Mapping Transfer Parameters of Radionuclides in Terrestrial Environments

Abstract

Safety assessment models for potential sites selected for underground repositories of high level and long-lived radioactive waste requires the prediction of phytoavailability of such radionuclides at the regional scale. In this context, the areas in which the radionuclides may be highly mobile or accumulate have to be well known, as they will contribute to a maximal dose for Man. The parameters controlling the phytoavailability of the radionuclides are mostly defined by experiments based on sieved-soil sample and a methodology is needed to extrapolate these parameters to the regional scale by taking into account the variability of the soil properties within the landscape. A mapping of three radionuclides phytoavailability (⁶³Ni, ⁹⁹Tc and ²³⁸U) was conducted here using the MapInfo mapping software. The distribution frequency of the phytoavailability parameters was represented over the 186 km² area of the French laboratory for the study of deep underground nuclear waste disposal, in Bure. Isotopically exchangeable pool of Ni, E_t and pH of the soil were the two phytoavailability parameters chosen respectively for ^63Ni and for ²³⁸U and were both measured on sieved samples coming from the soil units defined at 1 : 50 000. The redox potential, E_h was the parameter used for ⁹⁹Tc and was measured in the field. For each radionuclide, class of soils were built according to their properties in term of radionuclides phytoavailability and maps of phytoavailability were drawn at 1: 25 000.

Results allowed a prediction of the phytoavailability of the ⁶³Ni, ⁹⁹Tc and ²³⁸U at a regional scale, based on the superimposition of the laboratory measurement of the parameters significantly controlling the mobility and the phytoavailability of the three radionuclides and knowledge of soils over the area. Critical areas were also determined where either accumulation in soils may be highest or accumulation in plants may be highest. They also allowed defining the most likely transfer parameters for the three radionuclides in the area of Bure.