Evaluation of Methodology for Quantifying Radiopharmaceuticals in Tertiary-Treated Sewage

The production and utilization of radioactive pharmaceuticals and radionuclides for medical diagnosis and therapy warrant consideration of their fate and their radiation hazard to the population after discharge. Some of these nuclides—198Au, 133Xe, 131I, 99mTc, 85Sr, 75Se, 57Co, and 51Cr—represent curies of activity from a single facility that are being released to the sewers over a year's time. Although their relatively short half-life (6 h to 270 days) and the large sewer dilution may reduce the activity in the waste by several orders of magnitude at the point of treatment, the unpredictable range of decontamination (0 to 90 percent) by normal sewage treatment practices means that some will remain in the final effluent. Even after tertiary treatment operations, many of these radionuclides will still be present. This tertiary treatment is necessary when sewage effluents are to be used as coolants for nuclear reactors, and the confirmation of these nuclides in these effluents becomes of major concern as possible radiation hazards. Methodology to identify and measure the activity of five radionuclides—131I, 85Sr, 57Co, 51Cr, and 75Se—has been developed and tested. The validity of the developed procedures has been checked with standardized traces solutions added to different sewage effluents and dried lime-treated sludges. Chemical yields agreed with radiochemical yields, and with 15 litres of sewage, or 10-g sludge, the chemical yields were atleast 70 percent. Since the normal activity levels of each of these nuclides will be low, nondestructive quantification becomes questionable. The required precision and accuracy can best be satisfied by collecting intermittent samples and by radiochemical analysis of large volumes. Analyses on the influents, sludges, and treated effluents will indicate the overall efficiency of the entire sewage treatment process, provide evidence of the fate of each nuclide discharged into the sewage stream, and indicate whether a potential radiation hazard exists.