Minor Actinides Burn up Enhancement in the European Sodium Fast Reactor through Moderator Material Addition

The minor actinides (MA) burn up optimization in the European Sodium Fast Reactor (ESFR) core was studied by adding different moderating materials in the Minor Actinides Bearing Blanket subassemblies (MABB SA) using the ERANOS neutron code package. These SA are of hexagonal shape and are composed of pellets inside of pins. These pellets contain a mixture of uranium dioxide (UO₂) and americium dioxide (AmO₂). If some of these pins are replaced by other identical ones containing moderating material instead of minor actinides, a shift in the spectrum towards lower energies is expected, which might enhance the burn up performance. The results of this work demonstrated that the use of compounds of hydrogen and magnesium as moderators produces a shift in the neutron spectrum, improving the percentage of minor actinide consumption. ZrH₂ moderator material was found to exhibit the best performances for this propose, followed by MgO and MgAl2O4, in that order. The use of SiC, BeO, TiC, LiO₂ and ZrC material produced no effect on the shift of the neutron spectrum in the present configuration. For safety reasons, it seems hardly realistic to use hydrogenous compounds in sodium fast reactors. So, compounds with magnesium are selected to be placed into the pins to improve the percentage of minor actinide consumption. Also, the decay heat generated by ²⁴²Cm, ²⁴⁴Cm and ²³⁸Pu and the total neutron flux in the reactor core were estimated. (author)