ML15313A154

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Topical Report, VEP-NE-1, Rev. 0.1-A, VEPCO Relaxed Power Distribution Control Methodology and Associated Fq Surveillance Technical Specifications, August 2003
ML15313A154
Person / Time
Site: Surry, North Anna  Dominion icon.png
Issue date: 08/31/2003
From: Flaig K, Nicholson A, Smith N, Twitchell R
Dominion Resources Services
To:
Division of Operating Reactor Licensing
Guzman R
References
VEP-NE-1-Rev. 0.1-A
Download: ML15313A154 (45)
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VEP-NE-1-Rev.0.1-A Page 2 CLASSIFICATION/DISCLAIMER The data, information, analytical techniques, and conclusions in this report have been prepared solely for use by Dominion (the Company), and they may not be appropriate for use in situations other than those for which they are specifically prepared. The Company therefore makes no claim or warranty whatsoever, expressed or implied, as to their accuracy, usefulness, or applicability. In particular, THE COMPANY MAKES NO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, NOR SHALL ANY WARRANTY BE DEEMED TO ARISE FROM COURSE OF DEALING OR USAGE OR TRADE, with respect to this report or any of the data, information, analytical techniques, or conclusions in it. By making this report available, the Company does not authorize its use by others, and any such use is expressly forbidden except with the prior written approval of the Company. Any such written approval shall itself be deemed to incorporate the disclaimers of liability and disclaimers of warranties provided herein. In no event shall the Company be liable, under any legal theory whatsoever (whether contract, tort, warranty, or strict or absolute liability), for any property damage, mental or physical injury or death, loss of use of property, or other damage resulting from or arising out of the use, authorized or unauthorized, of this report or the data, information, and analytical techniques, or conclusions in it.

VEP-NE-1-Rev.0.1-A Page 3 PREFACE This topical report (Mod 1) presents a modified version of the Relaxed Power Distribution Control Methodology provided in VEP-NE-1-A, Revision 0 published in March 1986. This report updates the references to the current 3-D PDQ Two Zone and enhanced NOMAD models as well as outlines the use of the NRC approved Studsvik Core Management System in the RPDC methodology. This report also refers to the COLR section of the plant Technical Specifications for the applicable thermal-hydraulic codes(s) and correlation(s) for DNB analyses.

VEP-NE-1-Rev.0.1-A Page 4 TABLE OF CONTENTS Page Classification/Disclaimer.............................................................................................................. 2 Preface........................................................................................................................................... 3 Table of Contents.......................................................................................................................... 4 List of Figures ............................................................................................................................... 5 List of Tables ................................................................................................................................ 5 Section 1 - Introduction ................................................................................................................ 6 Section 2 - Condition I Analysis................................................................................................. 12 2.0 Analysis of Axial Shapes Which Result from Normal Operation ............................. 12 2.1 Axial Shape Generation ............................................................................................. 12 2.2 LOCA Delta-I Limit Formation ................................................................................. 20 2.3 Loss of Flow Thermal/Hydraulic Evaluation............................................................. 23 2.4 Final Normal Operation Delta-I Limit ....................................................................... 23 Section 3 - Condition II Analysis................................................................................................ 24 3.0 Analysis of Axial Shapes Which Result from Condition II Events ........................... 24 3.1 Determination of Accident Pre-Conditions................................................................ 25 3.2 Condition II Accident Simulation .............................................................................. 25 3.3 Overpower Limit Evaluation...................................................................................... 27 3.4 DNB Evaluation ......................................................................................................... 29 Section 4 - Other Safety Analyses .............................................................................................. 30 Section 5 - FQ Surveillance ........................................................................................................ 31 Section 6 - Conclusion ................................................................................................................ 34 Section 7 - References ................................................................................................................ 35

VEP-NE-1-Rev.0.1-A Page 5 LIST OF FIGURES Page Section 1 1.0-1 Typical CAOC Limits ................................................................................................. 10 1.0-2 Typical Variable Axial Flux Difference Limits .......................................................... 11 Section 2 2.1-1 Typical RPDC BOC Xenon Oscillation ...................................................................... 14 2.1-2 Typical RPDC EOC Xenon Oscillation....................................................................... 15 2.1-3 Typical Rod Insertion Limits....................................................................................... 17 2.2-1 Typical LOCA Delta-I Limits...................................................................................... 22 Section 3 3.3-1 Typical Maximum Power Density Flyspeck ............................................................... 28 Section 5 5.0-1 Typical N(z) Function................................................................................................. 33 LIST OF TABLES

................................................................................................................................................. Page Section 2 2.1-1 Typical Conditions Analyzed for Normal Operation Under RPDC ........................... 19

VEP-NE-1-Rev.0.1-A Page 6 SECTION 1 - INTRODUCTION In response to Loss-of-Coolant Accident (LOCA) Emergency Core Cooling System (ECCS) criteria that imposed new requirements on local power peaking, Westinghouse developed the Constant Axial Offset Control (CAOC) power distribution control procedure [1]. The CAOC strategy restricts axial power skewing in the reactor core during normal operation to within a band of +/-5% delta-I around a target value, determined at all-rods-out equilibrium conditions.

Delta-I is defined as delta-I (%) = 100 * (Pt - Pb) where Pt and Pb are the fractions of rated full-core power in the top and bottom halves of the core, respectively. This +/-5% limit on axial power skewing reduces the magnitude of axial xenon oscillations which, in turn, decreases the magnitude of any power peaking during abnormal operation. A typical CAOC delta-I band is shown in Figure 1.0-1. The CAOC target value varies with burnup as the all-rods-out equilibrium delta-I changes.

Much of the low power operational flexibility of CAOC was originally centered around the use of the part length rods as a means for axial power distribution control [1]. Full length rods and boron were to be used mainly for reactivity control associated with changes in power. Since the requirement for removal of part length rods was imposed, full length rods have had to be used to help control the axial power distributions. As a result, it became more difficult to maintain the axial power distribution within the +/-5% delta-I band at low powers. This is especially true near end-of-cycle when the soluble boron concentration has been reduced to a very low level to compensate for the effects of fuel depletion and fission product buildup. Should a trip occur during this portion of the cycle, a plant may not be able to return to full power easily because of difficulty in meeting the delta-I limits. There is insufficient reactivity available from boron dilution to allow the full length rod movement required to offset the buildup of xenon and, at the same time, maintain delta-I within its band. As a result, delta-I limits could be exceeded at low

VEP-NE-1-Rev.0.1-A Page 7 power levels, requiring the plant to remain below 50% power in order to meet the "one hour in twenty-four"* requirement in the plant Technical Specifications.

Some Westinghouse CAOC plants with available full power margin to their LOCA Overall Peaking Factor (FQ) license limits have transformed this margin into operating flexibility through delta-I "band widening." In the past [2], Surry had a delta-I band width of +6, -9%

about the target value. This method of gaining operational flexibility does provide some additional full power delta-I operating space, but offers only minimal relief for post-trip return to power at end-of-cycle conditions.

This operational restriction on delta-I imposed by CAOC can be eased by the implementation of a variable delta-I band control strategy that takes credit for the full power delta-I margin available from standard band widening while also providing for an increasing delta-I band with decreasing power. The widened delta-I band is formed by maintaining an approximately constant analysis margin to the design bases limits at all power levels. This is in contrast to CAOC operation which has large amounts of margin available at reduced power. For North Anna and Surry, which have LOCA-limited total peaking factors, this variable delta-I band would be selected such that the margin to the LOCA FQ*P*K(z) limit would remain approximately constant for all power levels. An example of a variable delta-I band is given in Figure 1.0-2.

  • The CAOC Technical Specifications impose no operational limit on delta-I while a plant operates below 50% power. However, in order to ascend above 50% power, the plant must not have exceeded the delta-I bands for more than one penalty hour of the previous twenty-four.

VEP-NE-1-Rev.0.1-A Page 8 The principal benefits of a variable band delta-I control strategy over CAOC operation are as follows:

1) The ability to return to power after a trip, particularly at end-of-cycle, is enhanced;
2) Control rod motion necessary to compensate for the CAOC +/-5% delta-I band restrictions is reduced to only that motion needed to maintain operation within a much wider band;
3) The reactor coolant system boration/dilution requirements are decreased, due, in part, to the reduced control rod motion;
4) The plant has enhanced operational flexibility.

The concept of widened delta-I limits at reduced power levels is not a new one. Combustion Engineering [3] and Babcock and Wilcox [4] have supported increased axial skewing at reduced power levels for their reload cores for several years. Westinghouse [5] has also developed and licensed a variable delta-I control strategy called RAOC (Relaxed Axial Offset Control) for application to reload cores.

Dominion has combined some of the concepts from the Combustion Engineering methodology

[3] with the current Dominion analysis techniques [1,6] to form an alternate methodology for variable band delta-I control. This methodology is called Relaxed Power Distribution Control (RPDC). The Sections that follow will discuss the Dominion procedure for generating the variable width delta-I band. They will also discuss the methods used to ensure that the margin to the design bases criteria, such as Departure from Nucleate Boiling (DNB), fuel centerline melt and Loss of Coolant Accident (LOCA) peak clad temperature is maintained.

This report also discusses the formulation of FQ Surveillance Technical Specifications. The CAOC radial peaking factor Fxy(z) surveillance is replaced by FQ(z) monitoring, using the

VEP-NE-1-Rev.0.1-A Page 9 measured value of FQ(z) augmented by a non-equilibrium operation multiplier, in order to verify compliance with the LOCA peaking factors. As will be seen in Section 5, FQ surveillance complements RPDC to form a consistent but more flexible plant monitoring scheme than that provided by the CAOC methods.

VEP-NE-1-Rev.0.1-A Page 10 FIGURE 1.0 TYPICAL CAOC LIMITS

VEP-NE-1-Rev.0.1-A Page 11 FIGURE 1.0 TYPICAL VARIABLE AXIAL FLUX DIFFERENCE LIMITS

VEP-NE-1-Rev.0.1-A Page 12 SECTION 2 - CONDITION I ANALYSIS 2.0 Analysis of Axial Shapes Which Result from Normal Operation The objective of a RPDC analysis is to determine acceptable delta-I band limits that will guarantee that margin to all the applicable design bases criteria has been maintained and, at the same time, will provide enhanced delta-I operating margin over CAOC. Because the RPDC delta-I band is an analysis output quantity rather than a fixed input limit, as in CAOC, axial shapes which adequately bound the potential delta-I range must be generated. These axial shapes must include the effect of all potential combinations of the key parameters such as burnup, control rod position, xenon distribution, and power level. Dominion has developed the methodology of Section 2.1 to generate the large number of axial shapes included in RPDC.

After the axial power shapes have been created, two separate allowable delta-I limits for normal operation are established: one based on LOCA FQ considerations and the other one based on a Loss of Flow (the limiting DNB transient) thermal/hydraulic evaluation. The methods used are described in Sections 2.2 and 2.3, respectively. These two separate delta-I bands are combined to form a composite delta-I limit as discussed in Section 2.4.

2.1 Axial Shape Generation The axial power distributions encountered during normal operation (including load-follow) are primarily a function of four parameters: the xenon distribution, power level, control rod bank position and burnup distribution. For RPDC, reasonable incremental variations that span the entire expected range of values must be considered for each of these parameters. The following method is used to create the axial power distributions needed for the development of the RPDC normal operation delta-I limits.

VEP-NE-1-Rev.0.1-A Page 13 2.1.1 Axial Xenon Distributions During Normal Operation The axial xenon distribution is a function of the core's operating history and, as a result, is constantly changing. In order to analyze a sufficient number of xenon distributions to ensure that all possible cases have been accounted for, a xenon "free oscillation" method similar to the one described in Reference 3 is used to form these distributions. By creating a divergent xenon-power oscillation, axial xenon distributions can be obtained that will be more severe than any experienced during normal operation, including load follow maneuvers.

To initiate a xenon-power oscillation, an equilibrium 1-D model [7,17] or 3-D model [16] of the reload cycle is perturbed. This perturbation will generally be in the form of a change in power, rod position, or both. However, since the core model may be inherently stable due to the presence of feedback mechanisms, these mechanisms must either be modified or bypassed to obtain a divergent oscillation. One way to accomplish this is to reduce the stability of the model by reducing the amount of Doppler (i.e., fuel temperature) feedback in the system. The divergent oscillation provides a spectrum of xenon distributions that will produce power distributions with delta-I values covering the expected delta-I range. The magnitude of the "free oscillations" should be such that the xenon distributions (when combined with normal operating conditions) produce axial power shapes with delta-I values that bound the expected operating limits.

The stability of the calculational model may vary with burnup or core loading. Therefore, the amount of perturbation and feedback modification necessary to achieve a divergent xenon oscillation may vary with cycle burnup or core loading. Typical examples are given in Figures 2.1-1 and 2.1-2 for beginning- and end-of-cycle, respectively. The Dominion NOMAD [7,17]

1-D diffusion code was used to perform these examples. These particular oscillations were initiated by reducing power, depleting for several hours and then returning to full power for an additional 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> of depletion.

VEP-NE-1-Rev.0.1-A Page 14 FIGURE 2.1 TYPICAL RPDC BOC XENON OSCILLATION

VEP-NE-1-Rev.0.1-A Page 15 FIGURE 2.1 TYPICAL RPDC EOC XENON OSCILLATION

VEP-NE-1-Rev.0.1-A Page 16 2.1.2 Power Level During Normal Operation For the normal operation analysis, power levels spanning the 50% to 100% range are investigated to establish the RPDC delta-I limits. This range is consistent with the current CAOC Technical Specifications which do not impose axial flux difference limits or require CAOC operation below 50% of full power.* The power levels used for RPDC analysis are selected at increments within the 50% to 100% range which are small enough to ensure an adequate number of power distributions are being analyzed; i.e. that all safety-related effects due to the power level are accounted for.

2.1.3 Control Bank Position During Normal Operation During normal operation, the control rod bank insertion is limited by the cycle-specific Core Operating Limits Report (COLR) rod insertion limits. Figure 2.1-3 gives a set of typical rod insertion limits. The insertion limits are a function of reactor power, and the rods may be anywhere between the fully withdrawn position and the variable insertion limit. In order to adequately analyze the various rod positions allowed, control rod insertions versus power level are selected which cover the range of rod insertions allowed for each particular power.

  • The CAOC Technical Specifications impose no operational limit on delta-I while a plant operates below 50% power. However, in order to ascend above 50% power, the plant must not have exceeded the delta-I bands for more than one penalty hour of the previous twenty-four.

VEP-NE-1-Rev.0.1-A Page 17 FIGURE 2.1 TYPICAL ROD INSERTION LIMITS

VEP-NE-1-Rev.0.1-A Page 18 2.1.4 Cycle Burnup The RPDC analysis is performed at several times in cycle life in order to provide limiting delta-I bands for the entire cycle. Typically, three cycle burnups, near beginning-of-cycle (BOC),

middle-of-cycle (MOC) and end-of-cycle (EOC), are chosen for the RPDC analysis. The MOC case is chosen to reflect the maximum middle-of-cycle radial peaking factors.

2.1.5 Combining Xenon Shapes, Rod Position, Power Level and Burnup The final power distributions used in the RPDC normal operation analysis result from combining axial xenon shapes, power levels, rod insertions and cycle burnups. At each selected time in cycle life, the xenon shapes are combined with each power level and rod configuration. A criticality search is then performed for each case using the NOMAD [7,17] or the SIMULATE

[16] code with normal feedback. Calculated axial power distributions are identified for use in the LOCA FQ and thermal/hydraulic evaluations discussed in Sections 2.2 and 2.3. The combinations of burnups, power levels, rod configurations and xenon distributions typically evaluated on a reload basis are summarized in Table 2.1-1. The conditions result in a delta-I range of approximately -60% to +50%, bounding the expected final delta-I envelope at all power levels. The combinations of rod insertions and power levels necessary for Surry and North Anna would be slightly different due to the difference in rod insertion limits between the two plants.

VEP-NE-1-Rev.0.1-A Page 19 TABLE 2.1-1 TYPICAL CONDITIONS ANALYZED FOR NORMAL OPERATION UNDER RPDC Cycle Burnups BOC, MOC, EOC Xenon Shapes 100 for each time in life Power Level Range (%) 50-100 Rod Insertions Range Versus Power: See Figure 2.1-3 (3 burnups) * (100 xenon shapes) * (30 power level/rod position combinations) = 9000 shapes

VEP-NE-1-Rev.0.1-A Page 20 2.2 LOCA Delta-I Limit Formation The FQ*Power for each shape is compared to the LOCA FQ*Power*K(z) limit at each power level to determine which axial shapes approach the LOCA limit, thereby establishing a preliminary allowable delta-I versus power band. This comparison replaces the traditional CAOC FAC analysis [1] and ensures that the margin to the LOCA FQ*Power*K(z) envelope is maintained during the cycle as long as reactor operation remains within the delta-I limits. A typical LOCA delta-I limit is shown in Figure 2.2-1.

A sensitivity study to examine the impact of a change in FQ on the width of the LOCA delta-I limits determined that a change of 1% increase in FQ results in less than a 1% decrease in delta-I at constant power. This conclusion is based on the analyses of a range of FQ values for Dominion plants using the methods just described.

2.2.1 FQ Using Standard 1-D/3-D Synthesis The axial shapes created in Section 2.1 using NOMAD are combined with Fxy(z) data using a standard 1-D/3-D FQ synthesis [1,7,8,17]:

FQ(z) = Fxy(z)

  • P(z)
  • Xe(z)
  • FNU
  • FQE
  • FGR where the following are non-dimensional parameters:

Fxy(z) = Fxy distribution calculated by 3-D PDQ Two Zone [8,17], dependent upon burnup, core height and rod position and power level.

P(z) = Axial power shape function generated by NOMAD [7,17]

Xe(z) = The radial xenon redistribution factor FNU = Nuclear uncertainty factor [7,17]

FQE = Engineering heat-flux hot-channel factor [9,10]

FGR = Grid correction factor [7,17]

VEP-NE-1-Rev.0.1-A Page 21 The axially varying radial xenon factor, Xe(z), compensates for increases to FQ(z) resulting from redistribution of the xenon in the radial plane due to rod movement. The radial xenon redistribution effect cannot be explicitly represented in a 1-D code and is therefore applied in the synthesis as an uncertainty factor. Xe(z) is calculated as follows:

max Fxy ( z )T Xe( z ) =

Fxy ( z ) E where Fxy(z)T is the Fxy(z) calculated from a transient resulting in xenon radial redistribution and Fxy(z)E is the Fxy(z) based upon an equilibrium xenon distribution. Fxy(z)T is calculated with a 3-D code by first pre-conditioning the radial xenon distribution for several hours with the core at reduced power and the control rods inserted sufficiently to drive delta-I to the negative edge of the expected band. By withdrawing the rods and increasing power a xenon transient is created. This transient will cause the xenon to redistribute radially as well as axially in the 3-D model. Fxy(z)T is calculated for each time step as this transient is followed in small time intervals. The maximum values of Fxy(z)T for the entire transient are used to determine Xe(z).

2.2.2 FQ Using 3-D Model The axial shapes created in Section 2.1 using SIMULATE are used directly [16]:

FQ(z) = Fq(z)

  • FNU
  • FQE where the following are non-dimensional parameters:

Fq(z) = Fq distribution calculated by 3-D SIMULATE [16], dependent upon burnup, core height and rod position and power level. (Includes xenon redistribution and grid effects).

FNU = Nuclear uncertainty factor [16]

FQE = Engineering heat-flux hot-channel factor [9,10]

VEP-NE-1-Rev.0.1-A Page 22 FIGURE 2.2 TYPICAL LOCA DELTA-I LIMITS

VEP-NE-1-Rev.0.1-A Page 23 2.3 Loss of Flow Thermal/Hydraulic Evaluation The Loss of Flow Accident (LOFA) represents the most limiting DNB transient not terminated by the Overtemperature Delta-T trip. In order to ensure the applicability of the current LOFA analysis, the entire set of axial power distributions formed by the RPDC normal operation analysis are evaluated against the 1.55 cosine design axial power distribution for the Loss of Flow Accident analysis with the applicable thermal-hydraulic code(s) and correlation(s) that are listed in the COLR section of the plant Technical Specification. The thermal/hydraulic evaluation methods used in this LOFA evaluation are similar to those of the CAOC techniques.

As a result of this LOFA comparison, a second set of delta-I versus power limits is formed.

These delta-I limits delineate the allowable operating band which will ensure that the margin to the DNB design base for LOFA is maintained. The impact of RPDC on other DNB transient events is discussed in Section 3.

2.4 Final Normal Operation Delta-I Limit The results of the LOFA delta-I limit generation are combined with the LOCA delta-I limits (Figure 2.2-1) to produce a set of limits which will ensure that the preconditions for both accidents are met. These generic limits will be verified on a cycle-by-cycle basis using the RPDC methods described in this report.

The LOCA FQ based delta-I limits are generally more restrictive than LOFA-based delta-I limits for Dominions plants. This will allow the plant cycle specific COLR to take advantage of the FQ versus delta-I sensitivity identified in Section 2.2.

VEP-NE-1-Rev.0.1-A Page 24 SECTION 3 - CONDITION II ANALYSIS 3.0 Analysis of Axial Shapes Which Result from Condition II Events One of the important features of any axial power distribution control strategy (RPDC, CAOC or any other) is the clear distinction between normal and accident conditions. The delta-I limits established in Section 2 and the cycle specific COLR control rod insertion limits (see Figure 2.1-

3) define conditions of normal operation. If the axial power distribution (as measured by delta-I) remains inside the pre-established band during all normal operation, and the control rods remain within the cycle specific COLR limits, then the margin to the design criteria of fuel centerline melt, DNB and LOCA peak clad temperature, will be maintained.

This Section examines Condition II or Abnormal Operation events, which may be the result of system malfunctions or operator errors and create reactor conditions that fall outside the bounds analyzed in Section 2. The RPDC analysis examines the more limiting of these Condition II events and confirms that the Overpower Delta-T (OPDT) and the Overtemperature Delta-T (OTDT) setpoints* have been conservatively calculated and ensures that margin to the fuel design limits is maintained. These setpoints are verified on a cycle-by-cycle basis.

  • The OPDT and OTDT setpoints were designed primarily to provide transient and steady state protection against fuel centerline melt and DNB, respectively.

VEP-NE-1-Rev.0.1-A Page 25 3.1 Determination of Accident Pre-Conditions Initial condition parameters for Condition II analysis are determined from the core conditions allowed by the normal operation delta-I versus power envelope. These conditions are a function of rod control cluster (RCC) position, boron concentration, xenon distribution, burnup and core power level. Any set of these conditions which produce an axial power distribution within the normal operation delta-I envelope established in Section 2 (Figure 2.2-1) can be a potential starting point for a Condition II accident. Each set of valid normal operation conditions is considered in the RPDC Condition II analyses.

3.2 Condition II Accident Simulation Three categories of credible accidents bound the range of abnormal operation events which must be considered in terms of their effect upon the axial power distribution or local power peaking.

These three accidents are rod withdrawal, excessive heat removal and erroneous boration/dilution. The rod withdrawal and boration/dilution events [1] are the most limiting Condition II events with respect to the impact of control rod position on the axial power distribution or local power peaking. In the excessive heat removal event the impact of temperature is investigated.

3.2.1 Uncontrolled Rod Withdrawal Event The rod withdrawal event [6] is an erroneous control rod withdrawal starting from a normal operation condition with the control banks operating in their normal overlap sequence. To perform the analysis of this accident, the xenon distribution and boron concentration are fixed at values allowed by the normal operation analysis. The lead control bank is then withdrawn in increments from the fully inserted to the fully withdrawn position. After each incremental movement a criticality search is performed with either NOMAD [7,17] or SIMULATE [16] and the axial power distribution is identified for use in the Condition II evaluation of Sections 3.3

VEP-NE-1-Rev.0.1-A Page 26 and 3.4. The analysis is limited to those cases producing power levels between 50% of rated power and the high flux trip limit.

3.2.2 Excessive Heat Removal Event The Excessive Heat Removal (or cooldown) event, like the rod withdrawal event, is an overpower accident. The accident assumes a decrease in the reactor core inlet temperature as a result of a sudden load increase, steam-dump valve opening, excessive feedwater flow or a turbine valve opening [6]. Since the control rods are assumed to be in manual control for this event, they will remain at their original position, which allows the reactor power to increase.

To simulate this accident, allowable normal operation xenon distributions, control rod positions and boron concentrations are provided as input to the NOMAD [7,17] or SIMULATE [16] code.

The inlet temperature is reduced and a criticality search is performed. The axial power distribution from each case is identified for use in the Condition II evaluation of Sections 3.3 and 3.4. Reduction of the inlet temperature is limited to 30oF, which has been shown to bound the results of the above accidents in the Surry and North Anna UFSARs [11,12]. Cases producing a power level greater than the high flux trip limit are excluded from consideration.

3.2.3 Boration/Dilution The Boration/Dilution event causes a movement in the control rods to compensate for the reactivity changes due to a change in soluble boron concentration as a result of inadvertent boration or dilution. In this analysis the control banks are assumed to be in automatic mode and to operate in a normal overlap sequence. The manual mode of operation could result in an overpower transient during a dilution incident. However, the consequences of this event are bounded by those of the rod withdrawal accident [6].

To perform the boration/dilution analysis, NOMAD reads each allowable xenon distribution from the Condition I analysis and runs a series of cases inserting the rods from fully withdrawn

VEP-NE-1-Rev.0.1-A Page 27 to the insertion limits in fixed increments. For SIMULATE, a restart case is read for each allowed xenon distribution from the Condition I analysis. For both NOMAD and SIMULATE, at each step a criticality search is performed. Once the rods reach the insertion limits, a rod position search is performed to determine the amount of control rod insertion necessary to compensate for the reactivity associated with a dilution of fifteen minutes. The rods are then stepped in from the insertion limits to the determined rod position, again performing criticality searches. All axial power distributions from the boration/dilution event are identified for the Condition II evaluation of Sections 3.3 and 3.4.

3.3 Overpower Limit Evaluation The maximum linear power density for each distribution produced by the Condition II accident simulations is determined using the 1-D/3-D FQ synthesis or the 3-D model techniques as described in Section 2.2 (with the addition of the densification spike factor S(z)). The results may be plotted in the "flyspeck" format shown in Figure 3.3-1, which shows typical results for the three limiting Condition II accidents described in Section 3.2.

The peak power density "flyspeck" is compared to the design basis limit for fuel centerline melt.

If necessary, the OPDT f(delta-I) function (which provides protection against this design limit) is modified to ensure that margin to the fuel centerline melt limit is maintained. If needed at all, this modification would be required only for very large values of delta-I. An alternative approach would be to maintain the margin to fuel centerline melt by restricting the OTDT f(delta-I) function beyond the DNBR requirement, effectively eliminating the need for the OPDT f(delta-I) function.

VEP-NE-1-Rev.0.1-A Page 28 FIGURE 3.3 TYPICAL MAXIMUM POWER DENSITY FLYSPECK

VEP-NE-1-Rev.0.1-A Page 29 3.4 DNB Evaluation The OTDT trip function and setpoints [13] provide DNB protection for Condition II accidents.

Part of this function, the f(delta-I) term, responds to changes in the indicated delta-I created by skewed axial power distributions. The axial power distributions formed by the RPDC Condition II accident simulations are evaluated to confirm that the assumptions [13] used to form the f(delta-I) term and the rest of the OTDT trip function remain valid. If the RPDC power distributions for any subsequent reload should be more limiting than those previously used to establish the OTDT trip setpoints, the OTDT setpoints will be reformulated using standard techniques [13] and the appropriate RPDC power distribution parameters.

VEP-NE-1-Rev.0.1-A Page 30 SECTION 4 - OTHER SAFETY ANALYSES No changes are required to the other safety analysis methods described in Reference 6 to incorporate the effect of the widened delta-I band resulting from the RPDC methodology. The CAOC methods used by Dominion employ a conservative method for incorporating the effect of skewed axial power distributions. However, as is the practice with CAOC, the accident analyses will be evaluated on a reload basis for RPDC to ensure that the key input parameters remain bounding. Should an accident analysis be determined to be impacted by a reload design, that accident will be re-evaluated or reanalyzed, as appropriate.

VEP-NE-1-Rev.0.1-A Page 31 SECTION 5 - FQ SURVEILLANCE Dominion instituted FQ Surveillance Technical Specifications as part of the RPDC implementation process. FQ Surveillance Technical Specifications [14,15] are a convenient method for overall power distribution monitoring during plant operation to ensure compliance with the specified LOCA FQ*K(z) limit. The FQ relationship is:

CFQ K ( z )

FQM ( z ) £

  • for P > 0.5 [5 - 1]

P N ( z)

CFQ K ( z )

FQM ( z ) £

  • for P £ 0.5 [5 - 2]

0 .5 N ( z )

where the nondimensional parameters are defined as FQM(z) = the measured plant FQ(z) at equilibrium conditions CFQ = the plant LOCA FQ limit K(z) = the normalized LOCA FQ(z) limit as a function of core height P = the fraction of rated thermal power N(z) = the maximum potential increase in FQM(z) resulting from non-equilibrium normal operation.

N(z) is a cycle dependent factor that represents the largest possible increase in FQ(z) that could result from changes in the power level and delta-I allowed during normal plant operation:

FQ( z ), max Condition I N ( z) = [5 - 3]

FQ( z ), equilibrium depletion The impact of control rod insertion and xenon transients, both axial and radial, are included in N(z). The FQ(z)'s in equation [5-3] are formed by the standard 1-D/3-D FQ synthesis or the 3-D

VEP-NE-1-Rev.0.1-A Page 32 model techniques as described in Section 2.2. N(z) is similar to V(z) given in Reference 15 and W(z) given in Reference 14. A typical N(z) function is given in Figure 5.0-1.

When FQM(z) exceeds the LOCA FQ*K(z)/N(z) limit, the delta-I versus FQ sensitivity discussed in Section 2.2 permits compensation by means of a reduction in the normal operation delta-I band.

VEP-NE-1-Rev.0.1-A Page 33 TOP AND BOTTOM 15 PERCENT EXCLUDED AS PER TECHNICAL SPECIFICATION BASES FIGURE 5.0 TYPICAL N(Z) FUNCTION

VEP-NE-1-Rev.0.1-A Page 34 SECTION 6 - CONCLUSION The RPDC methodology takes advantage of the large amounts of margin to the design bases limits available at reduced power levels in CAOC and forms wider delta-I limits at all powers.

The RPDC methodology may be summarized as follows:

1. A full range of normal-operation axial power shapes is obtained by combining the key parameters upon which each shape is dependent: xenon distribution, boron concentration, core power level and control rod position. A xenon "free oscillation" method is used to create the many and varied axial xenon distributions required for this analysis.
2. These axial power profiles are analyzed to determine which shapes result in an approach to the LOCA and LOFA limits.
3. A final normal operation delta-I limit is established by conservatively bounding both the LOCA and the LOFA limits.
4. Conditions which yield shapes within the final normal operation delta-I limit are used as initial conditions for the bounding Condition II accident simulations.
5. The resultant transient shapes are analyzed and the overpower and overtemperature trip function/setpoints are specified to ensure that margin to fuel design limits is maintained.
6. A N(z) function is formulated based on calculated Condition I Fq's to support the implementation of Fq Surveillance Technical Specifications.

All neutronics calculations are performed with NRC approved codes. All DNBR calculations are performed using the applicable thermal-hydraulic code(s) and correlation(s) that are listed in the COLR section of the plant Technical Specification.

The RPDC methodology presented in this report allows the Dominion nuclear units to operate with additional operational flexibility while at the same time ensuring that the design bases limits are met with an appropriate margin.

VEP-NE-1-Rev.0.1-A Page 35 Section 7 - References

1. Morita, T., et al.: "Topical Report -Power Distribution Control and Load Following Procedures," WCAP-8385, Westinghouse Electric Corporation, Pittsburgh, PA (September 1974).
2. Letter from A. Schwencer (NRC) to W. L. Proffitt (VEPCO), dated April 4, 1978.
3. "C-E Setpoint Methodology," CENPD-199-NP Rev. 1-NP, Combustion Engineering Inc.,

Windsor, CT (March 1982).

4. Hanson, G.E.: "Normal Operating Controls," BAW-10122, Rev. 1., Babcock & Wilcox, Lynchburg, VA (April 1982).
5. Miller, R. W., Pogorzelski, N. A. and J. A. Vestovich: "Relaxation of Constant Axial Offset Control," NS-EPR-2649 Part A, Westinghouse Electric Corp., Pittsburgh, PA (August 1982).
6. Bordelon, F. M., et al.: "Westinghouse Reload Safety Evaluation Methodology,"

WCAP-9272, Westinghouse Electric Corp., Pittsburgh, PA (March 1978).

7. "The VEPCO NOMAD Code and Model," VEP-NFE-1A (May 1985) and Supplement 1 (September 1996).
8. "The PDQ Two Zone Model," VEP-NAF-1 (July 1990).
9. Letter from Virginia Electric and Power Company (L.N. Hartz) to U. S. Nuclear Regulatory Commission, Virginia Electric And Power Company, North Anna Power Station Units 1 And 2, Proposed Technical Specifications Changes And Exemption Request Use Of Framatome ANP Advanced Mark-BW Fuel, Serial No.02-167, March 28, 2002.
10. Letter from C. O. Thomas (NRC) to E. P. Rahe, Jr. (W), Acceptance for Referencing of Licensing Topical Report WCAP-8691 (P) /WCAP-8692 (NP), dated December 29, 1982.
11. North Anna Power Station Units 1 and 2 UFSAR.

VEP-NE-1-Rev.0.1-A Page 36

12. Surry Power Station Units 1 and 2 UFSAR.
13. Ellenberger, S. L., et al.: "Design Bases for the Thermal Overpower delta-T and Thermal Overtemperature delta-T Trip Functions," WCAP-8745 (March 1977).
14. Miller, R. W., et al.: "The FQ Surveillance Technical Specification," NS-EPR-2649 Part B, Westinghouse Electric Corp., Pittsburgh, PA (September 1982).
15. Holm, J. S., and R. J. Burnside, "Exxon Nuclear Power Distribution Control for Pressurized Water Reactors - Phase II," XN-NF-77-57(A), Exxon Nuclear Co., Bellevue, WA (May 1981).
16. "Qualification of the Studsvik Core Management System Reactor Physics Methods for Application to North Anna and Surry Power Stations," DOM-NAF-1-Rev. 0.0-P-A (June 2003).
17. Letter from United States Nuclear Regulatory Commission (Scott Moore) to Virginia Electric and Power Company (David A. Christian), Virginia Electric and Power Company

- Acceptance of Topical Report VEP-FRD-42, Revision 2, Reload Nuclear Design Methodology, North Anna and Surry Power Stations, Units 1 and 2 (TAC NOS. MB3141, MB3142, MB3151, and MB3152), June 11, 2003.

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