Assessment of In-Fact Contaminated Concrete Inside Nuclear Power Plants to Reduce Waste Amounts After Decommissioning - 23220

The German project 'KOBEKA' focuses on determining radionuclide penetration into concrete structures within the containment, both fundamentally scientific and practically at the nuclear power plant Unterweser. The unit was shut down in 2011 and since then it has been under decommissioning and dismantling. The present project aims at assessing potential concrete areas which might have been subjected to and, consequently, penetrated by contaminated water during operation. Today, concrete within distinct areas must, by law, be considered contaminated irrespective their in-fact loading with radionuclides. This project aims at pronouncedly reducing the portion of concrete that has to be treated as nuclear waste as compared to claiming concrete contaminated by mere zonal categorizations. An alternative to core drilling in the power plant structure is developed to reduce hole diameters to two or three centimeters only. New measurement techniques and devices for sampling and scanning these tiny, deep wells are developed. A tube-capable probe for depth-resolved determination of dose rate, moisture and porosity is set up. In parallel, a method for laser-based continuous and discontinuous concrete sampling over the entire borehole length is explored. Laboratory test measurements using the fiber optic radiation sensor have been performed, demonstrating that it is possible to detect a contamination with a ¹³⁷Cs source with a sensitivity of 100 % and a specificity of 95 % in concrete that featured a constant background radiation. Concrete samples extracted from the power plant are being characterized for their microstructural, physical and chemical properties. Results serve as the basis for numerical modelling of radionuclide transport processes as well as to produce most similarly composed concretes for testing the newly developed physical measurement and ablation devices. (authors)