AVR prototype pebble bed reactor: a safety re-evaluation of its operation and consequences for future reactors

Abstract

Abstract The AVR pebble bed reactor (46 MWth) was operated 1967 – 1988 at coolant outlet temperatures up to 990°C, which are suitable for process heat applications. Also because of a lack of other experience the AVR operation is a basis for future HTRs. This paper deals with insufficiently published but unresolved safety problems of AVR and draws conclusions for future pebble bed HTRs: Although the AVR was operated only for about 4 years at coolant temperatures > 900 °C its primary circuit is heavily contaminated with dust bound metallic fission products (90Sr, 137Cs), which create major problems in current dismantling. The end of life contamination reached several percent of a single core inventory. The AVR contamination was mainly caused by inadmissible high core temperatures, and not – as presumed in the past – by inadequate fuel quality only. The high core temperatures, which cannot be easily measured in pebble beds, are probably caused by the insufficiently examined pebble bed mechanics. Safety of future reactors requires a gas tight containment and in addition elaborate R&D or reduction of requirements concerning temperatures and burn-up.