The integral fuel rod behaviour test IFA-597.3: Analysis of the measurements

The objective of the IFA-597.3 experiment is to study fission gas release (FGR), thermal behaviour and pellet-cladding mechanical interaction (PCMI) in high burnup commercial BWR fuel. Two fuel rods have been further irradiated from 59 MWd/kgUO2 to 61.5 MWd/ kgUO2. One fuel rod (8) is instrumented with a fuel centreline thermocouple and a bellows pressure transducer. The other fuel rod (7) is instrumented with a cladding elongation transducer. The pellet-cladding gap in the rods is tightly closed at power. This causes delayed measurement of fission gas release. The fuel centreline temperature is above the Halden threshold for 1 % fission gas release during the whole irradiation period. An extra increase in normalised fuel centreline temperature observed in rod 8 is due to the thermal resistance created by the additional grain boundary fission gas porosity and a radial redistribution of the power generation within the pellet. The high degree of PCMI in rod 7 decreases during the first part of the irradiation, showing relaxation of cladding stress and strain due to pellet-cladding slip and fuel densification or creep. During the rest of the irradiation period there is a gradual increase in normalised length at a rate similar to, or possibly in excess of, that predicted by solid fission product swelling of the fuel. (author)