Effect of fuel nuclide composition on the fuel lifetime of the RITM-200 reactor unit

•Thorium-based fuel extends the fuel lifetime by 600 effective days.•Increasing the fuel diameter of thorium fuel extends the fuel lifetime by 450 effective days.•Standard RITM-200 fuel does not require a diameter extension. This paper describes the methodology used in determining the fuel lifetime of the RITM-200, a small modular 200 MWth pressurized water reactor (PWR), under operating conditions. Three different dispersed fuel compositions, namely (238U + 235U)O2, (232Th + 235U)O2, and (232Th + 233U)O2, were examined for the calculation of the fuel lifetime. Switching the fuel composition from (238U + 235U)O2 to (232Th + 233U)O2 extends the fuel lifetime by 44% and the fuel burnup by 60% at the design values of the fuel element diameter. The dependency of the fuel lifetime and fuel burnup on the diameter was also determined. This study shows that the most efficient fuel composition is (232Th + 233U)O2 with the fuel diameter increased to 7.9 mm as it extends the fuel lifetime of RITM-200 by 78% and its burnup by 50% compared to the design values of the fuel diameter.