MEASUREMENT OF FISSION-PRODUCT GASES IN THE ATMOSPHERE

The ability to quickly detect and assess the magnitude of releases of fission-produced radioactive material is of significant importance for ongoing operations of any conventional nuclear power plant or other a ctivities with a potential for fission product release. In most instan ces, the control limits for the release of airborne radioactivity are low enough to preclude direct air sampling as a means of detection, es pecially for fission gases that decay by beta or electron emission. It is, therefore, customary to concentrate the major gaseous fission pro ducts (krypton, xenon and iodine) by cryogenic adsorption for subseque nt separation and measurement. This study summarizes our initial effor ts to develop an automated portable system for on-line separation and concentration with the potential for measuring environmental levels of radioactive gases, including Kr-85, Xe-131,Xe-133,Xe-135, C-14, H-3, S-35, I-125,I-131, etc., without using cryogenic fluids. Bench top and prototype models were constructed using the principle of heatless fra ctionation of the gases in a pressure swing system. This method remove s the requirement for cryogenic fluids to concentrate gases and, with suitable electron and gamma ray detectors, provides for remote use und er automatic computer control. Early results using Xe-133 tracer show that kinetic chromatography, i.e., high pressure adsorption of xenon a nd low pressure desorption of air, using specific types of molecular s ieves, permits the separation and quantification of xenon isotopes fro m large volume air samples. We are now developing the ability to measu re the presence and amounts of fission-produced xenon isotopes that de cay by internal conversion electrons and beta radiation with short hal f-lives, namely Xe-131, 11.8d, Xe-133m, 2.2d, Xe-133, 5.2 d and Xe-135 , 9.1 h. The ratio of the isotopic concentrations measured can be used to determine unequivocally the amount of fission gas and time of rele ase of an air parcel many kilometers downwind from a nuclear activity where the fission products were discharged.