Summary and Interpretation of the CABRI REP-Na Program

The CABRI REP-Na program was performed in the sodium loop of the CABRI reactor by the French Institut de Radioprotection et de Sûreté Nucléaire. The objective was to study the behavior of high-burnup UO₂ and mixed-oxide (MOX) fuel during a reactivity-initiated accident (RIA) and involved eight tests with UO₂ and four with MOX fuel. Failures of some UO₂ and MOX fuel rods at enthalpy levels ranging from 125 to 472 J/g (30 to 113 cal/g) demonstrated the need for further development of the present safety criteria pertaining to fuel behavior. Detailed interpretation of the test data led to identifying the deleterious influence of a high clad corrosion level with hydride concentrations on clad failure, the contribution of grain boundary gases on fission gas release, and potential clad loading, mainly in MOX fuel during the first phase of the transient without significant clad temperature increase.

Questions still remain concerning the transient fission gas behavior and its impact on clad loading during the entire transient, the rod behavior with high clad temperature and internal pressure, and the postfailure phenomena (fuel ejection, fuel/coolant interaction with finely fragmented solid fuel). These issues will be addressed by the CABRI International Program tests under typical pressurized water reactor conditions.