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Fission product release from accident tested irradiated High Temperature Reactor fuel pebbles

The Cold Finger Apparatus (KüFA) is used to examine the combined effects of Depressurization and Loss of Forced Circulation accident scenarios on High Temperature Reactor fuel pebbles, constituted of a graphite matrix containing sub-mm-sized fuel particles with tri-isotropic coating. A fuel pebble is heated in He atmosphere for several hundred hours, mimicking accident temperature transients up to 1800 C. Volatile fission products released from the fuel condense on a water cooled stainless steel plate (“Cold Finger”) positioned above the tested pebble in the furnace. Released fission gases are collected downstream in a charcoal trap cooled with liquid nitrogen and gamma monitored with a NaI detector. Exchanging plates frequently during the experiment and analyzing plate deposits by means of High Purity Germanium gamma spectroscopy allows a reconstruction of the fission product release as a function of time and temperature. In order to achieve a good quantification of the release, a careful calibration of the setup is mandatory. The analysis of condensation plates from recent KüFA tests shows that fission product release quantification is possible at high and low activity levels. An especially tailored collimator allows plate scanning with high spatial resolution, thus yielding information about the fission product distribution on the condensation plates.