Some applications of Ge(Li) gamma-ray spectroscopy to fuel element assay

Abstract

It was the object of this work to study the gamma rays emitted by the products of the interaction of thermal neutrons with the nuclei of U-238, Th-232, U-235 and Pu-239 during and after irradiation and to explore some applications mainly to fuel element assay. An irradiation facility and a Ge(Li) detector cryostat were constructed for this purpose. A new method of assaying a fuel rod containing a mixture of plutonium and uranium oxide, based on the difference in the observed yield of the fission products 1-135 and Sr-92, has been developed. The energies and intensities of the thermal neutron capture gamma rays for U-238 and Th-232 were determined. Four new lines have been found in the energy region previously unexplored for U-238. For Th-232, 66 certain lines were found, compared to 7 lines in the literature. Many prompt gammas emitted 'by the highly excited fission products following the fission of U-235 and Pu-239 were resolved in the energy region above 1.4 MeV. For U-235 fissions, 57 lines were found, and for Pu-239, 51 certain lines were recorded. The use of prompt gammas for assaying fuel rods was investigated. An accuracy of about ± 7% was obtained for the analysis of U-238 content; ± 10% to ± 20% accuracy was obtained for U-235 analysis in the range of 1% to 2% enrichment; and ± 35% accuracy for the analysis of 0.25% Puenriched rods. It has been found that Ge(Li) detectors can be operated as fast neutron detectors and used to determine the relative neutron yield. With this method, the enrichment of uranium rods can be found with an accuracy of ± 1% to ± 2% in the range from 116 to 2% enrichment. Finally, some considerations were given to the use of prompt gamma rays for measuring the initial conversion ratio C and the neutron yield parameter [eta].