Keeping Up with the Heat: Accident Tolerant Fuel and Cladding

By: John Taylor

Almost seven years ago, disaster struck the nuclear industry when a tsunami severely damaged the Fukushima-Daiichi Nuclear Power Plant. In the wake of the accident, the global nuclear industry sought more ways to improve reactor safety in emergency situations. To improve the resiliency of its nation’s own reactors, the U.S. Department of Energy currently supports Westinghouse and the GE subsidiary Global Nuclear Fuel (GNF) to develop accident tolerant fuels (ATF).

Accident tolerant fuels can withstand a loss of coolant accident (LOCA) for a significantly larger amount of time than standard fuel designs, while also preserving and advancing fuel performance under typical reactor conditions. The goal of these programs is to enhance reactor safety by improving cladding and fuel properties (physical, chemical, and thermal), as well as reducing the leakage of fission products into the coolant stream.

One ATF design is Westinghouse’s EnCore fuel, which replaces the traditional uranium oxide pellet with silicon-based materials. According to Westinghouse, their uranium silicide pellet has a thermal conductivity 550% higher than the traditional fuel pellet, meaning that it retains less heat when coolant is lost. The EnCore fuel also has a Uranium density 17% higher than that of the traditional UO2 fuel, meaning that a reactor can have a longer fuel cycle or reduced fuel loading. Another component of the EnCore ATF is the chromium-coated zirconium cladding that is used with the uranium silicide pellets.

Westinghouse claims that their EnCore cladding lives a longer life due to its reduced oxidation and hydrogen buildup under normal operating conditions, which also contributes to its ability to withstand extended exposure to high temperatures (1300-1400 degrees Celsius). These designs are currently undergoing irradiation in test reactors at Idaho National Laboratory, and Westinghouse intends to begin manufacturing lead test rods (LTRs) this year, with the intent of putting the first rods and assemblies in reactors by 2022.

Another company involved in developing ATF is Global Nuclear Fuel. They are working alongside Southern Nuclear and Exelon Generation to test their newest fuel design. The components they have developed include a new iron-chromium-aluminum cladding, IronClad, and a coated zirconium fuel cladding, ARMOR. The ARMOR coating is a material that can be applied to any zirconium cladding to increase its resilience to debris that manages to enter the core, and both IronClad and ARMOR provide better resistance to damage from oxidation than traditional cladding.

Both cladding advancements will be implemented at Georgia Power Company’s Plant Hatch during their refueling outage this spring. Global Nuclear Fuel plans to also implement their ATF cladding at Exelon Generation’s Clinton power station in 2019.

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