.JPG)
Nuclear power is the only base load (continuous) non CO2 producer.
Nuclear power is likely to play a substantial part in future power requirements as fossil fuels are phased out. Eventually it is itself likely to be phased out if fusion power becomes possible or if green systems, solar wind etc and the means of power distribution via electricity and hydrogen are developed..
Nuclear power stations.
A nuclear reactor with (or without) a heat exchanger generates steam which then drives a turbo alternator to generate electricity:-
A coal fired power station has a large quantity fuel input and handling equipment but in contrast the amount of uranium fuel in a nuclear power station is small. However a nuclear reactor is more complex than a fossil fired boiler.
Nuclear fission.
Uranium atoms split to produce heat in a thermal reactor with a moderator.
In the fission process atoms of uranium, U235, U233 or plutonium Pu239 are split and the process releases heat (due to lost mass.) 50 years ago the UK built gas (CO2) cooled 'thermal reactors.' In a 'thermal reactor' a 'moderator' is used to slow down the neutrons so as to increase the fission level. These gas cooled reactors had a graphite moderator core. Un enriched uranium was used as fuel (U235 being the fissionable content in the ore.) These were followed by the Advanced Gas cooled Reactor using enriched uranium as fuel. There are several of these two types remaining (ref 6).
Control rods which absorb neutrons are used to adjust and control the level of reaction and therefore the level of power output.
Neutrons are released naturally from the uranium isotope U235. When the density of a mass of uranium is made to increase it reaches a 'critical' mass when a 'chain reaction' builds up and more neutrons are released.
Uranium fuel rods are inserted into the core of a nuclear reactor. Boron carbide (or silver-indium-cadmium-hafnium) control rods in the reactor absorb neutrons to control the level of fission. To get the reaction going , the control rods are raised. and when a 'critical' level is reached, the neutron flux becomes self sustaining and heat is produced. Power level is controlled by raising control rods, so increasing the level of neutrons and lowering them to maintain stability at the new power level. To shut down, control rods are fully reinserted. Boron mixed with the water is also used in some reactors to assist in control.
Considerable heat continues for some time in the shut down state. Therefore a reliable auxiliary system for providing a large amount of power to remove this heat by circulating coolant in this shut down state, independent of the electricity grid or transmission system is required to maintain safety. This consists of batteries, diesels or gas turbines with a highly involved level of standby (3 or 4 times) in case of failure of parts of the system at the same time. Some advanced designs can provide natural cooling inherently in the shut down state.
PWRs most common in world (see sketch above)
The US developed the Pressurized Water and Boiling Water cooled 'thermal' reactors (PWRs and BWRs). Water acts as moderator and coolant; the U235 content in uranium is enriched by 3.5% to 5%; and the fuel consists of Uranium oxide pellets in sealed Stainless Steel cans. There are 362 BWRs and PWRs worldwide (ref 6). Load factors on these reactors have increased from 65% early on up to 90% and working life extended from 20 years to 40 to 60 years.
CANDU and Graphite moderated BWRs.
The Heavy water (deuterium) reactor CANDU was developed in Canada, which can use un enriched uranium, as heavy water is a good moderator; there are 40 worldwide. The CANDU reactor has horizontal pressure cylinders (300 to 600) which pass through the 'Callandria' which contains heavy water, the moderator. Each cylinder contains the moderator/coolant and the fuel tube bundles. The fuel tubes are zircalloy full of ceramic fuel pellets. Fuel bundles are 10 cm dia and 50 cm long. There are two shutdown systems, vertical control rods in the callandria and also a system for injecting gedolinium nitrate into the fluid. If U235 or Pu235 fuel is used to start, thorium 232 can be used to 'fission' to U233 and continue the reaction.
The Graphite moderated boiling water reactor was developed in Russia; there are 12.
PWRs are used in very large ships, submarines, tankers, ice breakers but with a greater amount of enrichment which extends the life of the fuel.
PWR Fuel assemblies
The BWR fuel is similar but each assembly is 'canned' with a thin tube surrounding it, there are 91 to 96 fuel rods per assembly and 368 to 800 assemblies. Each BWR is back filled with helium at 3 atmos pressure.
The thermal fission reaction
Natural uranium consists of a small amount of U235 which is 'fissionable' and a much greater quantity of U238 (which is not.) The reaction is as follows:-
U235 + n (a neutron) --> X + Y (fission products) + energy (= lost mass) + 2.5 n
The figure '2.5 n' means that out of 6 fissions 6 X 2.5 = 15 neutrons are emitted. 6 maintain the chain reaction, 5 are lost and 4 breed Pu 239 (from U238.)
U 238 is a non fissionable isotope and in much greater quantity in uranium. As U235 is fissioned, some U238 in the uranium is converted to Pu239 which is then also fissioned extending the life of the fuel :-
U238 + n --> U239 --> Np239 --> Pu 239 (fissionable).
Thorium is present in greater quantity than uranium and non fissionable but can also become fissionable if in a reactor it catches a neutron or "breeds":-
Th 232 + n --> U233 (fissionable) .
more details on nuclear fission
Fast Breeder Reactors (FBR's)- fuel use could be extended by 60 times, waste reduced.
While producing power from fast fission, the Fast Neutron Breeder Reactor can also produce more useable fuel, say 20% more each use than it uses by converting U238 to fissionable Pu239 - plutonium thus, after processing, extending considerably the use of uranium as a fuel, probably by 60 times (ref 145) and decreasing very considerably the long term storage waste.
Simplified diagram of fast neutron reactor :-
In contrast to the thermal fission process, for every 6 fissions of Pu239 in a fast reactor, 17 neutrons are emitted, 6 go on to maintain the chain reaction, 7 neutrons breed PU239 from U238 and four are lost. Thus more fissionable fuel can be created than is used up.
A 'fast' reactor has no 'moderator' (graphite, water, heavy water) and therefore water cannot be used as a coolant. Also the heat is more concentrated so a coolant such as Liquid sodium, or sodium/potassium, lead, lead-bismuth at 500 - 550 C is suitable which does not require a pressure vessel. Control rods are boron and/or cadmium.
A blanket of mainly U238 is wrapped around the core and spare neutrons convert this to PU239 to maintain the reaction which with used Uranium is reprocessed as further fuel.
Due to cost, abundance of cheap fossil fuels at the time and various handling and leakage problems, plans to build large numbers of fast reactors were abandoned or put on hold in the 1960s and 1970s but there is a renewed interest today.
Of the 18 FBRs 6 are still in operation including the 250 MWe Phoenix in France since 1973; the 40 MW FBTR India since 1985; 500MW reactors under construction at Kalpakkam; the BN 600 in Russia since 1981; and the 280 MW reactor at Monju in Japan
Long life fission products in high level waste can be fissioned in a FBR to products with a shorter life, so easing disposal. (Ref 6)
The amount of Pu239 fuel obtainable from weapons disposal could when reprocessed (as MOX) supply all the worlds needs for several decades.
Safety aspects
Deaths per terra watt year for electrical generators between 1970 - 1992 are put at 342 for coal power stations, 85 for natural gas power stations, 883 for hydro stations, and 8 for nuclear generators (ref 6 p115).
While overall statistics show that nuclear reactors have resulted in less deaths, less accidents than coal power stations, the one bad accident that happened at Chernobyl in 1986 in the Ukraine, which released radioactive contamination over a wide area had a profound effect on sentiment worldwide; but these reactors were considerably more risky than those used in the west.
There were two other serious incidents. A release of radioactivity happened in 1957 at Winscale, UK where no-one died. This was a primitive UK reactor which had an 'open' cooling circuit. Cooling gas blew straight to the atmosphere and with no containment. The Three Mile Island meltdown accident in the US 1979 was contained within the containment vessel with no escape of radioactivity.
Reprocessing fuel.
Fuel is replaced cyclically over a 3 year period in power station thermal reactors, longer for ships with higher enrichment. Reprocessing leads to less material that has to be disposed of in storage, more use of fuel and can help with disposal of weapons grade plutonium. Plutonium is fabricated with uranium oxide as 'MOX' fuel which can be burnt in existing reactors.
Areva and also the University of Texas are developing a process for destroying 99% of nuclear waste. A Compact Fusion Neutron source drives a fusion- fission hybrid. The hybrid burns long lived waste with neutrons.
Uranium reserves large depending on cost of extraction.
The electrical energy reserves of uranium is around 2000 EJ at today's commercial prices up to 7000 EJ depending on cost. Thorium reserves are about the same as Uranium. If FBRs were to be developed commercially uranium resources would extend to 400,000 EJ. If it were possible to extract Uranium from sea water, these figures would be 700 times higher.
In 2006 the total installation of nuclear generating capacity was 368 GW (1GW = 1000 MW) or 15% of the world's generating capacity; or 2650 TW hrs (1TWhr = 1000 GWhr) (ref 6) which is 18% of world power generated.
Waste from Nuclear power .
To get a perspective on waste, compare nuclear with coal generation (ref 6):-
3 tonnes black coal -->> 8 MW hrs electricity + 300 kg fly ash + 8 tonnes CO2 +SO2 + particles .
30-70 kg uranium ore- - >> 230g U3O8- ->> 30g enriched uranium (+200g depleted tails) - - -->> 8 MW hrs electricity + 30g spent fuel - -->> 20 ml high level waste - - >> 6g glass for permanent storage.
Disadvantages of nuclear power are the work in decommissioning power plants and the long term storage of radioactive waste. On the plus side, they do not run on fossil fuels and no CO2 is generated in operation. (116 costs.)