MOX fuel, short for mixed-oxide fuel, is a mixture of uranium and plutonium oxide. Most reactors use uranium fuel. As uranium fuel burns, some of it is converted into plutonium, so all operating reactors have plutonium in their core.
Safety implications differ depending on the amount of MOX fuel that is used. For example, at Japan's Fukushima Dai-Ichi plant, about 6 percent of the fuel in reactor #3 was MOX fuel, which contains about 200 kilograms of plutonium. This amount is a small enough that it likely made no significant difference in the amount of plutonium that escaped into the environment. However, the use of MOX fuel can lead to increased safety problems when it is a larger fraction of the fuel (e.g. 30 percent or more, as is done in France). In such a case the MOX fuel can increase both the likelihood that certain types of accidents will occur and the public health consequences should one occur.
Increased risk of nuclear terrorism
The manufacture, transportation, and storage of MOX fuel increase the risk of nuclear terrorism. In fact, MOX fuel is as great a terrorist and proliferation concern as plutonium itself. MOX fuel does not contain the highly radioactive components that make spent fuel dangerous, and the plutonium can be separated from the uranium by a straightforward chemical process.
There are two sources for the plutonium used in MOX fuel. First, spent fuel can be “reprocessed” to separate the plutonium and remaining uranium from the highly radioactive elements in the fuel. Second, there are significant amounts of plutonium from U.S. and Russian dismantled nuclear weapons. The Union of Concerned Scientists is opposed to reprocessing because it increases the risk of nuclear terrorism, and recommends that plutonium from dismantled weapons be “immobilized” by mixing it with radioactive waste for disposal in a geologic repository.