The factors affecting MTC of thorium–plutonium-fuelled PWRs

Plutonium loading in a plutonium–thorium (Pu–Th) mixed oxide (MOX) fuelled pressurized water reactor (PWR) core is typically constrained by large maximum radial form factors (RFF) and positive moderator temperature coefficient (MTC). The large form factors in higher Pu content fuels stem from the large differences in burnup, and thus reactivity, between fresh and burnt fuel, while positive MTC can potentially be the result of the high soluble boron concentrations needed to maintain criticality for such reactive fuel. The conventional solution to these problems is the use of burnable poisons (BPs). While BPs are able to reduce RFF, the positive MTC is not entirely due to a large critical boron concentration (CBC) requirement. In fact, analysis shows a positive MTC in Th–Pu fuel is mainly caused by fissioning in the epithermal–fast energy range. A reduction in epithermal–fast fissioning through the use of certain BPs and the strategic employment of loading patterns that encourage leakage are more effective in attaining negative MTC, as a reduction in CBC has a negligible effect on MTC. This paper examines the contributions to positive MTC by isotope and energy and identifies characteristics of BPs that are able to mitigate positive MTC in a Pu–Th MOX PWR core.