Modeled concentrations in rice and ingestion doses from chronic atmospheric releases of tritium

The expansion of nuclear power programs in Asia has stimulated interest in the improved modeling of concentrations of tritium in rice, a staple crop grown throughout the far east. Normally, the specific activity model is used to calculate concentrations of tritium in the tissue water of edible plants to assess ingestion dose from chronic releases. However, because rice, like other grains, has much lower water content than most crops, the calculation must also account for organically bound tritium. This paper reviews ways to calculate steady-state concentrations of tritium in rice, including the methods of Canadian and United States regulatory models, and the assumptions behind them. Concentrations in rice and resulting ingestion doses are compared for the various methods, and equations for calculating concentrations are recommended. The regulatory models underestimate doses received from ingestion of rice contaminated with tritium since they do not account explicitly for organically bound tritium. The importance of including organically bound tritium is illustrated in a comparison of doses from rice, leafy vegetables and milk for an Asian diet. Dose factors from tritium for these foods are estimated to be 135, 47, and 20 nSv y(-1)/(Bq m(-3)), respectively. Assuming known air concentrations, tritium concentrations in rice, calculated with the recommended equations, are uncertain by less than a factor 2 when tritium concentrations in the rice paddy water are known, and by less than a factor of 2.3 when concentrations in paddy water are unknown.