Improvement of minor actinides transmutation performances in fast reactors using fissile material

In Fast Reactor systems the heterogeneous minor actinides transmutation is a promising solution to transmute minor actinides and reduce the long-term radiotoxicity burden associated with nuclear waste without impairing core operations. In this approach, minor actinides are loaded in dedicated targets using uranium dioxide as support matrix at the core periphery. However, due to their location those targets are exposed to a lower flux level and fuel temperature inside is lower than for standard fuel. This has a negative effect on transmutation performances as those depend on the neutron fluence on the targets, as well as on fuel behavior under irradiation due to limited fuel restructuration and potential high solid swelling coming from important helium production (alpha decay of minor actinides). Additionally, plutonium breeding in the blankets leads to a consequent shift in power in the blankets during irradiation, from 0.5 MW up to 1.5 MW per assembly which has a potentially significant impact on core thermal hydraulics. To address these concerns, the use of small quantities of fissile material in the blankets is discussed here. Several options such as various plutonium or uranium isotopic vectors are investigated in terms of impact on minor actinides transmutation performances. Impacts on fuel behavior, fuel cycle and core power redistribution are also investigated. In a first time, transmutation performances are analyzed and it is found that the down blending of 5 %vol of weapon-grade plutonium doubles the total americium consumption of the core without significant impacts on core feedback coefficients such a sodium void worth. Introduction of a degraded plutonium isotopic vectors leads to improvements in transmutation rates ranging from 73 to 55% depending on the amount added, with ²³⁵U yielding similar results. The use of reprocessed uranium as support matrix for the blankets and the relative impacts on assembly dose rate are also characterized. Fuel cycle impacts are also limited in terms of target decay heat and neutron source due to a competition between an increase in curium production from a higher fluence and a decrease in capture cross sections from a faster spectrum. The effect of fissile addition in the blankets on fuel temperature and core radial power profile is also investigated. Small power redistribution towards core periphery is observed with power variations ranging from 2 to 3 MW during irradiation. This redistribution could be smoothened by core fissile content adaptation in order to limit the increase in total plutonium inventory. The impact on plutonium breeding in the blankets is also investigated and it is found that using degraded plutonium to speed up the transmutation process has a positive impact on both the plutonium isotopic vector and proliferation by breeding ²³⁸Pu and ²³⁹Pu. Finally, this solution is compared to the alternate approach based on the addition of moderating material in the blankets such as ZrH₂ or MgO to increase transmutation performances. (author)