Study of short-lived fission products using the gas jet method.

Abstract

The Gas Jet Facility at the Ford Nuclear Reactor was used to study the short-lived (less than one minute half-life) fission products of <super>235 </super>U. Gamma-ray spectroscopy was used to identify the fission products. Initial experiments were designed to better understand the nature of the volatile fission products formed in the target chamber and transported in the carrier gas stream. Trap studies indicated that bromine nuclides preferred polar trap materials. Benchtop and online extraction experiments indicated that about 77% of the selenium formed in fission was extracted by heptane, possibly forming alkyl selenides by reaction with ethylene or products derived from ethylene. Another series of experiments determined that the most effective carrier gas stream was a mixture of nitrogen and ethylene. The final set of gas jet experiments studied the nature and yields of volatile selenium and bromine fission products of <super>235</super>U using a semi-automatic batch solvent extraction technique (BEQ). Volatile products generated in the target chamber by the interaction of fission products with the carrier gases were bubbled through a heptane and water mixture. The resulting gamma-ray spectra of the heptane and water phases consisted of gamma rays from the nuclides of interest (<super>84</super>Se, <super>85</super>Se, <super> 86</super>Se and <super>86</super>Br, <super>87</super>Br, <super>88</super>Br), activation products, and neutron reactions with the Ge in the detectors. The activities due to the selenium isotopes <super>84</super>Se, <super>85</super>Se, and <super>86</super>Se and bromine isotopes <super>86</super>Br, <super> 87</super>Br, and <super>88</super>Br were calculated. The weighted average organic fractions of all runs of the selenium isotopes remained essentially the same: <super>84</super>Se, 0.77 +/- 0.02; <super>85</super>Se, 0.74 +/- 0.01; <super>86</super>Se, 0.73 +/- 0.02. These selenium values were used to resolve the fractions of the bromine nuclides which come from two major sources, viz., directly from fission and from the beta-decay of selenium. The organic and aqueous fractions of direct bromine were calculated assuming that indirect bromine from selenium in heptane stayed in the organic form. The average calculated values of the inorganic fractions of bromine formed directly from fission in this study were: <super>86</super>Br, 0.91 +/- 0.09; <super>87</super>Br, 1.02 +/- 0.13; <super>88</super>Br, 1.00 +/- 0.04. Using information available in the literature, a model was developed that supported this assumption.