Q: (i) What is meant by ‘mixed oxide’ fuel? (ii) Compare the pros and cons of oxide fuel against metallic fuel. (iii) Why are mixed oxides used in commercial reactors rather than mixed metals?
A:
(i) Mixed oxide (MOX) fuel is nuclear fuel that contains a mixture of plutonium dioxide (PuO) and uranium dioxide (UO). Typically the plutonium content is around 5—10% by weight, with the remainder being (usually depleted or natural) uranium oxide. MOX allows plutonium separated during reprocessing to be recycled as reactor fuel.
(ii) Oxide fuel (UO/MOX) compared to metallic fuel (U metal):
| Property | Oxide fuel (UO/MOX) | Metallic fuel (U metal) |
|---|---|---|
| Melting point | Very high (~2,865 °C for UO) — a major safety advantage | Lower (~1,132 °C for U metal) |
| Chemical stability | Excellent — does not react readily with water or air | Reactive — uranium metal corrodes in water and can ignite in air |
| Thermal conductivity | Relatively low — limits heat transfer from pellet centre to cladding | Higher — better heat transfer |
| Uranium density | Lower (fewer uranium atoms per unit volume because of the oxygen) | Higher (more uranium per unit volume) |
| Radiation stability | Good — retains structural integrity under irradiation | Prone to swelling and dimensional change under irradiation |
| Cladding compatibility | Good | Can have compatibility issues |
(iii) MOX fuel can be used in reactors that were originally designed for UO fuel (most PWRs and BWRs) with relatively minor modifications. This allows stockpiled separated plutonium from reprocessing to be consumed, reducing the proliferation risk of holding large inventories.
A metallic alloy of uranium and plutonium can be used as nuclear fuel, and some experimental fast-reactor fuels have been metallic alloys (e.g. EBR-II in Idaho). However, metallic fuels have the disadvantages listed above (lower melting point, corrosion, dimensional instability), and they are not compatible with the standard thermal-reactor designs that dominate the global fleet. For these reasons oxide fuels (including MOX) are overwhelmingly preferred for commercial power reactors.
End of Chapter 3