Patent Number: 047175281
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, particularly FIGS. 1 and 2, which graphically show the ideal behavior achieved by the present invention as well as the behavior of the prior art control rod system. In FIG. 1, the percent axial offset is illustrated as a function of control rod insertion in a reactor core. Axial offset is defined as the relative power in the top half of the core minus the relative power in the bottom half of the core divided by the total relative core power. Thus, FIG. 1 shows a comparison of axial offset response for the prior art control rod system and a response for one possible sequence of groups of the present invention. In the ideal situation, axial flux distribution within the core (or percent axial offset) would vary monotonically and linearly with power level for any given core conditions. Therefore, in FIG. 1, the ideal control behavior shown as curve A is a straight line. Curve B shows the response of the insertion sequence of the prior art control system with a fixed overlap between the banks of control rods having eight control rods per bank as described in the Description of the Prior Art, above. In comparing curves A and B, it can readily be observed that there exists a large deviation between the actual response and the ideal response. Curve C depicts the response of a control rod system having four rods per group and as arranged in a reactor core as shown in FIG. 3. Curve C is the response of the insertion of the first bank or group of control rods with the other banks or groups in an overlapped relationship. Thus, curve C shows the response of the inventive control rod system with the spacing or overlap between groups varying between 0 and 100% of travel. The improvement and the near linear response is obvious. Sufficient work has been done to suggest that even better responses are possible. In FIG. 2, the independent parameter of control rod insertion has been interchanged or replaced by core power level. The ideal behavior or response is again shown by curve A. Curve B shows the response of the prior art grouping of eight control rods per bank with a fixed overlap insertion sequence. The departure from the ideal response is again apparent. Curve C shows the response of the inventive control rod grouping and control system. Its close approximation to the ideal behavior and the improvement over the prior art are also apparent. FIGS. 1 and 2 thus show that in accordance with the inventive system, full length control rods can be used to effectively control the flux distribution and power level within a reactor core. FIG. 3 schematically shows the core locations of control groups each containing four control rods in a reactor core having one hundred and ninety-three fuel assemblies. As can be seen, each of the rods within a group of four are symmetrically located within the core cross section. The "C" followed by a number indicates a control rod and its insertion sequence. Thus, the four control rods designated "C1" are the first group of rods to be inserted into the reactor core. Each group of rods is independently controlled relative to any other group and spacing between groups can vary between 0 to 100% of travel. The prior art control rod bank makeup and the fixed overlap between banks causes undesirable flux peaking factors as a result of control rod insertion or removal, this, in turn, constrains the fuel loading patterns of fuel reload cycles. Also, the prior art arrangement allows a considerable variation in bank worth when an unusual fuel management scheme is necessitated by a utilities' energy requirements. The ability to redefine groups of control rods and their insertion sequence as provided by the present invention provides the following advantages: constant reactivity worth for control functions (integral rod capability, i.e., a proper choice of control rod groups and overlap are made available); flexibility in the control of flux peaking; flexibility in ejected rod severity under hot full power operation (i.e., a choice of controlling groups is made available); and, control of core radial and axial burnup shadowing is achieved (by changing the controlling groups of rods during the fuel cycle). The ability to control each group of control rods individually implies also the capability to trip each group individually. A study was done using only those groups of control rods typically assigned to shutdown banks of control rods in order to evaluate the range of partial trip capability as a result of flexibility provided by the present invention. FIG. 4 shows these trip group locations in the core cross section of FIG. 3. Table 1 shows the reactivity worth of each group of trip control rods at the beginning of life (BOL) and at the end of life (EOL) of an equilibrium fuel cycle. Table 1 indicates the possible interchange of shutdown rods for control rods as suggested in the above-stated advantages of the present invention. Table 2 shows the power levels achievable when selected groups of rods are tripped into the core starting at full power with all the rods out of the core. TABLE 1 ______________________________________ REACTIVITY WORTH OF TRIP GROUPS IN AN EQUILIBRIUM CYCLE REACTIVITY WORTH % AK GROUP BOL EOL ______________________________________ T1 .298 .336 T2 .297 .336 T3 .459 .502 T4 .459 .502 T5 .498 .545 T6 .498 .545 T7 .591 .602 T8 .381 .445 ______________________________________ TABLE 2 ______________________________________ POWER LEVELS REACHED FROM FULL POWER BY THE INSERTION OF TRIP GROUPS POWER LEVEL REACHED TRIP GROUPS INSERTED BOL EOL ______________________________________ T1 .868 .862 T8 .833 .818 T3 .803 .792 T7 .751 .751 T2 + T8 .699 .666 T3 + T8 .646 .605 T5 + T6 .596 .555 T6 + T7 .549 .509 T6 + T7 + T8 .428 .339 T2 + T3 + T7 + T8 .378 .262 T2 + T3 + T4 + T5 + T6 .284 .152 T1 + T2 + T4 + T5 + T6 + T7 .210 .050 ______________________________________ As can be seen by reference to Table 2, a continuum of partial trip power levels may be realized by the proper choice of groups. If the groups of rods typically assigned to the control banks were included in the evaluation of partial trip capability, the range could be extended even further and/or smaller increments between power levels may be achieved. The commands to move the individual groups of control and/or trip rods are generated by a rod control system comprising a rod control strategy computer. The output of the control computer is transmitted to the rod control logic system which converts the move commands into current pulses in the appropriate control rod drive mechanism coils. The rod control strategy computer is schematically shown in FIG. 5 and is made up of several microprocessor based computers. The rod motion strategy processor 10 receives input for reactivity changes from the nuclear steam supply system and generates the necessary motion commands to move the individual control rod groups. The rod motion strategy processor selects from the various combinations of groups of control rods, that combination which will minimize the power distribution peaking factor subject to the following constraints: the total control rod worth available for immediate insertion, comprising the shutdown margin, is kept above a given limit which limit varies as a function of the operating power level; the core axial power distribution is maintained within prescribed limitations to prevent problems associated with xenon redistributions; each combination of rod groups is assigned a predetermined insertion and withdrawal speed limitation which is dependent upon the distribution of the groups in the rod control system power cabinets. The unlimited flexibility provided by these constraints allows for optimal fuel economics which, of course, is very advantageous considering the present day fuel costs. A somewhat less flexible approach or a strategy which is more restrictive is the use of two or more preselected groups of control rods to adjust for required reactivity changes. The preselected groups may be changed at predetermined times compensate for the use of this restrictive strategy in minimizing the power distribution peaking factors. There are other strategies which may also be used which includes the use of plant operating personnel. For example, the rod motion strategy processor could analyze the data input and recommend the various combinations of rods to achieve optimum control of the reactor which recommendation would be effectuated manually by the plant operating personnel. The power distribution calculator 11 continuously determines local power density throughout the reactor core. The reactor core is divided into a large number of coupled reactor regions bounded by core axial and radial coordinates. For example, the horizontal across the flats dimensions of the fuel assemblies may comprise the radial coordinates of a region while the axial coordinates may comprise the planes fixed by the multisection excore NIS detectors. The local power density of each region is determined from point kinetic calculations based on the effective level of reactivity for that cell, the neutron leakage from and to adjacent regions, and the amount of neutron absorber existing within that region. The neutron absorber includes such items as the presence of a control rod, the presence of a burnable poison in the reactor coolant, the amount of xenon buildup in the fuel assembly, as well as any other like poisons or neutron absorbers in that region. The effective level of reactivity is calibrated on a predetermined time schedule, for example, weekly, biweekly, etc., to achieve agreement with the incore flux maps which are commonly being generated. Also, the average region power on each reactor plane is normalized to the measured power level for that plane as obtained by the excore NIS detectors. From these calculations, the total core power distribution, and power peaking factors are obtained as well as the predicted peaking factor changes for motion of each control rod group. The calculated power distribution from the power distribution calculator and the then current control rod positions, as measured by the digital rod position indication system, (DRPI) are used by the rod group worth calculator 12 to determine the instantaneous differential and integral reactivity worth of each control rod group. The worths as calculated by the group worth calculator 12 are used by the rod motion strategy processor 10 as input in its determination of the optimal motion commands for the individual control rod groups. The rod group integral reactivity worths as determined by the rod group worth calculator 12 are used by a partial trip processor 13 to respond to requests for immediate power reductions. The partial trip processor 13 selects the appropriate rod groups (either the trip rods or a combination of trip rods and control rods) and transmits commands to drop those rods into the core by deenergizing their respective control rod drive mechanisms. This partial trip capability improves the overall plant availability by keeping the plant on line following certain large component malfunctions, such as loss of a feedwater pump or a reactor coolant pump. The partial trip capability also has the potential for significantly reducing the capacity requirements of a steam dump system, i.e., condenser size. A burnup distribution control processor 14 maintains a history of the power distribution of the rodded fuel assemblies relative to the adjacent fuel assemblies and generates commands to remove the rods from any of the fuel assemblies which have been operating at relatively lower power levels. The rod motion strategy processor 10 replaces the removed rods with other different rods to maintain total core balance. Thus, over the long term, core burnup will be evenly distributed. Additionally, processor 14 reduces the potential for pellet clad interaction (PCI) failures due to rapid power increases by keeping the entire core at a uniformly high "precondition" power. The five modules making up the rod control strategy computer are interconnected by a shared memory bus arrangement 16 which may be controlled by a communications processor 15. In addition to moving data from one module to another, the communications processor 15 receives operator control inputs and transmits information from the five modules to the operator display system. The partial trip processor 13 and the rod motion strategy processor 10 output command signals to move the appropriate control rods in an appropriate amount. The output signals are received by a rod control logic system 17. The logic system 17 causes the intended control rod drive mechanism lift, movable gripper and stationary gripper coils to activate, in the proper manner, and to actually move the intended group of four control rods. FIG. 6 schematically shows the bus arrangement whereby power to the rod control apparatus arranged on a data or logic bus, is itself controlled and bus arranged for distribution to the rod control apparatus. The logic bus controller 20 comprises part of the control rod logic system 17 and receives input from the rod motion strategy processor 10 and the partial trip processor 13. Moving cabinet 21 contains the power circuitry for the lift coils and the movable gripper coils for all of the control rods in the core. The holding cabinets 22, 22.sub.2. . . 22.sub.n contain the power circuitry for the holding apparatus (stationary gripper coils) for the respective group of four control rods controlled by that particular power cabinet. Logic bus 23 in conjunction with bus controller 20 supplies signals to the individual holding cabinets 22 directing which particular cabinet is to be connected to power bus 24 to move the particular group of control rods selected by, for example, the rod motion strategy processor 10. Simultaneously, the power to the remaining control rod groups is withheld from their respective holding cabinets. It is to be understood that the bus controller 20 can command that the moving cabinet 21 power circuitry be shared among more than one holding cabinet 22 at any particular time so that, if it is desired, more than one group of rods may be simultaneously moved. FIG. 7 schematically illustrates the embodiment of FIG. 6 but in another manner. Moving cabinet 21 contains the power circuitry 25 and 26 for the lift coils and the movable gripper coils of the control rods. The lift 25 and the movable gripper 26 circuitry is distributed to the individual holding cabinets 22 by the illustrated bus arrangement. The power circuitry for the stationary gripper coils 27 of the control rods groups is contained within each of the respective holding cabinets 22. The inverted triangles designated by the numeral 28 in each of the holding cabinets schematically designates the selection logic provided by logic bus controller 20 and logic bus 23 of FIG. 6. In the manner illustrated, the moving circuitry for all the control rods in the core can be contained in a single housing or cabinet 21. In actual practice, the moving circuitry would be redundantly supplied in two cabinets. In this case, one cabinet could serve as a space and automatically replace the primary moving cabinet in event of a malfunction in that cabinet. Alternatively, both cabinets could be normally on line with the rod groups distributed between the two; in this case, malfunction of either cabinet would shift its rod groups to the remaining good moving cabinet. The advantage of the latter arrangement would be to allow .about.30% faster reactivity control from the rods, due to detailed consideration of control rod drive mechanism operation. Similarly, the bussed power arrangement allows for failure of one or more holding cabinets on line without dropping of the associated control rods. This is accomplished through an "insurance" bus bar in the power bus duct. All holding cabinets normally contribute to the insurance bus. When a cabinet malfunctions, it automatically switches its rod group onto the insurance bus, to replace the cabinets normal holding function. While the invention has been described, disclosed, illustrated and shown in certain terms or certain embodiments or modifications which it has assumed in practice, the scope of the invention is not intended to be nor should it be deemed to be limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.