The Initial PWR Core and Subsequent Reload Patterns

10.23. The fuel loading pattern for the first core only for a typical Westinghouse four-cooling-loop plant containing 193 fuel assemblies (§13.9) is shown in Fig. 10.3. By means of this pattern, we can see some of the challenges associated with reload batch design. In this case, fuel assemblies of three different enrichments are used. The low and intermediate enrich­ment assembles are arranged in a checkerboard pattern in the central portion of the core while the high enrichment assemblies are placed about the periphery. This approach utilizes the reactivity differences between adjacent fuel assembles in the central region to flatten the power density distribution. Since the fresh, highly enriched fuel is placed in the region of lowest neutron importance, where the leakage is highest, its tendency to cause local power peaking is mitigated. However, fast-neutron leakage from peripheral fresh fuel assemblies leads to vessel embrittlement (§10.26).

10.24. The general modified scatter-loading pattern may then be con­tinued for subsequent burnup cycles by discharging after one burnup period the fuel of lowest enrichment and reinserting the remaining fuel in the central region in a modified checkboard pattern in accordance with design considerations to be discussed further in §10.43. Fresh fuel assemblies would be placed about the periphery and after the next burnup period, the fuel having the initial intermediate enrichment would be discharged and the procedure continued in a manner similar to that described above. Such an approach is also known as a modified out-in type of loading. Although in this idealized scheme, the discharged fuel will be that having

maximum burnup, this might not be so in actual practice when various feed enrichments may be used for special purposes.

10.25. After a number of burnup cycles, a pattern such as that shown in Fig. 10.4 for a one-eighth core might evolve. We can see that there have been substantial deviations from the initial core interior checkboard pattern as a result of changes in batch size and feed enrichment in earlier burnup cycles. In the figure which is based on calculations using a two-dimensional nodal model (§10.42), we show “region number,” which corresponds to batch number. Since three regions of different enrichment were loaded for the initial burnup cycle, the figure shows the reload pattern for Cycle 7. Also of interest is the beginning-of-cycle (BOC) relative assembly power and the BOC exposure in GW • d/t, where the average cycle burnup has been 10 GW • d/t. We see that there are significant differences in burnup among assemblies of the same region as a result of neutron flux variations during previous cycles. Therefore, the reload core designer searches for an arrangement that will yield a reasonably uniform burnup for assemblies that are eventually discharged from the core while staying within assembly power peaking constraints [5].

10.26. During recent years, several trends have affected PWR reload core design. As reactor vessels approach the end of their planned 40-year operating life and consideration is being given to extended service, it is desirable to minimize the fast-neutron fluence reaching the interior surface to reduce the increase in the brittle-to-ductile transition temperature (§7.12). This can be accomplished by so-called “low-leakage” fuel loading, in which fresh fuel is loaded in the central region and older fuel about the periphery. A loading that is partially of this nature is shown in Fig. 10.5. In this case,

extending burnup through a fourth cycle is also an objective. Some twice — burned assemblies are loaded toward the >utside. Since the neutron flux is intentionally lower toward the outside, we see that such assemblies experience relatively low burnup during the operating cycle. It is not easy to meet the low-leakage objective since the power peaking introduced by the centrally loaded fresh fuel must be carefully controlled by the use of burnable solid absorbers, which will be discussed shortly. Another advan­tage of the low-leakage concept is improved neutron economy which tends to lower the feed enrichment and, in turn, fuel cycle costs. Advanced fuel rod designs, such as those using axial blanket pellets of natural uranium oxide, have also been found to improve neutron economy.

10.27. A second trend is to extend the burnup cycle from approximately 12 months to 18 months, a step that yields operating economies by in­creasing plant availability (the fraction of calendar time that the plant is available for energy generation). However, to provide enough initial reac­tivity to operate for this longer period, it is necessary to raise the enrichment of the fresh fuel and use burnable solid absorbers to control power peaking.

10.28. Another trend is to extend the fuel assembly total burnup from perhaps three annual burnup periods to four. Fuel utilization is thereby improved and the number of discharged assemblies that must be managed is reduced. Again, in this case, some increase in feed enrichment is needed requiring burnable absorber use for power peaking control. These trends apply only to once-through cycles, as practiced in the United States, not when plutonium recycle is applied, as in Europe.