Reevaluation of the burnup of spent fuel pins by the activity ratio of 134Cs/137Cs

A total of 215 burnup values among 10 different spent fuel pins of a Taiwan Research Reactor have been reevaluated by use of an activity ratio of 134Cs/137Cs. These burnup values have then been compared to both previous studies and a theoretical estimate from the ORIGEN-II code. The computed estimate was based on an interactive approximation of simplified balance equations for an integrated flux, and a series of one ground burnup dependent microscopic cross section library derived by a semiempirical test. The library file of the ORIGEN-II code appears to overestimate the σa (133Cs) value, leading to a predictive curve of burnup value of 134Cs/137Cs which is 24-35% less than the measured data. The reevaluated burnup of 7%-235U enriched fuel pins deviates from others at a burnup of over 9000 MWD/MTU, indicating the rough approximation of this newly developed model. However, one still has a reliable and available model for both natural and enriched uranium fuel pins. This is possible because of the excellent results obtained here in comparison to the destructive method for natural uranium level fuel pins under 9000 MWD/MTU. (author)