Mixed Lots

Mixed oxide, or MOX fuel, is a blend of oxides of plutonium and natural uranium, reprocessed uranium, or depleted uranium which behaves similarly (though not identically) to the low enriched uranium feed for which most nuclear reactors were designed. more...

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MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors that predominate nuclear power generation.

Thorium-plutonium fuel has some attractive characteristics such as much lower production of and more complete burning of transuranics and has been used in a few reactors to date, but is usually referred to specifically as thorium-plutonium rather than MOX to avoid confusion with uranium-plutonium MOX fuel.

One attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be disposed as nuclear waste, and would remain a nuclear proliferation risk.

Overview

In every uranium-based nuclear reactor core there is both fission of isotopes such as uranium-235 (U-235), and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238 (U-238). Most of the fuel mass in a reactor is U-238. This can become plutonium-239 (Pu-239) and by successive neutron capture plutonium-240 (Pu-240), plutonium-241 (Pu-241), plutonium-242 (Pu-242) and other transuranic or actinide isotopes. Pu-239 and Pu-241 are fissile, like U-235. Small quantities of uranium-236 (U-236), neptunium-237 (Np-237) and plutonium-238 (Pu-238) are formed similarly from U-235.

Normally, with the fuel being changed every three years or so, most of the Pu-239 is "burned" in the reactor. It behaves like U-235, with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically about one percent of the spent fuel discharged from a reactor is plutonium, and some two thirds of the plutonium is Pu-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. A single recycling of plutonium increases the energy derived from the original uranium by some 12%, and if the uranium-235 is also recycled by re-enrichment, this becomes about 20%. With additional recycling the percentage of fissile (usually meaning odd-mass number isotopes) in the mix decreases and even-mass number neutron-absorbing isotopes increase, requiring the total plutonium and/or enriched uranium percentage to be increased. Today in thermal reactors plutonium is only recycled once as MOX fuel, and spent MOX fuel, with a high proportion of minor actinides and even-mass plutonium isotopes, is stored as waste.

Re-licensing precedes the introduction of MOX fuel into existing nuclear reactors. Often only a third to half of the fuel load is switched to MOX. The use of MOX does change the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it. More control rods are needed. For more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility but so far have always operated on fresh low enriched uranium. In theory the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh Uranium fuel.

Read more at Wikipedia.org