Nuclear Power Plants | The Canadian Encyclopedia


Nuclear Power Plants

Nuclear Power Plants generate electricity from nuclear energy.

Nuclear Power Plants generate electricity from nuclear energy. As in all thermal-electric generating stations (see Electric-Power Generation), heat is used to boil water into steam, which turns a turbine and drives a generator, producing electricity. A conventional thermal generating station obtains heat by burning coal or other fuels; a nuclear power plant obtains heat from the fission of nuclear fuel in a nuclear reactor. There are many ways of applying the basic principles of fission to the design of actual reactors. In Canada, a unique design known as the Canada Deuterium-Uranium (CANDU) is used. Other countries employ various other designs. A number of auxiliary plants also are needed for nuclear-power generation. All current power reactors use uranium as fuel. Uranium is relatively abundant, being present in most rocks and soils and in the oceans. Currently ores that contain about 0.1% uranium by weight, or greater, are economic to mine. After mining and milling, the uranium is a yellow powder (yellowcake); after further chemical treatment, it becomes black (uranium dioxide).

CANDU Reactors

To make fuel pellets for a CANDU reactor, the uranium dioxide is compressed, then baked at high temperatures to yield hard, insoluble, ceramic cylinders about 14 mm in diameter by 20 mm long. To make one fuel element, a 500 mm long stack of pellets is loaded into a metal tube (made of the zirconium alloy, Zircaloy), which is sealed at each end by welding. For present CANDU reactors 37 elements are assembled by further welding to form a fuel bundle, with individual elements held apart from each other. This fuel bundle is the first basic building block for the reactor. Uranium is a very concentrated energy source. A fuel bundle 500 mm long, 100 mm in diameter and weighing 22 kg could be carried in an overnight bag. When put in a CANDU reactor, it can produce as much energy as burning about 400 t of coal or 2000 barrels of oil.

In the reactor, 12 bundles are placed end-to-end in a tube through which water coolant is pumped. Since the water is at nearly 300°C, it develops a pressure of about 100 atmospheres; the tubes are therefore known as pressure tubes. Each pressure tube, with its contained fuel and coolant and with end fittings to get the coolant in and out, constitutes a fuel channel, the next larger building block for a CANDU reactor. The reactor core consists of several hundred fuel channels positioned in a carefully calculated grid and passing horizontally through a tank, or calandria, containing heavy water as a moderator. Heavy water is a compound of hydrogen and oxygen, having a higher proportion of the heavy hydrogen isotope deuterium than does natural water. The presence of the heavy water and the particular arrangement of channels are essential for fission to occur in the uranium. This arrangement contributes to the safety of the reactor: if the reactor were to be seriously damaged one or both of these conditions would probably be affected and the fission process would stop automatically. This is an example of what is known as a fail-safe feature.

The coolant from the fuel channels is piped to steam generators, where the heat from the fuel is used to boil water in a secondary circuit. The resulting steam drives the turbine and turns the generator to produce electricity. The reactor coolant, now at a lower temperature, circulates back to the reactor in the closed primary circuit.

When a fuel bundle has to be replaced (after about a year and a half in the reactor), remotely controlled fuelling machines are clamped to each end of its fuel channel. Fresh fuel is pushed in from one end and the used fuel is deposited in the machine at the other end. A used fuel bundle, which looks much the same as a fresh one, retains all its wastes sealed within it. Used bundles are stored in a water-filled tank, like an extra-deep swimming pool, in a building adjacent to the reactor. The water cools the bundles and absorbs the radiation they emit. The ability to change fuel without having to shut down the reactor makes the CANDU design unique among current commercial reactors, and contributes to their exceptionally high capacity factors, ie, the electricity actually generated during some period, expressed as a percentage of what is theoretically possible.

To control the power level of the reactor, control rods are moved into or out of the reactor core. They are contained in tubes which penetrate the top of the calandria and pass between fuel channels. A reactor control system is used much as is an accelerator in controlling the speed of a car. However, unlike the car's accelerator, the control rods in the reactor can also bring things to a stop, ie, shut down the chain reaction. In addition to control rods there are 2 independent systems, each capable of shutting down the reactor quickly. These can be compared to 2 independent braking systems in a car, although the shutdown systems, unlike brakes, are neither needed nor used in normal operation. They are called upon only if some other system fails.One type consists of rods similar to control rods but capable of being inserted into the reactor core more rapidly; the other consists of perforated horizontal tubes in the calandria through which a liquid can be squirted into the heavy-water moderator. Control rods and shutdown systems both work by introducing into the reactor materials (eg, cadmium or gadolinium) that absorb neutrons strongly. Adding absorbers slows down, then stops the fission chain reaction; withdrawing them allows the reaction to start up again.

The fuel in an operating reactor (and even when discharged) is highly radioactive, ie, it emits gamma radiation similar to medical X rays. To protect the station operators, the reactor core is surrounded by heavy shielding, typically of reinforced concrete about 1 m thick. To protect the public against the possibility of radioactive releases which might occur in the event of an accident, the whole reactor and its primary coolant circuit are located within a sealed containment building, a massive concrete structure. No dwellings are allowed within a radius of about 1 km; thus any escaping radioactive material would be diluted and dispersed before reaching the public.

Other Commercial Reactors

CANDU reactors are moderated and cooled by heavy water; the moderator and coolant are in separate circuits. Another general type of power reactor, known as a light-water reactor, uses ordinary or "light" water for moderator and coolant, without any separation. All the fuel is immersed in water under pressure, contained in a single large pressure vessel. Since light water is not a good enough moderator to sustain a fission chain reaction in natural uranium, the uranium fuel for light-water reactors has to be artificially enriched in uranium-235. The light-water reactor, first developed in the US, has 2 subtypes: the pressurized-water reactor and the boiling-water reactor. In the first, the cooling water in the pressure vessel is maintained at a high enough pressure to prevent boiling. Thus, just as in the CANDU design, steam for the turbines is produced in a secondary circuit with heat being transferred from the primary to the secondary circuit in steam generators. In the boiling-water reactor the coolant is under less pressure, so that boiling occurs. After separation from entrained water, the steam passes directly to the turbine. This procedure has the benefit of eliminating the cost and temperature drop associated with the steam generator, but the presence of radioactive coolant in the turbine makes maintenance more difficult.

Another type of power reactor, originally developed in the UK and France, uses graphite as moderator and a gas as coolant, hence the term gas-graphite reactor. The earliest of these used uranium metal contained in magnesium-alloy cans as fuel and carbon dioxide as coolant. The UK's design is called the Magnox Reactor after the particular magnesium alloy developed for the fuel cans. Graphite, being intermediate between light and heavy water as a moderator, enables natural, unenriched uranium to be used. This type is no longer competitive. In the UK it has been superseded by the advanced gas-cooled reactor which uses graphite and carbon dioxide but, by changing the fuel to uranium dioxide in stainless-steel cans, is able to take the coolant to higher temperatures. This system gives a higher thermal efficiency, ie, more electricity can be obtained from the same amount of heat. There is insufficient operating experience with advanced gas-cooled reactors to assess how well they will compete with established water-cooled reactors.

Several countries are investigating a high-temperature gas-cooled reactor that promises even higher temperatures. Here the carbon-dioxide coolant is replaced by noncorrosive gaseous helium, and the fuel consists of myriads of tiny particles of uranium carbide individually coated with graphite and embedded in a graphite block or sphere. The concept is technically attractive but, in the absence of any full-size commercial plants, the economics are largely unknown.

The former USSR developed 2 types of reactor for use in central power stations: the VVER (pressure-vessel-type water-water reactor) and the RBMK (channel-type water-graphite boiling reactor). The VVER is very similar to the US design of pressurized-water reactor; the RBMK is a unique design. It has hundreds of fuel channels generally similar to those in the CANDU, but these are in a moderator of hot graphite, not heavy water.

Fuel Recycling

All currently commercial nuclear-power reactors consume only about 1% of the uranium fed to them. As long as uranium is relatively abundant and cheap, the present procedure, the "once through" fuel cycle, which involves storing the used fuel discharged from the reactor, is the simplest and most economic. Used in this way, the world's known resources of economically recoverable uranium have an energy content comparable to the world's recoverable resources of conventional oil. When the richer uranium ores have been exploited and leaner ores have to be mined, it may make economic sense to recycle the used fuel to obtain more of the energy potentially available. Recycling would involve dissolving the used fuel, removing the true wastes (about 1% of the total fuel weight) and fabricating the residues into fresh fuel for reactors. Fuel recycling is an essential component of any proposal to extract appreciably more energy from our nuclear-fuel resources.

The best-known application for fuel recycling is in the liquid-metal fast breeder reactor, a radically different design that is not yet available commercially. "Liquid-metal" refers to the coolant, usually a molten alloy of sodium and potassium. "Fast" refers to the speed of the neutrons in the reactor core. Since fast reactors do not incorporate a moderator, the neutrons are not slowed down much from their speed at birth in the fission process. "Breeder" refers to the fact that more fissile material is bred from fertile material than is consumed by fission. Often, and misleadingly, this type of reactor is said to produce more fuel than it consumes. However, the essential characteristic of this reactor type is that it consumes much less nuclear fuel (normally uranium) than current reactors. Thus the cost of the electricity produced is largely independent of the cost of the uranium.

Fuel recycling would greatly extend the world's nuclear fuel resources for 2 reasons. Since electric utilities could afford to pay a higher price for uranium, mining leaner ores would become feasible and much more uranium could be economically recovered. For any particular amount of uranium mined, a larger fraction would be consumed and converted to energy. Together, these factors mean that nuclear-fission energy, with fuel recycling, becomes a nearly inexhaustible energy source. In Canada, the same principle of largely decoupling electricity costs from fuel costs is possible in the existing and commercially proven design of CANDU reactors, by exploiting fuel recycling and switching from uranium to thorium (another naturally occurring nuclear fuel) as feed. Thus, Canada could be assured of the same indefinite supply of energy without having to introduce a new reactor type.

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