10.51.

Although the general principles of core management for BWRs are similar to those for PWRs, the differences in core design have an important effect on the specific approaches used. Hence, a description of the core arrangement from the fuel assembly management viewpoint is helpful. An important feature is the placement of a group of four assemblies with each cruciform control element as shown in Fig. 10.6. Since BWR assemblies are about 40 percent smaller than PWR assemblies and the core

Number of Bundles Number of Control Blades

is larger, there are many more of them that require management, i. e., about 750 compared with about 190 for a PWR.

10.52.

In PWRs, all the fuel rods (about 260) in an assembly have the same enrichment, but this is not the case in BWRs in which the assemblies are smaller. In order to compensate for the water gap effect when the control blades are withdrawn, the fuel rods in the corners of each assembly have a low enrichment. In fact, there are four levels of enrichment in each assembly, as indicated in Fig. 10.7. The numbers 1, 2, 3, and 4 represent decreasing proportions of uranium-235 designed to even out the power density distribution. In the initial core, the enrichments range from about 1.7 to 2.1 percent, but in subsequent cores they are approximately from 2.5 to 3 percent. The number 5 shows the locations of two “water rods,” which contain water but no fuel. The introduction of extra moderator in this manner increases the neutron flux near the center of the assembly; the result is an increased and more uniform burnup. From two to five of the most highly enriched fuel rods include a few percent of gadolinium oxide (Gd203) to serve as a burnable poison (§5.197).

10.53. Soluble boron cannot be used in a BWR. Therefore, shim control is provided by partial insertion of control elements with resulting pertur­bation of the neutron flux around neighboring assemblies. Such reactivity control elements, in selected patterns, are generally initially inserted deep in the core. Therefore, control management is closely related to fuel man­agement. Since operation of the recirculating pumps can be used to adjust the reactor power within limits (§5.199), some movement by other sets of control elements is available for power shaping. The Haling principle (§10.49) provides a reference for planning.

10.54. BWR loading patterns follow low-leakage strategies, where either blanket or lowest reactivity fuel is located at the core periphery. According to one general strategy, fresh and once-burned fuel would then be mixed in interior positions except for the so-called control cell locations (Fig. 10.6). However, because of multienrichment fuel assembly loadings and the influence of control rod movement strategy, specific practical patterns tend to be complex and will not be represented here. Also, as in the case of PWR management, deviations from a given configuration are likely after a number of burnup cycles.