Radioxenon from Spontaneous Fission Released by Spent Nuclear Fuel Reprocessing Facilities and Its Possible Impact on International Monitoring System Observations

Abstract

It is well known that significant amounts of radioxenon radionuclides are released from Medical Isotope Production Facilities and to a lesser extent from Nuclear Power Plants (NPP). These emissions cause a background in the atmosphere that is often detected by noble gas systems of the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission for nuclear explosion monitoring. In addition to those facilities, the operation of a Spent Nuclear Fuel (SNF) reprocessing plant may possibly also contribute to the IMS observations, but this has not yet been investigated. Even after long cooling time, the short-lived radioxenon isotopes are present in spent fuel due to spontaneous fission with the isotopes 244 Cm and 240 Pu being the main contributors. The SNF reprocessing process can promptly release the whole radioxenon inventory if there is no retention system. The aim of this work is to investigate the possible radioxenon emission during SNF reprocessing caused by spontaneous fission of heavy elements. Two independent methods are applied to determine the radioxenon releases. One approach is to use the published release of 131 I as a proxy. The other is to analyse the parameters of reprocessed spent fuel to determine the content of 244 Cm and 240 Pu and with this information to estimate the radioxenon inventory. It turns out that the estimated maximum release of 133 Xe is of the order of GBq/day which is almost as high as the average discharge on an NPP site. Assuming the absence of an effective retention system that prevents the release of radioxenon into the environment, the results of the calculations show that industrial scale reprocessing plants should be considered as a weak but not negligible source of radioxenon.

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