Patent Number: 062597566
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing, there is schematically illustrated in FIG. 2 a nuclear reactor core, generally designated 10, with a control blade pattern indicated by the square boxes disposed in the core 10. Each box of FIG. 2 represents a control blade 12 and an associated fuel assembly, generally designated 13, comprised of four fuel bundles 14 as illustrated in FIG. 1. The fuel bundles 14 are only partially illustrated with each bundle having fuel rods 16, e.g., in a 10.times.10 array, and associated vertically spaced spacers 18 as is conventional, only one spacer 18 being illustrated in each fuel bundle 14. Fuel channels 19 surround each fuel bundle 14 and define a cruciform opening between the fuel assemblies. As will be appreciated, each control blade 12 is cruciform in cross-section and is generally receivable within the core of the nuclear reactor in the cruciform openings between the four fuel bundles 14, the blades being movable from below the fuel assemblies, i.e., withdrawn positions, to positions within the cruciform openings adjacent the fuel assemblies. Hence, each box illustrated in FIG. 2 and represented by any one of the control blades A1, A2, B1 and B2 includes a single control blade 12 and four associated fuel bundles 14 arranged in quadrants defined by the cruciform spaces therebetween. Control blade sequences are generally divided into two groups known as A sequence blades and B sequence blades. Thus, the A and B control blades constitute first and second main groups A and B, respectively, of control blades. The first and second main groups A and B form a checkerboard pattern throughout the core as illustrated in the plan view of the core of FIG. 2. From a review of FIG. 2, it will be appreciated that the first main group of blades A is symmetric relative to the center of the core. The second main group of control blades B is asymmetric relative to the core. It will also be appreciated from a review of FIG. 2 that the respective main groups of blades A and B are each further divided into two sub-groups. For example, main group A is divided into sub-groups A1 and A2. Main group B is divided into sub-groups B1 and B2. A control blade of the sub-group A2 always lies at the center of the core. From a detailed review of FIG. 2, each first sub-group A1 of the first main group A is flanked by sub-groups B1 of the second main group B in a first or X direction. Each sub-group A1 of the first main group A is also flanked by second sub-groups B2 of the second main group B in a second or Y direction normal to said first or X direction. Also, each sub-group A2 of the first main group A is flanked by second sub-groups 82 of the second main group B in the first or X direction. Each second sub-group A2 of first main group A is also flanked by first sub-groups B1 of the second main group B in the second or Y direction. Similarly, each sub-group B1 of the second main group B is flanked by a sub-group A1 of the first main group A in the first or X direction. Each sub-group B1 of the second main group B is also flanked by the second sub-group A2 of the first main group A in the second or Y direction. Finally, each second sub-group B2 of the second main group B is flanked by a second sub-group A2 of the first main group A in the first or X direction and by the first sub-group A1 of the first main group A in the second or Y direction. Thus, it will be appreciated that each sub-group at any one control blade location within the core is located alternately in both X and Y directions. In each control cell formed by the single control blade 12 and four associated fuel bundles 14, the control blades 12 are movable from a withdrawn position below the core to either a shallow position or a deep position or both, depending upon their status as sequence A or B blades. The movement of the control blades relative to the fuel assemblies is indicated by the double-ended arrow in FIG. 1. Blades whose tips are inserted more than two-thirds into the core are referred to as deep blades. Blades inserted less than one-third into the core are referred to as shallow inserted blades. Deep blades are used to control the total reactor power, as well as the global radial power shape. Shallow blades are used to control the reactor axial power shape. Generally, blades are not inserted into the middle third of the core because they would tend to create axial power distribution problems. In accordance with a preferred embodiment of the present invention, and instead of using the first and second main groups A and B, respectively, of blades alternately as in the prior art, the first and second main groups A and B are at times employed simultaneously with the second main group B only used as shallow blades and the first main group A used either as deep or shallow blades. Moreover, the sequences are constructed to enable each fuel bundle a period of operation in an uncontrolled state without its associated control blade that is at least twice as long as the previous period of operation in a controlled state with its associated blade. That is, each fuel bundle is operated with its associated control rod withdrawn from the core for a period of time at least twice as long as the previous period of operation with its associated control rod inserted into the core. Additionally, each sub-group A1 and A2 of the first main group includes operational sub-groups A1' and A2', respectively. Consonant with a preferred embodiment of this invention, the sequence pattern for control of the core repeats at least every three consecutive time periods and any one blade is fully withdrawn for at least two consecutive periods after it has been inserted. With these criteria, there are a number of possibilities for control blade sequence patterns to optimize the BWR power control. For example, a control blade sequence pattern is illustrated in the chart below. CHART I Deep Shallow Time Period Blades Blades Not Used (Withdrawn) 1 A1 A1' A2, A2', B1, B2 2 A2 B2 A1, A2', B1 3 A2' B1 A1, A2, B2 4 A1 A1' A2, A2' B1, B2 5 A2 B2 A1, A2', B1 6 A2' B1 A1, A2, B2 7 A1 A1' A2, A2', B1, B2 8 A2 B2 A1, A2', B1 9 A2' B1 A1, A2, B2 As can be seen from Chart I, during the first time period, the first sub-groups A1 of the first main group A have their control blades inserted as deep blades. The first operational sub-groups A1 ' of the first sub-groups A1 have their blades inserted to shallow depths. The remaining blades of sub-groups A2, A2', B1 and B2 are totally withdrawn and not inserted. At the end of the first time period of reactor operation, the blades of A1 and first operational sub-group A1 ' are withdrawn, while the second sub-groups A2 of the first main group A are inserted into the core as deep blades. The blades of the second sub-groups B2 of the second main group B are also inserted into the core as shallow blades. The remaining blades of sub-groups A1 and B1 and operational sub-groups A2' are or remain withdrawn as applicable. The reactor is then operated for the predetermined time period 2. At the end of the second time period of reactor operation, the blades of the operational sub-groups A2' are inserted into deep positions, while the blades of the sub-groups B1 are inserted into shallow positions. The remaining blades of sub-groups A1, A2 and B2 are either withdrawn or remain withdrawn as applicable. The reactor is then operated for the third predetermined time period. At the end of the third time period, the preferred pattern preferably repeats itself, although it will be appreciated that the pattern may extend to periods beyond three consecutive time periods before repeat. Note that with this pattern, each fuel bundle is operational in an uncontrolled state without insertion of its associated control blade for at least two consecutive time periods before the associated control blade is inserted. Stated differently, any one control blade is fully withdrawn for at least two consecutive time periods after it has been inserted. Moreover, it will be appreciated that the control blades of the second main group B are used only as shallow blades, not as deep blades, and are movable only between positions withdrawn or into shallow positions apart from a scram condition. It will be appreciated that other sequences of operation of the control blades using the criteria stated above may be used. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.