The Spectral Shift Control Reactor as an option for much improved uranium utilisation in single-batch SMRs

Abstract The Spectral Shift Control Reactor (SSCR) uses a mix of D 2 O and H 2 O to moderate and cool the reactor. Initially, a high proportion of D 2 O is used, such that the reactor is substantially under-moderated, with excess neutrons being primarily captured in 238 U, breeding 239 Pu. Towards the end of the cycle (EOC), the coolant is predominantly H 2 O, thermalising the neutron spectrum and increasing reactivity. Recently, small modular reactors (SMRs) have gained significant interest as a means of providing a power source that requires little maintenance and refuelling. This motivates long cycles and reduced batch operation. For a single-batch reactor, there is typically a 33% penalty to uranium utilisation compared to a 3-batch reactor. Lattice calculations demonstrate the potential of the SSCR to greatly improve uranium utilisation in single-batch reactors over a range of enrichments. A relatively ‘wet’ lattice is employed which further improves uranium utilisation. Cases with 5% and 15% fissile loading are considered, for which it is respectively possible to achieve 47% and 39% increases in natural uranium utilisation using the SSCR relative to a ‘reference’ light water reactor. In the latter case, if 25% thorium is mixed into the fuel, the improvement in uranium utilisation increases to a total of 49%. Hence, in both cases, it is possible to in effect eliminate the penalty of using a single fuel batch. The ‘wet’ lattice introduces substantial thermal-hydraulic challenges due to the significantly higher fuel pin heat flux.