The IMF/MOX comparative test, IFA-651.1: results after the second cycle of irradiation

The IFA-651.1 inert matrix fuel (IMF) / MOX comparative test has successfully completed its second cycle of irradiation. The IMF consists of plutonium, erbia as a burnable absorber and Yttria-stabilised zirconia (YSZ) matrix. The test rig contains one cluster of three IMF rods and three MOX rods. Two of the IMF rods and two of the MOX rods contain fuel fabricated using a dry attrition milling process. The remaining IMF and MOX rods were fabricated using a wet co-precipitation process and the short binder less route (SBR), respectively. The rods have been irradiated in the HBWR conditions - heavy water coolant of 235 degrees Celsius at 34 bar. At the end of the second cycle, which had a total operation time of 143 days, the assembly average burnup was ∼104 kWd/cm3. The assembly average linear heat rate (LHR) reached 30 kW/m in a power ramp at an assembly average burnup of ∼ 60 kWd/cm3, and the LHR level was held. In the power upramp, fission gas release (FGR) was detected through rod internal pressure measurements in each IMF rod, while there was no FGR in MOX rods. Detailed plots showed that significant FGR occurred when the fuel temperature in the IMF rods exceeded the FGR activation threshold temperature established by Vitanza et. al. for UO2. The applicability of the Vitanza threshold to IMF has not been identified, but the threshold was highly consistent with the test results for both the IMF and MOX rods. The FGR rate reached ∼ 9%, ∼ 18% and ∼ 14% for IMF rods 2, 4 and 5 respectively after crossing the Vitanza threshold.