Corrosion properties of modified PNC1520 austenitic stainless steel in supercritical water as a fuel cladding candidate material for supercritical water reactor

The supercritical water-cooled reactor (SCWR) has been designed and investigated because of its high thermal efficiency and plant simplification. There are some advantages including the use of a single phase coolant with high enthalpy. Supercritical Water (SCW) has never been used in nuclear power applications. There are numerous potential problems, particularly with materials. As the operating temperature of SCWR will be between 553 K and 893 K with a pressure of 25 MPa, the selection of materials is difficult and important. The PNC 1520 austenitic stainless steel has been developed by Japan Atomic Energy Agency (JAEA) as a nuclear fuel cladding material for a Na-cooled fast breeder reactor. Austenitic Fe-base steels were selected for possible use in supercritical water systems because of their corrosion resistance and radiation resistance. The PNC 1520 austenitic stainless steel was selected for possible use in supercritical water systems. The corrosion data of PNC 1520 in SCW is required but does not exist. The purpose of the present study is to research the corrosion properties for PNC 1520 austenitic stainless steel in SCW. The SCW corrosion test was performed for the standard PNC 1520 (1520S) and the Ti-additional type of PNC1520 (1520T) by using a SCW autoclave. The 1520S and 1520T are the first trial production materials of SCWR cladding candidate material in our group. Corrosion and compatibility tests on the austenitic 1520S and 1520T steels in supercritical water were performed at 673, 773 and 600• •with exposures up to 1000 h. We have evaluated the amount of weight gain, weight loss and weight of scale after the corrosion test in SCW for 1520S and 1520T austenitic steels. After 1000 h corrosion test performed, the weight gains of both austenitic stainless steels were less than 2 g/m² at 400• •and 500• •But 1520T weight increases more and weight loss than 1520S at 600• •The SEM observation result of the surface after 1000 h corrosion of an test (oxide layer) shows the surfaces were covered with 10μm oxide layer after exposed of 600• •1000h with 1520T and 1520S The grown-up grain that covers the surface of 1520T at 600• •was specified with magnetite by XRD results. The results showed that at 573 and 400• •the surface of 1520S was formed with Fe-Cr-Ni alloy and magnetite. By increasing the temperature to 600• •the surface of 1520T was covered with magnetite formed in SCW and dissolution of the steel alloying elements has been observed. In view of corrosion, 1520S may have a more possibilitythan 1520T to adopt a SCW reactor core fuel cladding. In view of grain boundary susceptibility, 1520T may have comparatively more possibility than 1520S.