Also in "traditional" fuel containing enriched uranium when fresh, plutonium is produced during reactor operation in such quantities that about 1/3 of the total energy is produced by splitting plutonium in such fuel as well. When discharged, about one percent of plutonium remains in the fuel (by weight), and this can be "recycled" into manufacturing of fresh fuel in such a way that the plutonium collected from about ten assemblies will give enough "starting material" for one new assembly. Such fuel, called "MOX fuel", is being used in a number of NPP:s around the world, but so far uranium has been so cheap that plutonium recycling has not been an economical breakthrough. At Fukushima Dai-ichi unit 3, the first batch of 34 MOX assemblies was loaded among the about 500 uranium fuel assemblies last fall.
MOX fuel has certain safety challenges, which prevent hasty transition to its use:
1. Plutonium recycling requires transporting of highly radioactive spent fuel from the NPP to a reprocessing utility and back as MOX fuel. Transport of radioactive materials requires special arrangements in order to guarantee safety,
2. Reprocessing is a demanding process, and reprocessing utilities (in Europe: La Hague in France and Sellafield in the UK) contain lots of radioactive substances in different forms, and preventing them from escaping into the environment is a specific safety challenge.
3. Fresh MOX fuel radiates in such quantities that it's more difficult to handle than uranium fuel, which can be manipulated with thin gloves (which, as it comes, are needed to protect the fuel, not the hand).
4. Concerning final disposal of spent fuel, MOX fuel produces somewhat more decay heat, which requires a prolonged intermediate storage period or a reduced density in the final repository. (On the other hand, in the hour/day scale, which is important for accident situations, MOX fuel produces less decay heat than uranium fuel.)
5. Plutonium alters the physical properties of the reactor (so called reactivity feedbacks) to a degree, and the accident analyses of the NPPs must therefore be repeated with the new parameters in order to verify that the criteria set for ensuring fuel integrity are fulfilled in all required cases.
6. The efficiency of control rods is reduced by introduction of MOX fuel, so their efficiency must be ascertained prior to transition to MOX use. (As a side note, comments about the EPR reactor being "designed for MOX fuel" are to a large degree based on the increased number of control rods - not to the reactor having anything MOX specific in itself.)
7. Due to the different reactivity behaviour and heat conductivity of MOX fuel, the gas gap between the fuel pellet and the cladding will apparently contain a somewhat larger amount of fission products at high burnups than uranium fuel. Therefore, in some possible accidents the failed fuel rods may have a larger initial release of fission gases at the early stages of the accident. The difference disappears if the accident situation lasts longer, because the total amounts of fission products are close to each other in uranium and plutonium.
8. In the current nuclear reactor types (light water reactors) plutonium can not be recycled indefinitely, because eventually too large amounts of the isotope Pu-240 will collect in the plutonium. Therefore large-scale plutonium recycling is connected to the development of the next generation reactors operating on fast neutrons, the research on which has during the last couple of decades been almost nonexistent. During the last years, increasing worries about the sufficiency of the world uranium resources have led to a gradual reopening of fast reactor research projects and programmes.Since the use of MOX fuel has several such extra challenges in comparison to regular uranium fuel, using it is naturally a more challenging operation than using uranium fuel, if the same safety level is to be maintained. However, when we're talking about an accident such as in Fukushima, there's no difference whether the fuel that is used in the reactors was initially based on enriched uranium or recycled plutonium, since the threat does not come from the isotopes of the fresh fuel but principally from the fission products (iodine, cesium etc.), and there's no significant difference in their numbers to either direction between uranium and plutonium fuel.
