A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts

摘要

The AVR pebble bed reactor (46 MWth) was operated 1967-88 at coolant outlet temperatures up to 990°C. A principle difference of pebble bed HTRs as AVR to conventional reactors is the continuous movement of fuel element pebbles through the core which complicates thermohydraulic, nuclear and safety estimations. Also because of a lack of other experience AVR operation is still a relevant basis for future pebble bed HTRs and thus requires careful examination. This paper deals mainly with some insufficiently published unresolved safety problems of AVR operation and of pebble bed HTRs but skips the widely known advantageous features of pebble bed HTRs. The AVR primary circuit is heavily contaminated with metallic fission products (Sr-90, Cs-137) which create problems in current dismantling. The amount of this contamination is not exactly known, but the evaluation of fission product deposition experiments indicates that the end of life contamination reached several percent of a single core inventory, which is some orders of magnitude more than precalculated and far more than in large LWRs. A major fraction of this contamination is bound on graphitic dust and thus partly mobile in depressurization accidents, which has to be considered in safety analyses of future reactors. A re-evaluation of the AVR contamination is performed here in order to quantify consequences for future HTRs (400 MW$_{th]$). It leads to the conclusion that the AVR contamination was mainly caused by inadmissible high core temperatures, increasing fission product release rates, and not - as presumed in the past - by inadequate fuel quality only. The high AVR core temperatures were detected not earlier than one year before final AVR shut-down, because a pebble bed core cannot yet be equipped with instruments. The maximum core temperatures are still unknown but were more than 200 K higher than calculated. Further, azimuthal temperature differences at the active core margin of up to 200 K were observed, probably due to a power asymmetry. Unpredictable hot gas currents with temperatures > 1100°C, which may have harmed the steam generator, were measured in the top reflector range. After detection of the inadmissible core temperatures, the AVR hot gas temperatures were strongly reduced for safety reasons. Thus a safe and reliable AVR operation at high coolant temperatures, which is taken as a foundation of the pebble bed VHTR development in Generation IV, was not conform with reality. Despite of remarkable effort spent in this problem the high core temperatures, the power asymmetry and the hot gas currents are not yet understood. It remains uncertain whether convincing explanations can be found on basis of the poor AVR data and whether pebble bed specific effects are acting. Respective examinations are however ongoing. Reliable predictions of pebble bed temperatures are at present not yet possible. [...]