[Extra Speed] The Little Book Of Stock Market Cycles
Lutero Chaloux
About 20 fast neutron reactors (FNR) have already been operating, some since the 1950s, and some supplying electricity commercially. Over 400 reactor-years of operating experience has been accumulated. Fast reactors more deliberately use the uranium-238 as well as the fissile U-235 isotope used in most reactors. If they are designed to produce more plutonium than the uranium and plutonium they consume, they are called fast breeder reactors (FBRs). But many designs are net consumers of fissile material including plutonium.* Fast neutron reactors also can burn long-lived actinides which are recovered from used fuel out of ordinary reactors.
[Extra speed] the little book of stock market cycles
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Several countries have research and development programmes for improved fast neutron reactors, and the IAEA's INPRO programme involving 22 countries (see later section) has fast neutron reactors as a major emphasis, in connection with closed fuel cycle. For instance one scenario in France is for half of the present nuclear capacity to be replaced by fast neutron reactors by 2050 (the first half being replaced by EPR units).
Russia is at the forefront of fast reactor development. It operates the only commercial-scale fast reactors and is building a 300 MWe demonstration lead-cooled fast reactor. It also put lead-cooled fast reactors into its seven Alfa-class submarines, which was not a conspicuous success but yielded 70 reactor-years of experience.*
Today there has been progress on the technical front, but the economics of FNRs still depends on the value of the plutonium fuel which is bred and used, relative to the cost of fresh uranium. Also there is international concern over the disposal of ex-military plutonium, and there are proposals to use fast reactors (as 'burners') for this purpose. In both respects the technology is important to long-term considerations of world energy sustainability.
Natural uranium contains about 0.7% U-235 and 99.3% U-238. In any reactor some of the U-238 component is turned into several isotopes of plutonium during its operation. Two of these, Pu-239 and Pu-241, then undergo fission in the same way as U-235 to produce heat. In an FNR this process can be optimised so that it 'breeds' fuel. Some U-238 is burned directly with neutron energies above 1 MeV. Hence FNRs can utilise uranium about 60 times more efficiently than a normal reactor. They are however expensive to build and operate, including the reprocessing, and are only justified economically if uranium prices are reasonably high, or on the basis of burning actinides in nuclear wastes.
The fast reactor has no moderator and relies on fast neutrons alone to cause fission, which for uranium is less efficient than using slow neutrons. Hence a fast reactor usually uses plutonium as its basic fuel, since it fissions sufficiently with fast neutrons to keep going.* At the same time the number of neutrons produced per plutonium-239 fission is 25% more than from uranium, and this means that there are enough (after losses) not only to maintain the chain reaction but also continually to convert U-238 into more Pu-239. Furthermore, the fast neutrons are more efficient than slow ones in doing this breeding, due to more neutrons being released per fission. These are the main reasons for avoiding the use of a moderator. The coolant is a liquid metal (normally sodium) to avoid any neutron moderation and provide a very efficient heat transfer medium. So, the fast reactor 'burns' and 'breeds' fissile plutonium.** While the conversion ratio (the ratio of new fissile nuclei to fissioned nuclei) in a normal reactor is around 0.6, that in a fast reactor may exceed 1.0.
Many core configurations are possible, but for maximum breeding, the conventional core plus blanket arrangement is best. If a breeding ratio of less than 1, or just a little more than 1 is wanted, then axial blankets which are included in the fuel pins will serve the purpose. The entire fuel pins are then reprocessed, and the newly-formed plutonium is mixed with the used fuel materials from the fissile zone of the pins. It is also possible to have a uniform core without separate U-238, and with stainless steel reflectors, but little breeding is then possible.
Theoretically any fast reactor can be operated over a spectrum from burner (with steel reflectors around the core) to breeder (with U-238 blanket around the core). Operation of the BN-600 reactor to burn weapons-grade plutonium from 2012, means that the breeding blanket of depleted uranium is removed and replaced by stainless steel reflector assemblies.
One effect of the 1980s halt to FNR development is that separated plutonium (from reprocessing used light water reactor fuel) which was originally envisaged for FNRs is now being used as mixed oxide (MOX) fuel in conventional reactors.
Fast neutron reactors have a high power density and are normally cooled by liquid metal such as sodium, lead, or lead-bismuth eutectic, with high conductivity and boiling point and no moderating effect. They operate at around 500-550C at or near atmospheric pressure. Fast reactors typically use boron carbide control rods.
Also fast reactors have a strong negative temperature coefficient (the reaction slows as the temperature rises unduly), an inherent safety feature, and the basis of automatic load following in many new designs.
Experiments on a 19-year old UK breeder reactor before it was decommissioned in 1977, and on EBR-II in the USA in 1986, showed that the metal fuel with liquid sodium cooling system made them less sensitive to coolant failures than the more conventional very high pressure water and steam systems in light water reactors. More recent operating experience with large French and UK prototypes has confirmed this. With loss of coolant flow they simply shut themselves down.
There is renewed interest in fast reactors due to their ability to fission actinides, including those which may be recovered from ordinary reactor used fuel. The fast neutron environment minimises neutron capture reactions and maximises fissions in actinides. This means less long-lived nuclides in high-level wastes (the fission products being preferable due to shorter lives). Also the fast neutron environment is required for fissioning even-numbered isotopes of uranium, not only U-238 but also others which may be significant in recycled uranium.
In February 2019 the US Department of Energy launched its Versatile Test Reactor (VTR) programme, set up under the Nuclear Energy Innovation Capabilities Act 2017 and run by the Idaho National Laboratory. The VTR project is to be a research facility for testing of advanced nuclear fuels, materials, instrumentation and sensors. It is to provide accelerated neutron damage rates 20 times greater than current water-cooled test reactors. GE Hitachi's PRISM will be adapted as a test reactor under this programme for R&D. Operation is planned by the end of 2025.
Reprocessing used fuel, and especially the blanket assemblies, is fundamental to the FBR fuel cycle. Typically the recovered plutonium from aqueous reprocessing is incorporated into the core as MOX fuel and any surplus deployed elsewhere. The general principles of this are described above. In France about 25 tonnes of fuel from Phenix was reprocessed and the plutonium incorporated into fresh fuel elements, some of it recycled three times.
However, with the transition from core and blanket designs to integrated core designs, it is likely that used fuel will be reprocessed using electrometallurgical processes (so-called pyro-processing) and plutonium will not be separated but will remain with other transuranics and some highly radioactive fission products. Pyroprocessing has several advantages for fast reactors which greatly simplify waste management and closing the fuel cycle.
A generalised picture of fast reactor fuel cycle is two-stage separation of uranium then transuranics, leaving most fission products as a small waste stream. Some or all of the uranium, and the transuranics (including plutonium and minor actinides), are recycled.
India's nuclear power program has been focused on developing an advanced heavy-water thorium cycle, based on converting abundant thorium-232 into fissile uranium-233. The first stage of this employs PHWRs fuelled by natural uranium, and light water reactors, to produce plutonium. Stage two uses fast neutron reactors burning the plutonium to breed U-233 from thorium. The blanket around the core will have uranium as well as thorium, so that further plutonium (ideally high-fissile Pu) is produced as well as the U-233. Then in stage three, advanced heavy water reactors burning the U-233 and this plutonium as driver fuels, but utilising thorium as their main fuel, and getting about two-thirds of their power from the thorium.
The 500 MWe Prototype Fast Breeder Reactor (PFBR) started construction in 2004 at Kalpakkam near Madras. It was expected to start up about the end of 2010 and produce power in 2011, but this schedule was delayed significantly. In 2014, 1750 tonnes of sodium coolant was delivered. With construction completed, in June 2015 Bhavini was "awaiting clearance from the AERB for sodium charging, fuel loading, reactor criticality and then stepping up power generation." Criticality was expected in 2018. The approved cost is Rs 5677 crore. It is not under IAEA safeguards. The reactor is fuelled with uranium-plutonium oxide. It has a blanket with thorium and uranium to breed fissile U-233 and plutonium respectively.
Metal (U-20Pu-10Zr) has very high thermal conductivity compared with oxide, but high swelling and melts at a relatively low 1160C. It is not compatible with lead coolant, due to solubility (in case of cladding failure). It is being researched in Russia, USA, and Japan, and is planned for early use in Russia where it is seen to have the best safety characteristics for lead-cooled reactors.
France operated its Phenix fast reactor prototype from 1973 to 2009, apart from a few years for refurbishing. It ceased generating power early in 2009 but ran until October 2009 as a research reactor. Closure of the 1250 MWe commercial prototype Superphenix FBR in 1998 on political grounds after very little operation over 13 years set back developments. Research work on the 1450 MWe European FBR has apparently ceased.
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