10.44. A number of considerations are clear to the experienced reload core designer. Although some of these have previously been mentioned, a summary here may be helpful. The feed batch size, cycle length, and discharge burnups are related by material balance requirements as illus­trated in Example 10.1.

Example 10.1. A PWR rated at 3000 MW(th) has a core consisting of 193 assemblies, each containing 450 kg of uranium (as oxide). If a plant capacity factor[17] of 0.8 is assumed, approximately what fraction of the core must be discharged per year if a burnup of 30 GW • d/t is achieved? Staggered batch loading is used.

Energy generated per year = (3 GW)(365)(0.8) = 876 GW • d

Reactor inventory = (193)(0.45) = 87 tonnes U

Energy generated per reactor inventory before discharge

= (30)(87) = 2610 GW • d

Core lifetime = —— = 3 years 876

If an annual refueling is assumed, one-third of the core would be discharged each year. For a 12-month operating cycle yielding a burnup of 10,000 MW • d/t (864 GJ/kg), should the discharge burnup be increased from a nominal 30,000 MW • d/t to 40,000 MW • d/t, the feed batch size would be reduced from one-third to one-fourth of the total core assemblies.

10.45. The feed enrichment must be high enough to provide the reac­tivity needed for the planned burnup cycle length. However, when de­signing a loading pattern, careful management of the fresh fuel assemblies is needed to avoid local power peaks. Power peaking may be looked upon as a result of the reactivity contributions of adjacent assemblies. Thus, it is helpful to balance the reactivity of fresh or once-burned fuel in interior or “inboard” positions with neighboring depleted fuel assemblies.

10.46. In practice, compromises are likely to be needed in the adjacent assembly reactivity balancing procedure as a result of limitations on po­sitions available. The use of solid burnable absorbers is then necessary to suppress the local power peaks selectively. However, such use of burnable poisons should be minimized. One approach that provides additional flex­ibility in locating fresh assemblies is to divide the fresh assemblies into two subbatches, each with a different enrichment. Since the use of a split batch permits assigning a lower enrichment to those assemblies that will even­tually be discharged after several operating cycles with lower-than-average exposure for the batch, there is some saving in fuel cycle costs.

10.47. The selection of assembly positions and determination of burn­able poison loading are carried out in conjunction with core neutronic modeling calculations to determine the acceptability of candidate patterns. However, an alternative promising approach is to develop an extensive data base of loading patterns which include appropriate specific design parameters. A computer search isolated from neutronics methods can then select a desirable pattern [12].