Optimisation of the chemical oxidation reduction process (CORD) on surrogate stainless steel in regards to its efficiency and secondary wastes

Abstract

Nuclear Power is a decarbonated technology of electrical energy generation. Using nuclear energy as a power source is currently considered as the best option in the fight against climate change. But the radioactive waste generated from nuclear power plants and their related facilities are matter 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 about 500,000 tons expected in France alone, including 130,000 tons from steam generators. Majority of these metals are made of Stainless steel 316 alloy or Inconel 600. Under the effect of the primary circuit thermal-hydraulic constraints and irradiation, these the resulting corrosion products may be 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 preferred due to the possibility of decontamination of difficult geometries and tube bends. In order to decontaminate these materials, it is important to dissolve the oxide layers chemically and a few micrometers of base metal where it could have diffused. An existing chemical method used to treat these materials is studied in this article, Chemical Oxidation Reduction Decontamination (CORD). Surrogate steel samples were prepared using high temperature induction heating and water vapour after sample preparation and cleaning. The oxide layer was characterised before treatment of the samples in the batch method at different concentrations and its effects are observed on the dissolution of the oxide layers. A protocol is being developed for the treatment of secondary waste effluents by multi-stage precipitation with a goal to reduce the total waste volumes and thus the volumes of ion exchange resins that would otherwise be needed.

Methods for the destruction of oxalic acid decontamination effluents

“…It is a multi-step process where potassium permanganate/permanganic acid is used in the oxidation step as seen in Eq. 13, followed by oxalic acid in the destruction and reduction step, which destroys the MnO 2 formed during the oxidation step, and also dissolves and complexes the iron oxides, as in Eqs 14, 15 respectively (Ocken, 1999;Rivonkar et al, 2022).…”

“…The final solution is therefore rich in oxalic acid and its destruction is a precursor to the treatment of CORD effluents, as it is seen to affect the efficiency of precipitation of chrome (Remoundaki et al, 2007;Rivonkar et al, 2022).…”

“…The concentration of oxalic acid was determined from the concentrations used during the CORD process (ranging from 5 mM to 18 mM) (Ocken, 1999;Rivonkar et al, 2022). A middle ground concentration of 10 mM oxalic acid (dihydrate, ACS reagent, ≥95.0%, Alfa Aesar) was used and a volume of 100 mL was used.…”

“…Mn and Fe ions tend to influence the reaction by accelerating the reaction rate in the absence of UV as seen in Eqs 3-5 (Saeed et al, 2013;Kim et al, 2019;Cai et al, 2021). Their influence was studied as they are present in solution after the CORD process (Rivonkar et al, 2022). The selection of 0.8 mM Mn 2+ and 0.14 mM Fe 2+ concentrations are based on a previous work (Rivonkar et al, 2022) using the CORD process before effluent treatment.…”

“…Their influence was studied as they are present in solution after the CORD process (Rivonkar et al, 2022). The selection of 0.8 mM Mn 2+ and 0.14 mM Fe 2+ concentrations are based on a previous work (Rivonkar et al, 2022) using the CORD process before effluent treatment. The Mn 2+ concentration is mainly coming from the KMnO 4 used in the CORD process whereas the Fe 2+ arises from the total dissolved iron after the CORD process.…”