Influence of 235U enrichment on the moderator temperature coefficient of reactivity in a graphite-moderated molten salt reactor

To optimize the temperature coefficient of reactivity (TCR) for a graphite-moderated and liquid-fueled molten salt reactor, the effects of fuel salt composition on the fuel salt temperature coefficient of reactivity (FSTC) were investigated in our earlier work. In this study, we aim to provide a more comprehensive analysis of the TCR by considering the effects of the graphite-moderator temperature coefficient of reactivity (MTC). The effects of $$^{235}$$U enrichment and heavy metal (HM) proportion in the salt mixture on the MTC are investigated from the perspective of the six-factor formula based on a full-core model. For the MTC (labeled “$$\alpha _{{\mathrm{TM}}}$$”), the temperature coefficient of the fast fission factors ($$\alpha _{{\mathrm{TM}}}(\varepsilon )$$) is positive, while those of the resonance escape probability ($$\alpha _{{\mathrm{TM}}}(p )$$), the thermal reproduction factor ($$\alpha _{{\mathrm{TM}}}(\eta )$$), the thermal utilization factor ($$\alpha _{{\mathrm{TM}}}(f )$$), and the total non-leakage probability ($$\alpha _{{\mathrm{TM}}}(\varLambda )$$) are negative. The results reveal that the magnitudes of $$\alpha _{{\mathrm{TM}}}(\varepsilon )$$ and $$\alpha _{{\mathrm{TM}}}(p )$$ for the MTC are similar. Thus, variations in the MTC with $$^{235}$$U enrichment for different HM proportions are mainly dependent on $$\alpha _{{\mathrm{TM}}}(\eta )$$, $$\alpha _{{\mathrm{TM}}}(\varLambda )$$, and $$\alpha _{{\mathrm{TM}}}(f )$$, but especially on the former two. To obtain a more negative MTC, a lower HM proportion and/or a lower $$^{235}$$U enrichment is recommended. Together with our previous studies on the FSTC, a relatively soft neutron spectrum could strengthen the TCR with a sufficiently negative MTC.