External radiation doses to important organs in biota: Monte Carlo dose model calculations

Specific goals The new radiological protection system for the environment increases the need to estimate radiation doses to non-human biota in their specific environment. Therefore, reliable and realistic dose models are needed in risk assessments of biota. Current dose models for biota (e.g. ERICA, 2007) have been developed mainly to be used as a screening tool for the assessor and little guidance exists for cases where dose limits or screening values are exceeded. The simplified screening dose models assume a homogenous distribution of radionuclides in the organism and calculate a whole-body dose. In addition, only three types of ecosystems (terrestrial, freshwater, and marine) are available in the existing dose models and differences in soil type and heterogeneous distributions of radionuclides in the soil profile are not considered. In this proposed research project, radiation doses to important organs such as skin, liver and gonads in reference organisms suggested by ICRP (2009) will be calculated with a Monte Carlo dose model that will be developed. Validation of the model will be based on: (i) dose calculation with this model for various biota and organs based on soil and sediment samples from a wetland site with previously determined radionuclide distributions (Cs-137, U-series, Th-series, K-40) in the soil profile and specific soil composition. The field site is a riparian wetland north of Gävle in Sweden, a type of ecosystem that fall between the terrestrial and freshwater ecosystems in existing screening dose models but that may accumulate high levels of radionuclides such as Cs-137 from a flooding stream (Stark et al., 2006). The dose data will be validated by comparison with in situ TL-dosimetry based dose measurement in animal phantoms. (ii) comparison of dose calculations with this model and the RES-DAD-BIOTA and ERICA models for generic soil compositions and radionuclide distributions.