Environmental impacts of radon from closed uranium mine sites in Japan

The purpose of this paper is to review the current status of the techniques which are used at the Japan Atomic Energy Agency (JAEA) to evaluate the environmental impacts of radon released from closed mine sites. The impacts of radon from the sites are evaluated at the JAEA by two way approaches: environmental monitoring and dispersion model use. The traceability on radon measurements is realized by relating directly to the gas-filled ionization chamber method, the secondary standard of the JAEA. Through the intercomparison experiments among the international reference institutes, it was confirmed that the reliability and consistency of the secondary standard has been retained since 1984. Environmental monitoring program of the JAEA basically consists of the passive monitoring of radon and the active monitoring of F value. From a viewpoint of conservative evaluation, the plume model assuming Gaussian distribution was utilized to calculate the impact of radon from the sites. Our assessments, both environmental monitoring and dispersion model use, showed that the effective doses caused by closed uranium mine sites are less than 1 mSv y-1, the effective dose limit for the public, outside of the sites. These results were supported by the supplemental monitoring, the continuous radon and its progeny monitoring and the time-integrated radon progeny monitoring. Additionally, they were also supported by the preliminary investigation with using the numerical dispersion model including the wind-field estimation model based on a k-4 turbulence model. (author)