Short-Term Changes in Values Reported by an Enrichment Monitor

Changes in the enrichment values reported by enrichment monitors for UF6 flowing in pipes have been observed when there was no actual change in the UF6 being monitored. One type of change involves a decrease by a few percent in the reported value over approximately one day when gas is put back in the pipe after a shutdown; this is particularly noticeable with HEU. The enrichment monitor uses the transmission of the 122-keV line from 57 Co to measure the amount of uranium in the pipe and the 186-keV line from 235 U to measurement the amount of 235 U present. The ratio of these two values gives the enrichment. The detector is NaI(Tl). The system is very sensitive to any changes in the transmission peak because the attenuation in the gas is only approximately 3%. We have set up a system at Los Alamos that duplicates what has been fielded and which allows for long-term experimentation. We have been able to duplicate the decrease in enrichment. By substituting an HPGe detector for the NaI(Tl) detector we discovered that the decrease is caused by buildup of 231 Th (half-life 1.06 days) from the decay of 235 U. The 231 Th has a weak line at 84 keV, which is close enough in the NaI spectrum to the 122-keV transmission to affect the calculated transmission peak area. The low-energy end of the background under the transmission peak is determined by two channels near the 84-keV peak. As the 84-keV peak grows in, it raises this end of the background, increases the background subtracted from the transmission peak, decreases the net transmission peak area, and thus decreases the calculated enrichment.