Practical judgment of exemption by applying isotope specific exemption levels for surface contamination

Abstract

In terms of applying the concept of exemption to safety standards for surface contamination, we have derived isotope specific exemption levels for surface contamination (Bq cm^–2) by developing an original dose assessment model for surface contamination to develop reasonable radiation protection systems that reflect the radiological properties of nuclides and avoid excessive regulatory procedures. These exemption levels can be applied to the radiation, waste and transport safety fields by assuming a universal scenario and by applying radiation protection systems consistent with the current International Atomic Energy Agency (IAEA) Basic Safety Standards, safety series no. 115 and safety guide RS-G-1·7. In the case of materials containing a mixture of nuclides, the exemption can generally be judged on the basis of whether the condition ΣD/C<1 is satisfied (where D represents an actual measurement result and C represents the exemption level), and the estimation of the surface contamination density of key nuclides such as ⁶⁰Co, which are easily measured and dominant nuclide components, can be practically applied to the judgment of exemption. In this study, the contribution of nuclides to the summation defined in terms of relative importance was assessed using the exemption levels for surface contamination derived in previous Central Research Institute of Electric Power Industry (CRIEPI) studies and the technical data of nuclide composition ratios contributing to contamination assumed to occur in Japanese nuclear power plants. Important nuclides, whose relative importance was >0·1, were extracted following the 10% summation rule described by the European Commission in its document Radiation Protection 134. It was found that ⁶⁰Co was the only important nuclide in most of the cases and other nuclides can be ignored according to the reasonable exemption levels for surface contamination derived in CRIEPI's previous study, where the α emitter was the most important nuclide in the case of estimating fuel damage by applying the exemption levels for surface contamination derived from the dose conversion factors given in IAEA TECDOC-1449. It is expected that the resulting reasonable exemption levels for surface contamination will be applied practically to future regulation and that consistent radiation protection systems will then be realised throughout the radiation, waste and transport safety fields.