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© 2000 The Japan Society of Nuclear and Radiochemical Sciences The distinctive feature of the JCO criticality accident is the continuous release of fission neutron without appreciable release of fission products. Hence, the estimation of total fission events (TFE) is the most important task for evaluating environmental impact. The sampling of uranium solution in the precipitation vessel (PV) in which the criticality accident took place, was performed by the members of JAERI to know TFE and the analytical results were reported to the Science and Technology Agency (STA) and the Nuclear Safety Commission (NSC). On Oct. 25, the Ministry of Education, Science, Sports and Culture decided to organize the Japanese university member to make research for the environmental impact due to the JCO accident and about 12 scientists and their coworkers started their research under the management of the group leader Prof. K. Komura, Kanazawa University. This research group has already performed five times the sampling of soil and plants in the JCO site. On the 2nd sampling on Oct. 26, the present authors carried a portable Ge detector and a 4096ch PHA to make the in-situ γ-ray spectrometry. The detector was set on the top of the northwest corner of a shielding wall made of sandbag heaped up around the Uranium Conversion Building (UCB) in which the PV was placed. Intense γ-ray signals mainly due to La that was in radioequilibrium with its precursor Ba (t1/2=12.75d) was recorded by the 4096ch PHA. The dead time ratio of the PHA system was about 20% and we noticed that the detector was at the nearest position practically allowed to access. Within about 30 minutes, three spectra were recorded and one of the spectra is shown in Figure 1. The main γ-ray at 1596 keV is very clearly observed with its sharp Compton edge, i.e., that implies the 1596 keV γ-ray comes from a single remote radiation source. Although scattering components are quite large at below about 500 keV and intense bremstrahlung are recognized up to about 2000 keV, many characteristic γ-ray peaks of La are observed. Rather weak γ-ray peaks of I and Zr-Nb are also observed. The reference data of the 66th meeting of NSC suggested that the majority of fission products was retained in PV. Therefore, TFE can be estimated without the sampling of the uranium solution in PV by passive γ-ray spectrometry of appropriate fission product radioactivity. Toshiaki Mitsugashira,*,a Takashi Nakanishi,b Riki Seki,c and Sadao Kojimad aThe Oarai-branch, Institute for Materials Research, Tohoku University, Narita-machi, Oarai-machi, Ibaraki-ken 311-1313, Japan bDepartment of Chemistry, Faculty of Science, Kanazawa University, Kadoma-machi, Kanazawa 920-1192, Japan cDepartment of Chemistry, University of Tsukuba, Tsukuba, Ibaraki-ken 305-8571, Japan dRadioisotope Research Center, Aichi Medical University, Nagakute-machi, Aichi-gun, Aichi-ken 480-1195, Japan

journal of nuclear and radiochemical sciences

Estimation of Total Fission Events in the JCO Criticality Accident by Passive ^|^gamma;-Ray Measurement of 140La in Radioequilibrium with Precursor 140Ba