Chapter 04 _ the Conventional Nuclear Power Reactors

8/2/2019 Chapter 04 _ the Conventional Nuclear Power Reactors

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Nuclear Energy: Peaceful Ways of Serving Humanity

Edited by Dr. Mir F. Ali Page: 1

04. The Conventional Nuclear Power Reactors

An overall picture of the worlds nuclear power reactors which, are operating, underconstruction, planned, and proposed, is presented under Figure 4-1. This represents anoverwhelming anticipated increase of 537 (almost 122 percent increase) nuclear power reactors.The number of operating power reactors reported on this graph, does not include over 240research nuclear reactors, which are located in 56 countries around the world.

In spite of the concerns associated with the word, nuclear which appears to signify a possiblerelease of radioactivity in routine or accidental situations, nuclear energy is indeed a well-established component of electricity supply in many of the Organization for EconomicCooperation and Development (OECD) countries for the following reason:

The recognized potential role of nuclear energy in long-term strategies designed toalleviate the risk of global climate change and more generally in sustainabledevelopment policies, is attracting renewed interest from policy makers and the publicaround the world.

This could be the result of the excellent efforts that are being made by interested parties tounderstand risk perception, and communicate with civil society on the issues at stake and


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Nuclear Energy: Peaceful Ways to Serve Humanity


incorporate the public in decision making in an effective way. These activities are essential forthe future of nuclear energy.

1. NUCLEAR POWER REACTORS:The main function of a nuclear reactor is to produce heat. The production of heat in a nuclear

reactor is no different from producing heat by using a boiler in a conventional coal, gas or oil-fired power station.

Whether from a conventional boiler or a nuclear reactor, heat is required to turn water intosteam. This steam is, needed to spin large turbines, which in turn drive generators that produceelectricity. A major difference between a nuclear power station and a conventional fossil-fueledstation is that there is no release of combustion products to the environment from a nuclearstation.

All nuclear reactors operate on the same basic principle, although there are different kinds of

nuclear reactors in use throughout the world. A nuclear reactor creates heat by splittinguranium atoms. This fission of uranium atoms is, called a nuclear reaction.

2. CONVENTIONAL NUCLEAR POWER REACTORS:Generation I nuclear power reactors were, developed in 1950-1960s, and none of those nuclearreactors are in operation today. These reactors refer to the early prototype and power reactors,such as Shippingport, Magnox, Fermi1, and Dresden.A generation II nuclear power reactor is a design classification for nuclear reactors, and refersto the class of commercial reactors built up to the end of the 1990s and this section covers thosenuclear power reactors.Thenuclear power reactorscovered under this category are, classified as Thermal Reactors.

These reactors are composed of fuel (Fission Material) with the following characteristics:

Moderating materials to slow neutrons to low velocities (to prevent capture by U238); Heavy-walled pressure vessels to house reactor components; Shielding to protect personnel; Systems to conduct heat away from the reactor; and Instrumentation for monitoring and controlling the reactors systems.

This type of reactor is mostly used for generating electricity. The first plutonium productionreactors were thermal reactors using graphite as the moderator. Here is a brief description of

each Generation II type based on the World Nuclear Association:2.1 Pressurized Water Reactors (PWR):These reactors were originally designed by Westinghouse Bettis Atomic Power Laboratory for military

ship applications, then by the Westinghouse Nuclear Power Division for commercial applications. The

first commercial PWR plant in the United States was Shippingport, which operated for Duquesne Light

until 1982.
http://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20typeshttp://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20typeshttp://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20typeshttp://www.world-nuclear.org/info/inf32.htmlhttp://www.world-nuclear.org/info/inf32.htmlhttp://www.world-nuclear.org/info/inf32.htmlhttp://www.world-nuclear.org/info/inf32.htmlhttp://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20types


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This is the most common type, with over 230 in use for power generation and several hundredsmore employed for naval propulsion. The design of the PWR originated as a submarine powerplant. PWRs use ordinary water as both coolant and moderator. The design is distinguished byhaving a primary cooling circuit which flows through the core of the reactor under very highpressure, and a secondary circuit in which steam is generated to drive the turbine. A PWR hasfuel assemblies of 200-300 rods each arranged vertically in the core. A large reactor would haveabout 150-250 fuel assemblies with 80-100 tonnes of uranium.

Water in the reactor core reaches about 325C; hence, it must be kept under about 150 timesatmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressurizer asshown in the diagram (Figure: 4-2). In the primary cooling circuit, the water is also the

moderator and if any of it turned to steam the fission reaction would slow down. This negativefeedback effect is one of the safety features of the type. The secondary shutdown systeminvolves adding boron to the primary circuit.


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The secondary circuit is under less pressure and the water here boils in the heat exchangers,which are thus steam generators. The steam drives the turbine to produce electricity and isthen condensed and returned to the heat exchangers in contact with the primary circuit.

2.2 Boiling Water Reactors (BWR):These reactors were originally designed by Allis-Chalmers and General Electric (GE).

The General Electric design has survived, whereas all Allis-Chalmers units are now shutdown.The first GE US commercial plant was at Humboldt Bay (Near Eureka) in California. Othersuppliers of the BWR design worldwide have included ASEA-Atom, Kraftwerk Union, andHitachi. Commercial BWR reactors may be found in Finland, Germany, India, Japan, Mexico,Netherlands, Spain, Sweden, Switzerland and Taiwan. Japan and Taiwan have the newest BWRunits.

The BWRs typically allow bulk boiling of the water in the reactor. The operating temperature of

the reactor is approximately 570F producing steam at a pressure of about 1,000 pounds persquare inch. Current BWRs have electrical outputs of 570 to 1,300 MWe. As this time the PWRdesigns are about 33 percent efficient.

The BWR design has many similarities to the PWR, except that there is only a single circuit inwhich the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in


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the core at about 285C. The reactor is designed to operate with 12-15 percent of the water in thetop part of the core as steam.

The steam passes through drier plates (steam separators) above the core and then directly tothe turbines, which are thus part of the reactor circuit. Since the water around the core of a

reactor is always contaminated with traces of radionuclides, it means that the turbine must beshielded and radiological protection provided during maintenance. The cost of this protectiontends to balance out in the savings due to the simpler design. Most of the radioactivity in thewater is very short-lived (mostly N-16, with a 7 second half-life), so the turbine hall can beentered soon after the reactor is shut down.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in areactor core, holding up to 140 tonnes of uranium. The secondary control system involvesrestricting water flow through the core so that more steam in the top part reduces moderation.

2.3 Pressurized Heavy Water Reactor (PHWR):

The PHWRs have been promoted primarily in Canada and India, with additional commercialreactors operating in South Korea, China, Romania, Pakistan, and Argentina. Canadian-designed PHWRs are, often called, CANDU reactors. Siemens, ABB (now part ofWestinghouse), and Indian firms have also built commercial PHWR reactors.

PHWRs have been popular in several countries because they use less expensive, natural (notenriched) uranium fuels and can be built and operated at competitive costs. The continuous


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refueling process used in PHWRs has raised some proliferation concerns because it is difficultfor international inspectors to monitor. Additionally, the relatively high Pu-239 content of thePHWRs spent fuel has also raised proliferation concerns. The importance of these claims ischallenged by their manufacturers. PHWRs, like most reactors, can use fuels other thanuranium and the ACR series of reactors is intended to use slightly enriched fuels. Particular

interest has been shown in India in thorium-based fuel cycles.

Heavy water reactors now in commercial operation use heavy water as moderators andcoolants. The Canadian firm, the Atomic Energy of Canada Limited (AECL), has also recentlyproposed a modified PHWR (the ACR series) which would only use heavy water as a moderator.Light water would cool these reactors. No successful effort has been made to licensecommercial PHWRs in the United States.

CANDU-specific features and advantages include: Use of natural uranium as a fuel:

oCANDU is the most efficient of all reactors in using uranium: it uses about 15% lessuranium than a pressurized water reactor for each megawatt of electricity produced;

o Use of natural uranium widens the source of supply and makes fuel fabricationeasier. Most countries can manufacture the relatively inexpensive fuel;

o There is no need for uranium enrichment facility;o Fuel reprocessing is not needed so costs, facilities and waste disposal associated with

reprocessing are avoided; ando CANDU reactors can be fueled with a number of other low-fissile content fuels,

including spent fuel from light water reactors. This reduces dependency on uraniumin the event of future supply shortages and price increases.

Use of heavy water as a moderator:o Heavy water (deuterium oxide) is highly efficient because of its low neutron

absorption and affords the highest neutron economy of all commercial reactorsystems. As a result chain reaction in the reactor is less possible with naturaluranium fuel; and

o Heavy water used in CANDU reactors is readily available. It can be, produced locally,using proven technology. Heavy water lasts beyond the life of the plant and can bere-used.

CANDU reactor core design:o Reactor core comprising small diameter fuel channels rather that one large pressure

vessel;

o Allows on-power refueling extremely high capability factors are possible;o The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the

fuel in the reactor core; ando Extends life expectancy of the reactor because major core components like fuel

channels are accessible for repairs when needed.

2.4 Advanced Gas-Cooled Reactors (AGR):


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These are the second generation of British gas-cooled reactors, using graphite moderator andcarbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5 percent, instainless steel tubes. The carbon dioxide circulates through the core, reaching 650C and thenpast steam generator tubes outside it, but still inside the concrete and steel pressure vessel.Control rods penetrate the moderator and a secondary shutdown system involves injecting

nitrogen to the coolant.

The AGR was developed from the Magnox reactor, is graphite moderated and CO2 cooled, and

two of these are still operating in UK. They use natural uranium fuel in metal form.

The newer Advanced Gas Cooled (AGR) Reactors use a slightly enriched uranium dioxide cladwith stainless steel. Carbon dioxide is the coolant gas used. Two key advantages of this designare:

Higher operating temperature with a higher thermal efficiency; and Not susceptible to accidents of the type possible with water cooled/moderated reactors.


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2.5 Light Water Graphite Moderated Reactor (RBMK):In 2003, several of these reactors were still operating in the Soviet Union, but there were noplans to build any more, and there is international pressure to close those that remain. TheRBMK was the culmination of the Soviet program to produce a water-cooled power reactorbased on their graphite-moderated plutonium production reactors. The first of these, AM-1 (For

Atom Mirny, Russian for peaceful atom) was designed to produce 5MWe (30MW thermal)and delivered power to Obninsk from 1954 until 1959.

18Ordinary (light) water absorbs neutrons readily and so removing water from the core (such ashappens when it boils and is replaced by steam), tends to increase the rate at which the nuclearreaction proceeds. In a water-moderated reactor this effect is countered by the reduction inmoderation, but in the RBMK, the moderating effect of the water is small compared to that ofthe graphite, so the overall effect is positive. This is called a positive void coefficient. TheRBMK as designed also had a positive power coefficient, meaning that an increase in reactor

power tends to further increase the rate of reaction. Large positive void and power coefficientscan produce runaway conditions and have not been permitted in other reactor designs;however, it was not possible to eliminate them from the RBMK if natural uranium fuel was tobe used.


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The RBMK was also intended to use recycled uranium from reprocessed PWR fuel which has alow remaining enrichment. In this configuration it was also unstable. These characteristicsbrought the RBMK to the worlds notice in 1986, when one of the four RBMK reactors atChernobyl exploded in the worst civilian nuclear accident to date.

Since that accident remaining RBMK have been operated with a reduced number of fuelelements containing more highly enriched fuel, enabling them to operate relatively safely butdefeating the original concept. Control systems have also been improved, in particular toeliminate the graphite tips on the control rods, which produced an immediate increase inpower when the rods were first inserted. This design feature is blamed for triggering the firstactual explosion when the emergency shutdown button was pressed in an attempt to shutdown the already out of control reactor during the Chernobyl disaster.

This chapter was published on Inuitech Intuitech Technologies for Sustainability on January18, 2011:http://intuitech.biz/?p=7840

Resources:

1. Nuclear Technology Exploring Possibilities Major Reactor Types:http://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20types

2. Canadian Nuclear Association Major Reactor Types:http://www.cna.ca/curriculum/cna_nuc_tech/reactor_types-eng.asp?bc=major%20reactor%20types&pid=major%20reactor%20types

3. World Nuclear Association Nuclear Power Reactors:http://www.world-nuclear.org/info/inf32.html
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Chapter 04 _ the Conventional Nuclear Power Reactors