Corrosion Behaviors Of Candidate Materials For Supercritical Water-cooled Reactor

The Supercritical water-cooled reactor (SCWR) is the only water-cooled reactor among the six GenerationⅣreactor concepts. It has advantages such as economy, continuity and sustainability, and it can be the main reactor type for large scale electricity production. The coolant exit temperature is up to 500~650℃, which is extremely corrosive to the metal materials. The present core components and fuel cladding materials may not be compatible in the SCWR environment. A serials of candidate materials screening tests for SCWR has been conducted on the major fire tube materials used in supercritical fossil fired plant and fuel cladding materials used in current pressurized water reactors (PWRs), covering ferritic/martensitic heat resistant steels (F/M), austenitic stainless steels, nickel base alloys and oxide dispersion strengthened steel (ODS). The research on the corrosion of candidate materials in SCWR environment is of great importance for the selection and development of materials for core components and fuel cladding in SCWR.The candidate materials, including F/M steel P92, austenitic stainless steel 304NG and AL-6XN, nickel base alloy C276 and ODS steel MA956 were exposed in supercritical water including static and dynamic state at temperature of 550, 600, 650℃, and pressure of 25MPa for 1000h respectively. The surface morphology, microstructure and element distribution of each candidate material is analyzed by Optics Microscope, Scanning Electron Microscope (SEM), Energy-dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD).The experimental results showed that, P92 had very high corrosion rate, AL-6XN and 304NG had low corrosion rate, C276 had lower corrosion rate, while MA956 had extremely low corrosion rate. A dual-layer structure was formed on F/M steel and austenitic steels. The outer layer was Fe-rich magnetite structure, while the inner layer was Cr-rich spinel structure. Cracks were formed on the surface of P92 steel at 600℃, a kind of nodular corrosion morphology appeared on 304NG stainless steel, a layer of granular corrosion product with Fe-rich magnetite structure was observed on AL-6XN, while a layer of very thin and stable oxide film was formed on the surface of C276 and MA956, but existing some pitting and spallation.The experimental results revealed that, the exposure temperature had a great effect on the weight gain of candidate materials in the supercritical static autoclave, and all the candidate materials were attacked most seriously at 600℃. While in the supercritical dynamic autoclave, the exposure temperature only affected remarkably the weight gain of 304NG stainless steel, but had little effect on the other materials. Under the same exposure temperature, the weight gain in the supercritical dynamic autoclave was greater than that in the supercritical static autoclave, which meant that the flow water could accelerate the corrosion rate of materials.In conclusion, forming a layer of dense, integrated and stable oxide film on the surface of candidate materials is much crucial to the corrosion resistance in SCW. Chromium and aluminum are the key elements to form stable oxide film, but it is easy to produce volatile corrosion product in SCW, causing the partial loss of protect oxide film and finally accelerating the corrosion rate of candidate materials.According to the present experimental results, MA956, C276 and AL-6XN have excellent corrosion resistance in SCW among all candidate materials. Therefore, the three kinds of candidate materials may be used as core components and fuel cladding materials in SCWR.