Validity of Conversion to Transuranic Fuel from Uranium Loaded Sodium-cooled Fast Reactor

From domestic policy of sodium-cooled fast reactor about fuel, the uranium fuel will be loaded in preliminary SFR and the TRU fuel will be prepared for next stage. The TRU fuel may be better to the core performance but it has downside of the kinetics as the general knowledge. In this paper, the impacts of core performance and safety feature are evaluated when the TRU fuel will be replaced the uranium fuel in the SFR. The impact when driver fuel will be changed to the TRU material in small-sized SFR is evaluated with the MESOF core. The U-Zr metallic fuel is loaded in the reference core. The alternative fuels are U-TRU-Zr metallic fuel, (U,TRU)N nitride fuel and (U,TRU)O₂ oxide fuel. At this moment, the design specifications are held. As a result, the critical fissile mass is reduced about 2/3 to the all of the TRU fuel options. It means that the cycle length will be able to be extended. The peaking factor and flux level are similar to the all of the fuel options. Meanwhile, the sodium void worth is moved to positive value to the all of the TRU cores but it is not a problem yet. Because the values are less than about 2$ and the maximum limitation is about 7$ in the metallic fuel core. However, as the result of quasi-static analysis, all of the TRU options has the lack of safety potential for the UTOP severe accident. It means that the TRU fuels are weak to the accident with control rod. In the case of the (U,TRU)O₂ oxide fuel, ULOF severe accident also the problem therefore oxide fuel is not suitable to the SFR. The impact when the TRU fuel will be replaced uranium fuel in mid- or large-sized SFR is evaluated with the KALIMER core indirectly. The evaluation way is that the core size is minimized from 600MWe to 300MWe and the impact as the core size is evaluated. At this time, the fuel is maintained U-TRU-Zr and the equilibrium cycle is assumed. The portion of neutron leakage is small in the case of the mid- or large-sized core. It makes small excess reactivity at BOC and small requirement of TRU contents. The control worth are more uniformly distributed for all control rods therefore it is safe relatively for the accident with control rod. However, the sodium void effect is about 2,000pcm at EOC. If the delayed neutron fraction is assumed 0.003 for the conservation, the sodium void effect will be about 6.7$ in the $ unit. It is closed to the limitation. The above results are shown that the transition from uranium fuel to TRU in small-sized SFR is not attractive option. If reduce the excess reactivity at BOC, the safety potential for the control rod accident will be improved. But it is like to throw off the advantage of cycle length extension of the TRU fuel. However, the transition from the uranium fuel to the TRU is reasonable in mid- or large-sized SFR. Only sodium void worth is problem but it can be overcame to reduce the fuel slug height