MOX
Mixed oxide fuel, commonly referred to as 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 (LEU) 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 LEU fuel. 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. Although MOX fuel can be used in thermal reactors to provide energy, efficient fission of plutonium in MOX can only be achieved in fast reactors.
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 normalising the global commercial use of MOX fuel and the associated expansion of nuclear reprocessing will increase, rather than reduce, the risk of nuclear proliferation, by encouraging increased separation of plutonium from spent fuel in the civil nuclear fuel cycle.
In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235 (235
92U
), and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238 (238
92U
). Most of the fuel mass in a reactor is 238
92U
. By neutron capture and two successive beta decays, 238
92U
becomes plutonium-239 (239
94Pu
), which, by successive neutron capture, becomes plutonium-240 (240
94Pu
), plutonium-241 (241
94Pu
), plutonium-242 (242
94Pu
) and (after further beta decays) other transuranic or actinide nuclides. 239
94Pu
and 241
94Pu
are fissile, like 235
92U
. Small quantities of uranium-236 (236
92U
), neptunium-237 (237
93Np
) and plutonium-238 (238
94Pu
) are formed similarly from 235
92U
.
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