Study on the behaviors of radionuclides in a soil-plant system

A project 'Development of a general model for radionuclide transfer in an environment on a local scale' has been progressing since 1991 by 5 facilities in Japan. Power Reactor and Nuclear Fuel Development Corp. takes part in the study on the behaviors of radionuclides in a soil-plant system and started an investigation on the behaviors of long half-life radionuclides such as 99Tc and 237Np, of which concentrations and behaviors in the soil have been still unknown. The following results were obtained. A highly sensitive quantification method for a radionuclide with a long half life, termed as ICP-MS method was developed and it was found useful for the determination of extremely low levels of nuclides in the soil environment such as 99Tc and 237Np etc. Field experiments and laboratory experiments revealed that the oxidation-reduction potential of the soil, which might influence the changes in the chemical forms of radioactive nuclides; 99Tc and 237Np has significant effects on their adsorption onto the soil. Previously, the transfer rate of 99Tc from the soil to plants was estimated to be within the range of 10-2 - 101 by an tracer experiment, but the present field experiment suggested that it is less than 2 x 10-3. In the 2nd stage of this project, development of a dynamic analyzing model applicable to the evaluation of radiation risk acompanying time-course changes such as Chernobyl accident is planned and further, the parameters of active transfer of nuclides are to be solved in the next step. (M.N.)