can be managed in a number of ways, such as increasing the number of control rods or their speed of insertion, adding so-called burnable poisons, which are materials of high neutron absorption cross-section that absorb the extra neutrons, in particular those in the resonance region, and other methods.
The plutonium content in spent fuel will generally be larger if MOX is used in place of LEU. The reason is that the unused plutonium from the MOX will add to the plutonium that is bred from the U238, which is contained both in LEU and MOX fuel. However the plutonium in spent fuel will of course be reactor-grade (RPu), that is, its isotopic composition will be changed, and the spent fuel will meet the “spent fuel standard” in that the RPu is no more accessible to potential diversion than the plutonium from thermal reactors burning LEU or natural uranium. It may appear that the potentially larger plutonium content of spent fuel is a negative factor arguing against the use of MOX for disposing of WPu. This is clearly not the case for two reasons: First the amount of RPu in spent fuel, if MOX made from WPu is used, is under all circumstances a tiny fraction of the RPu now contained in the world ’s inventories from commercial nuclear fuel. Second, the total amount of plutonium contained in spent fuel, if MOX fabricated from WPu is used, is less, relative to the total amount of the original WPu put into the fuel, plus the plutonium produced in the spent fuel had the same amount of electric power been generated from LEU. In short, the plutonium content of the spent fuel is not a useful discriminant among alternate disposition approaches.
Some currently operating reactors are already designed for using MOX for all their fuel elements and others not now burning MOX have been demonstrated to be usable for full MOX cores. In the former category is the American pressurized water reactor (PWR) designed by Combustion Engineering, called the System-80. It incorporates the additional control rods and increased neutron absorber in the coolant required to permit full MOX operation. Also the Canadian deuterium-uranium (CANDU) reactors have been operated on experimental basis with MOX and the contractors operating CANDU conclude that the design safety margins in CANDU using full MOX loading should not be significantly different from the margin in the current system fueled by natural uranium.
Actual commercial experience with MOX operations exists only in Europe since reprocessing of spent fuel is not practiced (licensed) in the United States or Canada and since use of MOX based on surplus WPu remains a matter for the future. In Germany, the first experience with the use of MOX was in boiling water reactors (BWRs) in 1966 with the experimental reactor Kahl (VAK). Commercial operation with PWRs with MOX started in the reactor Obrigheim (KWO), which operated until 1980. Since 1981 MOX fuel has been used in additional PWRs. In total about 67.5 tons of plutonium has been processed in more than 100,000 fuel rods containing MOX. Extensive experience with a large variety of parameters, such as fuel composition and varying degrees of burn-up, has accumulated. These German reactors have been licensed for MOX fractions up to 50 percent. Table B-1 gives an overview of these reactors. The actual percentage of MOX use has been considerably less than that shown in the table. The reason is that not enough fuel was available. In principle, it could also be possible to license the BWRs for