Variation in background concentrations and specific activities of 36Cl, 129I and U/Th-series radionuclides in surface...

Assessment of the potential environmental impacts of nuclear fuel waste involves multiple lines of argument, one of which is an evaluation of the possible increments to background concentrations of certain radionuclides. This is especially relevant for radionuclides such as (3)H, (36)Cl and (129)I where there is continuous cosmogenic or geogenic production. However, for (36)Cl and (129)I and certain U/Th-series radionuclides, data are scarce because the analysis methods are complex and costly. The present study used accelerator mass spectroscopy (AMS) to measure(36)Cl and (129)I in river waters throughout Canada. Radiochemical methods were used for (3)H and the selected U/Th-series radionuclides, and stable element concentrations were also determined. There were distinct differences in concentrations among the sites. Stable Cl and I tended to be at higher concentrations near the ocean or population centres. The (3)H was high in regions with power reactors. The (226)Ra, (235)U and (238)U concentrations were high in areas with known U mineralizations, as expected. The (36)Cl and (129)I concentrations were generally homogenous in mid-latitudes, but the (129)I concentration was lower in the one arctic site sampled. Because the stable and radioactive isotopes of Cl and I varied in response to different factors, the resulting specific activities were especially variable. Both Cl and I are homeostatically controlled in animals, thus it follows that dose from (36)Cl and (129)I will depend more closely on specific activity than concentration, and therefore the environmental increments of interest are in the specific activities rather than simply concentrations.