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

•A PWR with mixed D2O/H2O moderator/coolant is investigated for SMR applications.•Heavy water concentration varied over the cycle to give ‘spectral shift’ operation.•Much wetter lattice than normal is neutronically favourable.•Taller fuel stack is thus needed to ensure acceptable MDNBR.•35–43% increase in uranium utilisation for single batch reactor is possible. The Spectral Shift Control Reactor (SSCR) uses a mix of D2O and H2O to moderate and cool the reactor. Initially, a high proportion of D2O is used, such that the reactor is substantially under-moderated, with excess neutrons being primarily captured in 238U, breeding 239Pu. Towards the end of the cycle (EOC), the coolant is predominantly H2O, 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.