The explosions that damaged a crippled Japanese nuclear plant during a disaster that forced mass evacuations in 2011 show what can happen when nuclear fuel overheats.
In response to the Fukushima Dai-ichi accident, the US government dramatically increased funding to develop tougher protective skins for nuclear fuel, hoping to spur innovation in designs that hadn't changed much in years. While the US Department of Energy was spending USD 2 million before the accident on future fuel designs, the funding reached as much as USD 30 million afterward.
Now scientists at multiple institutes are in the middle of developing designs that could start finding their way into test reactors as soon as this summer, followed by larger tests later on.
The goal is to create nuclear fuel that that is more resistant to damage and melting in extreme situations and less prone to a chemical reaction that makes its metal wrapping brittle and produces explosive hydrogen gas. If researchers succeed, their work could give plant workers more time to keep an accident from spiraling into a meltdown that releases harmful radiation. The work is no cure-all to prevent accidents, but it's a way of reducing risk.
"It's basically buying time for the reactor," said Andrew Griffith, the Energy Department's director for fuel cycle research and development. "It's basically an insurance policy."
Scientists in the government- and industry-funded efforts are experimenting with multiple solutions before narrowing their focus on the most-promising technologies.
Nuclear fuel has remained similar for decades. Uranium dioxide is compressed into a pellet about the size of a fingertip. Those pellets are stacked into fuel rods up to 15 feet (4.5 meters) long and placed in a tube, called cladding, made from zirconium alloy. That metal cladding resists corrosion in a reactor, holds up against heat and serves as a barrier that keeps radioactive elements in place without cutting too much into the energy produced by a nuclear plant. Nuclear fuel is supposed to withstand accident conditions, but the disaster at the Fukushima Dai-ichi plant shows how it can fail when pushed to extremes.
After an earthquake, tsunami waves crashed over the plant's seawall and disabled the electrical gear needed to run reactor cooling systems. When the cooling systems and backups stopped working, the reactors overheated. As water levels dropped, the metal cladding around the fuel reacted with steam and oxidized, producing hydrogen gas. Scientists blame that escaping hydrogen gas for causing multiple explosions that damaged the facility.
