Uranium is the fuel for all commercial power reactors. Uranium ores are mined, then refined and processed to produce uranium-oxide powder, that is pressed and fired to form hard, insoluble ceramic pellets about 1 cm long by 1 cm diameter. These are sealed into metal tubes which are assembled into bundles ready for insertion in the reactors.
Uranium is a very concentrated source of energy. A single fuel bundle for the Canadian design of power reactors, known as CANDU (CANada Deuterium Uranium), is 50 cm long by 10 cm diameter and weighs only 23 kg. Thus it could be carried in an overnight bag, but in a CANDU reactor it can produce as much heat as burning 400 tonnes of coal or 1700 barrels of oil. This heat produces enough electricity to supply more than 100 average homes for a year.
Twelve of these bundles are placed end-to-end in each pressure tube of the reactor, a tube 10 cm diameter that also contains flowing water to cool the fuel and remove the heat. The water is at nearly 300C and so develops a pressure of about one hundred times the normal atmospheric pressure. The tube is made strong enough to contain this pressure, hence its name.
Fission, and hence heating, can occur in natural uranium only if three conditions are simultaneously satisfied:
These requirements can contribute to the safety of the reactor. If it were to be seriously damaged, either accidentally or deliberately, one or more of these requirements would probably be affected, hence shutting off the fission process automatically.
The most efficient moderator is heavy water (deuterium oxide), which is used in CANDU reactors. By using this highly efficient moderator CANDU reactors are able to "burn" natural uranium without any special treatment (enrichment) beyond normal refining and fabrication.
For all practical purposes, nuclear energy is produced as heat. In a conventional generating station, combustion of coal, oil or natural gas produces heat that boils water into steam that turns a turbine that drives a generator. In the nuclear plant, heat from a nuclear reactor boils the water, and the rest is the same as the conventional plant.
In practice, a CANDU reactor consists of a large tank of heavy water (the calandria ), penetrated by several hundred fuel channels. Cooling water from the pressure tubes within the fuel channels (heavy water like the moderator) is taken to the steam generator ( boiler ), which produces steam in a separate circuit to drive the turbines that generate electricity as in other generating plants.
To change fuel that has delivered its energy, after one to two years in the reactor, two fuelling machines (Sketch a) connect to the pressure tube, one at each end. Fresh fuel bundles from one machine are pushed into the tube, forcing out used fuel bundles into the other machine which, when disconnected, removes these bundles to storage.
CANDU fuelling machines are simple in principle but highly complex in practice. The machines can be thought of as very large six-shooter revolvers. The one at the loading end has pairs of fuel bundles in each chamber of the cylinder (generally more than six) except one that is empty: the one at the receiving end has all its chambers empty originally. The muzzles of the two "revolvers" are clamped to opposite end-fittings of the pressure tube to be refuelled. The end-plugs in the end-fittings are then unplugged and withdrawn into empty chambers. The cylinders of both machines are rotated to bring other chambers into line with the pressure tube. The new fuel bundles are pushed into the pressure tube, thereby pushing used fuel bundles into the receiving machine. And so on until all the new bundles are loaded. The end-plugs are reinserted and the machines disconnected from the end-fittings. The machine with used fuel is subsequently disconnected from the reactor face for discharge of these bundles to underwater storage.
All these operations have to be performed remotely by operators outside the thick shielding surrounding the reactor. Also, the operations must be performed while maintaining coolant flow over the fuel and without leakage of expensive heavy-water coolant at high temperature and pressure. There was considerable scepticism that they would work as intended until the Nuclear Power Demonstration Reactor was brought into operation in 1962. They are key components of all CANDU reactors in that they permit on-power refuelling, a feature not available in other power-reactor designs. This, in its turn gives CANDU the advantage of an increased capacity factor and improved fuel utilization, as well as contributing to the safety of CANDU reactors.
To control the power level of the reactor control rods (Sketch b) are driven into or out of the calandria, in much the same way as the accelerator is used to control the speed of a car. Control rods contain materials that absorb neutrons without producing new ones. Thus the further they are inserted into the calandria the more neutrons they absorb and the fewer remain to continue the chain reaction. For the reactor, unlike the car, the control rods alone can bring things to a stop, i.e., shut down the reactor.
To provide extra protection if it is ever needed there are two independent shutdown systems (Sketch c), each capable of shutting down the reactor quickly. These can be compared to two independent braking systems in a car, if it is remembered that the shutdown systems, unlike brakes, are neither needed nor used in normal operation; they are there only to take care of unplanned events. One shutdown system consists of vertical rods, similar to control rods but faster acting: the other consists of horizontal tubes, permanently in the calandria, through which a liquid containing neutron absorbers can be squirted into the moderator rapidly.
The fuel in an operating reactor (and even when it is discharged) is highly radioactive, i.e., it emits radiation similar to medical X-rays. To protect the plant operators from this radiation the reactor core (the calandria and all it contains) is surrounded by heavy shielding, typically reinforced concrete about one metre thick. To protect the public against the possibility of radioactive releases that might occur in the event of an accident, the whole reactor and its heat transport system are located within a sealed containment, a massive reinforced-concrete building. To provide further protection, there are no dwellings allowed in the immediately surrounding area of about one kilometre radius, the exclusion zone.
The used fuel consists of over 98 percent of the original uranium, still unconsumed, about 0.8 per cent of the fission products that cause most of the radiation in the relatively short term, i.e., in the first few hundred years, and about 0.4 per cent of plutonium and very small amounts of other transuranics that together cause most of the radiation in the longer term. Thus the used fuel contains much uranium and plutonium that could be used to make new fuel, but the presence of the highly radioactive fission products and transuranics prevents this being done easily.
If fuel continues to be consumed in the present manner, once through then treated as waste, 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 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. Fuel recycling would involve dissolving the used fuel, removing the true wastes and fabricating what remains into fresh fuel. Doing this would roughly double the amount of energy obtainable from the original uranium, but would still leave most of it unconsumed.
Internationally, the best known application of recycling is in the liquid-metal fast breeder reactor, a radically different design that has been demonstrated to be technically feasible but that is not 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, its essential characteristic is that it consumes much less uranium than current reactors. This approach extends the availability of known uranium to hundreds of years.
Another, simpler approach to achieving a similar extension of resources is available to users of CANDU reactors. The use of heavy water as moderator makes these reactors very efficient in their fuel consumption. If this characteristic were to be combined with the unique nuclear properties of thorium as a fertile material, much of the advantage of the fast breeder reactor could be obtained: it would not produce quite as much fissile material as it consumed but it would greatly increase the amount of fertile material consumed and hence the energy produced. Thorium is an element similar to uranium, but contains no fissile isotope, and is often found in association with it. Globally, it is more abundant than uranium. The feasibility of using thorium-based fuels in CANDU reactors has been demonstrated on an experimental scale. Thus, as uranium becomes scarcer and more expensive, utilities with CANDU reactors can accommodate by recycling the fuel and modifying the fuel feed without having to develop a complex new technology, as expected to be necessary for those with light water reactors (see Chapter 4 ).
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