Oxide-Metal Ratio Dependence of Central Void Formation of Mixed Oxide Fuel Irradiated in Fast Reactors

Abstract

Based on thermal computation results obtained using an irradiation behavior analysis code, we have evaluated the effect of oxide-metal ratio on fuel restructuring from the results of postirradiation examinations for the B14 irradiation test fuel, which was a mixed oxide fuel and was irradiated in the experimental reactor Joyo. The thermal computation results showed that fuel restructuring in the stoichiometric oxide fuel was accelerated, though the fuel temperature in the stoichiometric oxide fuel was evaluated as lower than that of the hypo-stoichiometric one. We explained this behavior as follows: first, the fuel temperature decreased due to the high thermal conductivity at stoichiometry; second, the pore migration velocity increased due to the increase in vapor pressure caused by the high vapor pressure of UO₃, which was derived from the high oxygen potential at stoichiometry. In addition, our results indicated that the central void diameter strongly depended on not only fuel temperature, but also vapor pressure.

Keywords:

• MOX fuel
• fast reactor
• fuel restructuring

Acknowledgments

The authors would like to express their appreciation to Dr. Saito of JAEA for his fruitful advice.