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For the reactor cores just before the accident of Fukushima Dai-ichi NPP (FDNPP) units 1 to 3, the core average activity ratios of  134 Cs/ 137 Cs,  238 Pu/( 239 Pu+ 240 Pu), and  242 Cm/( 243 Cm+ 244 Cm) and the weight ratios of  235 U/ 238 U,  240 Pu/ 239 Pu, and  241 Pu/ 239 Pu were evaluated applying the nuclide inventories calculated with CASMO5 based on the detailed assembly specifications and the assembly-axial burnup histories and correcting the calculated inventories with the biases of the inventory calculations. The obtained activity and weight ratios were compared with those evaluated with the preceding inventory calculations with ORIGEN2.2. The present  134 Cs/ 137 Cs activity ratios were smaller by 3% to 5% than those with ORIGEN2.2. The measured results of the activity and weight ratios in the samples taken from the contaminated water in the plant facilities and the soil, dust, and others inside and outside of FDNPP were distributed around the calculated core average values, while some of them were deviated from them. The  240 Pu/ 239 Pu weight ratios can be used to distinguish the radioactive materials originating from unit 2 from those from unit 3 assuming that the measured results represent the core average values.

journal of nuclear science and technology

Activity and weight ratios of cesium, uranium, plutonium, and curium isotopes based on elaborate inventory calculations of Fukushima Dai-ichi NPP units 1 to 3