To evaluate the radioactive tracer dilution by gamma spectroscopy as a safeguards technique for monitoring the mass of liquid salt in molten salt systems, the gamma spectroscopy data of salt samples were acquired and analyzed from pyroprocessing facilities in Idaho National Laboratory. The facilities include an electrorefiner for pyroprocessing spent oxide fuels (ER-PyroSOF) and Mark-IV ER for processing the metallic fuels from Experimental Breeder Reactor-II (EBR-II). The primary focus of the study is on the uncertainty analysis of the gamma activity of 154Eu isotope in the molten salts. The gamma activity of the Mark-IV ER salt is significantly higher than that of the ER-PyroSOF salt, 2.3E5 µCi/g versus 2.4E3 µCi/g, on average. The gamma activity of Mark-IV ER salt mainly comes from 137Cs, and it has about 2.1E2 µCi/g of 154Eu. The gamma activity of ER-PyroSOF salt mainly comes from 241Am and 137Cs and it has about 9 µCi/g of 154Eu. The relative gamma activity uncertainty is consistently at 3% for 137Cs for both Mark-IV ER and ER-PyroSOF salts, but for 154Eu, it is 8% for ER-PyroSOF salt and 7% for Mark-IV ER salt. The current gamma activity uncertainties for 154Eu are deemed too high for safeguards purposes. The main reason for the higher uncertainty for 154Eu is believed due to the lower count in gamma spectroscopy. To improve the gamma activity uncertainty of isotope 154Eu, a longer data acquisition time (12 hrs vs 4 hrs) and a higher count rate gamma detector for gamma spectroscopy were tested and it was found that the uncertainty for 154Eu was significantly improved from 7% or 8% to 3%.
