Overview of the VERDON-ISTP Program and main insights from the VERDON-2 air ingress test

•2 VERDON experimental circuits for fission products release and transport studies.•Innovative thermal gradient tubes for fission product transport studies.•Overview of the 4 VERDON-ISTP tests: test matrix and main results.•Air ingress topic: fission product release under mixed steam-air conditions.•Ruthenium release and transport under steam-air conditions in the primary circuit. In order to reduce the uncertainties concerning source term assessment under LWR severe accident conditions, the International Source Term Program has been launched. Four main R&D research axes have been addressed in this program: iodine behaviour in the RCS and containment, study of the boron carbide effect on fuel degradation and FP release, study of the air effect on fuel and FP behaviour, FP release from high burn-up UO2 and MOX fuels. As far as the source term quantification is concerned, four VERDON tests have been defined and were performed in the VERDON laboratory at the CEA Cadarache Centre. We present an overview of these four tests, devoted to the study of FP release from high burn-up UO2 and MOX fuels. We focus on the VERDON-2 air ingress test performed on MOX fuel with a specific loop dedicated to FP release and transport. It addressed two issues: Ru release under mixed steam-air conditions following lower head failure and transport/re-volatilization of Ru and volatile FPs in the primary circuit. The latter was performed thanks to the implementation of thermal gradient tubes exposed at different phases of the sequence. In this configuration, the VERDON facility combines the ability of the analytical approach to identify and understand the physical phenomena involved, with a semi-integral approach, which allows the results to be applied to source term assessments for reactor application. This VERDON-2 test has shown a rather unexpected low release of ruthenium from the MOX sample and no re-volatilization from the thermal gradient tubes. On the contrary, a significant re-volatilization of iodine has been observed when switching to mixed steam-air conditions, as well as a low, but measurable amount of iodine transported under gaseous form downstream of the thermal gradient tubes.