Evaluation of Volatile and Gaseous Fission Product Behavior in Water Reactor Fuel Under Normal and Severe Core Accident Conditions

As the noble gases play a major role in establishing the interconnection of escape routes from the interior to the exterior of nuclear reactor fuel, a realistic description of the release of volatile fission products (VFPs) must a priori include a realistic description of fission gas release and swelling. In addition, as the VFPs are, in general, quite soluble in the fuel matrix and are known to react with other elements to form compounds, a realistic description of VFP release must include the effects of VFP chemistry on VFP behavior. The steady-state and transient gas release and swelling subroutine, FASTGRASS, has been modified to include a mechanistic description of the behavior of VFPs (iodine, cesium, CsI, CS₂M0O₄, and CS₂UO₄). Phenomena modeled are the chemical reactions between the VFPs, VFP migration through the fuel, and VFP interaction with the noble gases. Calculations were performed with FASTGRASS to describe the release of noble gases, iodine, cesium, and CsI from light water reactor fuel during steady-state and power-ramping conditions. Key issues that are addressed in the analysis are the effects of (a) VFP chemistry, (b) various assumptions concerning mechanisms of VFP migration through solid UO₂, (c) fission gas behavior, and (d) an accident scenario on the chemical form of the released iodine and the rate of iodine release from water reactor fuel.