increase the rate at which WPu can be loaded into reactors of a given size—but it does not reduce the amount of plutonium still remaining in the fuel when it is finally spent.

The quantity of energy generated by a given batch of fuel before it is considered spent is called the "discharge burnup" (or, alternatively, the "discharge exposure" or "discharge irradiation”). This is customarily measured in megawatt-days of thermal energy per metric ton of initial heavy metal (MTIHM or just MTHM), where "initial heavy metal" refers to the quantity of uranium, plutonium, and (sometimes) thorium and heavier elements in the fuel at the time it is first loaded into the reactor. Typical average discharge burnups for commercial LWRs are in the range of 25,000 to 40,000 MWd/MTHM, with the most recent fuel designs achieving the higher figures.¹² Canadian heavy-water moderated reactors (called CANDU, for Canadian deuterium-uranium) using natural uranium fuel achieve discharge burnups of about 7,000 MWd/MTHM; LMRs with fuel enrichments of 20-30 percent U-235 or plutonium achieve figures in the 100,000-MWd/MTHM range; and HTGRs are being designed to use fuel with enrichments above 90 percent to achieve discharge burnups of 500,000 MWd/MTHM and higher.

If every gram in an initial metric ton of heavy metal in fuel were actually fissioned, at 1 MWd/g the total burnup would be 1,000,000 MWd/MTHM. Correspondingly, each 10,000 MWd/MTHM of burnup represents the fission of about 1 percent of the heavy metal atoms initially present. Thus, for example, low-enriched uranium (LEU) fuel that achieves a discharge burnup of 33,000 MWd/MTHM starting with a U-235 content of 3.3 percent has fissioned altogether about 33 kg or 3.3 percent of the heavy atoms initially present in each ton of fuel, much of the fission occurring in the initial 33 kg stock of U-235, but a significant contribution coming from the fission of plutonium produced during the reactor's operation by the absorption of neutrons in U-238. Thus, for example, a 1,200-MWe LWR fueled with LEU at 3.3 percent U-235, and running at a capacity factor of 75 percent with a discharge burnup of 33,000 MWd/MTHM and thermal efficiency of 33 percent, would load annually about 30 MTHM (containing 1,000 kg of U-235), and would discharge, in spent fuel (after three years), about 250 kg/yr of U-235 and 300 kg/yr of plutonium, two-thirds of it the fissile Pu-239 and Pu-241 isotopes. A breeder reactor with the same electrical output would discharge 1,200-2,000 kg of plutonium per year, depending on fuel characteristics and operating mode.

If the fuel is not LEU but plutonium-based to start with, and the burnup and the conversion ratio are about the same, the plutonium content of the spent fuel necessarily will be higher. Even in a plutonium fuel that contains no fertile material from which new plutonium is produced in operation, the spent fuel will