Keeping the Clearance Levels of NPP Building Structures Validation by Measurement of the Nuclide-Specific Dose-Rate

Abstract

Clearance levels for building structures normally are defined surface-specific (Bq/cm2) by the projection of the specific activity onto the surface. This proceeding led to the actual release criteria for buildings in the EC, which were also used for the actual draft of the new German RPO. The practical ability of collimated in-situ spectrometry verifying the keeping of these revised nuclide-specific clearance levels and the corresponding radiological averaging area of 1 m2 will be demonstrated. A completely different approach performing clearance measurements in building structures with poor remaining artificial activity was sketched in an earlier work, by performing a set of in-situ measurements without collimation. For this purpose, the spectrometer is calibrated for the measurement of the gamma dose rate at a very low level. The direct measurement of this value and its comparison with the fundamental, dose-based release criteria is a large advantage of this procedure. The sensitivity of a commonly used high-purity germanium detector is sufficient for this purpose with short measuring time; or in other words the detection limits are much smaller than 1 nSv/h. Therefore this presentation is dealing also with a complete new concept for the release of building structures of NPP’s. It is not supported by the actual release criteria basing on conversion factors from effective dose to surface-specific activity. The concept shown here, does not need this conversion, the effective dose is evaluated directly. On the other hand, it might be a basis of a fast, simple procedure in the projects of the next decades. The basic theoretical work is finished. In future, a practical check in an actual decommissioning project has to be done and the experiences gained by this work have to be documented.