Fission Product Release and Fuel Behavior of Irradiated Light Water Reactor Fuel Under Severe Accident Conditions: The ST-1 Experiment

High-burnup uranium dioxide reactor fuel was heated in-pile at ∼2490 K in a reducing atmosphere (33% H₂ in argon) for 16 min. Fission product aerosols and vapors released from the fuel were collected on a series of sequentially opened filters; the fractions of the original fuel inventory collected on the filters were f _Cs = 0.56, f _I = 0.38, f _Ba = 0.078, f _Sr = 0.053, f _Eu = 0.064, and f _Te < 0.002. The measured release rates for nonvolatile fission products were much higher than predicted by existing release codes, whereas tellurium release was much lower. Posttest examination of the fuel indicates extensive fuel/clad interaction, fuel swelling, and infiltration of the fuel by a zirconium-rich metallic melt; this melt kept oxygen potentials in the fuel very low. The low oxygen potentials and fuel disruption may account for the discrepancy between release codes and the test release results.