Cooling time determination of spent nuclear fuel by detection of activity ratio 144Ce/137Cs

Activity ratio of two radioactive primary fission products which had sufficiently different half-lives was expressed as functions of cooling time and irradiation histories in which average burnup, irradiation time, cycle interval time and the dominant fissile material of the spent fuel were included. The gamma-ray spectra of 36 samples from 6 spent PWR fuel assemblies irradiated in Kori unit-1 reactor were obtained by a spectrometric system equipped with a high purity germanium gamma-ray detector. Activity ratio 144Ce/137Cs, analyzed from each spectrum, was used for the calculation of cooling time. The results show that the radioactive fission products 144Ce and 137Cs are considered as useful monitors for cooling time determination because the estimated cooling time by detection of activity ratio 144Ce/137Cs agreed well with the operator declared cooling time within relative difference of ±5% despite the low counting rate of the gamma-ray of 144Ce (about 10-3 count per second). For the samples with several different irradiation histories, the determined cooling time by modeled irradiation history showed good agreement with that by known irradiation history within time difference of ±0.5 year. From this result, it would be expected to be possible to estimate reliably the cooling time of spent nuclear fuel without the exact information about irradiation history. The feasibility study on identification of and/or sorting out spent nuclear fuel by applying the technique for cooling time determination was also performed and the result shows that the detection of activity ratio 144Ce/137Cs by gamma-ray spectrometry would be usefully applicable to certify spent nuclear fuel for the purpose of safeguards and management in a facility in which the samples dismantled or cut from spent fuel assemblies are treated, such as the post irradiation examination facility. (Author)