Pie/modelling of fission gas release from a Ringhals high burn-up rod bumped in Halden

An ABB Atom fuel rod irradiated in Ringhals 1 (BWR) to a peak burn-up of 68 MWd/kgU has been re-fabricated at Kjeller and irradiated (bumped) in the Halden reactor. The fission gas release was 20%. The gas composition corresponds to the calculated composition of the generated fission gas at a burn-up of about 80-85 MWd/kgU. Ceramography and SEM investigations show that the release is from the central region, out to about 50-60% of the fuel radius. From thermocouple readings Halden inferred a peak centerline temperature at axial maximum power position (solid pellet) close to 1 350 deg C. The fuel centre shows normal changes for this temperature and there is a rim zone about 150 micron wide. Code calculations were performed for the estimation of temperature and FGR. Using burn-up corrected fuel thermal conductivity it was possible to reach temperature and FGR values close to the measured ones; without the correction the calculated values were far too low. (author)