Although the nuclear industry has continuously improved the properties and reliability of fuel assemblies (FA) for light water reactors (LWR) since its existence, the fuel rods always were based on UO2 fuel and zirconium alloy cladding. This materials system works well under operation condition and has resulted into less and less failure rates. The advanced zirconium alloys (Zry) like M5® from Framatome and ZIRLO™ from Westinghouse provide excellent corrosion resistance and mechanical behavior at operation conditions. However, during accident scenarios with high temperatures, strong oxidation in steam-containing atmospheres leads to degradation of the mechanical properties of the cladding tubes as well as production of heat and hydrogen. At temperatures beyond 1200°C the chemical heat release due to the exothermic zirconium-steam reaction may exceed the residual decay heat and thus strongly affect the accident progression. The released hydrogen bears the risk of hydrogen detonations when coming in contact with oxygen/air.
Latest since the Fukushima Daiichi accidents the nuclear community started thinking about alternatives to the UO2/Zry system with one main focus on reducing heat and hydrogen release during severe accident scenarios, and thus increasing coping time for accident management measures (AMM), while retaining or even improving the fuel assembly properties during normal operation. Further issues include of course reasonable costs, licensing as well as front end and back end performance.
Today, the development of accident tolerant fuel (ATF), advanced cladding and other structure material is pushed by industry and research organizations worldwide mainly in the US, Asia and Europe. Furthermore, ATF research is coordinated by international organizations like the IAEA, OECD-NEA and the European Commission. So, recently, NEA published an extensive (>300 pages) state-of-the-art report on ATF prepared by an international expert group (EGATFL) including scenarios and metrics, cladding, fuel, and technology readiness level evaluation. An excellent overview paper on ATF cladding development (in the opinion of the author) with many references was published by Terrani in 2018.