Benchmarking release, circuit and containment codes against Phebus FP

PHEBEN was a concerted action with eleven partners, benchmarking modeling codes against fission product release, circuit transport, containment aerosol physics and containment chemistry data from the in-pile experimental programme Phebus-FP. The codes gave a satisfactory overall picture of the first two tests FPT-0 and FPT-1, with generally good predictions of containment thermal hydraulics and aerosol physics. Release models performed quite well, although the predicted release of semi-volatiles was too great and the observed effect of changing fuel geometry (rod-like to melt pool) is not considered. In the circuit, codes now predict correctly that most elements are in aerosol form, but while the zone of greatest deposition is well predicted, the amount of deposition is not. Containment chemistry models for silver/iodine reactions, which can trap iodine in the sump water, so reducing its presence in the atmosphere and potentially reducing the source term, have been successfully developed and validated. Overall, the focus of severe accident research is being significantly affected by Phebus results and effort in this area is expected to continue.