Issues on exemption levels for package surface contamination simply derived from IAEA TECDOC-1449 'Radiological aspects of non-fixed contamination of packages and conveyances'

Abstract

The present IAEA Transport Regulation on surface contamination (with limits specified in terms of Bq cm^–2) is determined from a simple radiological model for the most hazardous radionuclides (Pu-239 for α emitters and Sr-90 for β emitters) and its extremely conservative model is applied for all other α and β emitters. In the International Atomic Energy Agency (IAEA) TECDOC-1449 report, the effect of radiation from non-fixed contamination on packages is evaluated and the dose conversion coefficients [(mSv/y)/(Bq/cm²)] are calculated for each radionuclide. In this study, exemption levels for surface contamination are calculated with the dose conversion coefficients specified in TECDOC-1449. The dose criterion for deriving the exemption level is chosen to be 0˙01 mSv/y, to maintain consistency with fundamental concepts adopted by the IAEA safety standards committees, namely, the Radiation Safety Standards Committee (RASSC), the Transport Safety Standards Committee (TRANSSC) and the Waste Safety Standards Committee (WASSC). The result of this study shows that the exemption levels for α emitters such as Pu-239 and Am-241 are extremely low. This is because the resuspension rate from the packages that are handled manually by workers and the inhalation scenario in TECDOC-1449 have been set conservatively. For materials containing a mixture of radionuclides, exemption can be judged on the basis of the satisfaction of the condition ∑ D/C , 1, where D represents an actual measurement result and C represents the exemption level. If these extremely low surface contamination exemption levels are adopted for practical use, a small amount of α contamination can lead to a large contribution to the sum in the above condition expression in the case of nuclear power plants that are assumed to have experienced fuel damage. Moreover, it can be recognised that the result for Co-60 is approximately equal to 0˙4 Bq cm^–2, but it is less than 0˙8 Bq cm^–2, which is equivalent to the regulation of the US Nuclear Regulatory Commission (NRC) regulatory guide NRC RG-1˙86 and the Japanese Nuclear and Industrial Safety Agency (NISA) guideline that specifies the level of no contamination in nuclear power plants in Japan. This is because the working scenario assumed in TECDOC-1449 is set to be conservative, such as the one in which a site worker continuously unloads the small remotely handled packages from the first conveyance and loads them onto the second conveyance. In this study, it is concluded that present regulations on surface contamination are generally very conservative and the conservatisms in the scenarios and parameters assumed in TECDOC-1449 can even justify the possibility of higher exemption levels for non-fixed contamination in the IAEA Transport Regulations.

Keywords:

• EXEMPTION
• SURFACE CONTAMINATION
• IAEA-TECDOC-1449
• DOSE COEFFICIENT