WHAT IS NUCLEAR FUEL?
Radioactive uranium powers most nuclear reactors. Uranium dioxide is compressed into pellets about the size of a fingertip. The pellets are stacked into a hollow metal tube typically made of a zirconium alloy. The tubes are up to 15 feet tall and are collected in bundles. These bundles are eventually loaded into a nuclear reactor.
IS THE FUEL SAFE?
Generally, yes. Federal regulators say modern fuel has a safe track record. Engineers settled on the current metal alloys protecting fuel pellets because they resist corrosion, are strong and stand up well to heat. They are designed to withstand accidents. Rules set by the U.S. Nuclear Regulatory Commission require that emergency cooling systems in a nuclear plant keep the cladding from exceeding 2,200 degrees Fahrenheit. The rules also set limits on how much of the cladding can undergo the chemical reactions that create explosive hydrogen gas and give off excess heat.
SO WHAT'S THE PROBLEM?
The disaster at the Fukushima Dai-ichi nuclear plant showed that severe accidents can expose nuclear fuel to far worse conditions. On March 11, 2011, a series of tsunami waves knocked out the power needed to run cooling systems at the Fukushima Dai-ichi nuclear plant. Without those cooling systems, the nuclear fuel at the plant overheated. As cooling water boiled away, the resulting steam reacted with the zirconium alloy cladding on the fuel. That created a chemical reaction which produced explosive hydrogen gas and even more heat. Investigators blame the hydrogen gas for fueling several explosions at the site that damaged buildings, injured workers and flung around radioactive debris. The explosions significantly complicated efforts to fix the plant. The chemical reactions produced even more heat in an already over-heated reactor, speeding along fuel meltdowns.
WHAT CAN BE DONE?
Researchers are trying a range of tactics to make nuclear fuel survive longer in extreme accidents. Their overriding goal is giving plant workers more time before an accident becomes a full-on crisis. Some scientists are trying to develop coatings for zirconium alloys that would prevent or limit the reactions that produce hydrogen gas and heat. Other scientists are seeking to scale back the use of zirconium by using different metals or switching to ceramics. Developing new coatings will take years, while introducing new materials could take a decade or more.
ARE THERE OBSTACLES?
Federal regulators would require detailed studies to prove any new materials are safe before allowing them in reactors for widespread use. Also, the nuclear industry spent years in a successful effort to reduce the time its plants are shut down because of technical glitches. Consequently, utility companies will want assurances any new materials are bug-free — not just safe — before using them. Adding new protective features to fuel will almost certainly raise costs. Some researchers have proposed offsetting the increased cost by boosting the amount of electricity the fuel can generate before it must be replaced. Other researchers say they think the government would have to require the use of accident-resistant fuels to make their use widespread.