Study of Existing Chemical Decontamination Methods of Radioactive Metals with a View on their Optimization

Nuclear Power is a decarbonated method of electrical energy generation. Using nuclear energy as a power source is currently our best option in the fight against climate change. But the radioactive waste generated from nuclear power plants and their related facilities is a cause of concern. Though the high-level and intermediate-level activity wastes are contained in small volumes (≤ 10%), significant volumes of lower activity wastes are generated. Metallic wastes are a major component of these radioactive wastes with 500,000 to 600,000 tons expected in France alone. 130,000 tons of this is expected from steam generators. Majority of these metals are made of Stainless steel 316 alloy or Inconel 600. These makeup majority of the primary circuit of a PWR plant. Under the effect of the primary circuit water and irradiation, these components corrode and the corrosion products maybe activated when close to the fuel, and be transported throughout the circuit. These products can be deposited on the surface of other metal components, causing contamination of the latter. The contamination can be adsorbed on the surface but can also diffuse in the oxide layers and sub-surface. The oxide layer is composed of an inner layer of Cr oxide under a layer of Ni and Fe oxide. Chemical decontamination is the preferred form of treatment due to the possibility of decontamination of difficult geometries and tube bends. To decontaminate these materials, it is important to dissolve the oxide layers chemically and a few microns of base metal where it could have diffused. Some existing chemical methods used to treat these materials are studied in this paper. These methods include Chemical Oxidation Reduction Decontamination (CORD) and Metal Decontamination by Oxidation using Cerium (MEDOC). These methods are studied and a few parameters to be optimized are identified to dissolve the oxide layer and also the diffused activity by subsurface dissolution. Surrogate steel and Inconel samples will be tested initially to optimize the processes before moving on to the real active samples. These samples are created by SORC, as a part of the PREDIS European project,using water vapor and high temperature after sample preparation and cleaning. The experiments will be carried out in the batch method with some optimizations before moving on to a loop system. The optimization steps will target reducing the volumes or treatment strategies for the effluent wastes while keeping in mind the Waste Acceptance Criteria (WAC) for nuclear waste. (author)