The nuclear reactors at the Fukushima Daiichi power station in Japan that were crippled by the March 11 earthquake and tsunami are a lot like reactors in the U.S. They are a common, if not exactly modern, General Electric design that harnesses nuclear fission to boil water and drive steam turbines to generate electricity. The same reactor designs and containment system are in use across the U.S., for instance at the Browns Ferry Nuclear Plant near Athens, Ala., and the Vermont Yankee facility in Vernon, Vt.

But reactor No. 3 at Fukushima Daiichi, one of the units that has experienced severe problems in the past two weeks, has one characteristic that differentiates it from its neighboring reactors and from any operating reactor in the U.S. Among the hundreds of standard nuclear fuel assemblies in its core, which rely on the splitting of uranium atoms to release energy, are some that contain a mix of uranium and plutonium. This so-called mixed oxide, or MOX, fuel was loaded into Fukushima Daiichi reactor No. 3 in 2010 and has found use in several other countries' power plants as well. And a big-budget U.S. government project is scheduled to begin producing MOX for domestic utilities in 2016.

But, as with most issues relating to nuclear energy, the use of MOX is a source of some controversy. Proponents say that burning MOX in nuclear reactors is a sensible way to dispose of weapons-grade plutonium from Cold War nuclear stockpiles, as the U.S. plans to do with 34 metric tons of surplus plutonium at its planned Mixed Oxide Fuel Fabrication Facility (MFFF) in South Carolina. (Some countries also reprocess spent nuclear power plant fuel to produce MOX.) Critics say that MOX is riskier than standard fuel and that there are better ways to dispose of excess plutonium.

"I think it's a magnificent solution," says David Jones, senior vice president for the back-end business group at Areva, a Paris-based nuclear fuel manufacturer with U.S. headquarters in Bethesda, Md. Areva is half of a partnership that is the prime contractor to the U.S. Department of Energy (DoE) on the $5-billion MFFF project. "You're taking something that was designed to be dangerous, and you're turning it into something that benefits society," Jones says.

Ordinary low-enriched uranium fuel contains primarily uranium 238, the most common natural isotope of the element, along with about 5 percent uranium 235, a rarer isotope that splits, or fissions, more readily. MOX fuel, on the other hand, substitutes plutonium 239 as the fissionable component, reducing the need for uranium 235.

"It's a fairly well established technology, especially overseas," says Jess Gehin, a nuclear science and engineering researcher at Oak Ridge National Laboratory, where MOX fuel rods have undergone testing. "All of our analyses show that it can be used without significant differences to uranium dioxide."

But Robert Alvarez, a senior scholar at the Institute for Policy Studies, a Washington, D.C., think tank, says that MOX is not the best way to irreversibly render plutonium unsuitable for weapons use. "If you really want to pursue the path of irreversibility, there are probably cheaper, easier ways to do it," he says. One way would be to blend the plutonium down to a low concentration and put it in the DoE's Waste Isolation Pilot Plant in the New Mexico desert. With the price tag attached to the MFFF, "it's certainly not something you'd think you could make money off," Alvarez says. "I kind of see it as a nuclear equivalent to a bridge to nowhere."

And Edwin Lyman, senior scientist for global security at the Union of Concerned Scientists in Washington, D.C., argues that MOX is more likely to cause nuclear accidents than ordinary uranium fuel and is liable to release more harmful material in the event of an accident. "Plutonium has different properties than uranium 235 that generally tend to degrade some of the safety systems in nuclear plants," Lyman says. For instance, because weapons-grade plutonium fissions more readily than uranium 235, reactors may need more robust control rods—neutron absorbers that shut down the nuclear chain reaction when inserted into a reactor's core. "You never get quite as much margin even after doing all that as you do with uranium," Lyman says.

Jones counters that MOX has a proved track record. "You'll hear some folks say that MOX is experimental," he says. "Over 6,000 MOX assemblies have been safely used in reactors around the world." Jones notes that MOX has passed muster with several different regulatory bodies in Europe and Japan, where the fuel has found use in dozens of nuclear power plants. "They found that it does not pose a significant, unacceptable level of risk," he says.

Lyman authored a study in 2001 in Science & Global Security showing that radioactive leakage from a meltdown with MOX fuel, which in addition to plutonium has higher levels of radioactive isotopes such as americium 241 and curium 242, would be deadlier than a low-enriched uranium meltdown. "Because plutonium is so much more radiotoxic than many of the other radionuclides, even if it's released in relatively small concentrations it can have an impact on the effects," Lyman says. He adds that it is not possible at the moment to identify how much the MOX fuel in Fukushima reactor No. 3 has contributed to the radioactive plumes emanating from the plant.

Oak Ridge's Gehin argues that the flavor of nuclear accident is more important than the flavor of fuel in the reactor. "The uncertainty of what would happen [in an accident] is not driven by MOX fuel versus uranium dioxide," he says. "It is driven in what happens in the event itself."

And Jones points out that low-enriched uranium fuel and MOX fuel become more similar as the fuel is consumed in fission reactions. "Folks try to cast a pall over MOX fuel because it has plutonium in it," he says. "We're trying to make sure people understand that uranium fuel, once it goes in the reactor, starts producing plutonium as well as fissioning plutonium and generating energy from it."

Even reactors loaded with straight uranium fuel, such as those in the U.S., end up with a mix of radioactive elements in the core, essentially a lower-plutonium version of MOX. "I don't know why people keep trying to make MOX an issue, because every reactor in the world burns MOX fuel," Jones says. "It goes in as uranium fuel, but once it starts going it has plutonium in it."

Even as the South Carolina fabrication plant progresses toward start up, the future of MOX fuel remains somewhat uncertain in the U.S. "The DoE still can't find a utility that's willing to take this stuff," Alvarez says. Duke Energy had signed an agreement with the DoE to load four of its reactors with MOX fuel, but the utility let the contract lapse in 2008. The federally owned Tennessee Valley Authority (TVA), which operates the Browns Ferry Nuclear Plant and two other nuclear facilities, has expressed some interest in trying MOX and may step up to take fuel from the MFFF. But Lyman questions whether even TVA will be a willing taker. "I don't see why any utility, even a government-owned one like TVA, would want to dabble with this stuff," he says.