Studies on Dissolution Behavior of the Surface Layer of Sodium-Exposed SS 316LN in Decontaminating Formulation Using PEMHS

The Prototype Fast Breeder Reactor (PFBR) is nearing completion at Kalpakkam, India. Sodium is the heat transfer medium for PFBR, and austenitic steel SS 316LN is the material of construction for the sodium circuits of the reactor. During reactor service, the inner surfaces of the sodium circuit pipelines undergo corrosion by interacting with liquid sodium, forming ferritic layers. Radioactive nuclides formed by the activation of corrosion products are deposited on the ferritic surface, resulting in a radioactive burden on maintenance personnel. Chemical decontamination is generally carried out by dissolving the surface ferritic layer on the inside surface of the sodium circuit. In this context, a study of the dissolution behavior of the ferritic layer on SS 316LN samples formed by exposure to liquid sodium at 823 K was carried out by monitoring the H₂ released during the chemical interaction with decontamination formulation. The decontamination chemical formulation was a mixture of sulfuric acid and phosphoric acid. This paper discusses the sample preparation, formation of the ferritic layer, and studies carried out on its dissolution behavior in decontamination formulation by monitoring the hydrogen released during the reaction using a proton exchange membrane–based hydrogen sensor.