New Free-Release and Sorting Monitors Developed for NPP A-1 Decommissioning, Slovakia

Abstract

A pilot free-release monitoring post with a 600 L container monitor was developed and metrological tested within the 2nd stage of NPP A1 decommissioning project. In order to reduce the volume of contaminated soil monitoring a conceptual design of fast sorting loader shovel monitor (loader’s spoon) was proposed and tested within the project as well. The free-release monitoring post makes use of a pair of electrically cooled lead shielded semiconductor detectors placed into a mounting rack ensuring measurements in either horizontal (container monitoring) or vertical (drum monitoring) counting geometry. For evaluation of measured HPGe spectra the Canberra – Packard ISOCS detection efficiency calculation code was used. A loader is used to change the measured side of the 600 L container. For metrological certification of this monitor a special prototype test container with 24 rod sources inside a regular grid was necessary to design and to use. The mentioned above vertical counting geometry together with an additional drum rotator ensures standard free release monitoring of materials in 200 l rotating drums. Successful metrological qualification of the both counting geometries at SMU Bratislava showed 20% accuracy class. A pair of NaI(Tl) detectors and a measurement and navigation frame ensuring loader shovel fixation in counting position are used for the fast sorting monitoring. The navigation of loader and its shovel to the counting geometry should be as fast as possible. The monitored results shall be indicated by a prompt light indication system (apart from storing on HDD). MCNP 5 calculation code was used for assessment of gross gamma 137Cs detection efficiency. The estimated MDA for a pair of 2″ × 2″ Na(Tl) detectors and 30 s acquisition time is about 90 Bq/kg. However, due to the counting geometry deviations from calculated values the uncertainty of measurements can be relatively high. Hence, the system is applicable for sorting monitoring only.