When an activity involving radiation is decommissioned, the operator must demonstrate that sufficient measures are taken to clean up and monitor facilities and adjacent land areas that may have been contaminated with radioactive substances. One question is what levels of residual contamination by radioactive substances can be accepted given the continued use of the premises or land.
The Assignment
The Swedish Radiation Safety Authority (SSM) identified the need for more background knowledge before making decisions on the clearance of land contaminated with radioactive substances. Therefore, SSM funded the assignment, which Kemakta carried out, to estimate the radiation doses that a certain contamination of radioactive substances in soil might cause. Similar to how the Swedish Environmental Protection Agency has developed general guideline values for soil contaminated with non-radioactive substances, this assignment was based on the general exposure scenarios for sensitive land use (KM) and less sensitive land use (MKM).
Within the assignment, calculations were made of radiation doses to humans from contamination with radioactive substances in soil. The assignment included an estimation of individual doses for the age groups 0-5 years, 6-15 years, and 16-70 years. Calculations were performed for the following radionuclides: H-3, Cl-36, Fe-55, Co-60, Ni-63, Sr-90, Tc-99, Cs-134, Cs-137, Np-237, Am-241, Pu-241, Pu-238, Pu-239, Pu-240, U-232 series with Th-228, U-235 series with Pa-231 and Ac-227, U-238 series with U-234, Th-230, Ra-226 and Pb-210, and Cm-244.
Model and Results
Individual doses are expressed as unit doses in microSieverts per year (µSv/year) at an assumed contamination level of 1 Becquerel per kilogram (Bq/kg) for individual radionuclides homogeneously distributed in the upper soil layer. The model calculations are intended for use in smaller land areas contaminated by radioactive substances. The model includes estimates of the distribution of contaminants in the environment, transport of contaminants, and exposure of humans through various exposure pathways. The exposure pathways included in the model are: external dose, oral intake of soil, inhalation of dust, ingestion of drinking water from a well on the contaminated site, ingestion of plants grown on the contaminated site, and ingestion of meat and milk produced on the site. The developed model can also be used to calculate radionuclide levels in groundwater and radon levels in indoor air resulting from soil contamination. The calculations were made for the times 1, 10, 30, 100, 300, and 1000 years after the assumed time of contamination. The results show that the highest unit dose for sensitive land use was calculated for Ra-226 with daughter radionuclides (2.1 µSv/year per Bq/kg). The next highest unit doses for sensitive land use were calculated for Co-60 (1.5 µSv/year per Bq/kg) and the U-232 series (1.2 µSv/year per Bq/kg). For less sensitive land use, the highest unit doses were calculated for Co-60 (0.67 µSv/year per Bq/kg) followed by Ra-226 with daughter radionuclides (0.53 µSv/year per Bq/kg), the U-232 series (0.42 µSv/year per Bq/kg), and Th-228 with daughter radionuclides (0.33 µSv/year per Bq/kg).
The results have been presented in the report (2023:08 Radiation Dose Consequences for the Clearance of Land Areas Contaminated with Radioactive Substances).
