Release of unstable fission products from defective fuel rods to the coolant of a PWR

An analysis method has been developed to predict the contents of unstable fission products in the coolant of a PWR containing defective fuel rods under steady-state operating conditions. It takes into account the relevant physical processes, such as fuel pellet oxidation, enhancement of fission gas diffusivity due to fuel oxidation, and migration of fission products from the fuel-to-clad gap to the coolant by a first-order rate process. With the shape of grains treated as tetrakaidecahedron, the transport of fission products from the fuel matrix to the grain boundaries and formation of fission gas bubbles on grain boundaries are described by White and Tucker's model. In addition, the fraction of fission gas bubbles on grain boundaries interlinked to open surfaces is determined with the use of the Monte Carlo technique. Transport of the fission products from the gap to the coolant is assumed to occur by a first-order rate process. Comparison with experimental data shows that the present method predicts reasonably well the R/B ratios for defective fuel rods with intermediate linear heating rates and underpredicts for fuel rods with very high or low linear heating rates. (orig.)