Analysis of Prediction Accuracy for Actinide Decay Heat of Spent MOX Fuel from Light Water Reactors

On the basis of the background of the global warming and energy resource problems, the programs for utilization of nuclear power are now being revised upward around the world. This situation leads to expansion in utilization of uranium-plutonium mixed oxide (MOX) fuels in existing thermal reactors. The MOX fuels include larger amounts of heavy actinide nuclides as the initial components than conventional uranium fuels. This means that the decay heat of spent fuels becomes much larger than that of uranium fuels. In the present study, prediction accuracy is evaluated for actinide decay heat in one of the typical cases of spent MOX fuels in light water reactors on the basis of covariance data in JENDL-3.3. There are few studies on burnup sensitivity analyses focused on the actinide decay heat of spent fuels; the present study provides a good starting point for future works. The present results show that the errors of actinide decay heat are 35.1±0.9 kW/t (2.7%), 3.27±0.12 kW/t (3.7%), and 2.37±0.05 kW/t (1.9%) at a 68% confidence level in the cases of MOX fuels of 0.1, 10, and 50 year cooling after reactor shutdown, respectively. In order to improve the prediction accuracy of the MOX decay heat, it is important to reduce the uncertainty of the cross section of actinide nuclides including the capture reaction in Pu-238, -239, -242, Am-241, -243, and the fission in Pu-241 followed by capture in Cm-244.