Passive gamma-ray spectrometry for the determination of total fission events in the JCO criticality accident ʹ99 in Tokai

The total number of fission events (TFE) in the JCO criticality accident was determined to be 3.43×1018 with a 3σ uncertainty of 0.73×1018 by passive observation of the gamma-rays of 95Zr, 103Ru and 140La–140Ba from outside the Uranium Conversion Building. The TFE value is in fairly good agreement with the analytical result from the uranium solution taken from the precipitation vessel, as reported in NSC-Nov, 1999. The agreement implies that the criticality accident took place in a small limited volume and that the majority of the fission products were retained in the precipitation vessel.