Fission gas release from high burnup fuel during power transients: experimental data for modelling

The present paper aims at presenting recent experimental programmes and results on fission gas release during power transients at high burnups obtained on French PWR fuel rods delivered by FRAMATOME Nuclear Fuel. This type of results are needed for modelling purposes, since the pressure increase due to fission gas release is one of the main limitations to burnup extension. The first experimental programme presented here consisted in power ramps performed in the Studsvik R2 reactor on four refabricated fuel rods pre-irradiated during 5 cycles in Gravelines (standard operation) and during 4 cycles in Cruas (grid regulation operation) Electricite de France (EDF) units. These ramps were undertaken with Pellet Cladding Mechanical Interaction (PCMI) analysis objectives. Given that no failure occurred on these rods, gas punctures and measures after ramps (1 to 12 hours holding time at maximum power) gave interesting global data on fission gas release for the calibration of models. The results are presented and discussed. Nevertheless, in order to develop accurate and mechanistic models, data on the fission gas release kinetics and on the gas localization in fuel are needed. With this aim, the French Atomic Energy Commission (CEA), FRAMATOME Nuclear Fuel and EDF undertook the REGATE experimental programme. This analytical programme performed in Grenoble, with SILOE reactor and LAMA hot laboratory facilities, consisted in calibrated power transients on four PWR fuel segments pre-irradiated in Gravelines unit up to 50 GW-d.tM~\ The maximum power levels ranged from 290 to 390 W.cm" , while the holding time at power ranged from 20 minutes to 6 hours. The increment of fission gas release during transients were determined by differences between gas puncture results after re-irradiation and non destructive determination of the amount of Kr released during PWR operation. The Kr technique and results are presented and discussed. Elsewhere, complementary data were obtained from PIE: the precipitation in bubbles of fission gas was characterized by optical microscopy and the localization of gas in the fuel was determined using the ADAGIO technique recently developed in Grenoble. This technique allows to quantify separately the gas retention in grain and at grain boundaries.