Recycling in an ordinary water reactor

Mixed oxide (MOX) fuel provides about 2% of the new nuclear fuel used today and is manufactured from plutonium recovered from used reactor fuel. Its use also provides a means of burning military weapons-grade plutonium to produce electricity. Reprocessing to separate plutonium for recycling as MOX is increasingly being seen as more worthwhile from an economic point of view as uranium prices increase.

European and Japanese reactors use MOX extensively. It is generally employed as about one third of the core of reactors in these areas, but some reactors will accept up to 50% MOX assemblies. The EPR or AP1000, among other advanced light-water reactors, will be able to accept complete fuel loadings of MOX if required. If up to 50% of MOX is used, the operating characteristics of a reactor are not changed. A plant must be designed from the outset to take this fuel though, or adapted as necessary (for example more control rods are needed). If more than 50% MOX is used, plants either need to be designed from the outset to accommodate this, or changed significantly.

One advantage of MOX is that the addition of small extra amounts of plutonium easily raises, the fissile concentration of the fuel, whereas enriching uranium with higher levels of U-235 is relatively expensive. MOX use is becoming more attractive since reactor operators are looking to burn fuel harder and longer, increasing burn-up from around 30 000 MW days per tonne, which was typical earlier in the century, to over 50 000 MWd/t nowadays. MOX use is also attracting more interest since reducing the volume of spent fuel is increasingly desirable (World Nuclear Association, MOX, Mixed Oxide Fuel, 2009).