An analysis method for the fuel rod gap inventory of unstable fission products under steady-state operating conditions

An analysis method was developed to predict the equilibrium inventory of unstable fission products in the fuel-to-cladding gap of intact fuel rods under steady-state operating conditions. With the shape of grain treated as tetrakaidecahedron (TKD), the fraction of fission gas bubbles interlinked to open surface at grain corners was determined with use of the Monte Carlo technique and the White and Tucker's model. The release of unstable fission products from the interior region of the fuel pellet to the gap was related to irradiation conditions such as burnup, temperature and external restraint pressure on fuel pellet. In the lower temperature region where temperature is lower than about 1200 .deg. C, most of the release of unstable fission products was predicted to occur via burnup-independent paths such as crack and/or open pores. On the other hand, the release of unstable fission products appeared to be closely related to the interlinkage of grain corner bubbles at temperatures higher than 1200 .deg. C. In spite of several simplifying assumptions adopted for the prediction of inter-linkage of grain corner bubbles, the present method yields results that shows reasonable agreement between predicted and measured results from the CONTACT experiments and FIO series tests. Furthermore, the agreement indicates that during steady-state operation bubble interlinkage at grain corners is the most important step in determining the release rate of unstable fission products. In the case of defective fuel rods, another analysis method was also developed to predict both the contents of unstable fission products in the gap and the amount released to the coolant of the PWR containing defective fuel rods under steady-state operating conditions. It takes into account relevant physical processes, such as fuel pellet oxidation, and enhancement of fission gas diffusivity. Transport of the fission products from the gap to the coolant is assumed to occur by a first-order rate process. The fraction of fission gas bubbles on grain boundaries interlinked to open surfaces is determined in the same way as used for intact fuel rods. Comparison of predicted results with experimental data obtained under irradiation conditions shows that although uncertainty exists in each physical process mentioned above, the present method predicts reasonably well the contents of unstable fission products in the coolant released from the gap inventory of defective fuel rods when the pellet temperature lies in the following range : it is neither so high as to cause grain boundary sweeping nor so low as to cause the gap to be filled with water or mixture of water and steam. On the other hand, if the linear heat rate is low enough to permit the presence of water in the gap or if the linear heat rate is so high as to induce grain boundary sweeping, the present method underpredicts the amount of unstable fission products in the gap and coolant under steady-state operating conditions