Mixed Lots

Mixed oxide, or MOX fuel, is a blend 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.

An attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be handled as a difficult-to-store nuclear waste product, and 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) and plutonium-242 (Pu-242) as well as other transuranic or actinide isotopes. Pu-239 is fissile, like U-235. Small quantities of uranium-236 (U-236) 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 and its fission releases a similar amount of energy. The higher the burn-up, the less plutonium remains in the spent fuel, but 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 17%, and if the uranium is also recycled this becomes about 30%. With additional recycling the percentage of fissile plutonium in the mix decreases requiring the total plutonium percentage to be increased.

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 has so far always operated on fresh slightly enriched uranium.

According to the AECL, CANDU reactors could use 100% MOX cores without physical modification. Atomic Energy of Canada Limited (AECL), reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium.

Read more at Wikipedia.org