Speaker
Description
Whenever 135Xe is observed, it must originate from a fresh release due to its short half-life (9.2 hours). An activity ratio of 135Xe to 133Xe above 5 is an indicator for a possible nuclear test release. Therefore, it is important to fully understand observation of very high activity ratios of 135Xe to 133Xe in atmospheric air and even 135Xe without simultaneous observation of 133Xe as they occasionally occur at a few IMS stations (AUX04, JPX38, SEX63, NOX49, USX75, USX77). However, the nuclear processes causing such observations remain unexplained. One hypothesis of a possible source is power ramping down and the restart of light-water reactors (LWRs). For the mentioned IMS stations, the field of regard in the past 24, 48 and 72 hours are studied to identify a possibly common source region. The time of possible releases is compared with begin and end of outages reported in the Power Reactor Information System. The hypothesis is tested whether LWRs can explain some or all occurrences of high 135Xe concentrations. This has significant bearing on event screening for nuclear explosion monitoring. Specifically, conclusions will be drawn on the application of the 135Xe to 133Xe screening threshold.
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