Year
2005
Abstract
The Enrichment Monitor, developed by the Los Alamos National Laboratory, uses a transmission source to verify the enrichment of UF6 gas flowing through process pipes in a down-blending facility. It depends on two measurements to determine the enrichment. The first measurement determines the amount of 235U in the gas, based on the intensity of the 186-keV gamma ray. The second measurement, a transmission measurement, determines the total amount of uranium in the gas. The enrichment is directly proportional to the ratio of these two measurements. In our earlier design, the pipes were made of steel, and the transmission source was 57Co (122 keV). However, the 57Co half-life is only 272 days, and in a NaI(Tl) spectrum, there was some interference from the 143-keV 235U line with the 122-keV 57Co line. We also observed time-dependent pileup effects in the data from this source. Our new design uses 241Am (60 keV) as a transmission source. It requires aluminum pipe but has certain distinct advantages in the Enrichment Monitor, notably a lower-energy transmission gamma ray (which results in greater attenuation by the UF6 gas) and a significantly longer half-life. However, there is also some potential for spectral interference in the NaI(Tl) spectrum. The 53-keV gamma ray from 234U (which is present in the UF6 gas) and the 63-keV gamma ray from the decay of 234Th (from deposits within the pipes) could cause interference with the 60-keV transmission gamma ray. Therefore, calculations have been performed to confirm that the 53-keV gamma ray would not cause measurable interference with the 60-keV gamma-ray transmission line. In addition, spectral measurements have been made at Oak Ridge National Laboratory for a NaI(Tl) detector and a high-purity germanium detector, using aluminum pipe and UF6 gas at representative operating parameters. The potential for interference with the 53-keV and 63-keV gamma ray is quantified. Other significant interferences, such as those resulting from fission products, were not observed. Under expected operating conditions, spectral interferences will not present an analysis problem for the instrumentation. The significantly longer half-life of the 241Am transmission source will not result in any time-dependent pileup effects in the data.