Selection of Joining Materials for Light Water Reactors

Important components in nuclear fission power plants are the nuclear fuel containing rods, meter long tubes made of Zr-based alloys. The main drawback of the Zr-based alloys are redox reactions with steam, with production of H2, as it happened in Fukushima. Following this accident, several Accident Tolerant Fuels (ATF) materials have been proposed and SiC fiber reinforced SiC matrix composites (SiC/SiC) are considered very promising materials for safe fission reactors because, in case of accidents, they do not have redox reactions with production of H2. However, SiC/SiC must be joined to obtain meter long tubes and their joining is still an issue: the aim of this final project is to help in the selection of some joining materials for these applications. Several joining materials and joining techniques are available to join SiC/SiC, but very few of them are suitable for the next generation fission reactors. The joints must be hermetic, high corrosion resistant in hydrothermal environment, high oxidation resistant in steam at high temperature and neutron radiation tolerant; it should also be obtained without applying any pressure. The focus of this final project is to investigate the suitability of two different materials to join nuclear grade SiC and SiC/SiC. Materials to be joined were Chemical Vapour Deposition (CVD) SiC, Nano Infiltrated Transient Eutectic (NITE) SiC/SiC and Chemical Vapour Infiltrated (CVI) SiC/SiC; the joining materials were a silica-alumina-yttria (SAY) glass-ceramic and a MoSi2/Si composite obtained by using the Mo-Wrap process. The above mentioned SiC-based materials were joined without applying any pressure by using the following joining materials: SAY, in argon flow at 1375°C for 20 minutes, then at 1255°C for 1h to obtain a glass-ceramic joint; MoSi2/Si, at 1450°C for 5 minutes in argon flow. The joints were tested in static autoclave at SCK-DEN (Studiecentre voor Kernenergie - Belgium), simulating the Pressurised Water Reactor (PWR) water chemistry (1000 ppm B and 2 ppm Li dissolved, at 330°C) to evaluate their corrosion resistance. Characterisation of joints and bulk joining materials were performed prior and post static autoclave tests using FE-SEM analysis, EDX Analysis, X-Rays Diffraction and micro-CT SCAN (the latter at Oxford University, UK). Static autoclave test was conducted for 14 days as pre-screening. SAY-joined samples exhibited a negligible weight loss and further analysis on morphology and composition does not exhibit changes after autoclave. Mo-Wrap joined samples exhibited a higher mass loss compared to SAY ones. SAY was then selected as joining materials to join composites materials: SiC/SiC tubes by CEA (Commissariat à l'énergie atomique et aux énergies alternatives, France) and NITE SiC/SiC (NITE Corporation, Prof. A. Kohyama, Japan). Characterisation of joined SiC/SiC tubes found that the butt joint is not a suitable configuration for SAY joining. Optimisation of the joint design is needed. Hot stage microscopy and creep tests on SAY suggested good high temperature resistance up to 1365°C. Further loop autoclave test at SCK-CEN (PWR-like water chemistry, 330°C, 30 days) exhibited failure at the interface for uncoated SAY joined SiC. Investigation is needed to understand the motivation offailure. This thesis found that SAY could be a promising joining material for the application, however more validation is needed and the use of protective coatings must be explored.