Enhancing Safety: The Pursuit of Accident-tolerant Fuel

It’s hard to imagine, but there once was a time when cars didn't have turn signals or seat belts. Today, the safety technology in the average car is astonishing, with equipment and computers that can sense potential impacts, control stability and apply braking. While there have been countless improvements to automobile safety over the years, manufacturers continue to develop new ways to further enhance safety. 

When the first nuclear power plants were built more than 50 years ago, technologies and practices were put into place to maintain operational safety. From the physical barriers in the plant construction to myriad  safety systems that allow the plant to stay in a safe condition in the rare event of an emergency, multiple systems and technologies work together to maintain safety.

The Westinghouse AP1000® nuclear power plants are taking safety to an even higher level with the addition of passive safety systems that engage automatically and without the need for human action in the rare event of an emergency. These passive systems are a significant step forward in further enhancing the safety of what was already highly safe. What else can be done to enhance nuclear plant safety even further?

One area that is being researched is the nuclear fuel.  The Westinghouse accident-tolerant fuel (ATF) program began in 2004, aimed at producing light water reactor fuel that provides a leap ahead in safety and performance, while also being economically attractive for nuclear power plant operators.

How could nuclear fuel withstand the high temperatures in a loss of coolant accident?  It’s a matter of finding materials that can take the heat even better than the currently used zirconium alloys.

“Nuclear fuel has cladding that contains the stacked fuel pellets,” explained Ed Lahoda, consulting engineer for Westinghouse on the project. “By exploring new materials for both the cladding and the pellets, we’re working toward developing a fuel that will be able to withstand and survive extreme events.”

The cladding options, which provide most of the enhanced accident-tolerant characteristics, are SiC/SiC composite ceramics and zirconium alloy wrapped and coated with SiC (“SiC/Zr”).  SiC is silicon carbide (also known as carborundum), one of the hardest substances known and is widely used in wheels for cutting stone.  SiC composites are structures made up of SiC fibers and SiC pieces that resist the shattering experienced by most ceramics. “These options are expected to provide increased degradation resistance at temperatures up to ~1800°C (3272°F),” Lahoda said.

The fuel pellet material options considered include uranium silicide (U3Si2) and a composite of uranium mononitride (UN) and U3Si2.  “These pellets all have a higher density and higher thermal conductivity than the uranium dioxide (UO2) that is currently used, providing safety improvement through less stored energy in the fuel,” Lahoda said. “They also offer significant economic gains for the nuclear plant operators. These features will make ATF economically attractive while featuring improved safety.”

The pursuit of accident-tolerant fuel is being carried out by an international, multidisciplinary team, funded by the U.S. Department of Energy.  The team members and their primary missions on this project include:
  • Westinghouse Electric Company LLC – Program lead
  • General Atomics – SiC/SiC composite cladding development
  • Argonne National Laboratory and Ceramic Tubular Products – SiC/Zr cladding development
  • Idaho National Laboratory – Production of U3Si2 and UN/U3Si2 as well as test rod assemblies for testing in the Advanced Test Reactor
  • National Nuclear Laboratory (United Kingdom) – U3Si2 powder and pellets
  • Los Alamos National Laboratory and Texas A&M University – Production of UN granules and UN/U3Si2 pellets
  • Massachusetts Institute of Technology – In-reactor testing of SiC/SiC and SiC/Zr cladding
  • Institute for Energy Technology (Norway) – Test rod assembly and testing
  • Southern Nuclear Operating Company and Exelon Nuclear – Customer-based evaluation of ATF
  • University of Tennessee, University of Wisconsin and Argonne National Laboratory – Coated Zr rods
  • Paul Scherrer Institute (Switzerland) – evaluation of SiC properties
  • United Technologies Research Center and Ceramic Tubular Products – SiC specialty experts 
The current program will culminate in the manufacture of test fuel rods by 2016.  These test rods will undergo a six-year exposure in the Advanced Test Reactor at Idaho National Laboratory and the Halden Reactor in Norway to develop the data required for licensing as a prelude to loading lead test rods in commercial reactors in 2022.

 “This is a long process and quite an international effort,” said Sumit Ray, Westinghouse director, Methods & Technology.  “When these tests are complete and finally implemented commercially, it will be just another example of how Westinghouse and our research partners are leading the way with moving nuclear technology forward.”