Transfer models used for environmental impact assessments are briefly reviewed focusing on a soil-plant system. The transfer models are categorized into two types based on their structures: equilibrium and dynamic models. The chronological development of each model type and the features of selected models are summarized. Trends and future needs for improvement of transfer models are discussed.KEY WORDS: transfer models, soils, plants, environmental impact assessments, equilibrium model, dynamic model, improvement
I . INTRODUCTIONRadionuclides released to the environment can reach humans through a variety of pathways. For evaluating and preventing radiological health risks, radionuclide behavior need to be traced accurately by numerical techniques. Mathematical models used for this purpose are referred to as 'environmental transfer models'. They allow us to evaluate the effects of radioactive releases on human health and to supply necessary information for making decisions on which countermeasures should be carried out. Many environmental transfer models have already been developed and some models have been widely used for environmental impact assessments required for regulatory compliance, facility design, and safety analyses. Others have been used for dose reconstruction of past fallout events. These models, however, do not have the same structures. In the following sections, selected transfer models used for environmental impact assessments are briefly summarized focusing on the soil-plant system, which is of primary importance because most of our foods come from terrestrial sources(').tions for which each model has been intended for use. The situations can be categorized into two types: normal operation and accidental release.For safety assessments under normal operation, a long-term scale model, i e . , an 'equilibrium model' is generally used. This kind of model is also known as a 'concentration factor method'(') or 'steady-state The equilibrium model calculates periodical, equilibrated concentrations of radionuclides in environmental components such as air, soil, crop, livestock, and humans under stable conditions. This model involves empirical and time-independent parameters and has a good practicality because of its technical simplicity in calculation. From the viewpoint of safety, conservative values have been chosen for the parameters so as not to underestimate actual negative effects.The chronology showing development of selected equilibrium models is given in Fig.1. The oldest equilibrium model referred to is HERMES4), which laid the foundation for many successor models such as FOOD(5), USNRC Regulatory Guide 1.109 model@), Miller's and IAEA Safety Series 57 model('). The USNRC model has been widely accepted and succeeded by AIRDOS-EPA@) 1. Outline and Boone's model("). United Nations A large part of the difference among existing models is due to the difference in situa-
II. TRANSFER MODELS* Isozaki, Hitachinah-shi 311-12.
