ML14339A749
Text
AFD B 3.2.3 McGuire Units 1 and 2 B 3.2.3-1 Revision No. 115 B 3.2 POWER DISTRIBUTION LIMITS B 3.2.3 AXIAL FLUX DIFFERENCE (AFD)
BASES BACKGROUND The purpose of this LCO is to establish limits on the values of the AFD in order to limit the amount of axial power distribution skewing to either the top or bottom of the core. By limiting the amount of power distribution skewing, core peaking factors are consistent with the assumptions used in the safety analyses. Limiting power distribution skewing over time also minimizes the xenon distribution skewing, which is a significant factor in axial power distribution control.
The analysis performed to develop the AFD limits involves the generation and evaluation of several thousand, three dimensional power distributions which consider burnup, reactor power, coolant temperature, control bank position, and xenon. The generation of conservative limits is assured through the generation of power distributions which are more severe than expected to occur during normal or transient operation. The selection of severe xenon distributions for the peaking analysis also adds another degree of conservatism to the analysis. Subsequently, power peaking factors and power distributions are examined to ensure that the loss of coolant accident (LOCA), DNB limiting transients in which the power distribution remains unchanged during the transient, and anticipated transient limits are met. Violation of the AFD limits invalidate the conclusions of the accident and transient analyses with regard to fuel cladding integrity.
Although the maneuvering analysis defines limits that must be met to satisfy safety analyses, typically a target operating band is used to control axial power distribution in day to day operation. This requires that the AFD be controlled within a narrow tolerance band around a burnup dependent target.
The constant target band operating space is typically smaller and lies within the maneuvering analysis operating space. Control within the constant target band operating space constrains the variation of axial xenon distributions and axial power distributions during normal operation and unit maneuvers. The maneuvering analysis calculations assume a wide range of xenon distributions and then confirm that the resulting power distributions satisfy the requirements of the accident analyses.
AFD B 3.2.3 BASES McGuire Units 1 and 2 B 3.2.3-2 Revision No. 115 APPLICABLE The AFD is a measure of the axial power distribution skewing to either SAFETY ANALYSES the top or bottom half of the core. The AFD is sensitive to many core related parameters such as control bank positions, core power level, axial burnup, axial xenon distribution, and, to a lesser extent, reactor coolant temperature and boron concentration.
The allowed range of the AFD is used in the nuclear design process to confirm that operation within these limits produces core peaking factors and axial power distributions that meet safety analysis requirements.
The maneuvering analysis (Ref. 1) uses a three dimensional nodal reactor model to calculate a set of power distributions at several points in the core life. These power distributions are calculated with abnormal xenon distributions to ensure predicted power distributions are conservative with respect to those expected to occur. Peaking factors from these power distributions are then evaluated against various thermal limits. This evaluation then confirms the adequacy of current power dependent AFD limits, rod insertion limits, and the F(I) penalty function, or provides the bases for establishing new limits. The development of operational AFD limits and the F(I) function of either the Overpower T or the Overtemperature T RPS trip functions are established such to exclude the power distributions that exceed the respective thermal limits.
The limits on the AFD ensure that the Heat Flux Hot Channel Factor (FQ(X,Y,Z)) is not exceeded during either normal operation or in the event of xenon redistribution following power changes. The limits on the AFD also restrict the range of power distributions that are used as initial conditions in the analyses of Condition 2, 3, or 4 events. This ensures that the fuel cladding integrity is maintained within respective limits for these postulated accidents. The most important Condition 3 and 4 event is the LOCA. The most important Condition 2 events include loss of flow, uncontrolled bank withdrawal, and boration or dilution accidents.
Condition 2 and 3 accidents simulated to begin from within the AFD limits are used to confirm the adequacy of the Overpower T and Overtemperature T trip setpoints.
The limits on the AFD satisfy Criterion 2 of 10 CFR 50.36 (Ref. 2).
LCO The shape of the power profile in the axial (i.e., the vertical) direction is largely under the control of the operator through the manual operation of the control banks or automatic motion of control banks. The automatic motion of the control banks is in response to temperature deviations resulting from manual operation of the Chemical and Volume Control System to change boron concentration or from power level changes.
AFD B 3.2.3 BASES McGuire Units 1 and 2 B 3.2.3-3 Revision No. 115 LCO (continued)
Signals are available to the operator from the Nuclear Instrumentation System (NIS) excore neutron detectors (Ref. 3). Separate signals are taken from the top and bottom detectors. The AFD is defined as the difference in normalized flux signals between the top and bottom excore detectors in each detector well. For convenience, this flux difference is converted to provide flux difference units expressed as a percentage and labeled as % flux or %I.
The AFD limits are provided in the COLR. The AFD limits do not depend on the target flux difference. However, the target flux difference may be used to minimize changes in the axial power distribution.
Violating this LCO on the AFD could produce unacceptable consequences if a Condition 2, 3, or 4 event occurs while the AFD is outside its specified limits.
APPLICABILITY The AFD requirements are applicable in MODE 1 greater than or equal to 50% RTP when the combination of THERMAL POWER and core peaking factors are of primary importance in safety analysis.
For AFD limits developed using maneuvering analysis methodology, the value of the AFD does not affect the limiting accident consequences with THERMAL POWER < 50% RTP and for lower operating power MODES.
ACTIONS A.1 As an alternative to restoring the AFD to within its specified limits, Required Action A.1 requires a THERMAL POWER reduction to
< 50% RTP. This places the core in a condition for which the value of the AFD is not important in the applicable safety analyses. A Completion Time of 30 minutes is reasonable, based on operating experience, to reach 50% RTP without challenging plant systems.
SURVEILLANCE SR 3.2.3.1 REQUIREMENTS The AFD is monitored on an automatic basis using the unit process computer, which has an AFD monitor alarm. The computer determines the 1 minute average of each of the OPERABLE excore detector outputs and provides an alarm message immediately if the AFD for two or more OPERABLE excore channels is outside its specified limits.
AFD B 3.2.3 BASES McGuire Units 1 and 2 B 3.2.3-4 Revision No. 115 SURVEILLANCE REQUIREMENTS (continued)
This Surveillance verifies that the AFD, as indicated by the NIS excore channel, is within its specified limits and is consistent with the status of the AFD monitor alarm. With the AFD monitor alarm inoperable, the AFD is monitored every hour to detect operation outside its limit. The Frequency of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> is based on operating experience regarding the amount of time required to vary the AFD, and the fact that the AFD is closely monitored. With the AFD monitor alarm OPERABLE. The Surveillance Frequency is based on operating experience, equipment reliability, and plant risk and is controlled under the Surveillance Frequency Control Program.
REFERENCES
- 1.
DPC-NE-2011PA, "Duke Power Company Nuclear Design Methodology for Core Operating Limits of Westinghouse Reactors".
- 2.
10 CFR 50.36, Technical Specifications, (c)(2)(ii).
- 3.
UFSAR, Chapter 7.