Analysis of long-lived radionuclidic impurities in short-lived radiopharmaceutical waste using gamma spectrometry

Large hospitals and biomedical research centers utilize decay-in-storage programs to minimize the volume of their low level radioactive waste. However, some medically useful radionuclides often contain small amounts of long-lived radionuclidic impurities which may complicate simple waste management procedures. We have evaluated the extent of this problem in low level radioactive waste involving 67Cu and (111)In over a 6-mo cycle of decay-in-storage by identifying the residual radionuclides in our dry waste using a multichannel analyzer. The multichannel analyzer was also used to quantify the radionuclide constituents of our liquid waste at the beginning of a decay-in-storage cycle. Radionuclides were identified by the presence of characteristic photopeaks of each isotope in the gamma spectrum and quantified by region of interest analysis. After a decay-in-storage cycle, long-lived 58Co, 57Co, and 56Co isotopes were observed in dry 67Cu waste and (114m)In identified in dry (111)In waste. The (114m)In was detected in dry (111)In waste containing initial (114m)In activity of 740 kBq (20 microCi), while the cobalt radionuclides were detected in dry 67Cu waste containing initial 58Co, 57Co, and 56Co activities of 444, 148, and 148 kBq (12, 4, and 4 microCi), respectively. Such dry low level radioactive waste was thus disqualified from short-term radioactive waste storage programs. The radionuclide constituents in the liquid waste were quantified in microCi mL(-1) and confirmed to be within the Nuclear Regulatory Commission set limits of 2 x 10(-4), 6 x 10(-4) and 6 x 10(-5) microCi mL(-1) for 58Co, 57Co, and 56Co, respectively, before disposal. The highest levels of long-lived isotopes that have been found in our liquid low level radioactive waste at the beginning of decay-in-storage were 5.5 x 10(-4), 4.8 x 10(-4), and 1.4 x 10(-4) microCi mL(-1) for 58Co, 57Co, and 56Co, respectively. Gamma spectrometry can be used to aid waste segregation and final management decisions on low level radioactive waste.