Applications of results from the Phébus-FP programme in the US regulatory process

•Phébus-FP test results are being used in the U.S. regulatory process.•Core degradation is predicted to taken longer.•Cesium is predicted to be in a less volatile chemical form.•Larger molybdenum and tellurium releases are predicted now.•The chemical form of iodine is an issue requiring further resolution.

Results of the Phébus-FP experiments are being used by the U.S. Nuclear Regulatory Commission to validate, update, and revise its mechanistic models of severe reactor accident phenomena. These updated models are being used to revise accident analysis source terms used in a variety of regulatory processes including plant siting, equipment qualification and control room habitability assessments. Important revisions include assumption of a less volatile chemical form for radioactive cesium, larger release of radioactive molybdenum and prolonged duration of the reactor core degradation process. The treatment of iodine release to the reactor containment remains problematical and requires additional research. This research should address the need to assure containment sump water pH be kept high throughout a reactor accident.