Burnup performance of MOX and Pu-ROX fuels in small pebble bed reactor with accumulative fuel loading scheme

Pebble bed reactors (PBRs) are among the promising designs for future nuclear power plants. There are several fuel management strategies for PBRs. The accumulative fuel loading scheme was introduced as the simplest fueling strategies for a PBR. In this study, mixed-oxide (MOX) and plutonium-rock like oxide (Pu-ROX) fuels were introduced as starting materials. The MVP-BURN code was used to calculate the burnup performance of these fuels. The optimum fuel composition of MOX fuel was 5 g HM/pebble when uranium-plutonium ratio was 70:30. In the case of MOX fuel, the maximum discharged burnup was 172 GWd/t over 6 years of operation time. On the other hand, the optimum fuel composition for Pu-ROX fuel was 3 g HM/pebble. The results showed that the maximum discharged burnup was 550 GWd/t with an operation time of 12.7 years. The maximum excess reactivity occurred in the initial condition for both fuels. Moreover, the results of initial excess reactivity for both fuels were quite similar. The use of burnable poison materials to compensate for the initial excess reactivity of both fuels was not effective. A negative temperature coefficient was achieved for both MOX and Pu-ROX fuels throughout the operation period.