Chemical analysis of transuranium nuclides in the uranium solution of the JCO criticality accident

Atom numbers of Np-239 and Pu-239, which had been produced by the neutron c apture reaction of U-238 in the uranium solution of the JCO criticality acc ident at Tokai-mura, Japan, were determined precisely by a radiochemical me thod. It was found that the measured atom ratio of Np-239/Pu-239 at termina tion of the criticality depends sensitively on the history of the neutron c apture reaction during the accident (about 20-hours continuance). The histo ry was evaluated by comparing the measured value and those calculated by a burnup calculation code. The percentage of the number of the neutron captur e reaction at the initial stage of the criticality during 25 min against th e total events was determined to be 24 +/- 6%.