Energy and Environment

The Pressurised Water Reactor (PWR)

A number of different civil nuclear reactor designs have been developed in the last half century but, currently, one type, the Pressurised Water Reactor (PWR) is dominant and any new commercially viable British civil nuclear plant to be built for the next two decades will be of this type. The PWR consists of a reactor pressure vessel, typically some 3 – 4 metres in diameter and 10 – 12 metres high. The top part of this vessel consists of a head which may be removed to load and reload the fuel assemblies. The control rods are operated though sleeves running through the head. The fuel assemblies contain pellets of uranium dioxide (UO2), enriched to about 3% to 5% U235 and the total mass of uranium oxide is about 100 tonnes for a 1GW(e) power plant. Water at high pressure (~14,000 kN/m2) is pumped though the pressure vessel and the reactor core and then forced through steam generators (water to water heat exchangers or boilers) at 310oC – 320oC where the heat is transferred to a secondary water/steam loop to generate steam at around 300oC and 4,500 kN/m2 for the steam turbine. The water inside the reactor vessel acts as both coolant and moderator. The temperature at which a PWR may be operated is limited by the need to keep the reactor water coolant liquid and by practical limitations on the construction of the reactor vessel. Consequently the thermal efficiently is limited to about 30 – 32%. There are two types of PWR currently available for a new British civil nuclear programme. These are the Areva Evolutionary Power Reactor (EPR) of nominal output 1.6 GW(e) with a claimed refueling cycle time of 24 months and the Westinghouse AP1000 of nominal output 1.1 GW(e) with a claimed time between refueling of 18 months.

The Advanced Gas Cooled Reactor (AGR)

In the UK, the dominant nuclear power plant currently in use is the Advanced Gas Cooled Reactor (AGR) of which there are seven stations each with two reactors; each reactor provides about 600 MW(e). These plants are planned to be progressively decommissioned between 2012 and 2023 and no new reactors of this type are likely to be built. The coolant is CO2 gas which is driven through “once through” boilers to produce stream and the moderator is provided by blocks of carbon graphite. The fuel, also UO2, is enriched to about 2.2% and the plant is operated at higher pressure and temperature than the PWR, giving steam to the turbine at around 16,000 kN/m2 and 570oC, with an overall thermal efficiency of approximately 42%.

The Fast Breeder Reactor (FBR)

In both the PWR and AGR reactor types, a certain amount of the dominant fertile U238 material is “bred” into fissile Pu239. However, as the amount of Pu239 produced is substantially less than the U235 consumed, in the longer term there is a requirement for a breeder reactor to breed more fissile material than it consumes. This is only possible with fast neutrons and hence the need to the Fast Breeder Reactor (FBR). There are a number of theoretically possible design types of FBR but, in practice, almost all development has been concentrated on the liquid sodium cooled type. There have been six industrial scale liquid cooled FBRs built, two in the USSR, one of which is still operating, one in the UK (the 250 MW(e) Prototype Fast Reactor at Dounreay), two in France (the 230 MW(e) Phénix, which continues to operate and the 1,200 MW(e) Superphénix, which was abandoned in 1997 when the French government withdrew financial support) and Japanese 280 MW(e) Monju which has been shut down for extensive modifications but is due to re-start during 2010. All these reactors have a core of mixed uranium dioxide and plutonium dioxide (approximately 75% UO2 mixed with 25% PuO2). The core sits in a pool of liquid sodium which is pumped through the core and then though intermediate heat exchangers in which a secondary loop of liquid sodium is heated and used in turn to heat water in stream generators to provide steam for the steam turbine at about 15,500 kN/m2 and 540oC, giving an overall thermal efficiency of around 40%. There are obviously complexities in managing the two loops of liquid sodium and ensuring separation the sodium and water/steam. Consequently production FBRs are likely to be some 25% more expensive to build than PWRs and, since uranium ore has remained relatively cheap and plentiful, there has hitherto been little incentive to launch a large scale programme. 

Uranium Ore

A typical PWR of 1 GW(e) output will have a reactor core containing about 100 tones of uranium dioxide fuel. However, this is enriched fuel which will have required about 450 tonnes of natural uranium, T(u), for its production. A PWR requires about 25% of its fuel replaced every year so, without reprocessing, this would require about a further 115 T(u) to be imported into the UK.

Uranium ore is found in various countries but the principal producers are Canada, Australia and Kazakhstan, each of which supplies about 20% of the total world output, and Namibia, which supplies about 10%.  According to the IAEA [2], in 2005, the total identified world stocks of uranium ore were 4.7 million tonnes and the annual demand was about 55,000 tonnes. The current world installed nuclear capacity is 370 GW(e) and this is expected to rise to between 450GW(e) and 530GW(e) in 2025, leading to an increase in annual world demand to somewhere between 80,000 and 100,000 tonnes. The identified reserves in Australia alone have since been estimated as 2 million tonnes [3].

The spot price of uranium ore increased from $40/Kg(u) to $200/Kg(u) between 2000 and  2004 but has since fallen back to about $165Kg(u). The increased demand for uranium ore is expected to stimulate prospecting and supply and the total easily available world supplies of ore available for mining at current prices are thought to exceed 35 million tonnes [3]. Consequently, we are not likely to “run out” of uranium ore for a further two or three centuries and, if we began to do so, the increasing price would stimulate the exploitation of further lower grade ores including, ultimately, the extraction of uranium from sea water which would be an inexhaustible resource.

Since, currently, only a small quantity of nuclear fuel is recycled and the foregoing assumes most uranium ore is used in a nuclear reactor only once. Two or three times as much energy could be extracted from each tonne of ore by recycling and reuse in a PWR type of reactor. Use in fast breeder reactor could increase the energy extracted by a factor of around 50. Recycling and reuse has the added advantage of greatly reducing the high level waste from fuel.

In summary, using uranium ore only once in a reactor gives world uranium supplies sufficient for about 200 years, recycling through a conventional PWR would extend this to 500 years and use in a breeder reactor would extent this to several thousands of years; extraction from seawater would give a virtually infinite resource. For the present, and in contrast to oil and natural gas, about half the world’s supplies of uranium ore are mined in friendly stable democracies.

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