Two Causes of Changes in Values Reported by an Enrichment Monitor

Both short and long-term changes in the enrichment values reported by enrichment monitors for UF6 flowing in pipes have been observed when there was not an actual change in the UF6 being monitored. One short-term change involves a decrease by a few percent in the reported enrichment value followed by a complete recovery over a few days. One long-term change involves a gradual decrease in the reported enrichment value over many months. Our enrichment monitor uses the transmission of the 122-keV line from 57Co to measure the amount of uranium in the pipe and the 186-keV line from 235U to measure the amount of 235U present. 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%. The short-term change is caused by either a small increase in the Full Width Half Maximum (FWHM) or a small shift in the position of the transmission peak probably caused by some instability in the gain stabilizer. The effect is more noticeable when the 57Co is weak. These changes shift the tails of the peak outside the region of interest, decrease the calculated transmission peak area, and thus decrease the calculated enrichment. The long-term change is caused by the fact that the transmission peak is decaying with a half-life of 271.2 days whereas the rest of the spectrum is essentially stable. This affects the background that must be subtracted from under the transmission peak and makes it difficult to determine the net area with sufficient accuracy. A spreadsheet model shows that as the 57Co decays the calculated net transmission peak area decreases too fast because the subtracted background is an increasing fraction of the peak area and the calculated enrichment decreases.