Assay of fission products in a gas stream.

Abstract

The gas jet facility at the Ford Nuclear Reactor (FNR) at The University of Michigan has been used to study short-lived light-mass fragments from thermal-neutron-induced fission of $\\sp{235}$U. The delivery rates of the noble gases have been determined by direct measurement using an on-line technique for nuclides with half-lives $<$60 s, and a batch technique for longer-lived nuclides. A mathematical model has been developed which describes the penetration of light-mass fragments and the attenuation of heavy-mass fragments through a thin barrier foil which covers the U target. The model allows calculation of fission rate and the release rates for other fission products. Using the Kr and Xe gamma ray measurements, published fission yields, and transmission factors predicted by the model, we have calculated the fission rate for the FNR gas jet's $\\sp{235}$U target. Reported relative gamma ray intensities have been normalized to absolute values. Comparison of the calculated release rate for $\\sp{142}$Xe with its observed gamma ray spectrum, using measured detector efficiencies, provided a normalization factor of 0.29. Bromine nuclei produced in fission react with ethylene carrier gas in the radiation field to form volatile compounds. Several experiments were performed to identify the nature of these compounds. On-line measurements using various traps indicated formation of polar species. Batch extraction using heptane/water showed 80% of Br in the aqueous phase. These data, combined with the results of tracer experiments, suggest the formation of HBr.