Overview of release phenomenology in Phebus FP and comparison with out-of-pile experiments

An estimate of the fission-product (FP) releases from fuel under reactor accident conditions is a key input to a source-term calculation. Numerous out-of-pile FP release experimental programs have been conducted over the years, including HEVA/VERCORS (France), HI/VI (U.S.A), VEGA (Japan) and HCE/UCE/MCE (Canada). These out-of-pile experiments have led to a good understanding of the fundamental phenomena affecting FP releases under accident conditions. One of the main objectives of the Phebus FP in-pile experimental program is to measure FP releases from fuel under conditions representative of Light Water Reactor (LWR) severe accident sequences. This paper presents a summary of the out-of-pile experiments and the fundamental phenomena affecting FP release from fuel under accident conditions. Releases determined in the Phebus FP in-pile tests conducted to date agree well with the results of out-of-pile tests for most volatile fission products. The extent of cladding oxidation makes a difference, particularly to the release kinetics. Releases for certain less-volatile fission-product elements, such as Mo and Ba, also show qualitative agreement for most tests. (author)