Inner-Side-Protected Cladding with Meter Scale for High-Temperature Oxidation Resistance via the Swaging-Drawing Process

Abstract

Nuclear accidents, such as the Fukushima nuclear accident, have highlighted the necessity for accident-tolerant fuel (ATF) cladding. Previous studies focused on coating the outside of Zr alloy currently used in nuclear reactors with an oxidation-resistant material in a vacuum environment. This limits the coating to the inside of the cladding and does not tend to achieve a uniform coating on meter scale cladding. In this study, a room temperature and non-vacuum-based swaging-drawing process was demonstrated as an alternative cladding manufacturing process. It enables both the inner and outer sides of the 2-m-long Zr alloy cladding to be uniformly covered with a 100-mu m-thick corrosion-resistant material (316-L stainless steel; SS316L), thereby minimizing its high-temperature oxidation and avoiding failures. After the swaging-drawing process, there was a gap of less than 1 mu m between outer SS316L and Zr alloy and a gap of about 12 mu m between inner Zr alloy and SS316L. The high-temperature oxidation properties of the resulting triplex Gachon ATF cladding tube (G-tube) were evaluated up to 1,200 degrees C in an atmospheric environment. Following heat treatment at 1,200 degrees C, the control cladding completely oxidized and ruptured, potentially causing leakage of radioactive material during application. In contrast, only 15% of the G-tube cladding manufactured by the swaging-drawing process was oxidized despite a gap, and the Zr alloy of the G-tube changed phase from alpha-Zr to alpha-Zr (O) and prior beta-Zr. The cladding microstructure, oxide layer, and oxidation mechanism were analyzed through microscopy, X-ray diffraction, and thermogravimetric analysis. As a result, it was confirmed that SS316L completely prevented oxygen diffusion into the bulk Zr alloy. In addition, there was no elemental diffusion between SS316L and the Zr alloy. These results demonstrate the feasibility of using room temperature, nonvacuum environment-based swaging-drawing process to fabricate structurally stable ATF cladding at extremely high temperatures.