Recycle Experience of Dismantled Cask Handling Crane by Surface Removal Sampling at Kori Unit No.1

Abstract

The Kori No.1, which began operation in 1978, replaced its cask handling crane in 2000. To prove the safety of recycling and reuse of crane scrap, a particular calculation method for surface contamination was used. Because surface radioactive contamination of steel is limited to a few-microns-thick layer, we can calculate the total(removable and fixed contamination) activity of the sample conservatively by this surface removal sampling means. If we multiply the ratio of total surface and the area of the selected surface by its activity, total activity of the scrap can be estimated. Conservatively, the sampled portion can be used as a representative sample of the scrap. Both the inner and outer part of the scrap was sampled separately, and gamma spectra were analyzed to check whether activation had occurred. Before sampling, the entire surface of the steel is scan surveyed by several kinds of GM and GP detectors. Contaminated parts were segregated, or decontaminated to the background. Almost one sample per one ton of steel was collected. Gamma spectra of 62 samples were analyzed by 100% efficiency HP Ge detector. Only 60Co was detected, and its highest activity was 0.01 Bq/g,. This level of activity is much lower than the ''clearance levels'' outlined in IAEA TecDoc-855.(4). The total alpha and total beta for 6 samples were measured in the laboratory by low background alpha, using a beta gas proportional counter. Activities were much lower than 0.005 Bq/g. A representative sample was taken from the complete mixture of 62 samples. Gamma activities of nuclides were measured to estimate the dose to the public. This study revealed that activities of nuclides were lower than 'clearance levels' if decontaminated until the lower limit of detection level of the portable field instrument. New surface removal sampling method was tested. This method allows us to easily calculate the specific activity for the solid material