"Dear Arjun"

Dear Arjun,
What is MOX fuel and why is it so costly?
Mystified in Mystic, CT

Dear Mystified,
Once upon an ancient time, there was a knight who always tricked his fellow knights into taking the wrong path in their search for the Holy Grail, so that he could find it first. But since he never actually did, he came to be known as the "foxy fool." The court jester soon began to mock him, making all the other knights laugh, thus giving them the heart to continue their quest. Soon the jester's energetic performance came to be called "MOX fuel."

After the modern nuclear knights discovered plutonium, they thought they had found the Holy Grail and gave MOX fuel a new meaning: "Mixed OXide fuel." MOX fuel is a mixture of uranium dioxide and plutonium dioxide. It can be used in nuclear reactors as a substitute for the more commonly used pure uranium dioxide fuel.

Power Reactors (For more information see IEER's report "The Nuclear Power Deception.")
Most power-production reactors in the world are light water reactors and use low enriched uranium dioxide as fuel (see Centerfold for reactor types). Nuclear power relies on energy released during the fissioning (or splitting apart) of two important isotopes: uranium-235 and plutonium-239.

Natural uranium contains three isotopes (or forms) of uranium, namely uranium-238, uranium-235, and uranium-234. Of these, only uranium 235 is fissile and can sustain a chain reaction. "Enriched" fuel has a higher percentage of uranium-235 than does natural uranium, which contains only 0.711 percent uranium-235. "Low enriched uranium" (LEU) used in light water reactors contains 3 to 5 percent uranium-235. Such fuel also contains very small amounts of uranium-234, which is far more radioactive than the other two isotopes, and hence important from a health and safety point of view.

Almost all the rest of uranium in reactor fuel is uranium-238. Since uranium-238 can be split only by fast neutrons, it provides essentially no energy in light water reactors or other reactors that use slow neutrons to sustain the energy-producing chain reaction However, when slow neutrons are absorbed by uranium-238, the resulting nuclear reactions convert the uranium-238 into plutonium-239, which can be split by slow neutrons. Such fissions result in a release of energy (and two lighter elements called fission products). In this way some of the energy produced in nuclear reactors containing natural uranium or low-enriched uranium as fuel comes from the plutonium-239 made during reactor operation itself. Other isotopes of plutonium, notably plutonium-240, -241 and -242, are also created in reactors. (This mixture of plutonium isotopes is simply referred to as plutonium below.) Plutonium is also produced in other reactors that contain significant amounts of uranium-238, which includes almost all civilian power reactors.

The MOX Mix
In order to operate efficiently and maintain chain reactions, old fuel rods (containing "spent fuel") must be removed from reactors before all the uranium and plutonium are used up. At this point, the fuel rods can be stored as waste or "reprocessed" to recover usable fissile materials. (See IEER's report "Risky Relapse into Reprocessing.") In reprocessing, the residual plutonium, consisting mostly of plutonium-239, can be separated from fission products and from residual uranium in the spent fuel. However, plutonium cannot be used by itself as a fuel in reactors designed to use natural or low enriched uranium; these uranium fuels contain only a small proportion of fissile material (in the form of uranium-235). Using only plutonium as a fuel would result in far too many fission reactions, and the accompanying release of energy would overheat the reactor and damage it. Therefore, to be used as a fuel, plutonium must either be used in specially designed reactors, or it must be mixed with far larger amounts of natural or slightly enriched uranium. This limits fissile material loading in light water reactors to about 5 percent or less. The plutonium loading could be increased to 6 to 7 percent if special neutron-absorbing materials are added. According to a 1994 National Academy of Science study on plutonium, the use of such materials "would require safety review."(1) MOX fuel can also be used as a fuel in other reactor types.

To make MOX fuel, plutonium is converted into the chemical form of plutonium dioxide (PuO2) and mixed with uranium dioxide (UO2). These are both powders. The mixture is then formed into ceramic fuel pellets. This is the final form of MOX fuel.

The Price of MOX
MOX fuel is both costly and a security risk. It is far more expensive to fabricate MOX fuel than LEU fuel because plutonium is thousands of times more radioactive per unit weight than low enriched uranium. Such high levels of radioactivity require special handling and safety procedures, which in turn raise the cost of MOX fuel. Moreover, unlike natural uranium or LEU, plutonium from civilian power plants can be used to make nuclear weapons. This makes security requirements at plants that handle plutonium far more complicated and expensive. For both these reasons, it is far more expensive to make MOX fuel than pure uranium fuel. In fact, given the low price of uranium that currently prevails, and that is forecast to prevail in the foreseeable future, MOX fuel is more costly than uranium fuel, even if the plutonium itself is free.

The end of the Cold War has accelerated the dismantling of nuclear warheads that contain weapon-grade plutonium (which contains 93 percent or more of plutonium-239). There is no consensus as yet in the US on what to do with this dangerous material. There is some interest in converting plutonium into MOX fuel, as a way of making it more resistant to proliferation. Since burning MOX fuel in reactors mixes plutonium with highly radioactive fission products, it is difficult to re-extract and use the plutonium in weapons. On the other hand, burning MOX fuel in the US would encourage other countries to continue reprocessing their spent fuel, and would tend to perpetuate the plutonium problem. Further, the liquid, high-level radioactive waste stream associated with reprocessing is more dangerous from an environmental and health perspective than standard low enriched uranium fuel for the time that such liquids must be storied in tanks (prior to vitrification).

Institute for Energy and Environmental Research

Comments to Outreach Coordinator: ieer@ieer.org
Takoma Park, Maryland, USA

Updated: September 1996