The Reactivity Temperature Coefficient Analysis in Light Water Moderated UO₂ and UO₂-PuO₂ Lattices

The contributions of different physical phenomena to the reactivity temperature coefficient (RTC) in typical light water moderated lattices have been assessed. Using the APOLLO2 code with the CEA93 cross-section library based on JEF2.2 data, we have analyzed the main French experiments available on the RTC: the CREOLE and MISTRAL experiments. In these experiments performed in the EOLE critical facility located at CEA/Cadarache, the RTC has been measured in both UO₂ and UO₂-PuO₂ pressurized water reactor-type lattices. Our calculations have shown that the calculation error in UO₂ lattices is <1 pcm/°C, which is considered as the target accuracy for reactor design calculations. On the other hand the calculation error in mixed oxide lattices is more significant in both low- and high-temperature ranges: An average error of -2 ± 0.5 pcm/°C is observed at low temperatures, and an error of +3 ± 2 pcm/°C is obtained for temperatures >250°C. Our analysis has shown that the negative error in the low-temperature range is linked to the thermal spectrum shift effect, which is strongly dependent on the thermal shapes of the cross sections of plutonium isotopes, whereas the positive error in the high-temperature range is mainly linked to the water density effects.