Induced nuclear reactions: processes – systems – and elements – With control of reactor – By movement of control element or by release of neutron...
Reexamination Certificate
2000-08-22
2001-07-10
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
With control of reactor
By movement of control element or by release of neutron...
Reexamination Certificate
active
06259756
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of operating a boiling water nuclear reactor and particularly relates to a control blade sequence pattern to optimize the BWR power control and hence obtain greater power and/or improved uranium fuel efficiency and economy.
Control blades typically containing boron carbide provide reactivity control, i.e., control the total reactive power and power distribution. The control blades are generally in cruciform cross-section and are inserted between four associated fuel bundles or assemblies containing arrays of fuel rods. The blades enter the bottom of the reactor core and are movable into positions within the reactor core for reactivity control and positions withdrawn from the core. In modern BWRs, control blades are divided into two groups, generally known as “A sequence” and “B sequence.” The control blades of the A sequence and the B sequence are referred to herein respectively as A or B control blades or A or B sequence blades. These two groups generally form a checkerboard pattern throughout and in plan view of the reactor core. An A control blade is conventionally at the center of the core. Thus, every other control blade in a first direction, i.e., an X direction, from the center of the core is an A sequence blade and every other blade in a Y direction, i.e., a second direction normal to the first direction, from the center of the core is an A sequence blade. Alternate blades in the X and Y directions between the A sequence blades are B sequence blades. With this typical arrangement, the A sequence blades are symmetric relative to the center of the core, while the B sequence blades are asymmetric.
Control blades are also designated as deep or shallow inserted blades. Blades whose tips are inserted more than two-thirds into the core are referred to as deep blades, while those inserted less than one-third into the core are referred to as shallow blades. Deep blades are used to control total reactive 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 tend to create axial power distribution problems. Control blades are, of course, movable to the deep and shallow positions from positions totally withdrawn from the core.
In conventional BWR operations, four basic control blade patterns, the control blades of which are designated at A1, A2, B1 and B2, are used to develop operating control blade pattern sequences throughout each cycle. The typical practice is to alternate use of two blade sequences during operation of the reactor. Each sequence of blades, for example, the A sequence, is used exclusively of the other sequence, e.g., the B sequence. Thus, only A sequence or B sequence blades, but not both, are inserted into the core at any given time. The non-inserted blades are disposed in withdrawn positions. The blades are moved in and out relative to the core on a scheduled basis to avoid power distribution difficulties within the fuel bundles.
When employing this operational practice, the asymmetry associated with B sequence blades, however, causes the power distribution within the reactor to become asymmetric and more highly peaked to various locations, i.e., the radial power shape of the core tends to be skewed toward one corner or another corner, i.e., non-symmetric. This asymmetric peaking must be accommodated and reduced by design of the reactor fuel loading, necessarily reducing fuel efficiency. Problems associated with the B sequence blade asymmetry, however, have been partly alleviated through the use of control cell core (CCC) loading patterns. With the CCC design, low reactivity fuel assemblies are placed in the control cells so that control rod motion occurs adjacent only relatively low-power fuel. That is, CCC loading schemes place relatively low power fuel bundles around A sequence blades, only some of which are used for power control. Fresh fuel bundles which have higher power must not be loaded next to these blades because the blades must be inserted for longer periods. With the CCC design, those certain A sequence control blades located in the low-power control cells are utilized for reactivity and power distribution control purposes throughout the entire cycle.
CCC loading, however, has limitations and drawbacks. As reactors are operated for long periods of time between refueling cycles, the number of fresh bundles inserted each cycle increases. When the fresh batch fraction is larger than 40%, the CCC loading becomes difficult and inefficient. Moreover, longer control periods associated with the use of CCC designs may contribute to a type of fuel failure associated with power peaking with the bundles. Accordingly, there is a need for a control blade sequencing pattern which avoids the asymmetry problem associated with the B sequence blades but does not require that the reactor fuel loading be limited as in the CCC design.
BRIEF SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, the blades of each of the A and B blade sequences, i.e., main blade groups A and B, are respectively divided into blade sub-groups, referred to as A1 and A2 and B1 and B2 blades. With a control blade A2 at the center of the core, every other control blade location in the core in the X and Y directions is an A2 blade. The first and second sub-groups of each main group are flanked by first and second sub-groups, respectively, of the other main group in the X direction. For example, the first and second sub-groups A1 and A2 of main group A are flanked by first and second sub-groups B1 and B2, respectively, of the main group B in the X direction, while the first and second sub-groups B1 and B2 of main group B are flanked by first and second sub-groups A1 and A2, respectively, in the X direction. The first and second sub-groups of each main group are flanked by second and first sub-groups, respectively, of the other main group in the Y direction. Thus, first and second sub-groups A1 and A2 of main group B are flanked by second and first sub-groups B2 and B1, respectively, in the Y direction, while the first and second sub-groups B1 and B2 of main group B are flanked by second and first sub-groups A2 and A1, respectively, in the Y direction. Thus, A1 and A2 control blades are symmetric relative to the center point of the core, whereas the B1 and B2 blades are asymmetric.
In accordance with a preferred embodiment of the present invention and contrary to prior practice, selected A and B control blades are inserted into the core at the same time but with the B sequence blades used only as shallow blades and the A sequence blades used as either deep or shallow blades. That is, the B sequence blades are movable only between positions withdrawn from the core and shallow positions within the core. The A sequence blades, however, are movable between withdrawn, deep or shallow positions. Further, the operating sequence is such to allow each fuel bundle a period of operation with its associated control blade withdrawn that is at least twice as long as the previous period of operation with the associated control blade inserted. The pattern repeats itself every three or more consecutive time periods and any one blade is fully withdrawn from the core for at least two consecutive time periods after it has been inserted enabling operation of the associated fuel assembly in an uncontrolled state for those two consecutive periods. That is, a different one of control blades A1 or A2 are inserted from a withdrawn position into a deep position at the beginning of each of at least three successive time periods, while a different one of the control blades A1, A2, B1 or B2 is inserted into a shallow position at the beginning of each such successive time period. Thus, selected non-inserted blades are maintained in withdrawn positions at the beginning of each such successive time period for at least two consecutive time periods.
In a preferred embodiment accordin
Reese Anthony Paul
Yeager Harold Hartney
Behrend Harvey E.
General Electric Company
Nixon & Vanderhye
LandOfFree
Control blade sequence patterns for optimization of BWR... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control blade sequence patterns for optimization of BWR..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control blade sequence patterns for optimization of BWR... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2518993