Simulated accident testing of a fuel element from the HFR-EU1bis irradiation campaign

The Cold Finger Apparatus (KühlFinger-Apparatur – KüFA) in operation at JRC-ITU is designed to experimentally examine the effects of Depressurization and LOss of Forced Circulation (DLOFC) accident scenarios on irradiated High Temperature Reactor (HTR) fuel pebbles. While remaining under an inert helium atmosphere, a HTR fuel pebble is subjected to heating schedules up to 1800 °C for several hundred hours. Fission gas release is monitored online during the experiment and volatile fission products are collected on condensation plates made of stainless steel positioned above the fuel sample on a “cold finger”, which is water-cooled to approximately 100 °C. Analysis of the substances deposited on the plates by means of gamma spectroscopy provides information on the fission product release as a function of time and temperature.The most recent KüFA test was performed on a fuel element irradiated in the High Flux Reactor (HFR) in Petten in the HFR-EU1bis campaign. The condensation plates feature dose rates up to 80 mSv/h and the lab background levels to some hundred μSv/h. Thus, effective collimation and background shielding is mandatory in order to perform a quantitatively accurate analysis of the samples by means of gamma spectroscopy. We present a detailed description of the experimental setup and the calibration procedure. The time-dependent fractional release of the volatile fission products 134Cs and 137Cs is shown. The results for the most recently tested fuel pebble HFR-EU1bis/5 are compared to data obtained for other fuel elements