The foregoing numbers lead to the widely used approximate rule of thumb that all reactors, irrespective of details of type and fuel, fission about one gram per day of heavy nuclei (and produce about one gram per day of fission products) per megawatt of average thermal output. This ratio—one megawatt-day of output per gram fissioned—means that a large modern power reactor of nominal 1,200-MWe capacity, thermal-to-electric generating efficiency of 33 percent, and annual average capacity factor of 75 percent—providing 1,200 MWe × 365 days × 0.75 / 0.33 MWe/MWt = 1 million MWd of thermal energy per year—will fission 1 million grams or 1 metric ton (MT) of heavy nuclei per year. A reactor of this size that derived all of its energy from plutonium, then, would fission a ton of plutonium per year, and this relation provides a helpful metric for the WPu quantities addressed in this report: 100 tons of WPu represents the amount of fissionable material consumed by 100 large power reactors in a year. In mid-1993, world nuclear power capacity was equivalent to more than 270 such reactors (see the latter part of this chapter).

The preceding figure does not mean, however, that it is feasible to destroy 100 tons of surplus WPu in the space of a year using a third or so of the world's power reactors. The technical reasons this is not possible (leaving aside logistic and institutional obstacles to such an approach) are that (1) most reactors unavoidably produce plutonium at the same time as, although in most reactor types at a slower rate than, they burn it; and (2) it is not possible, in general, to burnup all of the plutonium (or other fissionable material) that is in a reactor at any one time.

The production of plutonium occurs, as described above, in consequence of the absorption of fission neutrons in the principal isotope of natural uranium, U-238. This process is called "fertile-to-fissile conversion," with U-238 correspondingly termed a "fertile" material. The customary index of the rate of production is the "conversion ratio," CR, defined as

CR = (fissile atoms produced) / (fissile atoms destroyed),

which can be calculated as a ratio of instantaneous rates or, more practically, in terms of total production and destruction over a period of time.⁹ The LWRs that dominate world nuclear-energy generation today have conversion ratios around 0.6, while the heavy-water reactors in commercial use in Canada and a few other countries have conversion ratios in the range of 0.7 to 0.8. Any reactor with a conversion ratio greater than unity is called a "breeder reactor," and the conversion ratio is then called the "breeding ratio"; reactors with a conversion ratio less than unity are called "burners" or "convertors.” Of the new fissile material produced in a reactor by the conversion of U-238 to Pu-239 (or, in the

FRONT MATTER (R1-R14)

Executive Summary (1-16)

Chapter 1: Introduction (17-25)

Chapter 2: Background (26-58)

Chapter 3: Criteria for Comparing Disposition Options (59-115)

Chapter 4: Reactor Options (116-213)

Chapter 5: Disposal of Plutonium Without Irradiation (214-249)

Chapter 6: Comparing the Options (250-396)

Chapter 7: Conclusions and Recommendations (397-418)