Mixed Oxide (MOX) Explained
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Mixed Oxide (MOX) Explained

By Dr. Zoomie

Dear Dr. Zoomie – every now and again there’s something in the news about something called MOX. Some people seem to like it and it seems to make others unhappy. Can you tell me what the story is?

This is one of those issues that, like you, most people don’t know much about – in fact, most of the population probably doesn’t know anything about it at all. Those who do follow the controversy tend to come down heavily in favor or vehemently against – not many are on the fence. With that as a bit of a prelude, here’s what the facts are – I’ll let you decide how you feel about the issue.

To start, MOX stands for Mixed Oxide. What this refers to is the fact that MOX fuel contains both uranium and plutonium mixed together with each metal in a chemical form called an oxide (rust, for example, is iron oxide). Since the uranium and plutonium are both fissionable, the reactor generates power by fissioning both of these elements instead of just the uranium.

Mixed oxide, or MOX fuel, is a mix of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

This plutonium can come from one of two sources. One possibility is that plutonium from former nuclear weapons – or from plutonium stockpiles – can be mixed in with standard uranium fuel. Alternately, plutonium can be extracted from spent reactor fuel (more on this in a moment) and mixed in with fissionable uranium to form reactor fuel. In either case, this is sort of like blending ethanol in with gasoline – they both burn so you can mix them together and still fuel your car.


As for where plutonium is created…well, this gets interesting. Whether the plutonium in the MOX fuel comes from weapons or weapons stockpiles, or from spent reactor fuel it was produced in the same way – inside a nuclear reactor. In fact, every operating reactor produces plutonium during its operation. Every nuclear reactor generates power by nuclear fission and the atoms most likely to cause these fissions are the lighter isotope of uranium, U-235. But if reactor fuel is, say, 6% U-235 (at the upper range for commercial reactor fuel) then the other 94% is U-238. The core of a reactor has a lot of neutrons flying around (the neutrons come from fission) and many of those neutrons will be captured by U-238 atoms to form U-239. U-239, in turn, is radioactive and it emits beta radiation to become fissionable plutonium (Pu-239). So normal fission results in the formation of Pu-239 in every single nuclear reactor on the planet. The question is what happens to the Pu-239 after it’s formed.

Some of the Pu-239 fissions as the fuel continues to produce energy. Some of it will capture another neutron to become Pu-240 (which also fissions) or can even capture more neutrons to form heavier forms of plutonium. But not all of the Pu-239 will fission or capture neutrons – a lot of it simply remains in the fuel until it’s removed from the reactor as spent fuel. And this is where some of the controversy starts to arise – since all spent reactor fuel contains Pu-239, a nation (or a sophisticated terrorist group) can chemically process the spent fuel to extract the plutonium. In truth, much of this will be marginally useable for nuclear weapons because of the Pu-240 and heavier isotopes. But the fact remains that it’s still there and it can be extracted by anyone with access to the right chemistry, handling equipment, and the rest of the necessary technology. This is, in fact, one reason that the US gave up reprocessing spent reactor fuel – since the plutonium presents a possible proliferation risk if the fuel is reprocessed, deciding not to reprocess the fuel means that the Pu-239 remains locked up in the fuel rods alongside the highly radioactive fission products, rather than being extracted for possible theft or diversion into nuclear weapons.

Another approach – no matter where the Pu-239 comes from (reactor fuel or nuclear weapons stockpiles) – is to deliberately mix the Pu-239 in with uranium fuel and to use it to produce energy. The thinking here is that, if the Pu-239 is sequestered away inside of spent fuel, it still exists and can be extracted at some time in the future; or if it’s locked in a secure bunker it still exists and can be stolen or formed into weapons by a nation that has decided it’s time to make new weapons again. On the other hand, according to this line of thought, if the Pu-239 is used to make reactor fuel, the actual atoms of Pu-239 are split and can never be fissioned again – if the plutonium is fissioned then it no longer exists; no more than carbon dioxide and water vapor can be reconstituted into the gasoline that they once were. When the Pu-239 is fissioned it is forever gone from this world and it will never again be a proliferation risk.

On the other hand, groups that oppose MOX fuel point out that, until the fuel is inserted into a reactor and fissioned, it poses even more of a security risk. The thinking here is that, normally, Pu-239 is either locked up inside of secure storage vaults or is locked up within dangerously radioactive spent fuel rods (which are, themselves, locked up behind multiple barriers). Either way, the plutonium is fairly secure.

By comparison, those who are opposed to the use of MOX fuel point out that it makes the plutonium much easier to steal. First, it’s already been separated from the dangerous fission products (if the source is spent reactor fuel). Second – and even more important – the fuel itself can be stolen when it’s in transit to the reactor in which it will be used. Typically, the most vulnerable time for any dangerous material is during shipping, when it is outside the normal security barriers and safety systems. No matter how much security is in place, materials in transit are never as secure as when they’re locked up behind multiple barriers in a hardened facility.

So this is the situation – we have (quite literally) tons of plutonium in various places around the world, and every nuclear reactor is producing more all the time. It doesn’t matter whether you are pro- or anti-nuclear power – the fact is that the world we live in has this material in it. That being the case, the question – and the controversy – is what to do with this material. Do we lock it up to achieve maximum security (but the plutonium continues to exist)? Or do we add it to uranium to form MOX so that it can be destroyed forever (but introducing potential vulnerabilities during transit)? There is logic to both positions – the question is whether the ability to gain useful energy, as well as removing the plutonium from the face of the Earth, is worth the added risk of theft.