Feasibility of 106Ru peak measurement for MOX fuel burnup analysis

Simulations were performed using ORIGEN-ARP to investigate 137Cs and 106Ru-106Rh as suitable fission products for non-destructive analysis of irradiated MOX. The simulations confirm that both 137Cs and 106Ru will provide a linear correlation with burnup when exclusively applied to MOX fuel assemblies. Moreover, 106Ru can also be used in conjunction with cesium to form a ratio almost independent of enrichment and power history. Simulations were conducted using three different uranium enrichments and one MOX enrichment over a burnup range of 20,000-60,000 MWD/MTHM. Comparison of the three uranium enrichments indicates the 106Ru ratio is consistent in predicting burnup with a maximum standard deviation of 0.046. Two MOX cases were simulated confirming operational history independence of 106Ru in predicting total burnup. The 106Ru burnup ratio also has the benefit of enabling distinction between UO2 and MOX fuel because of its significantly larger (∼11 times) fission yield from 239Pu. To investigate the detectability of 106Ru and other cesium peaks, data was collected using a HPGe detector at the Missouri S and T nuclear reactor (MS and TR) beam port. Gamma spectra were obtained immediately following reactor shutdown with the most promising spectrums obtained 3-5 h after shutdown. Even for relatively high enrichment (∼20%) fuel at MS and TR, cesium peaks were prominent and easily discernable from the intense Compton continuum. The 106Ru peak was weak, though still distinguishable from the background, suggesting that with an appropriately designed collimator, suitable detector and electronics it might be feasible to reliably measure 106Ru in even UO2 fuel. For MOX and LEU LWR fuels one would expect a more intense 106Ru signature.