Materials

Abstract

This chapter deals with fuel cladding and thermal insulating materials. The current status regarding development of fuel cladding materials is described in Sect. 4.1. Zr-modified 15Cr20Ni austenitic alloy (1520Zr alloy) has been developed for fuel cladding application of the supercritical-pressure light-water cooled reactor by improving the austenitic alloy PNC1520 to provide the necessary high temperature strength and compatibility with high temperature water environments. Excellent creep strength was confirmed for the 1520Zr alloy. The oxidation property in supercritical water and SCC susceptibility in subcritical water of 15Cr20Ni based austenitic alloys have been preliminarily examined using PNC1520 alloy and the essential resistance to those degradation modes was demonstrated. The oxidation kinetics of three types of 15Cr-20Ni austenitic stainless steels (1520 SSs) in supercritical water at 700°C under 24 MPa are described in Sect. 4.2. The cladding tube-shaped 1520 SSs showed very low oxidation kinetics and no spalling. It was considered that the tube-shaped 1520 SSs show good performance for fuel cladding application in the SCWR from the viewpoint of oxidation kinetics. The high oxidation resistance of the tube is due to a protective Cr-rich oxide layer formation on the outside surface of the tubes. It was considered that Cr diffusion within the outside surface layer of the tubes is accelerated as a result of grain refinement and/or an increase of dislocation density due to a high degree of cold work. The authors of the Sects. 4.1 and 4.2 are Yutaka Watanabe and Hiroshi Abe. Section 4.3 describes the thermal insulating materials that are used on the water rod walls. They are required because of the large temperature difference between the coolant inside and outside. Yttria-stabilized zirconia (YSZ) was developed for this purpose. Evaluations showed that 8 mol% YSZ with a density above 40 % is suitable for the thermal shielding material for the Super LWR and Super FR. Section 4.3 was written by Yoshiaki Oka based on the results of Kazuya Sasaki and Takayuki Terai.