Year
2002
Abstract
In previous reports we described an enrichment monitor for UF6 gas flowing in a pipe. The total amount of uranium is determined by measuring the attenuation by the gas of 122-keV gamma rays from a 57Co source. The amount of 235U is determined by measuring the 186-keV counting rate. The ratio gives the enrichment. We chose a NaI(Tl) detector because, unlike a germanium detector, it does not require cooling, and because the resolution is not good enough to be considered intrusive. In this report we describe three possible improvements to the UF6 enrichment monitor. First, the attenuation can be enhanced by using 60-keV gamma rays from an 241Am source. This is especially important if the gas pressure is low. For steel pipe the source strength must be greater than the strength of the 57Co source because of the increased attenuation in the pipe wall. An 241Am source has weak higher energy lines that may interfere with the 186-keV line in low-resolution detectors because of differential attenuation of the 60-keV line relative to these higher energy lines. For aluminum pipe the strength of an 241Am source is comparable with that of a 57Co source for a steel pipe and differential attenuation does not significantly enhance the higher energy lines. We present some sample results. Second, the NaI(Tl) detector can be replaced with a high-pressure xenon (HPXe) detector that can improve the resolution by approximately a factor of 3, which will reduce the interference of lines and also reduce the background continuum under the lines of interest. HPXe detectors are much more stable than NaI(Tl) detectors as demonstrated by an HPXe detector operated on the MIR space station for approximately 6 years. We present a sample spectrum. Third, we describe another approach for determining the total amount of uranium that does not involve the use of a gamma-ray transmission source. In this approach the density of the gas is determined by measuring the velocity of sound in the gas. This approach requires a pipe penetration for a combined transmitter and receiver. A gamma-ray detector is still needed to determine the amount of 235U. We describe the advantages and disadvantages of this approach.