The Vegetation-to-air Concentration Ratio in a Specific Activity Atmospheric Tritium Model

Abstract

Specific activity models are frequently used to estimate the concentration of tritium oxide in vegetation. In such models, a single value represents the ratio (R) of the specific activity of tritium oxide in vegetation to the specific activity of atmospheric tritium oxide. Federal agencies such as the Nuclear Regulatory Commission and the Environmental Protection Agency have not established a consensus default for R. Literature on this topic suggests that a site-specific distribution of R should be developed when feasible. In this study, a distribution of R is established for the Savannah River Site. Environmental tritium concentrations in air and vegetation measured on and around the Savannah River Site over a 9-y period form the basis for the analysis. For dose assessments of chronic atmospheric tritium releases at the Savannah River Site, R is best parameterized by a normal distribution with a mean of 0.54 and one standard deviation of 0.10. The Nuclear Regulatory Commission default for R is approximately equal to the Savannah River Site site-specific estimate. Based on the results, the default value for R recognized by the Environmental Protection Agency overestimates tritium concentrations in vegetation and, therefore, doses from foodstuff consumption pathways at humid sites. For the Savannah River Site, the magnitude of the error is on the order of a factor of 2. This consideration may be important if an estimated dose approaches an as-low-as-reasonably-achievable or regulatory threshold. Conversely, without the benefit of site-specific data, ingestion doses may be underestimated in regions with dry climates.

Development of an Ingestion Pathway Model for AXAIRQ

“…For predictive models, Anspaugh et al (1973) use a conservative uptake coefficient of 0.5. Studies at SRS indicate a site applicable uptake coefficient of 0.54 (Hamby and Bauer 1994) The annual average absolute humidity determined for the SRS is 11.4 mL;rm3 (Arnett 1996).…”

Savannah River Environmental Report for 2006

“…The major cause of this difference is the CAP88 code's use of 100-percent equilibrium between tritium in air moisture and tritium in food moisture, whereas the MAXDOSE-SR and POPDOSE-SR codes use 50-percent equilibrium values, as recommended by the Nuclear Regulatory Commission [NRC, 1977]. A site-specific study indicated that the 50-percent value is correct for the atmospheric conditions at SRS [Hamby and Bauer, 1994].…”

MAXINE: A Spreadsheet for Estimating Dose from Chronic Atmospheric Radioactive Releases

“…The concentration of tritium in vegetation, C T V , is determined by (Hamby 1990) Studies Hamby 1993, Hamby 1993) have shown that dose estimates for the vegetation consumption pathway are sensitive to the parameters in this model. Therefore, a site-specific value was determined for the plant-tritium-to-atmospheric-tritium model (Hamby and Bauer 1994).…”