FAST NEUTRON REACTORS | NUCLEAR FUEL CYCLE


Left:


The ODEK site includes the BREST-300 lead- cooled fast neutron reactor under construction


mention of nuclear energy as a low-carbon energy source. However, the discussions held during the conference showed a growing interest on the part of the participating countries in the use of nuclear energy as a low-carbon source. Even so, the existing structure of nuclear power, based on thermal neutron reactors, even when scaled up, cannot solve the problems of long-term energy supply for mankind because of its inherent resource and technical limitations when operating in an open nuclear fuel cycle. First, the reserves of natural uranium, like the reserves of any other fossil primary energy source, are finite at reasonable prices. At the current rate of consumption, these reserves are estimated to last for 100-150 years. Another limitation is the accumulation of used nuclear fuel (UNF) and the need to decide on its ultimate management. Even the processing of UNF from thermal neutron reactors with the subsequent use of regenerated uranium and the resulting plutonium does not solve the problem, but only slightly postpones the exhaustion of fuel resources. The reason for this is that plutonium-based fuel – mixed uranium-plutonium oxide (Mox) fuel can be used in a thermal reactor only once, since a change in the isotopic composition of plutonium during irradiation leads to the accumulation of other isotopes of plutonium, which prevents its further use in thermal reactors. Therefore, used Mox has to be stored until fast reactors are available. Gradual changes in global nuclear energy structure and its eventual transition to fast neutron reactors based on a closed fuel cycle is a good solution to these problems. Moreover, this transition could be possible initially based on a two-component nuclear energy phase comprising simultaneous use of thermal and fast neutron reactors operating in a combined closed fuel cycle. This phase would facilitate smooth transition to a different structure of nuclear energy, as well as assist in solving the problem of accumulated UNF, the complete disposal of high-level radioactive wastes using controlled storage period lasting hundreds rather than thousands of years, and resolve the problem of separated plutonium now held in storage. Fast neutron reactors of Russian design, analogues of the BN-1200M currently under development and the BREST-OD-300 experimental demonstration reactor already


under construction, have practically no restrictions on fuel resources due to the use of the main isotope of natural uranium, uranium-238. It should be noted that Russia has already accumulated about 1 million tonnes of depleted uranium hexafluoride, in other words, U-238 of near-nuclear purity, which can be used to produce fuel for fast reactors. It is estimated that this amount of U-238 is equivalent of ~8∙10²² J of energy. In comparison, mankind annually produces ~6∙10²0 J of energy. This means Russia’s depleted uranium alone carries energy which is 100 times greater than this annual production level. Fast reactors are virtually omnivorous and can use plutonium from thermal neutron reactors multiple times, even without separating it from a mixture with uranium and other minor actinides. Fast reactors will also be able to transmute their own minor actinides as well as those isolated from VVER UNF. This will ensure a reduction in the radioactivity of radioactive waste, making it much easier to handle because of the absence of minor actinides. This reduces the radioactivity of wastes in less than 200 years to a level equivalent to natural uranium. However, a large-scale transition to nuclear power based


on fast reactors and a closed fuel cycle raises concerns about an increase in the risk of nuclear proliferation. The response to these concerns should be newly developed technical and institutional measures, closely linked to the verification procedures carried out by the International Atomic Energy Agency (IAEA) within the framework of the implementation of safeguards agreements with non- nuclear-weapon States. As for technical solutions, we can use the example of the pilot demonstration power centre (ODEK) being built in Russia with a BREST-OD-300 lead-cooled fast reactor. This reactor does not structurally provide a blanket – a radial or axial plutonium breeding zone – and the reactor breeding coefficient is close to one, i.e. there is no excess plutonium production. The reactor also does not produce weapons- grade plutonium – it is impossible to make any serious explosive device from this plutonium and certainly not a nuclear weapon. During the processing of the used fuel, a product will be produced from which new fuel will be made. This product is characterised by incomplete purification from fission products, and will contain uranium, U


Fast neutron reactors have practically no restrictions on fuel resources due to the use of the main isotope of natural uranium, uranium-238


www.neimagazine.com | September 2022 | 23


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