10.15. The useful lifetime of fuel in a reactor is measured by the burnup, expressed as the total amount of thermal energy generated per unit quantity of heavy element charged to the core. In LWRs, the freshly loaded fuel consists of plutonium-free uranium oxide. Hence, the burnup is expressed as the thermal energy produced during the lifetime of the fuel per unit mass of uranium initially charged to the reactor even though significant plutonium is produced during this time. Alternative methods for expressing burnup, especially in irradiation effects studies, are (1) number of fissions per m3 of fuel and (2) percentage of heavy-element atoms that have under­gone fission. As seen in Example 1.2, for a reactor with uranium dioxide fuel, e. g., a LWR, a burnup of 2.6 TJ/kg U (or 30,000 MW • d/t) is equivalent to about 7.4 x 1026 fissions/m3 of fuel and to about 3 percent of atoms fissioned as uranium-235 plus plutonium-239 and plutonium-241.

10.16. There are several reasons why reactor fuel must be discharged long before the fissile and fertile materials are consumed. In the first place, accumulation of fission products and of the isotopes of heavy elements, which act as neutron poisons, and depletion of the fissile species, e. g., uranium-235 and plutonium-239, can decrease the reactivity to such an extent that the operational requirements of the reactor will no longer be satisfied. Furthermore, materials performance levels may deteriorate with continued exposure so that they become unacceptable. For example, leak­age of fission products into the coolant is not tolerated. The relevant fuel performance considerations are described in §7.170 et seq.

10.17. Generally, a high burnup is desirable so that unit costs associated with the preparation of the fuel could be spread over more units of energy generated. When fuel reprocessing was considered, cost studies showed that the increased burnup benefits leveled at a burnup in the vicinity of 2.6 TJ/kg as a result of the influence of enrichment charges [2]. However, with present “once-through” (no reprocessing) practice in the United States there is an incentive to extend the burnup in LWRs until the fuel materials performance levels are no longer acceptable. On the other hand, as burnup is extended, the higher enrichment needed makes the in-core fuel man­agement design more difficult (§10.26 et seq.). In addition to more efficient resource utilization, extended burnup reduces the number of discharged fuel assemblies that must be stored per amount of generated energy. Ex­perimental burnups on the order of 4.3 TJ/kg have been obtained in some LWRs [3].