New Nuclear Reactors

There are very many types of nuclear reactors, both actual and potential, and continual studies are in progress to design reactors that are safer and more efficient, reliable and economical. Reactors may be classified according to their purpose, their fuel and moderator and the energy of the neutrons when they cause another fission. A reactor may be built for experimental purposes, or to produce fissile material or to generate useful power. The fuel can be uranium, or uranium enriched with a higher proportion of fissile uranium isotopes, or with plutonium or thorium. As already mentioned, the neutrons in the reactor will cause fission only if they are slowed down, and this is done by including in the reactor a moderator. The moderator can be any material that slows the neutrons down efficiently by elastic collisions without capturing too many of them. The lighter the material the more efficiently it slows the neutrons, but many of the lighter nuclei are excluded because they readily capture neutrons. This leaves ordinary water, deuterium in the form of heavy water and graphite as the preferred choices. Reactors with a moderator are called thermal reactors. Fermi's first experimental reactor used natural uranium as fuel and graphite as a moderator because at that time heavy water was too expensive and ordinary water captures too many neutrons. Subsequently, large reactors were built to produce the fissile plutonium 239, which is formed from the non-fissile uranium 238 by neutron capture followed by the successive emission of two electrons. The plutonium differs chemically from uranium, and may therefore be separated from the uranium by relatively easy chemical methods.

It is not possible to build a reactor with natural uranium and ordinary water because the hydrogen nuclei in water combine with neutrons to form deuterons. Too many neutrons are absorbed in this way and the chain reaction cannot develop. If however the uranium is enriched to about 3% fissile uranium 235 or with plutonium 239 a chain reaction can occur with ordinary water as a moderator. These are the light water reactors that are widely used to generate nuclear power.

If the uranium is enriched to 10%, a moderator is not needed. Such reactors are called fast reactors and they can be designed to produce more fissile material than they consume, and hence are referred to as breeders. The main fuel is plutonium 239 and sufficient fissions occur to sustain the chain reaction and also to convert some of the uranium 238 into plutonium. One type of fast reactor contains 80% uranium 238 and 20% plutonium 239 in the interior, surrounded by a blanket of uranium 238. About three neutrons are emitted when plutonium 239 fissions, and of these one is required to sustain the chain reaction, leaving two to account for losses and to breed more plutonium 239. Another type of fast reactor uses a blanket of thorium 232, which is converted into the fissile uranium 233.The spent fuel rods from all these reactors are sent to a reprocessing plant such as the Thermal Oxide Reprocessing Plant (THORP) at Sellafield to extract the fissile material in the form of uranium-plutonium mixed oxide (MOX) reactor fuel. Japan has a large reprocessing plant at Rikkahao that is planned to reprocess about 430 tonnes of spent nuclear fuel to produce 2.3 tonnes of plutonium. This will be made into MOX fuel for use in their light water reactors (Nuclear Issues, January 2006).

The power density in a fast reactor is substantially higher than in a normal power reactor, so liquid sodium, which cools effectively without moderating the neutrons, is used to remove the heat. Prototype fast reactors have operated for a number of years in several countries. Their great advantage is that they enable over 50% of the uranium to be used to generate power instead of the 0.7% of natural uranium. They can either produce or burn plutonium, depending on the neutron density, and can also be used to burn nuclear wastes.

Fast reactors have a strong negative temperature coefficient and so are inherently safe. Due to the low level of corrosion and the ease with which components can be replaced they are likely to have a longer life than thermal reactors, possibly up to seventy years. They also conserve raw material and reduce the quantity of waste products.

Since fast reactors can burn the uranium 238 remaining in the spent fuel rods from thermal reactors, there is a vast store of energy waiting to be used. It is estimated that the depleted uranium now stored in Britain contains the energy equivalent of all the 500 years' supply now in our reserves. Fast reactors are also the most efficient way to use the plutonium and highly enriched uranium from the decommissioning of nuclear warheads. They cost more to build than thermal reactors, and so at present it is uneconomic to build them because uranium is plentiful. Ultimately it will become too costly to extract all the uranium we need for thermal reactors and then we can turn to fast reactors. It is thus unwise to abandon the development of fast reactors.

Nevertheless many countries, including the USA, Britain and France, have severely curtailed or cancelled their fast reactor programmes. Germany's fast reactor at Kalkar, costing £5B, was completed but never allowed to operate, a victim of politics. The 1200 MWe French fast reactor Super Phenix at Creys-Malville started to operate in 1985, but was subsequently closed down by the then minister for the environment, the leader of the Green Party in the French Parliament. Like so many political decisions, such actions may make sense in the short term, but in the long term may prove disastrous. When fast reactors are needed, they will not be able to compete with countries like Japan that are still developing them. India has no option but to accelerate its development of fast reactors to use the thorium that is plentiful there. It has had 13 MW fast reactors operating since 1985, and plans to build a prototype 500 MW fast reactor. China is building an experimental fast reactor of 25 MWe, to be followed by one of 600 MWe (Nuclear Issues 27, August 2005). The French have now decided to build both a sodium-cooled and a gas-cooled fast reactor to be ready for commercial use by 2035-2040. Russia has a 600 MW fast reactor at Beloyarsk and is building another of 800 MW (Nuclear Issues 28, December 2007).

Existing reactors are continually being improved and several new types of reactors are being studied. In the present thermal reactors the spent fuel rods still contain about 96% of the original uranium together with 1% plutonium, as well as the unwanted fission products. The utilisation of uranium can be improved by using uranium-plutonium mixed oxide (MOX) fuels. In France 1150 tonnes of spent fuel is being reprocessed in this way and converted into stable oxide form as a strategic reserve. One tonne of plutonium in the form of MOX fuel has an energy equivalent of two million tonnes of coal. This fuel will be used in the next generation of boiling water and pressurised water reactors.

Reactors with uranium as fuel and heavy water as the moderator were first built in Canada, and developed to form the CANDU power reactors. They are very reliable and economical in fuel since the moderator captures very few neutrons. The neutrons which are captured by the uranium 238 convert it to the fissile plutonium 239, and the fuel burn-up is nearly compensated by the plutonium 239 production, thus increasing time of operation before the fission fragments have to be removed. It is also possible to use a mixture of uranium 235 and thorium 232, and then the neutrons captured by the thorium 232 convert it to uranium 233 which is a more efficient fissile material than either uranium 235 or plutonium 239. The uranium 233 emits more neutrons per fission and makes it possible to use over 90% of the fissile material in the reactor.

Among the new reactors is the AP1000 already approved by the US Nuclear Regulatory Commission. This is essentially an improved version of the very successful pressurised water reactors (PWR) already operating in many countries. The improvements include a high degree of passive operation, using natural effects such as air circulation and gravity feed of cooling water, and a simplified design with fewer valves, pumps and pipes, and ventilation and cooling units. It occupies far less space and produces only 10% of the waste compared with the older plants (Nuclear Issues 26, September 2004). These reactors cost $US 1400 per kW (Speakers' Corner, October 2001).

Plans for the future include systems with self-sustaining fuel cycles such as sodium metal-cooled fast reactors, very high temperature reactors, supercritical water-cooled reactors, lead alloy-cooled reactors, gas cooled fast reactors, and molten-salt reactors. Some of the more promising of these projects may become available around 2030 (Goddard 2006).

Plans are also being made for other types of reactors, such as the pebble-bed reactor developed in South Africa following a German design. This is a relatively cheap low-power reactor that is inherently very safe. A Chinese energy consortium plans to build a 195 MW gas-cooled reactor that could be operating by 2010. This would put China in the leading place in a technology that could offer a new meltdown-proof alternative to water-cooled nuclear power stations. The world's only test pebble bed reactor is operating in the Institute of Nuclear and New Energy Technology in Beijing and provides the new technology for the planned reactors. In the United States, Westinghouse is considering leading a consortium to build a $500m PBMR in Idaho.

Another possibility is the accelerator-driven reactor that consumes its own waste, as discussed in the previous section. Detailed studies of its feasibility and cost are being made in Switzerland and in Los Alamos in the USA.

The continuing operation of nuclear reactors depends on the supply of uranium. The present economically recoverable uranium amounts to about 3.1 million tonnes and at the current rate of use amounts to about 70,000 tonnes per year, so that present resources are enough to about 44 years. It is estimated that highly probable deposits amount to at least four times that amount. Furthermore there are large deposits of uranium that are at present not economically recoverable, but can be used if needed. Doubling the price increases the recoverable reserves by a factor of about four. Introducing the fast reactor increases the uranium reserve by a factor of about sixty. Since the cost of fuel is only about 10% of the total running costs of a reactor, an increase in the cost of uranium would not appreciably affect the cost of nuclear power. There are also large amounts of uranium in granite (4 ppm) and seawater (0.003 ppm) contains about 4.5. billion tons.

The amount of uranium in sea water is so large that it is virtually inexhaustible, but it is not practicable to extract it (Norman, Worrall and Hesketh 2007). A technique for extracting uranium from seawater has been developed in Japan, at a cost of $100 per kilogram compared with the current cost of $20 per kilogram. The use of uranium obtained in this way would not appreciably affect the cost of nuclear power, but there are formidable difficulties in making the large amounts needed for a reactor programme. The Japanese method required 350 kg of absorbent material and collected 1.6 kg of uranium per year whereas a 1 GW reactor requires 160,000 kg per year. In addition even larger amounts of other salts in seawater would be produced. It is therefore doubtful if it is practicable to scale up the process to give the quantity of uranium needed.

In addition to uranium, there are four times more reserves of thorium. Although it is not itself fissile it can be converted in a reactor to fissile uranium 233. The Indian nuclear power programme plans to use the thorium-rich monazite sands in Kerala. They hope to have the first reactor using uranium-plutonium fuel with a blanket of thorium to breed uranium 233 in operation by 2010 (Nuclear Issues 26, January 2006; Ibid. 29, November 2007). Since all the thorium can be burned the total available energy is comparable to that obtainable from fast reactors (Mackay 2008, p. 106).

There is thus no danger of the development of nuclear power being curtailed by shortage of uranium or thorium in the foreseeable future.

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