Monitoring the 235U enrichment of uranium deposited on pipes in centrifuge plants operating at low pressures using a high resolution gamma-ray spectrometer

Methods are described for determining the 235U enrichment of uranium deposited on pipes in centrifuge plants operating at low pressures using a γ-ray spectrometer. Results of measurements made on eight centrifuge cascades that had been in operation for several years are given. It is shown that the ratio of the number of 84 keV γ-rays emitted by 231 Th, a daughter of 235U, to the number of 63 keV γ-rays emitted by 234Th, a daughter of 238U, enables the 235U enrichment of the deposited uranium to be determined without the inspector needing to know the operating pressure of the centrifuge. It is possible to determine the 235U enrichment of the uranium deposited on product pipes to within +-5% relative of the average declared value in a measurement time of 4 hours when using a high resolution γ-X semi-conductor detector. The presence of directly deposited thorium fluoride on the pipes has been established. As the ratio of the number of 231Th/234Th atoms deposited is a measure of the enrichment of the UF6 gas currently being produced, it also enables gross misuse of the centrifuge plant during the period of inspection to be detected. (author)