Evaluation of elevated temperature process for the decontamination of stainless steel 304LN surface

Abstract

Deposition of activated corrosion products on out-of-core surfaces and ensuing radiation field build-up in nuclear reactors calls for remedial measures like decontamination to minimize exposure of service personnel carrying out routine maintenance jobs. Decontamination of stainless steel structural materials in nuclear power plants pose challenges due to the inherent difficulty in dissolving chromium containing oxides like ferrites or chromites. Stainless steel, SS 304LN, is the proposed structural material for the main heat transport system of Indian Advanced Heavy Water Reactors (AHWRs) and it is important to be equipped with an effective decontamination process to mitigate the radiation field build-up in AHWRs. This paper discusses the studies to develop an elevated temperature (ET) decontamination process for SS304LN surface. Oxide covered SS304LN specimens were prepared under simulated AHWR coolant chemistry conditions. The oxide film formed on the specimen surface was Ni and Cr substituted single phase ferrite spinel. The oxide film could be completely dissolved from the SS304LN surface in the nitrilotriacetic acid (NTA) based ET process at 180 °C in a single step with an acceptable and low corrosion rate of 7.7 × 10−3 μm/h. The efficiency of the ET process was compared with the known NTA based conventional low temperature dilute chemical decontamination process and a multi-step, multi-cycle process comprising of alternate oxidation and reduction processes. The ET process proved to be more effective and apt for decontamination of SS304LN surface.

Keywords

• SS304LN
• Corrosion
• AHWR
• Elevated temperature decontamination
• Chromium enriched spinel