Contributions to the 37 Ar background by research reactor operations
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Contributions to the operations
37
Ar background by research reactor
A. G. Fay • S. R. Biegalski
Received: 2 July 2012 / Published online: 23 August 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract 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 9 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. Keywords CTBT Treaty monitoring Radioargon On site inspection Effluent monitoring Noble gas
A. G. Fay (&) S. R. Biegalski Nuclear Engineering Teaching Laboratory, The University of Texas at Austin, 1 University Station, R9000, Austin, TX 78712, USA e-mail: [email protected]
Introduction In order for the Comprehensive Nuclear-Test-Ban Treaty to enter into force, a verification regime must be in place to ensure the compliance of member states. The use of radionuclides for monitoring purposes has traditionally focused on radioxenon collection and measurement by the International Monitoring System (IMS). Recent work has shown that radioxenon may not be the only radioactive noble gas useful for monitoring purposes. Radioargon isotopes would be produced in an underground nuclear explosion in sufficient quantities to be collected and measured by an On-Site Inspection (OSI) team [1, 2]. Development on current 37Ar measurement systems has achieved a minimum detectable concentration (MDC) of 45.1 mBq m-3 [3]. Estimates suggest a lower limit on the MDC of 20 mBq m-3 for a field system and 0.02 mBq m-3 for a laboratory system [1]. At these concentrations, it is important to account for sources of 37Ar background that may be present at the OSI location. Argon is the third most abundant component of air at a volume fraction of 0.93 %. The radioisotopes 37Ar and 41 Ar, among other argon isotopes, are incidentally produced in a reactor by the activation of air. Typical off-gas systems collect activated air from the water coolant and release it f
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