Different Approaches for Clearance of Metals - a Comparison - 19079

摘要

Large quantities of contaminated and potentially contaminated metals are generated during the lifetime of nuclear installations, especially when they are dismantled and demolished. In many countries, clearance for recycling is considered Best Practice for the management of metals which are contaminated or potentially contaminated. In some countries, such as Sweden, it has become a legal requirement to recycle metals and other materials when it is safe to do so and the cost is reasonable. Therefore, multiple management routes are available with the aim of establishing a graded approach. These include general (unconditional) clearance, conditional (specific) clearance of surface contaminated metals, conditional clearance of metal ingots after treatment in a licensed facility and in some cases, recycling inside the nuclear sector. Kemakta Konsult AB recently derived rounded clearance limits for the conditional clearance of scrap metal for re-melting from the decommissioning of Swedish nuclear sites, on behalf of the Swedish Radiation Safety Authority (SSM). The calculations used the RP89 exposure scenarios, but the parameters were adjusted to reflect normal practice in the Swedish scrap metal and recycling industries. The study found that, for most of the 39 radionuclides judged to be relevant, the rounded clearance levels were the same as or higher than those in RP 89. However, longer transport times in Sweden compared to the ones used for RP 89 resulted in lower rounded clearance levels for three gamma-emitting radionuclides, including Co-60, the most important radionuclide in terms of dose exposure. Cyclife Sweden AB has, through more than 40 years of operation of the metal treatment facility at the Studsvik industrial site, significant experience of the practical implementation of clearing metal ingots as well as metallic strap, in total about 50,000 tonnes. A previous study confirms very low radiological impact to the receivers of the ingots and to the general public under the current procedures and conditions for clearance. The production of steel ingots for recycling in the Cyclife plant involves pre-treatment (segmentation and decontamination) followed by melting. Certain nuclides, like Cs-137 and alpha nuclides, are transferred to the secondary waste (slag and filter dust) in the melting process. The non-separated nuclides are homogenized in the ingot. The radionuclides that are present in the ingots are expected to remain in the metal during re-melting at metal works. The general approach of together with specific information for typical ingots has been used to further verify the radiological implications of applying conditional clearance of ingots. This paper provides a background, present the studies performed, give an overview of the findings and the overall result including a comparison between different approaches.