Ways for Efficient Decommissioning of Reactor Components and Concrete Shielding

Due to Germany’s nuclear phase-out, the decommissioning of nuclear power plants (NPPs) and final disposal of structural materials becomes an increasingly important task. The project WERREBA (German acronym for Ways for Efficient Decommissioning of Reactor Components and Concrete Shielding) aimed at the reliable determination of radionuclides produced by neutron activation, the activity as a function of time since shutdown and investigating subsequent radionuclide mobility. In the scope of the project, activity measurements and calculations were carried out for samples of the reactor pressure vessel and the concrete shielding of unit 2 of the Greifswald NPP shut down in 1990 during the German reunification.
Both measurements and calculations show that the highest specific activity of the RPV is found in a small region adjacent to the reactor core. Several decades after shutdown, Cobalt-60 (half-live time 5.27 y) is the dominating nuclide. A prolongation of the interim storage time by several years, i.e. caused by the delayed start of the operation phase of the Konrad repository, will therefore lead to a significant reduction of the activity of the structural materials. The specific activity decreases by 4 to 5 orders of magnitude with increasing distance to the reactor core. It is expected that a specific clearance or even unrestricted clearance will be possible for parts of the RPV after several decades of interim storage time.
Unit 2 of the Greifswald NPP is a first-generation VVER-440 (Russian acronym for pressurized water reactor with light water as coolant and moderator) which features an annular water tank. A neutron radiation field calculation using the radiation transport code MCNP reveals that the maximum neutron fluence in the concrete component is located in the floor just below the RPV. The concrete structures closest to the reactor core are shielded efficiently against neutron radiation by the annular water tank.
Measured and calculated specific activities of Europium-152, Europium-154 and Cobalt-60 for the cement screed at the position of the maximum neutron fluence are surprisingly low compared to recently published calculations and measurements for VVER-440 of the second generation. This is attributed primarily to the distinct design of the first generation VVER-440/230 compared with the second generation VVER-440/213 without annular water tank.