Contributions to the 37Ar background by research reactor operations

Radioargon has been identified as a useful nuclide for verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty. Use of 37Ar to identify a nuclear explosion requires quantification of contributions to the 37Ar background at a potential measurement site. A method of estimating 37Ar release activities using isotopes of radioxenon and radioargon has been developed in this paper. Numerical solutions to the system of equations describing air-activation in a reactor were used to determine ratios of release activities for 135Xe/133Xe, 133mXe/131mXe, and 37Ar /41Ar as function of irradiation time and off-gas residence time prior to measurement and release. Published radioactive noble gas effluent data for the High Flux Isotope Reactor, HFIR (ORNL) from the year 1996 to 2010 were compiled as a test data set to predict the 37Ar release on a yearly basis. An average 37Ar release rate of 1.86 x 1010 Bq per year was calculated. The estimated release rate was used as a source term for atmospheric transport to run a test case for 37Ar release over a typical HFIR operation cycle. Results showed that ground-level concentrations of 37Ar did not exceed the minimum detectable concentration for a 37Ar field measurement system beyond the immediate vicinity of the release point. (author)