Accident-tolerant fuel (ATF) claddings for LWRs

ATF materials became worldwide a hot research topic after the Fukushima Daiichi accidents. Karlsruhe Institute of Technology (KIT) is involved in various activities on the development and especially high-temperature characterization of ATF cladding materials. Most of these activities are embedded in international collaborations in the framework of IAEA, OECD-NEA, EU as well as cooperation with partners from industry. With the bundle experiment QUENCH-19, the worldwide first large-scale test of ATF cladding under severe accident conditions was conducted in 2018. This test involved FeCrAl cladding produced and supplied by ORNL. Approx. 100 times less hydrogen was released compared to reference test QUENCH 15 with ZIRLO cladding tubes until the end of the reference test scenario, and a coping time of at least 2000 seconds was confirmed. MAX phase coatings for Zr alloys based on the systems Ti/Cr/Zr + Al + C were produced using a unique two-step procedure based on magnetron sputtered elementary nano-layers and subsequent annealing procedure. Most promising results were obtained with Cr2AlC coating. High-temperature oxidation tests in steam atmosphere with these MAX phase coatings as well as a number of most promising ATF cladding types including various other coated Zr alloys, FeCrAl, and SiCf-SiC composites were conducted. Some interesting results with illustrative examples will be presented in this paper.

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.

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