Measurement and analysis of decay heat of fast reactor spent MOX fuel

Decay heat measurements have been conducted in the experimental fast reactor JOYO to accumulate experimental data and verify the analysis method. We developed the measurement system based on the calorimetric method as a non-destructive examination. The calibration for heat measurement considering heat balance and the correction for heat loss due to radiation penetration were evaluated. Using this system, the decay heat of the spent MOX fuels, with burn-ups of approximately 66 GWd/t and cooling times of between 40 and 729 days, was measured in the JOYO spent fuel storage pond. An appropriate coolant flow rate has allowed us to measure decay heat between 150 W and 2000 W with an error of 1.7–6.0%. The decay heat was calculated using the ORIGEN2 code with the JENDL-3.2 cross section library and the JNDC-V2 decay data and fission yield data library. The irradiation condition used as an input to ORIGEN2 was calculated using the JOYO core management code system. The ratios between the calculated and experimental values were between 0.96 and 0.90. A systematic discrepancy was observed between the calculated and measured values, which was larger than the experimental error. The discrepancy appears due to the production and transmutation calculation uncertainty of minor actinides and long-lived fission products.