The 2011 Fukushima Daiichi event demonstrated the need for improved nuclear energy safety, thus justifying the global investments in the development of accident-tolerant fuel (ATF) cladding materials that are expected to outperform commercial zircaloy fuel cladding materials both during nominal operation conditions and high-temperature transients/accidents. This lecture describes the systematic approach used within the H2020 IL TROVATORE project towards the accelerated development of innovative accident-tolerant fuel (ATF) cladding materials based either on binary carbides (SiC) or nanolaminated ternary carbides known as the MAX phases. Accelerated material development entails the continuous communication between application-driven material design, material production, and material performance assessment (esp. material compatibility with the coolant & radiation tolerance). Moreover, it demands the development of reliable, high-throughput candidate material screening tools, such as the combined use of in-situ and ex-situ ion/proton irradiation to assess response to irradiation and/or the establishment of multiscale modelling approaches to predict in-service material behaviour.
This lecture will attempt to demonstrate the scientific & technical (S&T) challenges involved in the accelerated development of two ATF cladding material concepts that rely on the use of carbides to address the requirements of the stringent ATF application. These two concepts are: (i) the SiC fibre-reinforced SiC matrix (SiC/SiC) composite material concept, and (ii) the MAX phase-coated zircaloy material concept. SiC/SiC composites as candidate ATF claddings have already claimed major R&D investments in Europe, the USA, and Japan, due to their remarkable refractoriness and pseudo-ductility that impart to these ceramic matrix composites (CMCs) a great potential for true accident tolerance during beyond-design-basis accidents (>>1200°C). Prior to the commercial deployment of SiC/SiC composite claddings for use in nuclear power plants (NPPs), important inherent shortcomings of SiC (i.e., inadequate compatibility with water/steam; dependence of radiation swelling on temperature & damage dose) and remaining technical challenges, such as joining, must first be fully addressed. These S&T challenges are the primary focus of the HORIZON SCORPION project and will be described in this lecture.
The MAX phases are ternary carbides/nitrides given by the M_n+1AX_n chemical formula, where M is an early transition metal, A is an A-group element, X is C or N, and n = 1, 2 or 3. Due to their nanolaminated crystal structure, the MAX phases are characterised by hybrid ceramic/metallic properties, while they are also widely known for their exceptional radiation tolerance, esp. at temperatures >600°C; however, smart architectural design can extend the radiation tolerance of these ceramic materials to lower temperatures, turning them – under certain conditions – into suitable coating materials for zircaloy claddings. This lecture will demonstrate the accelerated development assessment of MAX phase coating materials for the coated zircaloy ATF cladding material concept. Coolant compatibility was assessed under nominal (PWR water, 330°C, 1 month) & transient/accident (steam, 1200°C, 1 h) operation conditions. Radiation response was evaluated by in-situ ion irradiation, using 6 keV He⁺ at 350-800°C to a maximum damage dose of 11.5 dpa. The studied MAX phase-based ceramics (211, 312 & 413 ternary compounds, and higher-order solid solutions) were produced in the (Zr,Nb,Ti,Cr,V,Hf)-(Al,Sn,Si)-C system.