A New Look at 99Tc Releases to the Atmosphere

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Abstract

Recent experimental data suggest that the uptake of 99Tc by vegetation from soils may be two to three orders of magnitude higher than the 0.25 value currently being used in radiological assessments. Following a survey of the literature, a concentration factor of 50 was applied to evaluate the dose from a 1.0 Ci/yr-release to the atmosphere by a hypothetical uranium enrichment facility. Doses to the GI tract and thyroid of an adult living 1600 m from the facility were 18 mrem and 80 mrem, respectively. These doses are delivered entirely through transport of 99Tc through food chain pathways. This assessment indicates a potential for 99Tc exposures to exceed recently proposed standards of the U.S. Environmental Protection Agency in 40 CFR 190. The previously assumed concentration factor of 0.25 would have produced corresponding doses of 0.13 mrem to the GI tract and 0.57 mrem to the thyroid. The results of this analysis demonstrate the need for additional research on the environmental behavior and dosimetry of 99Tc. In particular, data are needed to elucidate the retention of 99Tc in soils and the uptake of 99Tc by edible vegetation in field studies simulating chronic exposure conditions. Data on the uptake and retention of 99Tc in humans are also necessary to improve the reliability of dose conversion factors for specific organs and various age groups.

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