ISSUES ON EXEMPTION LEVELS FOR PACKAGE SURFACE CONTAMINATION SIMPLY DERIVED FROM IAEA-TECDOC-1449 “RADIOLOGICAL ASPECTS OF NON-FIXED CONTAMINATION OF PACKAGES AND CONVEYANCES”

The present regulation on surface contamination [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 TECDOC-1449 report published by the International Atomic Energy Agency (IAEA), the effect of radiation from non-fixed contamination on packages is evaluated and the dose conversion coefficients [(mSv/y)/(Bq/cm 2 )] are calculated for each radionuclide. In this study, exemption levels for surface contamination are calculated with the dose conversion coefficients 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 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 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 manual packages and the inhalation scenario in TECDOC-1449 have been set conservatively. For materials containing a mixture of radionuclides, the exemption can be judged on the basis of the satisfaction of the condition Σ 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 recognized 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 NRC RG-1.86 and the 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 2 TECDOC-1449 is unrealistic, such as the one in which a site worker continually unloads and fixes packages. In this study, we conclude that the scenarios and parameters assumed in TECDOC-1449 should be more realistic to derive practical and reasonable exemption levels for package surface contamination.