lead them away from the enrichment chamber. On the molecular scale, gas centrifuges and gaseous diffusion are “brute force” methods of enriching uranium because, in effect, they use a lot of energy to process large amounts of uranium hexafluoride, achieving relatively minor levels of enrichment at each step. By comparison, AVLIS is more elegant in that it effectively sorts a uranium vapor, atom by atom, into the two major isotopes. In addition, less chemical processing is required at the uranium mill, the uranium is present in a less dangerous chemical form, and there is less waste generated. However, AVLIS requires the highest level of technology and is proving difficult to scale up from technology demonstrations to full production. Nonetheless, its advantages are such that it will likely replace the other forms of uranium enrichment in coming years.

g) Reactor fuel fabrication Finally, the enriched uranium is ready to be made into fuel for a nuclear reactor. To do this, the UF6 must be chemically converted into uranium oxide, which is more chemically stable and less hazardous than the uranium hexafluoride. Once this oxide conversion is completed, the enriched uranium oxide powder is mixed with zirconium or some other metal and pressed into small pellets, each about 1 cm in diameter and a few cm long. These fuel pellets are loaded into stainless steel rods, and the rods are then assembled into bundles called fuel assemblies. It is these fuel assemblies that are loaded into the reactor core.

h) Reactor re-fueling After a period of time, usually 12-18 months, the reactor fuel can no longer sustain a nuclear chain reaction efficiently because so much of the U-235 has been fissioned. During the time the fuel is in the reactor it is periodically moved from place to place within the reactor core to help maximize the life of the fuel assembly while maintaining desired characteristics within the core itself. For example, new reactor fuel has a higher concentration of U-235 so it will produce more power than an older fuel bundle, or it will produce power in a less neutron-rich environment. Such bundles are often placed near the edge of the reactor core, where there are fewer neutrons, because this helps to balance power production in the reactor core. As the fuel burns out, it will be shifted closer to the center of the core, experiencing higher neutron levels and producing the


same amount of energy. After a sufficiently long time the fuel will no longer produce enough energy to warrant keeping it in the core, so it is removed and more new fuel is added. The process of moving and replacing reactor fuel is complex and carries with it the potential for very high radiation doses to workers. For this reason, reactor re-fueling is done as infrequently as possible, and is never done unless necessary. In addition, the reactor must be shut down to perform the refueling, removing it from a nation’s electrical power generation system. Shutting the reactor down means that workers can enter the reactor compartment to perform needed maintenance, but it also means that the reactor is not generating power or income. Because of this, these “outages” are tightly scripted to get the most work possible done with the least amount of expense and in the shortest time possible. In the US, most planned reactor outages are scheduled for the spring or autumn, when neither air conditioning nor heating is necessary, reducing electrical power demands as much as possible.

i) Reactor fuel reprocessing After removal from the reactor core, some nations send the spent fuel to a large chemical processing facility to recover as much unfissioned uranium as possible. Although the spent fuel rods are dangerously radioactive when they are first removed from the core, this radioactivity fades relatively quickly and they can be safely handled after several months. In the reprocessing facility, the fuel pellets are removed from their cladding and are dissolved in nitric acid. After much chemical processing, the facility is left with uranium, plutonium, and waste. The plutonium and waste will be discussed in other sections of this chapter. The uranium is recycled, enriched again, and made into new reactor fuel. The United States reprocessed reactor fuel until the mid-1970s, at which time the practice was halted. The reason the US stopped reprocessing reactor fuel is because of the plutonium; President Carter felt that the chance for this plutonium to be obtained by terrorists or by nations trying to develop nuclear weapons was too great. At present, the US does not recycle reactor fuel but, instead, considers it all to be waste. The fate of spent reactor fuel and other radioactive wastes is discussed in the following section.

j) Radioactive waste Radioactive waste is any radioactive material that serves no useful purpose and that is not in its natural state. For example, uranium that is found naturally in soil is not radioactive, nor is it considered waste when it is producing energy in a nuclear reactor. However, once removed from the reactor, when it is no longer useful, the remaining uranium contained within a fuel rod is considered radioactive waste and must be properly disposed of. Similarly, items made radioactive by bombardment with neutrons become radioactive waste when they are removed from the system, and items contaminated with radioactivity are also considered radioactive waste. Contrary to popular beliefs, the majority of radioactive waste is not glowing green deadly liquid. In fact, most radioactive waste is gloves, tools, paper towels, rags, and other items that are mildly radioactive or mildly contaminated. There are, of course, wastes that are dangerously


radioactive, but they constitute the minority of the volume that is sent to radioactive waste disposal facilities around the world. Most nations have strict regulations regarding the packaging, shipping, disposal, and accountability of radioactive wastes. Typically, radioactive wastes are placed into a large metal drum or box, and trained technicians keep accurate and detailed records of the exact contents of each waste container. These records are used to determine the appropriate way to ship the wastes, and are kept on hand by the driver until the waste is delivered to the final disposal facility. Along the way, the waste may be sent for processing to reduce its volume, liquid wastes may be solidified by mixing them with concrete, or other actions may be taken to make final waste disposal as safe and economical as possible. Waste treatment and disposal depends on many factors, including the physical form of the waste (solid or liquid) and the level of radioactivity present (high-level or low-level radioactive waste). Some of these factors are described in the following sections.

i) Waste treatment and processing Many options exist for treating or processing radioactive wastes prior to ultimate disposal. For example, radioactive liquids may be mixed with concrete to solidify them, making leakage less likely. Some materials, including many solvents, paper, plastics, and wood can be incinerated and the ashes shipped for disposal – this reduces the volume of the waste by a factor of ten or more. Metals may be melted, and the ingots can then be disposed of in a more compact form, and some materials are vitrified (turned into glass) and disposed. Some materials may be chemically processed to remove the radioactivity, and other materials are simply compacted by enormous hydraulic presses to reduce the final volume as much as possible before burial. Reducing the volume of radioactive material is a prime consideration in many of these treatment techniques because no landfill has unlimited space available, and radioactive waste landfills are even more restricted than many others.

ii) Waste disposal – low level radioactive waste (LLRW) Low-level radioactive waste is generally waste that poses little or no health risk due to its radioactivity. It may or may pose other hazards due to its chemical or physical properties; for example, dropping a 1000 kg container of LLRW on a person will hurt that person, even if they receive virtually no radiation dose from it! In general, most nations have at least one specially designed repository for the disposal of LLRW. In most cases, these repositories must adhere to strict siting guidelines that are intended to minimize the chance for radioactivity to escape into the environment. Such sites are also typically ringed with monitoring stations designed to detect even the slightest leakage of radioactivity, and they are inspected frequently by regulatory agencies. In the early years of LLRW disposal some sites did experience the release of minor amounts of radioactivity into the environment. However, in all cases (except for a few in the former Soviet Union) these releases were not sufficient to cause lasting harm to the environment, to workers, or nearby residents.


In some nations, LLRW disposal facilities are not unlike other properly designed landfills. In such facilities, soil is excavated and the resulting pit is lined with compacted clay to isolate it from the groundwater. Beneath this clay there may be monitoring lines, to detect leakage through the clay if it occurs. When the waste containers are placed in the pit, it is capped with more compacted clay and sometimes a polymer sheet designed to keep rain out of the pit. In other nations, LLRW facilities are constructed above ground. In these facilities, concrete vaults are built atop a compacted clay foundation. These vaults are filled with an orderly array of waste containers. As with the below-ground waste facilities, the environment is monitored on a continuing basis, including looking for evidence of radioactivity in the groundwater. The vaults may be covered with soil when they are filled, or they may be left uncovered to facilitate inspecting for damage over the years. In any event, both forms of waste disposal have generally proven to be effective at isolating radioactive waste from the environment safely.

iii) High-level radioactive waste, TRU, and spent reactor fuel Aside from low-level radioactive wastes, nuclear reactors also produce high-level radioactive waste (HLRW), trans-uranic (TRU) waste, and spent reactor fuel. These are all treated differently than LLRW because of the unique hazards each poses. HLRW is radioactive waste that is dangerously radioactive and may remain so for many years. It can include resin from the water purifying units used to ensure water purity in the reactor, or nuclear reactor components that have become radioactive after years or decades of neutron bombardment. HLRW is often placed in special casks that provide additional shielding. It may be stored in this manner at the site where it was generated, or it can be shipped for disposal in a special HLRW facility. In most nations, HLRW is disposed of in deep geologic repositories; tunnels and chambers drilled into solid rock, often hundreds of meters deep. These are thought to be capable of safely holding the wastes until they are no longer radioactive. By neutron capture, some uranium atoms will be transmuted into plutonium, americium, and other elements beyond uranium on the periodic table. These transuranic elements are often very radioactive and many are chemically toxic as well. In addition, some isotopes of plutonium are fissionable and can be made into either nuclear reactor fuel or nuclear weapons. Although Pu, contrary to popular belief, is not the “most toxic substance known to man”, it is both radioactive and toxic and it must be treated with care. However, the greatest risk posed by Pu is the potential for it to be seized by a terrorist group or a rogue nation, made into a nuclear weapon, and that weapon used in an act of terrorism or war. This is one of the reasons the US stopped reprocessing spent nuclear reactor fuel; separating Pu from spent reactor fuel is difficult, dangerous, and expensive, and it is thought that few nations have developed the ability to do so at this time. In the US, TRU wastes are now disposed of at the Waste Isolation Pilot Plant (WIPP), located near Carlsbad New Mexico in the US desert Southwest. WIPP is located within a deep underground layer of salt that, over time, will engulf the waste containers, keeping them safe for millennia.


Spent reactor fuel, in many nations, is reprocessed and the reprocessing waste is disposed of as HLRW. The US and some other nations, however, dispose of spent reactor fuel, which can remain highly radioactive for several years after it is removed from the nuclear reactor core. The US is in the process of developing a spent reactor fuel disposal facility inside of a mountain in the Nevada desert, not far from Las Vegas. Until that time, spent reactor fuel is stored at the nuclear reactor plant where it was generated. When first removed from the reactor core, radioactive decay of fission products generates so much heat that the spent fuel is kept in a large tank of water, called a spent fuel pool. The water helps to keep the fuel cool and also shields the radiation coming from the fuel assemblies. However, due to the long delay in opening the Yucca Mountain facility, many US reactor plants have filled their spent fuel pools, so they have begun moving old, “colder” spent fuel to land-based storage containers. Here, the spent fuel can be safely maintained for many years, until it can be sent to Yucca Mountain as a final repository. Other nations have a variety of methods for dealing with spent fuel, including some or all of the options mentioned above.

k) Summary The nuclear fuel cycle is a long and complex series of events that involves every facet of mining and processing uranium to make fuel for nuclear reactors and dealing with the wastes at all steps of this process. Although the details vary between countries, the overall scheme remains the same – uranium must be mined and enriched, fuel must be manufactured and installed into nuclear reactors, and the inevitable waste products must be treated and disposed of in a safe manner that is protective of the environment. Radiation exposure from the nuclear fuel cycle Source Mining, milling,

enrichment, fuel fabrication

Reactor operations

Fuel reprocessing

Waste transportation, processing, disposal

Total

Annual dose (person-Sv)

180 157 100 1.3 438

4. Kinds of Nuclear Reactors The basic physics behind uranium fission is the same, regardless of the manner in which it happens. Each atom of U-235 is virtually identical, each requires the same conditions to fission, and each behaves roughly the same when it fissions. There are some differences between atoms due to quantum mechanical uncertainties, but generally speaking, the process of fissioning an atom of U-235 is the same for all U-235 atoms. However, the manner in which fission is initiated, controlled, and the energy utilized varies between types of nuclear reactors. In this section, we will look at some of the most common nuclear reactor designs in terms of how they work, their advantages and disadvantages, and how many are in use in the world. First, however, we will briefly discuss the safety features present in virtually all nuclear reactors plants because it is these safety features that keep a nuclear reactor accident from becoming a catastrophe. It must be noted that, in the case of the Chernobyl nuclear reactor, some of these safety features were not in place or were bypassed; major factors that led to the severity of that accident. It must also be noted that the accident at the US Three Mile Island plant, while very damaging to the


reactor core, resulted in very low radiation dose to the workers and surrounding population, a fact that is often not appreciated.

a) Nuclear Reactor Safety Features Virtually all nuclear reactors require safety features to ensure the following:

• The reactor will automatically shut down if the core integrity is threatened • The reactor can be manually shut down if necessary • The reactor core will remain cool • Major releases of radioactivity will be isolated from the environment

Reactors are shut down by inserting neutron-absorbing control rods into the core. In virtually all nuclear reactors, the control rods are withdrawn from the reactor core by energizing a motor that pulls them out to the required position. As the rods are pulled, a powerful spring, the scram spring is compressed. Once in position, powerful latches are activated electrically and they hold the control rods in place against the pressure of the scram spring. The control rods are typically powered from the same source of electricity as the other important reactor plant systems. If electrical power is lost, the electromagnet holding the control rod latches will lose power and the latches will spring open. With nothing to hold them in place, the control rods are forced into the core by the scram spring, shutting down the reactor. The automatic shutdown system is also designed to activate in the event that certain reactor plant parameters become serious. For example, if the core is producing too much power, if the core temperature is too hot, or if flow through the reactor core drops below a certain level, the safety systems will scram the reactor to keep it from damaging itself or endangering workers, the environment, or the public. This same system can also be activated manually if an operator turns a scram switch. Even shutdown, the decay of radioactive fission products will produce large amounts of heat that can damage the reactor core if not removed. For this reason, most nuclear reactors have a few systems to make sure that the reactor core is kept full of coolant (usually water) and that this coolant is circulated through a heat exchanger to transfer this heat away from the reactor. In most cases, these systems are designed so that they will automatically activate if needed. In newer reactors, they are also designed to take advantage of basic laws of physics so they will operate properly even in the total absence of electrical power. For example, a tank to keep the reactor plant full of water will be located high above the reactor so that gravity will ensure water flows into the reactor. Other systems use natural circulation and convection to circulate water through the reactor core and into heat exchangers, taking advantage of the fact that hot water, being less dense than cold water, will rise into a heat exchanger located above the reactor and the cooled water will descend through another pipe back into the reactor core. Together, these emergency systems help to ensure that temperatures in the reactor core are kept low enough that the fuel is not severely damaged and the radioactive fission products are safely contained. The main reason for all of these systems is to make sure that the reactor fuel keeps its integrity so that the fission products are kept safely within the fuel matrix. However, it is always possible for the core to be damaged in spite of these systems, so a final defense is to isolate the fission products from the environment by using multiple layers to contain them. The first layer of containment is the reactor fuel itself, and as long as the fuel is intact, the fission products are


safe. If the fuel is breached, as happened at Three Mile Island, the fission products will flow into the reactor plant, and the piping itself acts as the second layer of containment. As long as the reactor system piping retains its integrity, the fission products will remain isolated from the environment. However, this too can fail, and there have been instances of broken reactor system components that release reactor coolant into the reactor compartment. Thus, the entire building in which the reactor is located constitutes yet another layer of containment, the final layer. In the US, the reactor containment buildings are designed to stringent criteria, ensuring their ability to withstand tornados, earthquakes, terrorist attacks, and even airplane crashes. As long as the outmost containment remains intact, the worst of the fission products will remain safely isolated from the environment. In fact, at Three Mile Island, virtually the entire reactor core was destroyed and the highly radioactive coolant leaked from the reactor plant into the containment building. However, because of the containment design, this radioactivity largely stayed put, and very little was inadvertently released to the environment. By comparison, the Chernobyl plant lacked a containment building and the accident there released enormous amounts of radioactivity that were tracked throughout the Northern Hemisphere.

b) Types of nuclear reactors There are a number of ways to use a self-sustaining chain reaction to generate electrical energy, and there are several competing designs for nuclear reactors. However, all commercial nuclear power plants have some points of similarity. In this section, we will first look at these similarities, followed by a brief description of the most important nuclear reactor plant designs, their strong and weak points, important design characteristics, and their “popularity” in the world nuclear energy picture.

i) General nuclear reactor plant design Regardless of details of engineering, all commercial nuclear reactor plants share a commonality of purpose; to produce electrical energy as effectively and efficiently as possible. To do so, they must accomplish the following tasks:

1. Maintain a self-sustaining nuclear chain reaction for prolonged periods of time 2. Contain fuel and fission products safely 3. Provide adequate margins of safety against accidents 4. Remove heat from the core 5. Convert the thermal energy of the reactor core into electrical energy

Of these, we have already discussed the first three in preceding sections and they will not be further discussed here. The fission process generates a tremendous amount of energy and this energy is deposited in the fuel in the form of heat. Left uncooled, fuel temperature would easily rise to the point of melting the fuel, destroying the reactor core as happened at Three Mile Island. However, this heat is the reason for building a nuclear reactor in the first place. The trick is to safely transfer the heat of nuclear fission to a place where it can be used to generate electricity. All commercial nuclear reactors do this by circulating some cooling fluid through the nuclear reactor core to transfer the heat of nuclear fission to a turbine, which spins to generate electricity. The fluid varies, and


different reactor designs use water (both heavy and light), liquid metal, liquid salt, and helium. The method of generating electricity varies as well; some reactors boil water directly to make steam which turns the turbine, some use the expansion of hot gas in the reactor core, some pass hot liquid through a heat exchanger to generate steam. In spite of these differences, though, all commercial nuclear reactors need to accomplish the same goal – to use the heat produced by nuclear fission to, in some way, cause a turbine to spin and generate electricity. In the paragraphs that follow, we will look at some of these variations.

(1) Pressurized water reactors Pressurized water reactors (PWRs) try to prevent water from boiling in the reactor core because, in this reactor design, the formation of steam impedes the removal of heat from the fuel and can result in damage to the reactor fuel. To accomplish this feat, reactor designers take advantage of the fact that, as pressure increases, the boiling temperature increases as well. Attached to the reactor plant is a small tank containing banks of heaters called the pressurizer. These heaters keep the pressurizer hotter than the reactor core and, by so doing, maintain the pressurizer at a high pressure. Since the pressurizer is directly connected to the reactor plant, this same pressure is transmitted to the rest of the water, keeping it high enough to prevent water from boiling in the core. Because water in the core does not boil it cannot be used directly to make steam, and it cannot produce electricity yet. Instead, the hot water from the primary plant (the part of the reactor plant containing all the water that has had direct contact with the reactor core) is pumped through a steam generator, where it passes through a bank of several thousand thin metal tubes. On the other side of these tubes is more water, and as the heat crosses the tubes it causes water in the steam generators to boil at high temperatures and pressures. This steam is then led to turbines where it causes them to spin at high speeds, turning an electrical generator and producing electricity. The major advantage of the PWR is that water passing through the reactor core never comes in contact with the turbines, condensers, and other equipment in the secondary (steam) plant. This greatly reduces the amount of radioactive contamination, which makes maintenance and other work performed on PWRs much easier than would otherwise be the case. In general, PWRs generate relatively small amounts of radioactive waste. In addition, the primary system piping acts as another barrier to help contain radioactive gases formed by nuclear fission, and PWRs tend to emit lower amounts of radioactivity into the environment than would otherwise be the case. On the negative side, PWRs are more complex than some other kinds of reactors because they have two separate systems that must interact in order to generate steam. This means that PWRs tend to have more components that can break, they are more complex to operate, and may be more expensive than comparable plants of other design. However, the PWR is possibly the most mature nuclear power plant design, and more PWRs have been built than any other type of commercial nuclear reactor. In addition, virtually every military nuclear reactor is of this design because of its low contamination levels.


(2) Boiling water reactors As the name suggests, boiling water reactors (BWRs) directly use the heat of nuclear fission to boil water in the reactor core. Unlike PWRs, BWRs are designed to tolerate boiling in the core without core damage. By so doing, BWRs dispense altogether with pressurizers, steam generators, and the separation of the primary and secondary systems, greatly simplifying the nuclear reactor plant. Instead, a single system removes heat from the reactor fuel, boils water directly in the reactor core, and the steam goes to turn the electrical turbine generators. After passing through the turbines, the steam is condensed to form water which is then returned to the reactor to begin the cycle again. The biggest advantage of the BWR is simplicity of design, followed by the lower cost to design and build a less complicated reactor plant. BWRs have the same safety systems as PWRs, and there seems to be no difference in the reliability of the two types of reactor plants, but they are easier and less expensive to design, build, and maintain. The chief disadvantage of BWRs is that, with no separation between the primary and secondary plants, the radioactive contamination is not isolated, so the turbines and piping systems become radioactively contaminated the first time the plant is operated. This means that some radioactive gases are present in the containment building, and contaminated particles are present throughout the turbines, condensers, and steam pipes. This contamination makes maintenance more costly and difficult, and is a distinct drawback.

(3) Liquid metal fast breeder reactors Any fluid can be used to transfer heat from the reactor core to the turbines, and some plants use liquid metals or liquid salt. Liquid metal-cooled reactors generally use liquid sodium to remove the heat of fission and transport this heat to a steam generator in much the same way that water is used for this purpose in a PWR. However, liquid metals do not slow neutrons down as efficiently as water does, so most liquid metal reactors use fast neutrons to fission uranium rather than thermal (or slow) neutrons that are used in water-cooled reactor plants. A breeder reactor is one in which the power-producing part of the core is surrounded by natural uranium. The purpose of this uranium is to capture the fast neutrons that escape from the reactor core. By so doing, the uranium is converted to plutonium through a series of reactions that proceed as follows:

238 239 239 239U n U Np Puβ β− −+ → → + → + The final product, 239Pu is fissionable and can be used for fuel in other nuclear reactors. Hence, a breeder reactor produces not only energy, but fuel for another generation of nuclear reactors as well. Liquid metal fast breeder reactors (LMFBRs) have a number of advantages. The primary one, of course, is that they produce at least as much fuel as they consume, making them a good long-term source of energy. In addition, using liquid metal as a heat transfer medium gives greater thermal efficiency because metals transfer heat more readily and more efficiently than water


does. So a given size of reactor core will produce more electrical energy than will a comparable water-cooled reactor. However, LMFBRs do have some distinct disadvantages. Most important is the fact that the 239Pu made in the reactor can be used to make nuclear weapons, making such reactors and their associated fuel reprocessing and fabrication facilities sensitive targets for terrorists or governments trying to obtain plutonium for illicit purposes. In addition, liquid metals, especially liquid sodium, are reactive and can cause problems in the event of a coolant leak into the steam system. In particular, the sodium-water reaction is highly exothermic, so any leakage of sodium coolant into a steam generator would be catastrophic. Finally, liquid metals must be kept hot at all times or they will solidify. This means that, theoretically, any event that would cause the reactor to scram could lead to the coolant crystallizing in the pipes, causing severe problems later on. In spite of these drawbacks, LMFBRs are considered a safe and reliable way to generate electrical energy and have been put into commercial use in France and Russia, with more on order in Japan and Russia.

(4) Gas cooled reactors Gas-cooled reactors and high-temperature gas cooled reactors (GCR and HTGCRs, respectively) use gas, usually helium, as the cooling medium instead of water. Using a gas to carry away the heat of fission removes the need for many of the expensive safety systems associated with water-cooled reactors and gives a reasonably high thermal efficiency if properly designed. GCRs work in a way analogous to LMFBRs and PWRs in that the cooling medium, in this case helium, is used to transport the heat of fission to a steam generator, where it transfers this heat to water in a heat exchanger, generating steam to turn turbines. One advantage of GCRs is their relative higher thermal efficiency, due to the favorable heat transfer characteristics of helium as compared to water and due also to their relative simplicity of design, which removes many of the power-consuming safety systems required for water-cooled reactors. For example, gas is less viscous than water and flows more readily, so in the event of an emergency, gas will circulate readily through the reactor core, driven simply by the fact that hot gas expands and rises. This means that the elaborate systems designed to pump water through a reactor core in an emergency are not required for a GCR. In addition, many GCRs (the HTGCRs) operate a higher temperatures than do water-cooled reactors, further enhancing their thermal efficiency. There are few negative aspects of this reactor design, other than that they have a shorter operational history than water-cooled reactors, so many of their quirks are not yet as well-known and the technology is not yet as mature as their competitors. However, these drawbacks are relatively minor and are rapidly fading as the world utilities gain more experience building and operating GCRs.

(5) Heavy water reactors Most of the hydrogen on earth (and in the universe, for that matter) consists of a simple proton orbited by an electron. In a small fraction of hydrogen atoms, however, a neutron is bound to the


proton. Although this deuterium has the same chemical properties as ordinary hydrogen, it is twice as massive and has slightly different atomic properties. Water molecules containing deuterium instead of hydrogen are therefore slightly heavier, and such water is called “heavy water”. Since heavy water is somewhat more efficient at slowing down fast neutrons than is light water, a class of reactors has been designed using heavy water for this purpose, and heavy water reactors are the only class of nuclear reactors in which natural uranium can sustain a critical chain reaction, thus producing power. Most heavy water reactors are moderated by heavy water and cooled by light water. This means that there are two separate water systems – the heavy water is contained within the reactor core and its primary purpose is to moderate, or slow down the neutrons from fission. A second light water cooling system then removes the heat of fission from the fuel and transports it to the steam generators for power production. The largest class of heavy water reactors are the CANDU (CANadian Deuterium Uranium) reactors, designed in Canada and forming a major part of the Canadian nuclear energy program. The major advantage of heavy water nuclear reactors is their ability to do away with the need for uranium enrichment to produce reactor fuel. By cutting out this entire step of the nuclear fuel cycle, the cycle as a whole achieves a greater efficiency because of the energy used in uranium enrichment. The primary disadvantages of heavy water reactors are the added expense of heavy water and the added complexity of design necessitated by having two separate systems for cooling and moderating the nuclear reactor. Light water reactors and gas cooled reactors use the same fluid for both purposes, and liquid metal reactors do away with moderation altogether. Requiring two complete systems for these two tasks makes for a more complicated plant. Having said that, it must also be noted that Canada has used such reactors quite successfully for several decades and this does not appear to detract from the efficacy of this design.

(6) Graphite-moderated reactors Light-water cooled graphite moderated reactors (LWGRs) are another reactor design that uses different media for cooling and neutron moderation. In this case, graphite, another super-efficient neutron moderator, serves to slow neutrons down for fission while water acts as the coolant and the rest of the reactor plant is similar to most other reactor designs. This also lets the reactor use natural uranium instead of enriched uranium for fuel. The very first reactors built relied on graphite moderation, as did the Chernobyl nuclear reactor. This does not imply that graphite-moderated reactors are primitive or inherently unsafe, but few of this type of reactor have been built and there is only one on order as of this writing. The primary advantages of the LWGR are its use of natural uranium fuel and that, unlike its fluid-moderated cousins, it cannot lose its moderator due to a leak. This is also its primary disadvantage; in other reactor designs, loss of coolant means loss of moderator, so the reactor shuts itself down and power production stops. However, a GMR can continue operating, even with a total loss of reactor coolant, continuing to produce power. Because of this, an accident in


which reactor coolant is lost can rapidly cause severe damage to a reactor in which uranium fission continues unabated, adding increasing amounts of heat to the fuel.

(7) Pebble bed modular reactors Pebble bed modular reactors (PBMRs) have recently gained much attention as a possible reactor design that is inherently safe, relatively inexpensive, radiologically “clean”, and easy to upgrade as a nation’s energy needs increase. The “pebbles” are spheres of uranium oxide fuel about 6 cm in diameter. However, instead of carefully loading them into fuel assemblies, they are stacked in the reactor with a graphite moderator and the coolant, usually helium, flows around them to remove the heat of fission. They operate at a high temperature, over 1600 C, and the heated helium then expands and passes through a turbine to generate energy. The advantages of the PBMRs are that they promise to be relatively simple to design and build, they produce little radioactive waste, and they are very efficient. In addition, they are designed to be inherently safe in that under even the most dire scenarios imagines, the fuel spheres are designed to remain intact and contain the fuel and fission products. The primary disadvantage to this design is that, as of this writing, only demonstration-sized PBMRs have been constructed. However, there is no reason to believe this technology cannot be scaled up to full commercial sized reactors, and the first commercial PBMRs are expected to go on-line within the next decade. Reactor type

Coolant Temp-erature (C)

Pressure (atm) 1

# in use 2

# on order

GWe generated

Annual Emissions (TBq)

Population dose (person-Sv)

PWR Light water 325 150-155 256 33 167.7 196 14.7 BWR Light water 290 70-75 92 6 61.6 42.8 58 GCR Helium 740 45-50 32 0 9.2 32.7 23.2 LMFBR Sodium 535 1 2 3 0.44 0.57 0.17 HWR Heavy water 310 100-105 43 9 12.4 3940 53.3 LWGR Light water 280 60-65 13 1 7.8 7.86 PBMR Helium 870 0 2 0 3 3

Totals N/A N/A N/A 438 54 259.2 4212 157.2 1 One atmosphere is equal to 14.5 psi, 101.325 kPa, or 760 torr 2 As of July, 2001 3 The first PBMRs are on order, but are not yet completed. Accordingly, the energy output

of these reactors cannot yet be reported. Sources: Nuclear News World List of Nuclear Reactors, March 2001 (Reactor plant statistics) UNSCEAR 2000 Report to the UN General Assembly (emissions) A Guidebook to Nuclear Reactors (operating parameters) Nuclear Engineering, Ronald Knief (operating parameters)

5. Environmental Issues Any discussion of the merits of nuclear power as a source of energy invariably includes a discussion of its environmental impact. On the one hand, anti-nuclear extremists claim that nuclear power plants and their waste are causing irreversible damage to the earth’s environment. On the other hand, pro-nuclear extremists claim that nuclear energy may be the best way to


reduce environmental degradation and even to undo some environmental damage. There is some merit to arguments made by both sides, as well as some fallacies. However, rather than attempt to resolve this debate, this section will simply attempt to lay out the scientific evidence in as unbiased a manner as possible. In particular, we will examine the environmental impacts of uranium mining, nuclear reactor operations, the biological risks of exposure to radiation, and radioactive waste and we will try to compare each of these areas to non-nuclear forms of energy production. With respect to accidents during reactor plant operations, it should suffice to note that, in about 50 years of nuclear reactor plant operations, there has been only one serious accident, Chernobyl, that has released damaging amounts of radiation to the environment, and the circumstances that made this accident so damaging are not likely to be repeated.

a) Uranium mining As noted in Section 3d, uranium is mined in several locations around the world, and uranium mining often results in the generation of large amounts of mine tailings. What was not mentioned is that uranium is a naturally occurring element that is found in, quite literally, virtually every bit of rock and soil on Earth. Uranium ore is simply a rock formation in which the uranium has become concentrated to an unusual degree. However, there is no doubt that mining uranium produces a great deal of waste rock, just as happens when mining copper, molybdenum, gold, and any other metal. With uranium mine tailings, the primary concern is the emission of radon, a radioactive gas, from the mine tailings, and many people worry that the radon can damage the health of nearby residents. For this reason, many US mines are required to take measures to reduce radon emissions from their uranium mine tailings. However, it must also be noted that the radon is rapidly diluted in the air, and are undetectable more than a kilometer or so from the tailings. Another concern with using uranium as a fuel is that it might run out. For this reason, the International Atomic Energy Agency has performed periodic assessments of the economically recoverable uranium reserves and they have determined that they are adequate to meet projected energy requirements for at least the next 50-100 years. By comparison, it is thought that petroleum reserves may run out in 50-60 years, natural gas in 70-90 years, and coal in 250-300 years. Some potential sources of energy, such as solar, wind, geothermal, and hydroelectric energy will not run out, but these forms of energy are likely to remain niche providers for some time for many reasons that are beyond the scope of this chapter. It is possible that new technologies will, at some point, make these options viable for large-scale energy production, just as it is possible that hydrogen fusion will become economically viable, but there is no way today to guess when that day will arrive.

b) Nuclear reactor plant operations Nuclear reactors unavoidably emit radiation and radioactivity when they operate. This radiation and radioactivity enters the environment and results in slight increases in the amount of radiation to which nearby residents and the environment are exposed. This section will address the amount of radioactivity released by nuclear reactors as compared to other methods of energy production and the next section will discuss the biological and environmental effects of this exposure.


It is inevitable that minor amounts of fission products will enter the reactor coolant, even in the absence of defects in the fuel cladding. Trace amounts of uranium are almost always present in the clad, and this uranium can fission too, and the fission products can enter the reactor coolant. In addition, the high levels of neutron radiation in the reactor core cause gases dissolved in the reactor coolant to become radioactive. Then, when the coolant is removed from the reactor plant for any reason, these radioactive gases can escape into the environment. However, this is expected, the potential release points are continually monitored, and the radiation exposure to the public is very small. In fact, in the US, dose to the public is limited to 0.25 mSv annually, which is less one tenth the exposure from natural sources of radiation. According to the United Nations Science Committee on the Effects of Atomic Radiation (UNSCEAR) in their 2000 report to the General Assembly, the total radiation dose from nuclear reactors worldwide is 157.2 person Sv per year and the total amount of radioactivity released by the world’s commercial nuclear reactors is about 4212 TBq per year. Although coal, petroleum, and natural gas are not nuclear, all of these sources of energy contain naturally occurring radioactive materials, and these are released to the environment when they are burned to generate electricity. According to the US Environmental Protection Agency (USEPA), the total radiation dose to members of the US population from these three sources of energy is greater than the dose from nuclear power plants (ref). A summary of these exposures is contained in the following table, and is compared to the total population dose from natural background radiation to the population of the US. As this table shows, nuclear energy exposes people to no more radiation, even on a per-megawatt basis, than do other forms of energy. It is also apparent that nuclear energy releases smaller amounts of greenhouse gases than does burning fossil fuels, and that uranium reserves are likely to last longer than those of other sources of energy currently available. Form of energy

TW produced 1, 2

Total population radiation dose (person-Sv)3, 4

Radiation exposure (person-Sv per TW)

Global reserves (years)1

Greenhouse gas emissions (Gtons of carbon)1, 5

Nuclear 0.60 157.2 262 50-100 0 Coal 2.89 14,161 4900 250-300 638 Petroleum 4.06 6.61 1.63 50-60 110 Natural gas 2.25 ~500 6 ~200 70-90 72 1 From International Energy Energy Annual, DOE 2001 2 1 TW is equal to 1012 watts of energy production 3 By comparison, radiation dose from exposure to natural background radiation is estimated to

be 144 x 106 person-Sv annually (UNSCEAR 2000) across the entire human population 4 Values for radiation dose from fossil fuel plants are calculated from values given in NCRP

Report #95 5 1 Gton is equal to 109 tons, or 1012 kg of carbon emissions 6 The majority of radiation dose from natural gas combustion comes from home use for

cooking and heating; power generation produces very little radiation exposure. The values given are for electrical power generation only; other uses of natural gas yield about 2000


person-Sv per year across the world’s population, and disposal of radium-containing wastes adds a further, unknown dose

c) Health Effects

The effects of exposure to high levels of radiation are well-known and unequivocal. Studies of the survivors of Hiroshima, Nagasaki, workers at Chernobyl, and people involved in accidents involving radiation or radioactivity show conclusively that exposure to high levels of radiation will cause cancer or radiation sickness, depending on the amount of radiation received. In general, after exposure to more than 1 Sv (100 rem) of radiation exposure, people will start to feel ill and their risk of cancer will increase by a factor of 2 or more. After exposure to more than about 4 Sv (400 rem), about half of those exposed will die of radiation sickness, and 100% of those exposed to doses of 9-10 Sv (900 – 1000 rem) will die. What is not known as precisely are the effects of exposure to lower levels of radiation, such as those in the vicinity of operating nuclear power plants. The reason these effects are not as well known is that the effects, if any, are very small so they are very difficult to measure with accuracy. In fact, for every report that concludes that this level of radiation exposure is harmful, there is another report showing there are no effects, and even some reports showing possible beneficial effects from exposure to low levels of radiation. The manner in which we respond to low levels of radiation exposure will likely not be known for many years. Most governments set their radiation regulations under the assumption that all exposure to radiation is potentially harmful, and they further assume that the risk of getting cancer from radiation is directly proportional to the amount of dose received. Called the Linear, No-Threshold (LNT) model, it is the most conservative of the major hypotheses in use because it assumes the greatest risk from radiation. Under the LNT model, a person receiving 1 mSv of radiation exposure per year in excess of background levels will have one chance in 10,000 of developing cancer as a result of that exposure. By comparison, the background cancer incidence in the US is about 1600 in 10,000. Other models, which suggest there may be a threshold level below which radiation exposure is not harmful, also suggest this risk may be even lower. There are a few conclusions that can be drawn from this discussion:

• Nuclear power plants produce less public radiation exposure than fossil fuel plants, so the cancer risk from fossil fuel plants is higher than from nuclear power plants

• Neither fossil fuel plants nor nuclear power plants emit enough radiation to constitute a cancer threat to the public, even under the most conservative scenario

• We all receive more radiation from natural sources (including naturally occurring radioactive potassium in our bodies) than from either nuclear or fossil fuel power plants

d) Radioactive waste

Another area of some controversy is the safe disposal of radioactive waste, in particular, spent reactor fuel. Many anti-nuclear activists are concerned that radioactive waste cannot be safely isolated from the environment and that it poses a long-term risk to the environment and to nearby residents. They are also concerned that spent reactor fuel, which is often highly radioactive, poses a risk to the population during transport and subsequent burial. On the other hand, the nuclear power industry feels it is quite possible to safely sequester both “routine” radioactive


wastes and spent reactor fuel for prolonged periods of time, during which the radioactivity present will decay to safe levels. Those opposed to nuclear energy are particularly concerned about the possibility that radioactive waste disposal sites will leak, letting radioactivity enter groundwater systems, the atmosphere, farms, and so forth, and that this radioactivity will both pollute the environment and cause lasting health effects to nearby residents. In support of their claims, they note the difficulty of designing a landfill to successfully isolate waste for a period of decades, and that many items of radioactive waste remain radioactive for centuries or longer. These arguments gain added strength when discussing the disposition of spent nuclear reactor fuel, which contains plutonium as well as some radioactive elements that remain radioactive for millennia. On the other hand, point out advocates of nuclear energy, virtually all radioactive waste sites are required to have elaborate containment and monitoring systems that will catch any radioactive leakage long before it can become a health concern. In addition, they note that the great majority of radioactive waste has a relatively short half-life and will decay within a few decades or, at most, a few centuries, so that most of the radioactivity that is put into a radioactive waste disposal facility will vanish long before it can escape into the environment. With respect to spent reactor fuel, although it will remain radioactive for thousands of years, the majority of radioactivity is again gone within a few centuries, so the fuel will be dangerous for only a relatively short period of time. Finally, the plutonium found in spent fuel, although dangerous, is no more dangerous (and no more toxic) than many items found in chemistry labs, and does not pose any special hazard. Most nations that operate nuclear power plants have built repositories for both radioactive waste and spent reactor fuel. Virtually all radioactive waste facilities are designed to allow continuous monitoring for leaks as well as environmental monitoring for accidental releases. Most, too, have strict waste acceptance criteria, to ensure that the waste received will remain safely within its package for decades or longer. And, in the case of spent reactor fuel or very highly radioactive waste, many nations have built or are in the process of constructing disposal facilities buried deeply underground in geologically stable areas that fully capable of safely containing the waste for millions of years. Finally, many scientists take some solace in noting that radioactive waste at the Oklo natural nuclear reactor (discovered in 1972) has remained stable for almost 2 billion years, in spite of being located in porous sandstone that has frequently been below the water table.

6. Conclusions For better or for worse, nuclear reactors provide much of the world’s energy needs, and provide an important role in powering naval vessels in several of the world’s most powerful nations. Like any source of energy, nuclear power plants have both good and bad, and we can only hope that the benefits we derive from the continued use of nuclear energy outweigh the negative. In many instances, this seems to be the case, but like any technology, some risks are unavoidable and will always remain. There are several points in favor of continued use of nuclear energy:


1) Nuclear power plants release no more radioactivity (and possibly less) than comparable fossil

fueled power plants 2) Nuclear power plants release far fewer greenhouse gas emissions than do fossil fuel plants 3) The world’s uranium reserves are likely to last for a much longer period of time than fossil

fuel reserves 4) Nuclear energy is more dependable than alternate sources of energy and relies on proven

technology, rather than hoped-for future breakthroughs 5) Nuclear power plants are no more dangerous than are fossil fuel power plants and have, in

fact, suffered fewer accidents than have other sectors of the power industry However, there are also some drawbacks to nuclear energy as a source of power: 1) Nuclear power plants are often more expensive to build than are other types of power plants 2) Spent reactor fuel can provide a source of plutonium for nuclear weapons production 3) Disposal of radioactive waste is neither simple nor inexpensive 4) In some nations, public opposition to nuclear power is organized and vociferous, making

political support of nuclear energy problematic at times Balancing these issues is difficult, even under the best of circumstances, and it is not the intent of this chapter to do so. However, it is reasonable to point out that nuclear power has been in use for over a half century, and (counting military nuclear reactors), nearly 1000 nuclear reactors have operated at one time or another for the past 50 years. In addition to the energy produced by these nuclear reactors, millions have benefited from the isotopes produced in them that are used in medicine and research. Against that, we have the indisputable tragedy of Chernobyl and the expense (even in the absence of risk) of Three Mile Island. It seems likely that nuclear energy can play a safe and important role in meeting the world’s increasing energy needs, in spite of the drawbacks noted above, but this is a decision that must be made by the world’s citizens and their governments. Bibliography: DOE; International Energy Outlook 2001; March 2001 (available on the World Wide Web at

www.eia.doe.gov/oiaf/ieo/index.html) DOE; International Energy Annual 1999; February 2001 (available on the World Wide Web at

www.wia.doe.gov/iea) Eisenbud and Gesell, Environmental Radioactivity from Natural, Industrial, and Military

Sources, Fourth Edition, Academic Press, 1997 Knief, RA; Nuclear Engineering, Second Edition, Hemisphere Publishing Company, 1992 NCRP, Report No. 95, Radiation Exposure of the US Population from Consumer Products and

Miscellaneous Sources, 1987 NCRP, Report No. 93, Ionizing Radiation Exposure of the Population of the United States, 1987 Nero, AV; A Guidebook to Nuclear Reactors, University of California Press, 1979 Nuclear News; Third Annual Reference Issue and World List of Nuclear Power Plants, March

2001


Ohio State University Fact Sheets on Radioactive Waste (found on the World Wide Web at http://ohioline.osu.edu/lines/ennr.html and at http://www.ag.ohio-state.edu/~rer/index.html)

UNSCEAR, Report to the General Assembly, Volumes 1 and 2, United Nations Science Committee on the Effects of Atomic Radiation, 2000

Documents

Published in the Encyclopedia of Life Support Systems, 2001 · Metric system multiples are used to describe large amounts of activity so that a kBq is 1000 Bq, an MBq is 106 Bq, a