(The) evaluation of the state of fuels in a PWR

Abstract

For a PWR, the prediction method based on an analytical model and visual technique is developed to estimate the escape rates of fission products through a defective cladding, the mass of tramp uranium in the primary coolant, and the number of defective fuels, by using the measured activities of $^{131}I$, $^{133}I$, and $^{135}I$ in the primary coolant in the steady-state operation. The analytical model represents the mass blance of fission products for three regions, i.e, the pellet, gap, and coolant. In the pellet region, the fission products produced by fission are proposed to be released into the gap by three mechanisms which are recoil, knock-out, and temperature-enhanced migration. In the gap, the inventory of fission products is determined by those released from the fuel and escaping into the coolant through the defective cladding. The concentrations of fission products in the primary coolant are dependent on the escape rates of the fission products, removal rates from the coolant due to the clean-up system, and release rates of fission products into the coolant by fissioning of tramp uranium. Based on the analytical model, the computer program, EDFIP (the Evaluation of Defective Fuels In PWRs), is developed. By using EDFIP, a map is drawn, which shows the relationships of the escape rates of fission products through the defective cladding and the mass of tramp uranium in the primary coolant circuit in terms of the activity ratios of $^{131}I/^{133}I$ and $^{135}I/^{133}I$. To test the validity of the present method, the results calculated by the present model are compared with those of the sipping tests conducted during the EOC 1 and 2 of KORI - 1. In the first cycle, it is estimated that there were 2 clad failures, and that the maximum mass of tramp uranium was about 102g which corresponds to release of 5 pellets from the fuel to coolant. The sipping tests conducted during the EOC 1 showed 2 defected assemblies. In the second cycle, the EDFIP code pred...