Impacts of Stable Element Intake on 14c and 129i Dose Estimates

Abstract

The purpose of this study was to evaluate and provide insights related to the influence of the intake of stable isotopes of carbon and iodine on the committed doses due to the ingestion of (14)C and (129)I. This was accomplished through the application of two different computational approaches. The first was based on the assumption that ground (drinking) water was the only source of intake of (14)C and (129)I, as well as stable carbon and stable iodine. In the second, the intake of (14)C and (129)I was still assumed to be restricted to that in the ground (drinking) water, but the intake of stable carbon and stable iodine was expanded to include that in other components of the diet. The doses were estimated using either a conversion formula or the applicable dose coefficients in Federal Guidance Reports No. 11 and No. 13. Serving as input for the analyses was the estimated maximum concentrations of (14)C or (129)I that would be present in the ground water due to potential releases from the proposed Yucca Mountain high-level radioactive waste repository during the first 10,000 y after closure. The estimated contributions of stable carbon and iodine through the consumption of ground water were based on analyses of samples collected in the Amargosa Valley, NV. The contributions through dietary intake were based on surveys conducted in the United States. Based on the accompanying analyses, it was noted that stable isotope intake has a significant effect on the estimated doses due to the intake of radioactive isotopes of the same element. While this is a well-known fact, this observation has international implications in terms of dose estimates for key radionuclides, such as (14)C and (129)I, a primary reason being the wide variations in the intakes of stable carbon and iodine in various countries. For this reason, analysts planning to apply the dose coefficients developed by the International Commission on Radiological Protection (ICRP) should either confirm that the average total intake in their country of stable isotope(s) of the radioactive isotope being evaluated is in reasonable agreement with the value assumed by the ICRP or suitably modify the ICRP dose coefficients to account for any differences. If such a procedure is to be implemented, there is a need for periodic updates of the dietary intakes of various stable elements in countries throughout the world. The importance of this is documented by recent surveys in Asia that revealed that their average total daily intake of stable iodine was less than half of the ICRP value for Reference Man. In this case, application of the ICRP dose coefficients, without modification, would underestimate the dose due to ingested (129)I by a factor of more than two. A related situation exists in the United States where the latest surveys indicate that the daily intake of stable iodine is 75% of the ICRP value.

Lauriston S. Taylor Lecture: Yucca Mountain Radiation Standards, Dose/Risk Assessments, Thinking Outside the Box, Evaluations, and Recommendations

“…Accounting for its specific activity (1.65 ϫ 10 11 Bq g Ϫ1 ) (Table 1) the total daily mass intake of this radionuclide would be 1.19 ϫ 10 Ϫ11 g. Since this would be diluted by a daily intake of 300 g of stable carbon ( 12 C ϩ 13 C), this means that the ratio of the mass of the daily intake of stable carbon to that for 14 C [(300 g d Ϫ1 ) Ϭ (1.19 ϫ 10 Ϫ11 g d Ϫ1 )] would be 2.52 ϫ 10 13 , or 25 trillion to 1. This places an upper bound on the dose rate that the intake of 14 C can yield (Moeller et al 2005).…”

“…THIS PAPER is based on observations and findings that have resulted from five years of studies of various facets of the Yucca Mountain (YM) high-level radioactive waste repository Ryan 2004, 2005;Moeller et al 2005Moeller et al , 2007. As the title implies, the primary topics that will be discussed are (1) the appropriateness and applicability of the radiation protection Standards promulgated by the U.S. Environmental Protection Agency (U.S. EPA 2005), and the accompanying Regulations promulgated by the U.S. Nuclear Regulatory Commission (U.S. NRC 2005); (2) insights gained in estimating the dose rates due to postulated releases of the eight regulated radionuclides, and assessments of their compliances with the Standards; and (3) the dominating influences and impacts of the relationships between dose and risk.…”

Factors Affecting Dose Estimates for Long-Term Performance Assessments: Case Study???armagosa Valley

“…Highlighting the importance of this observation is that surveys indicate that the estimated average daily intake of stable iodine in the U.S. has decreased from 300 g in the early 1970's to about 150 g today (NRC 2004). These observations make it mandatory that periodic assessments and updates be made of the dietary intakes of various stable elements in the U.S., as well as other countries of the world (Moeller and Ryan 2005). Nonetheless, because the assessment for 14 C indicated neither conservatism nor non-conservatism, and the estimate of the non-conservatism associated with the assessment for 129 I was so small, neither is currently a significant source of conservatism or non-conservatism with respect to dose estimates associated with radionuclide releases from the proposed Yucca Mountain repository.…”

“…These are 14 C, 99 Tc, 129 I, 226 Ra, 228 Ra, 237 Np, 239 Pu, and 241 Am (Garrick 2003). Topics previously addressed include a review and evaluation of the applicable regulations, with emphasis on their apparent inconsistencies; the sensitivity of the dose estimates as a function of the source of the relevant coefficients; and differences in the estimated doses to various age groups (Moeller and Ryan 2005). Supplementing these efforts were papers on the effects of the intakes of stable carbon and iodine on doses from 14 C and 129 I , and on the importance of 14 C and 228 Ra in terms of releases from the proposed repository (Moeller et al 2006).…”

RADTRAN 6 technical manual.

“…(1) the quantity of radioactive material taken up by the agricultural commodity ingested; e.g., 1.2 percent of 137 Cs deposited on grass could be transferred to milk (Saricks, et al, 1989), (2) the chemical form and activity of ingested radioactive material in any particular organ or tissue (the ingestion dose conversion factor); for example, the bone surface dose from dissolved 137 Cs is 1.4 x 10 -8 Sv/Bq 1 (ICRP, 1996) , (3) the type and energy of ionizing radiation ingested (e.g. see ICRP, 1983), and (4) the extent to which a target organ is saturated with stable (non-radioactive) isotopes of the radionuclide in question (Moeller, et al, 2005). The ingestion dose model assumes that every radioactive atom is ingested by someone and therefore contributes to a collective, societal dose.…”