[en] Non-destructive gamma-ray spectrometry was carried out on the spent fuel assemblies of the whole core of JPDR-I which was a BWR. These data were analyzed by considering power distribution, spatial variation of neutron spectrum, and history of reactor operation. The burnup and the Pu/U atom ratio in each assembly were derived from the non-destructively measured distributions of ¹³⁷Cs activity and ¹³⁴Cs/¹³⁷Cs activity ratio, respectively, by using calibration curves established for fuel specimens of a standard assembly of the core. The results were compared with calculational ones based on the operational data, and good correlations were found between them. The total amount of plutonium build-up in the core estimated from the non-destructive measurements agreed quite well with the amount obtained from reprocessing. (author)