Limitations on upper bound dose to adults due to intake of 129I in drinking water and a total diet-implications relative to the proposed Yucca Mountain high level radioactive waste repository.

The purpose of this report is to comment on the potential annual doses due to the intake by adults of I, an important radionuclide in the proposed high-level radioactive waste repository at Yucca Mountain. An often overlooked, but significant, factor is that, in this case, the ground water, which would be the primary transport vehicle for any releases, contains relatively high concentrations of stable iodine (127I); in fact, the median concentration in the ground water in the vicinity of the proposed repository is 5.0 microg L-1. In comparison, the maximum concentration of 129I in the ground water, due to potential releases of 129I during the first 10,000 y following closure of the repository, is estimated to be approximately 3.7 x 10(-7) Bq L-1 (approximately 10(-5) pCi L-1). This would result in a 127I to 129I ratio in the water of almost 90 million to one. Assuming no other sources of these two isotopes were being consumed, this would place an upper bound on the annual committed thyroid dose of 1.2 x 10(-1) mSv (1.2 x 10(-1) mrem), less than one thousandth of the Ground Water Protection Standard of 4 mrem y-1. When the additional intake of stable and radioactive iodine in other components of the diet is considered, the overall ratio of 127I to 129I would be more than 2 billion to one. The would place an upper bound on the annual committed effective dose of approximately 2.5 x 10(-8) mSv (approximately 2.5 x 10(-6) mrem), less than one millionth of the Individual Protection Standard of 0.15 mSv (15 mrem).