Fact Sheet 6

For so-called ‘thermal’ reactors such as the PWR the chain reaction process described in Fact Sheet 2 and suggested by this diagram is, in fact, an over-simplifi cation.

Fission neutrons are emitted at speeds approaching that of light. Unexpectedly, however, they are much more eff ective at causing further fi ssions if they are moving relatively slowly at so-called ‘thermal’ speeds around 2200 metres per second. � ey therefore have to be slowed down (‘moderated’) very considerably between being ‘born’ and before they can go on to cause further fi ssions.

In the PWR, this slowing down is performed by the water surrounding the fuel. � e fi ssion neutrons undergo repeated collisions with hydrogen nuclei in the water and lose energy each time. After perhaps twenty such collisions they are moving suffi ciently slowly to cause further fi ssion.

In this diagram, fi ssion neutrons are ‘born’ in a fi ssion event in the fuel ‘pin’ on the right. One of them is shown undergoing multiple collisions with hydrogen nuclei in the water fl owing upwards past the fuel pins. Finally, when it is moving suffi ciently slowly, the neutron causes a fi ssion in the left-hand fuel pin. In this case two more neutrons are generated to continue the chain reaction process.

� e three essential components of the core of a ‘thermal’ reactor are thus the uranium fuel to generate the heat, the ‘moderator’ to slow the neutrons down between fi ssions, and a coolant to transfer the heat energy from the very hot fuel in the core to the power station turbo-generator. In the case of the PWR reactor system, water performs the functions of both the moderator and the coolant.

� e fuel in some early reactors was natural ‘as mined’ uranium but is now usually enriched uranium. � e moderator can be solid carbon (in the form of graphite) or ordinary ‘light’ water or ‘heavy’ water (see below). � e coolant can be water or carbon dioxide or helium gas. � e possibility of using these materials in diff erent combinations has given rise to the variety of ‘thermal’ neutron reactor types on the market.

HOW DOES A NUCLEAR POWER REACTOR WORK?� ere are many types of nuclear power reactors. Most of them, 265 of the 430 reactors now generating 14% of the world’s electricity, are Pressurised Water Reactors (PWRs). � ere are several types of PWR. Examples are the two 900 megawatt (MW) reactors of French design operating at Koeberg, 28 km north of Cape Town. Reactors to be built in the future in South Africa will be later versions of the same basic type.

� e intention here is to describe how a PWR works as exemplifi ed by Koeberg and then to indicate briefl y how other current reactor types diff er from it.

Nuclear generation is still at an early stage of development. Reactor evolution from the earliest power reactors to those likely to be built into the foreseeable future will be described in NIASA Fact Sheet 11.

FUEL, MODERATOR, COOLANT

As discussed in Fact Sheet 2, energy is released when uranium-235 nuclei contained in the nuclear fuel split. � e reactor is so designed that billions of nuclei split every second in the chain reaction process illustrated below.

In a reactor producing 1000 megawatts of electricity, there are about 100 billion billion fi ssions every second.

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Koeberg Nuclear Power Station, 2 x 900 MW PWRs

Fact Sheet 6In passing through the reactor core, it is heated to around 325ºC but is prevented from boiling, as in a pressure cooker, by the high pressure in the primary circuit.

On leaving the pressure vessel the primary circuit water passes through thousands of inverted U-tubes in the steam generators. Only one U-tube in one steam generator and only one pump are shown in this schematic diagram but there may be two, three (as at Koeberg) or four steam generators depending on the reactor power. In a large reactor, each steam generator, like the reactor pressure vessel, weighs about 500 tons and contains U-tubes with a total length of over 100 km.

� e top of the reactor pressure vessel is removable. It is unbolted and removed every eighteen months or so to allow partial refuelling.

In other types of American-designed PWR the steam generators are taller to accommodate straight rather than U-shaped tubes. In the Russian VVER type the steam generators are horizontal.

� e entire primary circuit is located inside the domed containment building to prevent a release of radioactivity to atmosphere in the event of leakage from the primary circuit.

PRESSURISED WATER REACTORPWR FUEL

� e natural abundance of the isotope uranium-235 in as-mined natural uranium is 0.7%. Essentially all the rest is uranium-238. See NIASA Fact Sheet 2. In PWR fuel the uranium-235 content is artifi cially increased to between 3 and 5%. Black ‘enriched’ uranium dioxide powder is compressed into cylindrical pellets and stacked inside pencil-like metal tubes some four metres long. � ese very long thin fuel ‘pins’ are held together in square bundles known as fuel assemblies.

Each of the 157 Koeberg fuel assemblies contains 464 kilograms of uranium.

Because uranium is only very mildly radioactive, new fuel assemblies are safe to handle. When they are eventually lifted out of the reactor after about four years in operation, however, they are intensely radioactive and have to be stored under several meters of water to protect operating personnel.

Up to 240 fuel assemblies, depending on the reactor power, standing packed together to form an upright cylinder constitute the reactor core. Because of the fi ssion process going on inside the fuel assemblies, the reactor core becomes extremely hot. � e core is shown in red in the following circuit diagram

PRIMARY CIRCUIT

Water fl owing upwards between the pins in the fuel assemblies in the core is driven around the primary reactor coolant circuit, shown in pink, by powerful pumps.

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Circuit Diagram: Courtesy, Virtual Nuclear Tourist

Reactor pressure vessel in transit. Photo curtesy of AREVA.

PWR Fuel Assembly

Fuel pellets.Courtesy of AREVA

Fact Sheet 6

SECONDARY CIRCUIT

Secondary system water (shown in blue in the circuit diagram) is pumped into the base of each of the steam generators and boils on contact with the hot inverted U-tubes.

� e rush of high pressure steam so created drives the turbine to rotate (in most systems at 25 revolutions per second). In turn, the turbine drives the generator to produce the electrical output of the power station.

Having passed through the turbine, the steam is collected and cooled in a massive ‘condenser’ immediately beneath it. � e steam is turned back into water and pumped back into the steam generators to begin the process again.

� e water circulating in the secondary circuit does not come into contact with the reactor.

COOLING WATER (CW) CIRCUIT

A considerable fl ow of cold water is needed to cool the condenser and so condense the secondary circuit steam. For this third coolant circuit (shown in green in the circuit diagram) cooling water may be taken from cooling towers as shown in the diagram, or from a large river, or from the sea. � e two Koeberg reactors take 80 cubic meters from the sea every second via the dredged harbour-like intake structure.

In condensing the steam, the cooling water is heated by about 10ºC before it is discharged back into the ocean just beyond the longer breakwater arm – as shown above.

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REACTOR CONTROL

� e power level in the reactor is controlled by raising or lowering control rods. When the rods are lowered into selected fuel assemblies in the core, boron and other materials contained in them absorb fi ssion neutrons and so slow down or stop the chain reaction process - as required. In an emergency, the control rods drop into the core and stop the reaction automatically.

Slow adjustments to the power level can be made by varying the concentration of boron dissolved in the primary circuit water. This adjustment is needed to compensate for aging of fuel over its four years in the reactor.

AUXILIARY SYSTEMS Many auxiliary systems are provided to support the three principal cooling systems already described. Among other functions, these auxiliary systems provide back-up cooling in the event of emergencies and for when the reactor is shut down for maintenance.

OTHER TYPES OF ‘THERMAL’ REACTORAs discussed, of the 430 power reactors now in operation, 265 are various types of PWR (including Russian designed VVERs). � e basic PWR concept was developed in the USA fi rst for submarine propulsion and then for land-based power generation. � e submarine USS Nautilus and the USSR’s nuclear icebreaker Lenin were launched respectively in 1954 and 1957.

Of other power reactors in the world, 94 are BWRs (boiling water reactors), 44 are PHWRs (Pressurised Heavy Water Reactors) developed in Canada and 18 are British GCRs (Gas Cooled Reactors).

BOILING WATER REACTOR (BWR)

� e BWR concept was also developed in the USA in the 1950’s. It diff ers from the PWR principally in that there are no steam generators. � e primary circuit water boils as it passes up through the fuel assemblies and the rush of high pressure steam drives the turbines directly.

Among other differences, the control rods are driven into the reactor core through the bottom of the pressure vessel rather than entering from the top. Moreover, the ‘pressure suppression’ arrangement for dealing with accidental leakage from the primary circuit make for an entirely diff erent containment building concept.

Koeberg Cooling Water intake structure

Mitsubishi Steam Generator in transit.

Fact Sheet 6GAS COOLED REACTOR (GCR)

� e world’s fi rst commercial nuclear power station was Calder Hall in the UK. Unit 1 (of 4) was commissioned in 1956. � e core in this type of reactor is a massive cylindrical block of carbon in the form known as graphite. � e fuel is stacked vertically in hundreds of holes (‘channels’) drilled down through the carbon core.

� e carbon surrounding the fuel acts as the moderator in this type of reactor. Fission neutrons are slowed to ‘thermal’ speeds by multiple collisions with carbon nuclei (rather than with hydrogen nuclei as in the PWR).

� e coolant is carbon dioxide gas driven up through the fuel channels. � e hot gas is passed through boilers to raise steam to drive the turbo-generators.

� e type of GCR now supplying rather less than 20% of UK electricity is the Advanced Gas Reactor (AGR).

PRESSURISED HEAVY WATER REACTOR (PHWR)

� is system was fi rst developed in Canada. � e moderator is heavy water (1) contained in a large tank called the calandria. � e fuel elements are supported in pressure tubes running horizontally through the calandria. � ey are cooled by an independant fl ow of ‘heavy’ water at high pressure driven through the tubes. As in the PWR, the very hot water so generated is passed through boilers (steam generators) to raise steam to drive the turbo-generator.

An advanced (‘Generation III+’) design known as ACR-1000 will use ‘light’ water as the coolant and slightly enriched uranium fuel.

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Both heavy water and graphite are better moderators than ordinary ‘light’ water. � is made it possible in the early versions of the PHWR (known as ‘CANDU’) and of the GCR (known as ‘Magnox’) to use relatively cheap natural (unenriched) uranium as the reactor fuel. Almost all later reactors have used enriched uranium – for reasons of economy.

(1) Heavy water is a form of water in which hydrogen atoms are replaced by deuterium atoms. Deuterium is an isotope of hydrogen having as its nucleus the single proton characteristic of hydrogen plus a single neutron

FAST BREEDER REACTORSBreeder reactors have operated from the outset. � e fi rst ever nuclear electricity was generated in Idaho in 1951 by EBR-1, Experimental Breeder Reactor – 1. Breeders make far more effi cient use of uranium fuel than do conventional ‘thermal’ reactors. � ey also make it possible to convert the relatively abundant metal thorium into reactor fuel.

Being rather more expensive to build and to operate than today’s thermal reactors, breeder reactors will come into use as the world’s reserves of uranium dwindle probably in the second half of this century.

Several diff erent types of breeder are on today’s drawing boards. � ey and other probable future nuclear developments will be discussed in more detail in NIASA Fact Sheet 11.

Deuterium atom

Nuclear Industry Association of South Africawww.niasa.co.za10 August 2012

Oldbury Advanced Gas Cooled Reactors

Point Lepreau PHWR.Photo Curtesy of New Brunswick Electric Power Commission.