Development of an atmospheric 85Kr automated sampler and analyzer

One of the potential interests of atmospheric 85Kr measurement is the detection of clandestine reprocessing activities as 10-35 TBq of 85Kr are emitted per kg of reprocessed Pu. To this goal, a system for Krypton sampling, purification, concentration and 85Kr measurement is currently under development. It has been designed to be fully automated, to perform Krypton trapping at room temperature, and to obtain at least 0.5 cm3 of Krypton for a 6-hour operating cycle. The prototype system is composed of four distinct stages that perform successively the following steps: i) air sampling, first purification and pre-concentration; ii) purification and concentration; iii) further concentration; iv) Krypton volume and 85Kr activity measurements. Thanks to a permeation membrane, the first-step purifies air from O2, H2O, CO2, and pre-concentrates Krypton by a factor of three. Air equivalent processed volume in 6 hours is about 0.6 m3. The second step consists in Krypton purification by room temperature trapping - high temperature (around 200 deg C) desorption on high specific area active charcoal beds in three columns. Choice of the active charcoal is of utmost importance. The further concentration stage allows reduction of the elution gas volume thanks to three in-line activated carbon columns. Final transfer into measuring cell is carried out by hot desorption of the accumulated Krypton for each 6-hour cycle. For the activity measurement, we modified a commercial proportional counter to adapt it to low-level 85Kr measurement. Counting efficiency is close to 70% and background is about 5 counts·minute-1. We tested the automated process, without the proportional counter, for many operating cycles. First tests show that we collect about 0.7 cm3 of stable Krypton (STP conditions). The whole system will be able to detect moderate 85Kr activity variations, typically a few tenth of Bq·m-3, above 1.4 Bq·m-3 background. (author)