Evaluation of the in-pile pressure data from instrumented fuel assemblies IFA-431 and IFA-432.

Abstract

This report includes results of the examination of the in-pile pressure data from instrumented test assemblies IFA-431 and 432. The pressure data have been used to estimate the fission gas release fraction as a function of fuel burnup. Included are comparisons of the estimated release functions and those predicted by three fission gas release models using the experimental temperature histories of the fuel rods. These comparisons show that fuel temperature is the primary factor in determining fission gas release and that burnupenhanced fission gas release is not important in U0 2 fuels irradiated to 1700 GJ/kgU (20,000 MWd/MTM). v • SUMMARY AND CONCLUSIONS This report includes the internal pressure data from fuel rods in two of the Nuclear Regulatory Commission/Pacific Northwest Laboratory's instrumented fuel assemblies. Three rods in each assembly were equipped with pressure transducers for monitoring fission gas release during irradiation. The experimental parameters were consistent with current light water reactor fuel and include fuel temperature, fuel density, fuel stability with respect to densification, and burnup. Instrumented fuel assembly 431 (IFA-431) was discharged after achieving burnups in excess of 350 GJ/kgU (4 GWd/MTM). Assembly 432 is still being irradiated, but current exposures in the IFA-432 rods are in excess of 1700 GJ/kgU (20 GWd/MTM). Meaningful pressure data was obtained from five of the six pressure transducers. Data from all five shows a pressure decrease during the initial stages of irradiation, which is due to the combined effects of fuel densification and helium loss from the fuel rod system. The relative contribution from each of these could not be established and, thus, a range of gas release fractions was calculated from the pressure data. In all cases, the estimated gas release was less than 15%. The estimated release fractions were compared to the predictions from gas release models currently being used in the GAPCON and FRAPCON series of steadystate fuel performance computer codes. Specifically, the GASREL (Beyer, et ale 1975), FGASRL (Reymann, ed., 1978), and ANS54 (Rausch and Panisko, 1979), subroutines for fission gas release were used in conjunction with the measured temperature/power histories of each rod. From these comparisons, the following has been concluded regarding fission gas release from U0 2 fuels. • Fuel temperature is the predominant factor in fission gas release. • Fuel density has only a minor influence on fission gas release.

Final data report for the instrumented fuel assembly (IFA)-432

“…Higher temperatures at the uoper position also shortened the useful lifetime of the upper TCs in the remaining fuel rods. for rods 1, 5, and 6, the upper TC failures were associated with temperature increases caused by fission gas release (Bradley et al 1979a). Data from the upper TC of rod 3 appeared to be questionable after 1933 GJ/kgU (22.4 GWd/MTU).…”

“…IFA-432 has provided a vast amount of well-characterized experimental data under conditions that realistically simulate light-water reactor (LWR) conditions. The data have been used extensively for analyzing fission gas release (Bradley et al 1979a;Bradley et al 1979b) and thermal and mechanical fuel rod performance(b) and for evaluating error propagation in stored energy calculations (Cunningham et al 1978;Cunningham et al 1980). .…”