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What Is MOX?

"MOX" refers to "mixed oxide nuclear fuel." The fuel consists of two types of oxygen-containing compounds able to undergo nuclear fission reactions specifically, uranium oxide blended with a small amount of plutonium oxide.

Whereas low-enriched uranium remains the primary fuel burned in nuclear reactors worldwide, MOX came into use in the 1980s as a way of disposing of surplus weapons-grade plutonium. There are some 260 tonnes of military plutonium in world stockpiles which, if they weren't used as fuel, would have to be disposed of as nuclear waste.

Another attraction of MOX lies in the fact that plutonium is much more "fissile" than low-enriched uranium: Its atomic nuclei undergo fission split into smaller parts, releasing heat with more ease. One kilogram of Pu-239, an isotope of plutonium, can produce sufficient heat to generate nearly 10 million kilowatt-hours of electricity.

One facility where purely-uranium fuel gets reprocessed to become MOX is the Fukushima Daiichi nuclear power plant in Japan. Reactor Unit 3 burns MOX fuel made of 94 percent uranium and 6 percent plutonium.

MOX fuel rods in a spent fuel pool at Fukushima are causing grave concern. The latest chapter in a catastrophic chain of events since the power plant was damaged by Friday's massive 9.0 earthquake, workers are unable to keep the MOX rods in the spent fuel pool sufficiently cool, and if they start to burn, plutonium, an especially dangerous radioactive substance, will be released into the environment.

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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the  Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.