Contributions towards the development of a packaging concept for the final disposal of spent HTGR pebble bed fuel

Abstract In accordance with decisions of the Government of the German Federal Republic on the treatment of spent fuel from nuclear reactors, the spent spherical fuel elements from the German pebble bed High Temperature Gascooled Reactors (HTGRs). AVR (Arbeitsgemeinschaft Versuchs Reaktor) and THTR (Thorium HochTemperatur Reaktor) shall be directly disposed of in the national repository. Owing to their radiological and thermal properties, the spent HTGR fuel may be treated similar to heat generating medium active waste for which the disposal concept is the emplacement in deep boreholes of a salt mine. Packaging concepts for the final disposal of spent HTGR fuel shall follow the general design and the radiological requirements for the borehole emplacement technique, but credit may also be taken of advantageous properties of the HTGR fuel elements. Especially their built-in barriers against fission product release, designed for stringent operational, upset or even accident conditions in service, can help in the development of a safe and economic packaging and disposal concept for spent HTGR fuel. Evaluation of the mechanical behaviour of HTGR spent fuel elements in experimental testing under near repository conditions has shown that the standard HTGR fuel particle can generally survive the crushing of the elements under rock pressure, but, for safety reasons, a backfill matrix such as quartz sand or cement should be introduced in the case of thin walled disposal canisters to prevent element breakage as a whole at rock pressure.