Mixed oxide fuel (MOX fuel) is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation. For example, a mixture of 7% plutonium and 93% natural uranium reacts similarly, although not identically, to low-enriched uranium fuel (3 to 5% uranium-235). MOX usually consists of two phases, UO2 and PuO2, and/or a single phase solid solution (U,Pu)O2. The content of PuO2 may vary from 1.5 wt.% to 25–30 wt.% depending on the type of nuclear reactor. One attraction of MOX fuel is that it is a way of utilizing surplus weapons-grade plutonium, an alternative to storage of surplus plutonium, which would need to be secured against the risk of theft for use in nuclear weapons. On the other hand, some studies warned that normalizing the global commercial use of MOX fuel and the associated expansion of nuclear reprocessing would increase, rather than reduce, the risk of nuclear proliferation, by encouraging increased separation of plutonium from spent fuel in the civil nuclear fuel cycle.
Overview In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235, and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238. Most of the fuel mass in a reactor is uranium-238. By neutron capture and two successive beta decays, uranium-238 becomes plutonium-239, which, by successive neutron capture, becomes plutonium-240, plutonium-241, plutonium-242, and (after further beta decays) other transuranic or actinide nuclides. Plutonium-239 and plutonium-241 are fissile, like uranium-235. Small quantities of uranium-236, neptunium-237 and plutonium-238 are formed similarly from uranium-235.
Normally, with low-enriched uranium fuel being changed every five years or so, most of the plutonium-239 is "burned" in the reactor. It behaves like uranium-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 plutonium-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. Reprocessing the plutonium into usable fuel 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%. Plutonium is only reprocessed and used once as MOX fuel; spent MOX fuel, with a high proportion of minor actinides and plutonium isotopes, is stored as waste.
Existing nuclear reactors must be re-licensed before MOX fuel can be introduced because using it changes the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it; for example, more control rods are needed. Often only a third to half of the fuel load is switched to MOX, but for more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The System 80 reactor design deployed at the U.S. Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility, but so far has 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. Fast neutron BN-600 and BN-800 reactors are designed for 100% MOX loading. In 2022, the BN-800 was fully loaded with MOX fuel for the first time. According to Atomic Energy of Canada Limited (AECL), CANDU reactors could use 100% MOX cores without physical modification. 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.
Spent MOX fuel The content of un-burnt plutonium in spent MOX fuel from thermal reactors is significant – greater than 50% of the initial plutonium loading. However, during the burning of MOX the ratio of fissile (odd numbered) isotopes to non-fissile (even) drops from around 65% to 20%, depending on burn up. This makes any attempt to recover the fissile isotopes difficult and any bulk Pu recovered would require such a high fraction of Pu in any second generation MOX that it would be impractical. This means that such a spent fuel would be difficult to reprocess for further reuse (burning) of plutonium. Regular reprocessing of biphasic spent MOX is difficult because of the low solubility of PuO2 in nitric acid. As of 2015, the only demonstration of twice-recycled, high-burnup fuel occurred in the Phénix fast reactor.
Applications
Reprocessing of commercial nuclear fuel to make MOX is performed in France and to a lesser extent in Russia, India and Japan. In the UK THORP operated from 1994 to 2018. China plans to develop fast breeder reactors and reprocessing. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States due to nonproliferation considerations. Germany had plans for a reprocessing plant at Wackersdorf but as this failed to materialize, it instead relied on French nuclear reprocessing capabilities until legally outlawing the transport of German spent fuel for reprocessing in 2005. The United States was building a MOX fuel plant at the Savannah River Site in South Carolina. Although the Tennessee Valley Authority (TVA) and Duke Energy expressed interest in using MOX reactor fuel from the conversion of weapons-grade plutonium, TVA (the most likely customer) said in April 2011 that it would delay a decision until it could see how MOX fuel performed in the nuclear accident at Fukushima Daiichi. In May 2018, the Department of Energy reported that the plant would require another $48 billion to complete, on top of the $7.6 billion already spent. Construction was cancelled.
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