Optimisation for the thorium fuel cycle of a D2O-cooled PWR-like SMR core

Sammanfattning

Under-moderation in PWRs is of real interest for nuclear fuel conversion. One promising approach is to replace the light water coolant with heavy water, which improves fuel conversion in a faster neutron spectrum. However, this change renders obsolete the previous reactivity management methods used in light-water PWRs and therefore requires the development of new solutions. One approach is to use the Spectral Shift Control (SSC) concept. This concept proposes to manage reactivity within the reactor by thermally adjusting its neutron flux. Based on studies carried out in the 1950s and tested in the 1960s with the BR3 reactor, one solution involves gradually adding light water to the coolant during the cycle, thereby softening the neutron flux. However, this method deteriorates the initial quality of the coolant, requiring additional costs to restore the initial concentration of heavy water between each cycle. An alternative consists of inserting moderator rods made of ZrH2 within the reactor to soften the core locally while preserving the initial state of the coolant locally. This method has shown promising conversion results, similar to the traditional method of adding light water while preserving the quality of the coolant for 180 days. This concept was tested and validated on a D2O-cooled PWR-like SMR core operating with a thorium-uranium cycle. At the same time, additional studies were carried out on alternative cycles, such as the thorium-plutonium cycle, to observe the benefits of under-moderation.