GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1030 Application of MSHIM Core Control Strategy For Westinghouse AP1000 Nuclear Power Plant

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1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1030 Application of MSHIM Core Control Strategy For Westinghouse AP1000 Nuclear Power Plant Masaaki Onoue 1, Tomohiro Kawanishi 1, William R. Carlson 2 and Toshio Morita 2* 1 Mitsubishi Heavy Industries, LTD., Minatomirai 3-3-1, Nishi-ku, Yokohama, , Japan 2 Westinghouse Electric Company, LLC, Post Office Box 355, Pittsburgh, PA , U.S.A Westinghouse has developed a new core control strategy, in which two independently moving Rod Cluster Control Assembly (RCCA) groups are utilized for core control; one group for reactivity/temperature control, the other for axial power distribution (Axial Offset) control. This control procedure eliminates the need for Chemical Shim adjustments during power maneuvers, such as load follow, and is designated MSHIM (Mechanical Shim). This core control strategy is utilized in the AP1000. In the AP1000, it is possible to perform MSHIM load follow maneuvers for up to 95% of cycle life without changing the soluble boron concentration in the moderator. This core control strategy has been evaluated, via computer simulations, to provide appropriate margins to core and fuel design limits during normal operation maneuvers (including load follow operations) and also during anticipated Condition II accident transients. The primary benefits of MSHIM as a control strategy are as follows; - Power change operation can be totally automated due to the elimination of boron concentration adjustments. Full load follow capability is achievable for up to more than 95% of cycle life. - Load follow operations performed solely by mechanical devices results in a significant reduction in the boron system requirements and a significant reduction in daily effluent to be processed. KEYWORDS: AP1000, Mechanical SHIM (MSHIM), Load Follow Operation I. Introduction The prime objective of core control is to simultaneously manage core reactivity and power distributions. To accomplish this objective, Westinghouse PWRs utilize RCCA movement supplemented by adjustments of the soluble boron concentration in the moderator (Chemical Shim). Westinghouse is developing a new generation nuclear power plant, referred to as the AP1000 (1). Anticipating future demand for operational enhancements to nuclear power demand requirements, the AP1000 is designed to provide extensive load follow operational capability without the need to adjust the soluble boron concentrations during such maneuvers. An advanced operational control strategy, designated MSHIM (Mechanical Shim) (2),(3) has been employed for this purpose in the AP1000. The MSHIM control system utilizes two independently operable groups of control banks for essentially simultaneous control of reactivity/temperature and axial power distribution. Reactivity/temperature control is provided primarily by a series of control rod banks which will be referred to as M-Banks. The M-Banks consist of several control banks moving with a fixed overlap. The bank worth and overlap are defined so as to minimize the impact on axial offset during control bank maneuvering and still retain the reactivity/temperature control required to meet the desired load changes. Axial power distribution control is provided by what will be referred to as the. In order to have a reasonable impact on axial offset with a small degree of bank motion, the must be relatively high worth bank. Studies have indicated that the above mentioned control strategy, which is also backfittable to operating PWR plants, permits daily load follow operation without the need to adjust boron concentration during repetitive daily load cycle operation. In order to achieve complete boron-adjustment free load follow operation for up to more than 95% of cycle life (including the periods of transition operation), reduced reactivity worth RCCAs, termed "gray rods", are utilized. By taking advantage of these new features, the MSHIM operational control strategy has been tested extensively for the AP1000 core. The results are very promising. This paper summarizes the results of this study. * Corresponding author, Tel , Fax , moritat@westinghouse.com

2 II. Features of AP1000 The AP1000 reactor core is comprised of 157 fuel assemblies (17x17 lattice design) with a 14-foot active fuel length. The fuel assembly configuration in the core is the same as in a conventional Westinghouse 3-loop plant, but the RCCA configuration is different due to an increased complement of RCCAs (from 48 to 69). Figure 1 shows the typical RCCA configuration in the AP1000. The 69 RCCAs are placed in a checkerboard pattern. 16 Gray RCCAs are employed in the MA through MD-Banks. These reduced worth gray RCCAs utilize 20 stainless steel rodlets and 4 conventional Ag-In-Cd rodlets mounted on a conventional RCCA spider assembly. 12 Black RCCAs are employed in the M1 and M2-Banks. The black RCCAs are the typical RCCA configuration, (i.e., a conventional 24 Ag-In-Cd rodlet spider assembly). The above six banks are designated as M-Banks. The primary objective of M-Bank RCCA configuration is to control reactivity/temperature. Before any anticipated load follow operation, MA and MB-Bank will have been fully inserted into the core. The initial M-Bank insertion permits compensation for both negative and positive reactivity insertions occurring during the power changing maneuvers. M-Banks move with an overlap relationship as shown in Figure 2 and are driven by a single variable for criticality as if they are conceptually in one control group. The first two moving Banks, MA and MB, operate together with 100% overlap relationship. The similarity between the MA+MB- Bank and MC+MD-Bank provides the capability for the interchanging of these banks during operation. This option would mitigate burnup-shadowing effects due to long term insertion of the MA and MB-Bank. Interchanging also reduces the magnitude of the rod stepping duty on the same control rod drive mechanisms. The is composed of 9 "black" RCCAs. As mentioned earlier, the prime function of the is to control axial power distribution. In order to have a reasonable impact on axial offset with a small degree of bank motion, the must be a relatively high worth bank. The worth of and M-Banks must be balanced such that a monotonic relationship between AO- Bank position and axial offset is achieved. It is crucial to control the axial offset to a pre-determined target value by using the (independent from M- Bank movement). The AP1000, therefore, utilizes a Constant Axial Offset Control (CAOC) strategy (4) about the target axial offset values. S4 MC S4 M2 S2 S2 M2 MB AO M1 AO MB M2 S1 S3 S3 S1 M2 S4 AO MA MD MA AO S4 S2 S3 S1 S1 S3 S2 3 M2 MC M1 MD AO MD M1 MC S2 S3 S1 S1 S3 S2 S4 AO MA MD MA AO S4 3 M1 M2 S1 S3 S3 S1 M2 MB AO M1 AO MB M2 S2 S2 M2 3 MD S4 MC S4 0.1 MC M-Bank MA : 4 MB : 4 MC : 4 MD : 4 M1 : 4 M2 : 8 (Gray) (Black) Total 69 Rods AO : 9 Shutdown-Bank S1 : 8 S2 : 8 (Black) S3 : 8 S4 : 8 Figure 1 AP1000 RCCA Configuration (Black) MA+MB Reactor Core Figure 2 AP1000 M-Banks Sequence and Overlap Ratios

3 The shutdown Banks are composed of 32 "black" RCCAs. The AP1000 core design increases the number of RCCAs relative to a conventional 3 loop plant. The increased complement of RCCAs contributes to reductions in boron system and waste processing requirements while enhancing shutdown margins over the range from cold shutdown to hot shutdown. III. MSHIM Load Follow Simulations MSHIM load follow operation has been demonstrated for the first cycle and equilibrium cycle of the AP1000. The reference cycle length is 18 months with a 95% capacity factor. For this demonstration, an automated simulation program has been developed which employs onedimensional diffusion theory. This program evaluates core characteristics during load follow operation, such as core average axial power distribution, M-Bank and position, and critical boron concentration. The prime objective of this analysis is to confirm that a variety of load follow power maneuvers can be performed without the need for soluble boron concentration changes. It is also confirmed that acceptable axial power peaking factors, F Z, can be achieved throughout these maneuvers. Various power demand schedules, as shown in Table 1, have been considered for this study. In this paper, the results of Profile 2 for each cycle at BOC and EOC, are presented. This power profile is a representative power demand schedule which is interpreted as 12 hours operation at rated power, followed by a 3 hour ramp to 50% rated power. Power is then maintained at 50% rated power for 6 hours, followed by a 3 hour ramp back to rated power. Table 1 Power Demand Schedules Profile Duration (hour) Power (%) extended low 9 power (2days) CAOC operation is employed for the AP1000 about predetermined target axial offset values. Setting these target axial offset values during load follow operation ensures power peaking factors will remain within the design limit. In the next section, the procedure used to set the target axial offset and results from MSHIM load follow simulations are described % -10% -14% -18% P eaking F Z Peaking F Z % -10% -14% -18% Figure 3 MSHIM Load Follow Simulation (First Cycle at 150MWd/t (BOC) Figure 4 MSHIM Load Follow Simulation (First Cycle at 19800MWd/t (EOC)

4 III-1. Operation Procedure To initiate a change between base load and load follow operation, the operation procedure is as follows: prior to load follow operation, the basic control rod position is position is defined as : a) MA+MB-Bank fully-inserted, b) MC-Bank slightly-inserted (by the slight overlap with MA+MB-Bank, c) slightly-inserted. The target axial offset for this condition is defined as TAO BASE. During base load operation the axial offset is maintained at this value. During load follow operation, the axial offset is maintained at a more negative target value prior to and during power maneuvering. The target axial offset value is approximately 8% more negative than TAO BASE in order to provide axial offset adjustment capability in both negative and positive directions. This target value is defined as TAO LF. and is achieved by control rod movement alone prior to initiation of load follow maneuvers. III-2. Simulation Results Figure 3 through Figure 6 show the results of MSHIM load follow simulation for the first cycle and equilibrium cycle at BOC and EOC, respectively. Each figure identifies the following key parameters: (i) Reactor Power (ii) Axial Offset and Delta-I (dashed lines) (iii) and Lead M-Bank (dashed line) (iv) MC, MD (dashed line) and M1 (second dashed line) Bank (v) Axial Peaking Factor, F z (vi) for Criticality During an initial transition day, the reactor power stays at the base load power level, but the axial offset is changed from TAO BASE to TAO LF. The Figures show a successful transition from base load to load follow operation by using the transition day operation prior to the initiation of load follow. In order to control the reactivity/temperature and the axial offset by using only RCCAs, the M-Banks and are inserted independently and simultaneously into the core to achieve both purposes. Generally, long cycle operation, like an 18-month cycle, results in a double-humped axial power distribution at EOC. Therefore, partial RCCA insertion occurring during load follow maneuvers generally results in more pronounced changes to the axial offset and axial peaking factor, F Z. Since MSHIM maintains tight control of the axial offset, this helps to alleviate peaking factor concerns associated with rod insertion. Another feature is the ability to maintain a constant boron concentration during load follow operation. Figures show that reactivity/temperature and power distributions are controlled well by using only RCCA maneuvering. In a conventional PWR plant, it is nearly impossible to load follow at the low boron concentrations encountered near EOC due to limited boron dilution capability. With MSHIM operation, it is possible to load follow for up to 95% of cycle life without the need to change boron concentration, as shown in the Figures. Full MSHIM load follow capability could be theoretically maintained as long Peaking FZ P eaking F Z 0% -4% -8% -12% -16% % -4% -8% -12% -16% Figure 5 MSHIM Load Follow Simulation (Equilibrium Cycle at 150MWd/t (BOC) Figure 6 MSHIM Load Follow Simulation (Equilibrium Cycle at 20000MWd/t (EOC)

5 as the MA+MB-Bank is fully inserted during base load operation. Simulation results show that acceptable MSHIM load follow operation in the AP1000 is achievable, without the need to change boron concentration, for up to 95% of cycle life. IV. Power Capability Evaluation The one-dimensional MSHIM load follow simulations described above showed that the AP1000 can perform various power changing operations without the need to change boron concentration. During the simulation studies, the power peaking factors are monitored only by the core average axial peaking factors. For more precise power peaking factor determination and validation of the reactor trip functions, comprehensive three dimensional power distributions and the associated safety related parameters need to be analyzed. The reactor conditions should cover all anticipated situations which could occur during MSHIM load follow operations (i.e., Condition I Normal Operation and Condition II accidents as well). A wide range of xenon distributions are generated from the MSHIM load follow simulations described in Section III. The xenon distributions incorporated into the three dimensional analysis reproduce the axial xenon distribution profiles observed during a variety of MSHIM load follow simulations outlined in Table 1. The generation of these xenon distributions is performed at different times in cycle life, and for each cycle of operation. By incorporating these xenon distributions into the three-dimensional models, a series of three-dimensional power distribution calculations can then be performed for the different combinations of reactor conditions as defined in Table 2. For each calculated power distribution, safety related parameters, such as the core peaking factors and DNBRs at various thermal/hydraulic conditions, are also evaluated. Table 2 Input to 3D Power Distribution Analysis Reactor Power M-Bank Xenon Shape Cycle Burnup 50% to 125% RTP ARO to 40% (With Overlap as shown in Figure 2) Zero to 50% From MSHIM Load Follow Simulations (Table 1) BOC, MOC, EOC The key concept of the three dimensional analysis methodology is to search for the maximum allowable reactor power (MAP) under the constraint of various safety criteria described below. These searches are performed for varying core operating configurations, such as cycle burnup, xenon distribution and RCCA positions. From the MAP Anticipated I band Figure 7 Maximum Allowable Power for Normal Operation (Condition I) First Cycle (BOC) Anticipated I band Figure 8 Maximum Allowable Power for Normal Operation (Condition I) First Cycle (EOC) Anticipated I band Figure 9 Maximum Allowable Power for Normal Operation (Condition I) Equilibrium Cycle (BOC) Anticipated I band Figure 10 Maximum Allowable Power for Normal Operation (Condition I) Equilibrium Cycle (EOC)

6 results, the power capability achievable during Condition I normal operation and the fuel integrity margin during Condition II accidents can be evaluated for AP1000 MSHIM operation. Three dimensional power distributions, which are determined using a standard Westinghouse design code system, are then used in performing integrated thermal/hydraulic and fuel rod design analyses. The standard calculational uncertanties associated with the use of this code system are included in the results. IV-1. Condition I Operation During Condition I normal operation, including various modes of the MSHIM operation, the following criteria need to be met at all times: Design Limit(118% State Point) Design Limit (90% State Point) Figure 11 DNBR Limited Power Relative to Design Limit First Cycle (BOC) i. the maximum linear power density must be within the (Loss of Coolant Accident) requirement, ii. DNBRs during (Loss of Coolant Flow Accident) must be greater than the safety limit DNBR, and iii.rccas are at or above the insertion limit. The maximum allowable powers for Condition I operation are shown in Figure 7 through Figure 10, at BOC and EOC for the Cycle 1 and Equilibrium Cycle cores respectively. The plottings are made as a function of the core average axial flux difference ( I = * Axial Offset), and each maximum power is determined by either or constraints. For the AP1000 plant, is more limiting for the upper positive range of I and dominates the lower negative range of I as shown in Figures 7 through Figure 10. As long as the reactor is operated in an operating space which is beneath the MAP data points, the required safety criteria are met. It is seen that full power operation is achievable over a rather wide range of intermediate I values. For excessive Ι, power reduction is required. In Section III, the axial offset or I swings occurring during the MSHIM load follow simulations have been evaluated. The maximum I ranges required for MSHIM operation are shown by a horizontal line in Figures 7 through 10. It is confirmed that MSHIM operation can be achieved without violating the primary Condition I safety criteria. Design Limit (118% State Point) Design Limit (90% State Point) Figure 12 DNBR Limited Power Relative to Design Limit First Cycle (EOC) Design Limit (118% State Point) Design Limit (90% State Point) Figure 13 DNBR Limited Power Relative to Design Limit Equilibrium Cycle (BOC) IV-2. Condition II Accident - - Adequacy of DNB Trip Function for Condition II Accident With regard to Condition II accident scenarios, the AP1000 plant has a reactor trip protection system to preclude fuel damage. The trip setpoints are defined for the AP1000 by using a conventional trip set point methodology in conjunction with a set of standard limiting axial power distributions. The required safety criteria are : Design Limit (118% State Point) 90% (Design Limit) Figure 14 DNBR Limited Power Relative to Design Limit Equilibrium Cycle (EOC)

7 i. DNBRs are greater than the safety limit DNBR values. Traditionally, two state point coolant inlet temperatures are used, one corresponding to 118% relative power, the other corresponding to 90% power, and ii. the maximum fuel centerline temperature is less than the fuel melting temperature. The plant trip setpoints are intended to be defined for each plant and should be applicable for all cycles of operation. Applicability of the generic setpoint methodology to each cycle of operation is confirmed on a cycle specific basis. The confirmation is performed by comparing the DNBR maximum allowable powers (MAPs) at the specified inlet temperatures corresponding to the 118% and 90% power statepoints relative to the cycle independent generic values. Traditionally, the powers are plotted as a function of the axial offset. Results of the cycle specific evaluation are shown in Figure 11 through Figure 14 at BOC and EOC for the first cycle and equilibrium cycle cores respectively. The solid and dashed lines were obtained using the standard sets of limiting axial power shapes, from which the reactor trip setpoints are determined. The cycle specific MAPs are plotted in these figures. It is seen that all the cycle specific MAPs are above the generic lines. This confirms adequacy of the generic DNBR analysis for the proposed MSHIM operations. V. Conclusion It has been demonstrated that the proposed MSHIM control system and control strategy is capable of: The elimination of soluble boron changes during power maneuvers results in a significant reduction of daily effluent, thus providing a cost saving to the utility by greatly reducing the amount of effluent to be processed. In addition, the potential cost savings become especially significant towards the end of cycle life, when reactivity control through soluble boron changes results in the generation of substantial quantities of total daily effluent. The elimination of soluble boron changes during power maneuvers also has the potential to make power change operation totally automatic. The MSHIM control system and control strategy identified in this paper has been shown to be highly beneficial to the AP1000 plant design. References 1. Winters, J. W.,"AP1000 Design Control Document", APP-GW-GL-700, April Morita, T., et al., "Load Follow Operation with the MSHIM Control System, ANS Topical Meeting Transaction No.2 Vol 56, April Morita, T., et al., "MSHIM Load follow Operation in the Westinghouse AP600 Plant Design", ANS San Francisco Winter Meeting, November Morita, T., et al., "Topical Report - Power Distribution Control and Load Following Procedures", WCAP-8403, September, 1974 a) performing MSHIM load follow operation for up to 95% of cycle life without changing soluble boron concentration in the moderator, b) meeting all applicable design limits over the entire range of I anticipated during MSHIM operation, and c) protecting the core from DNB during Condition II accidents via the Over Temperature trip function.