Concepts and methods for the clearance measurements of building structures of nuclear facilities under decommissioning

At the end of every decommissioning project large amounts of cleared building structures and outdoor areas remain on the site ('green field option'). The rising number of nuclear facilities under decommissioning in Germany requires the development of concepts and methods for measuring procedures for the clearance of building structures under decommissioning. Especially in the past it was common practice that building structures were released by analysing samples randomly taken from the surface or measuring the surface activity by large proportional counters. In the last years in-situ gamma spectrometry became a major part in release measurements. We have designed and constructed a high resolution in situ gamma spectrometer based on a 44% p-type HPGe-detector with a collimator made of tungsten within the scope of a project funded by the Federal Ministry for Education, Science, Research and Technology and the State Ministry for Regional Development and Environmental Protection of Bavaria. The system satisfies the demands for release measurements referring to practical and technical aspects and was calibrated according to the standard method, which was qualified within the scope of our former research work. In addition we have generated strategies for clearance measurements and calibration techniques for the use of uncollimated in situ spectrometry in buildings. To get knowledge about the distribution and extent of contamination we have performed measurement campaigns in different zones of nuclear power plants and nuclear fuel fabrication facilities under decommissioning. Different measurement techniques have been applied and compared during the project. The results will be concluded in our presentation. The concepts we have developed for measuring procedures combining the different measuring techniques concerning radiological criteria, economical and actual efforts will be explained and presented. We analysed the influence on the radiological effects by varying the areas over which release measurements are averaging on the basis of exposure scenarios like direct radiation or incorporation. Maximal averaging areas based on the 'de-minimis e.g. 10 μSv-concept' (IAEA-safety series No.89) resulted from this investigations. The results of the measurements were integrated into a Monte-Carlo computer code we developed. This code is able to simulate the release measurements, applying different techniques including their systematic and statistic uncertainties. Using real contamination scenarios from the campaigns mentioned above we got information on the efforts, for example the number of measurements necessary for a tolerable systematic uncertainty of random samples in comparison to overall collimated in-situ measurements. Considering the results above we worked out concepts and methods for clearance measurements (unrestricted release) of building structures and the site of nuclear facilities under decommissioning. The results will be explained and concluded in our presentation. (author)