Source Term of Iodine and Noble Gas Fission Products in the Fuel-to-Sheath Gap of Intact Operating Nuclear Fuel Elements

The extent to which fission products are released from reactor fuel and escape from the primary system is determined primarily by their volatility and chemical behaviour. In order to study such short-lived fission-product release behaviour in intact operating CANDU-type fuel, several experiments at Chalk River were performed using a sweep gas technique. The sweep gas technique was used to determine the physical mechanisms of gas release for unoxidized fuel through the measurement of an instantaneous release rate. However, from a safety analysis perspective, it is also important to present the data as an integrated source term in the fuel-to-sheath gap which is immediately available for release after sheath failure. The iodine species have diffusion behaviour in the UO2 fuel matrix similar to that of noble gas. Assuming similar release processes, the release behaviour of iodine can also be modelled from the noble gas kinetic data.