Directional Distribution of Radiation around an Accident at a Uranium Fuel Factory in Tokai-mura, 1999

JUN TAKADA 0 SHINJI SUGA 0 KAZUHIDE KITAGAWA 0 MASAYORI ISHIKAWA 0 SEIJI TAKEOKA 0 MASAHARU HOSHI 0 HIROMITSU WATANABE 0 AKIHIRO ITO 0 NORIHIKO HAYAKAWA 0 0 Research Institute for Radiation Biology and Medicine, Hiroshima University Kasumi 1-chome , Hiroshima 734 - 8553, Japan (Received, July 19, 2000) (Revision received, September 25, 2000) (Accepted, December 18, 2000) Tokaimura / Criticality accident / Radiation / Directional dependence / Residential areas A beta-ray survey was carried out on concrete walls of the boundary and buildings after a criticality accident at a factory of JCO Co. Ltd. at Tokai-mura. A remarkable distribution of beta counts was observed on the walls depending on the complex internal and external structures of buildings surrounding a precipitation vessel containing uranium 23 days after the accident. The directional distribution function, based on the beta counts on the walls, was consistent with data concerning the neutron dose rate measured in several directions during the accident, suggesting an anisotropic neutron distribution to the residential area. - A criticality accident, which occurred at the uranium conversion facilities of JCO Co. Ltd. in Tokai-mura, Japan on September 30, 1999, caused radiation exposure to not only workers, but also residents1). The critical state, which occurred at 10 : 35 am, was terminated by draining off cooling water around the precipitation vessel (PV) at 6 : 30 am on October 1. The evacuation of people within a 350 m zone began at 15 : 30 on the same day1). During the accident, neutrons and gamma rays exposed people around the factory. The gamma ray and neutron dose rates were reported to be 0.84 mSv/h (gamma) at 11 : 36, 0.5 mSv/h (gamma) and 4.5 mSv/h (neutron) at the boundary of the factory at 19 : 09 on Sep. 30, as the maximum values1, 2). This neutron exposure to residents was completely different from that in the Chernobyl atomic power plant accident, and nuclear weapon tests where there was no direct neutron exposure to the population around the site3). A dosimetry study for residents is urgently important under such circumstances. Japan Atomic Energy Research Institute (JAERI) reported external doses as a function of distance2, 4). We focus here on the directional dependence of radiation from the criticality accident due to the complex structures of the buildings between the source and the residential area. The enriched uranium and fission products were kept in the PV (450 mm in inner diameter) of the building where the accident occurred during our measurements. Sand-bag walls were built on Oct. 2 for radiation protection of the residents located around the facility. Several concrete walls were built instead of sand-bag after our measurement on Oct. 26, 1999. A radiological survey was conducted mainly for beta counting on the wall of the boundary and of the building after the accident under the present condition. MATERIALS AND METHODS Beta counting was conducted at about 1-m height on the walls using a ZnS (Ag) plastic sintilator (Aloka TCS-352, active area of 72 cm2). The detection efficiencies for beta rays (Emax = 310 keV) and gamma ray (Eav = 1253 keV) of Co-60 are 14.3 and 1.3 (%/2), respectively. Each measurement was done for one minute. The main measurements were carried out on the boundary wall, which was made of concrete with 3 cm thick exists, the length between northeast and southwest was 880m, as shown in Fig. 1 on Oct. 23 25. The values at three points with 80 cm intervals, which were measured at each site of the boundary wall, were averaged. The sites on the outer wall of facility where measurements were made on Oct. 26 are shown in Fig. 2. The number of measurements was one for each site in this case. The Fig. 1. Local map of the facility for uranium fuels factory in Tokai where the accident took place. Beta counting was carried out on walls B1, B2 and B3 at the boundary of the factory. Fig. 2. Measurement sites on the outer wall of the buildings around the critical uranium source. The number of high beta count sites (more than 10 kcpm) is indicated by circles. The sites of maximum beta counts on walls B1 and B2 are marked by bars. The values of No. 911 of the C building were estimated to be more than 10 kcmp from the corresponding gamma dose rates (see Table 1). entire wall is also made of the same material and thickness (autoclaved light concrete with 10 cm thickness, Siporex, Sumitomo Metal Mining Co., Ltd.,). A clear distribution of beta counts was observed on the western B1 and northern B2 walls, as shown in Fig. 3. In this figure, the net count was evaluated from the gross counts subtracted by the background value of 137 cpm, which was assumed to be average one for data beyond 300 m from the northwestern corner of the wall. Moreover, each point was recalculated by smoothing with neighboring points (ni = (ni 1 + ni + ni+1)/3). The maximum net values were about 3 sigma (12 cpm) of the raw background counts in the B1 and B2 walls. On the other hand, the level of net counts on the eastern wall (B3) was about within one sigma of the background count. Therefore, the distribution of beta count rate on this B3 wall was not clear due to the distant area about 500 m east away from the uranium source. The distribution of beta counts in the B1 wall had a maximum at 85 m from the northwestern corner with full width at half maximum (FWHM) of 40 m. The count rates sharply Fig. 3. Net beta counts on walls B1 and B2 plotted as a function of the distance from the northwestern corner on October 23 25 1999. The data were smoothed by 3 continuous points. decreased toward southern side from this site. This may have been due to the shielding of the E-building for radiation, as shown in Fig. 2. The distribution of the beta count rate at the B2 wall had the maximum value at 50 m from the northwestern corner with a FWHM of 29 m. An obvious distribution of beta count rates was also observed on the outer wall of several buildings, as shown in Fig. 2 and Table 1. The most remarkable site was No. 7 of the Dbuilding with a beta count of 31 kcpm. The values were about 30-times higher than those on neighboring sites of both sides on the same wall. There was no concrete wall but two iron doors between site No. 7 and the uranium source in the C-building. Neutrons passing through the iron door from the source might strongly irradiate the wall at site No. 7. We notice that site No. 7 of the D-building is on one line between the source in the C-building and the maximum site of the B1 wall. Moreover, a remarkable distribution of beta counts was observed on the outer walls of the C-building of the facility where the accident occurred. Sites No. 7 and No. 8 at the northeastern wall of the C-building exhibited more than 10 kcpm. This wall has no glass windows. The distance between the source and the residents is more than 400 m, and several buildings exist in the northeastern direction. In the northweste (...truncated)