The autonomous transportable on-demand reactormodule (ATOM), a small modular reactor (SMR), hasbeen recently developed at center for autonomous smallmodular reactor research (CASMRR). An innovativeburnable absorber concept, centrally-shielded burnableabsorber (CSBA), was successfully utilized in the core toachieve a soluble-boron-free (SBF) operation. For furthercore development, in this paper, several heavy reflectordesigns are introduced (i.e., ZrO2, PbO) to reduceneutron absorption and leakage, and consequentlyimprove the core cycle length compared to the currentstainless steel reflector. In addition, CSBA loadingscheme is also proposed and optimized for each design tominimize the burnup reactivity swing and power peakingfactor, so that the SBF operation can be assured.Moreover, the accident-tolerant-fuel (ATF) claddings arealso investigated to replace the conventional Zircaloy-4cladding in the current ATOM core. All calculations inthese neutronic-coupled thermal hydraulic assessments ofthe ATOM core are simulated using a Monte Carlodiffusion hybrid code system.
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