Neutronic analysis of mixed H2O/D2O moderated SMART reactor fuel assembly with varying fractions of D2O during the fuel burnup

The neutronic behavior of nuclear reactors is being investigated by considering different fuel, cladding, and neutron-moderating materials. In the present manuscript, two different assembly types of SMART small modular reactor with different enrichments and different numbers of IFBA rods are considered, and the effects of mixed heavy/light water moderator, with varying fractions of heavy water during the burnup, on the assembly cycle burnup are investigated. It is observed that, to extend the cycle burnup we have to use a higher fraction of D2O at the Beginning of the Cycle (BOC) whereas it reduces toward the End of the Cycle (EOC). A higher fraction of heavy water causes the neutron spectrum to shift to the resonance region, resulting in a higher capture rate of the fertile materials. This, in turn, causes an increase in the conversion ratio. on the contrary, toward the EOC, by increasing the light water fraction, the neutron spectrum becomes softer. This also causes an increase in the fission rate of fissile materials. Finally, a certain improvement in the cycle burnup is observed. Moreover, by implementing the proposed method on an assembly containing (Th+U)O2 fuel, the UO2 mass saving is calculated.