Study for Nuclide Transfer Ratio of Particles Generated by Thermal Cutting

In dismantling of nuclear facilities, secondary products would disperse into air and water. Thus, it is necessary to assess the effective dose for the public due to the gaseous and liquid radioactive wastes discharged. Especially for the cutting of a highly activated reactor core structures, the material may release special nuclides, some of which can be metal with low melting temperature and relatively high vapor pressure. In this case, the vaporized metal easily transfers to the particles instead of leaving in the dross and would make significant impact to the work place and environment. A pressure-tube type reactor often utilizes zirconium alloy as core structure, which may contains nuclides with high vapor pressure such as radioactive tin or antimony caused by activated tin. In this study, the radioactivity transfer ratio for thermal cutting in air has been investigated by cold tests. The tests have been carried out in an air-tight clean house and used a dust sampling system. The test piece was zirconium and niobium 2.5% alloy with some impurities, which was non-radioactive spare material for the pressure tube of Fugen NPS. Plasma cutting method was carried out with 200 A electric current. The particles were separated by one micrometer size with membrane filters and the material was investigated with an Energy Dispersion X-ray Analyzer (EDX) and the shape of the particle was observed with a Scanning Electron Microscope (SEM). The results showed that there was almost no tin in the dross because most of the tin was evaporated and transferred to aerosol. However, the ratio of zirconium, niobium and iron in the aerosol was not as high as that of tin. The vaporization pressure of antimony is higher than that of tin. The tin can be easily activated by neutron and becomes radioactive antimony. Because Zircalloy-2 or 4 contains ca. 1 percent of tin, irradiated Zircalloy contains significant radioactive antimony (Sb-125). Preparatory analyses showed that the Sb-125 to the environmental impact was comparable with other nuclides. For this reason a test for taking data of the transfer ratio of antimony by using irradiated material has been also carried out.