Recently, small modular reactors (SMRs) have gained significant interest for 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 multi-batch reactor. This paper concerns the application of the principle of spectral shift to allow a long single-batch fuel cycle to be maintained without penalising the fuel cycle cost. Three designs are considered: (1) the spectral shift control reactor, where D_2O and H_2O are mixed together in the reactor and the fraction is varied over the cycle; (2) an application of the DUPIC fuel cycle to SMRs; (3) the pressure tube variable moderator concept, where a calandria is steadily filled with moderator over the cycle. The improvement in fuel utilization for (3) is relatively modest, and is largest for (2). Both (1) and (2) require use of heavy water, which is expensive. All designs require a different core configuration, or alternatively, in the case of (2), re-cladding the fuel between use in the SMR and use in the HWR.
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