ML20153H337
| ML20153H337 | |
| Person / Time | |
|---|---|
| Issue date: | 02/20/1986 |
| From: | Berkow H Office of Nuclear Reactor Regulation |
| To: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| References | |
| NUDOCS 8602280769 | |
| Download: ML20153H337 (2) | |
Text
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February 29, 1986 Mr. W.'L'.-Stewart, Vice President Nuclear Operations Virginia Electric and Power Company Richmond, Virginia 23261
Dear Mr. Stewart:
SUBJECT:
ACCEPTANCE FOR REFERENCING 0F LICENSING TOPICAL REPORT VEP-NE-1, "VEPC0 RELAXED POWER DISTRIBUTION CONTROL METHODOLOGY AND ASSOCIATED FQ SURVEILLANCE TECHNICAL SPECIFICATIONS" We have completed our review of the subject topical report submitted by the Virginia Electric and Power Company (VEPCO) by letter dated December 10, 1984.
We find the report to be acceptable for referencing in license applications to the extent specified and under the limitations delineated in the report and the associated NRC evaluation, which is enclosed. The evaluation defines the basis for acceptance of the report.
We do not intend to repeat our review of the matters described in the report and found acceptable when the report appears as a reference in license applications, except to assure that the material presented is applicable to the specific plant involved. Our acceptance applies only to the matters described in the report.
In accordance with procedures established in NUREG-0390, it is requested that i
VEPCO publish accepted versions of this report, proprietary and non-proprietary, within ti. ee months of receipt of this letter. The accepted versions shall incorporate. this letter and the enclosed evaluation between the title page and the abstract-The accepted versions shall include an -A (designating accepted) following the report identification symbol.
Should our crittria or regulations change such that our conclusions as to the acceptability of the report are invalidated, VEPC0 and/or the applicants referencing the topical report will be expected to revise and resubmit their respective documentation, or submit justification for the continued effective applicability of the topical report without revision of their respective documentation.
Sincerely, DISTRIBUTION:
CentralfilejREmch SSPD Reading JDosa PDR MDunenfeld Original signed by DCS LEngle Herbert N. Berkow, Director PNoonan TChen Standardization and Special HBernard Projects Directorate HBerkow Division of PWR Licensing-B
Enclosure:
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- j WASHINGTON, D C. 20555 February 20, 1986 Mr. W. L. Stewart, Vice President Nuclear Operations Virginia Electric and Power Company Richmond, Virginia 23261
Dear Mr. Stewart:
SUBJECT:
ACCEPTANCE FOR REFERENCING OF LICENSING TOPICAL REPORT V6P-NE-1, "VEPC0 RELAXED POWER DISTRIBUTION CONTROL METHODOLOGY AND ASSOCIATED FQ SURVEILLANCE TECHNICAL SPECIFICATIONS" We have completed our review of the subject topical report submitted by the Virginia Electric and Power Company (VEPC0) by letter dated December 10, 1984.
We find the report to be acceptable for referencing in license applications to the extent specified and under the Ifmitations delineated in the report and the associated NRC evaluation, which is enclosed. The evaluation defines the basis for acceptance of the report.
We do not intend to repeat our review of the matters described in the report and found acceptable when the report appears as a reference in license applications, except to assure that the material presented is applicable to the specific plant involved. Our acceptance applies only to the matters described in the report.
In accordance with procedures established in NUREG-0390, it is requested that VEPC0 publish accepted versions of this report, proprietary and non-proprietary, within three months of receipt of this letter.
The accepted versions shall incorporate this letter and the enclosed evaluation between the title page and the abstract. The accepted versions shall include an -A (designating accepted) following the report identification symbol.
Should our criteria or regulations change such that our conclusions as to the acceptability of the report are invalidated, VEPC0 and/or the applicants referencing the topical report will be expected to revise and resubmit their respective documentation, or submit justification for the continued effective.
applicability of the topical report without revision of their respective documentation.
Sincerely,
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H rbert
. Berkow, Director Standardization and Special Projects Directorate Division of PWR Licensing-B
~
Enclosure:
As stated
FEB 3W SAFETY EVALUATION REPORT Report
Title:
Vepco Relaxed Power Distribution Control Methodology and Associated F Q Surveillance Technical Specifications Report Number:
VEP-NE-1 Report Date:
October, 1984 INTRODUCTION The Virginia Electric and Power Company (Vepco) has developed the relaxed power distribution control (RPDC) methodology to replace the constant axial offset control (CAOC) strategy currently employed at its Surry and North Anna reactors. Associated with the RPDC methodology is direct monitoring of the maximum peaking factor (F ) relative to plant limits; this replaces the present q
Fxy Technical Specifications.
The analyses performed in support of relaxed power distribution control, and sample generic F surveillance Technical q
Specifications are described in the subject report.
Additional information considered in this review is given in Ref. 1.
SUMMARY
OF TOPICAL REPORT
'The constant axial offset control (CAOC) strategy currently employed by Vepco was developed by Westinghouse (W) in order to meet power peaking limits imposed by loss of coolant accident (LOCA) analyses.
The CAOC procedure requires the maintenance of the axial flux difference (AI) within a specified, constant band about a target axial offset defined at equilibrium conditions. While mainten-ance of AI within these limits insures that the F is bounded by a specified q
limit, CACO is unnecessarily restrictive, particularly below full power where significant margin to peaking limits exits.
These restrictive AI limits have a negative impact on operational flexibility, especially in the ability to return to full power quickly following a reactor trip near end-of-cycle (E0C). The development of the relaxed power distribution control approach by Vepco was motivated primarily by this limitation.
4 ENCLOSURE gjl Q qg g g
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Under RPDC the AI vs. power operating domain is typically broader than that permitted under CAOC (even with band widening), with the width of the band increasing with decreasing power levels.
(Similar variable width operating bands are employed by all three PWR vendors in their axial power di_stribu-tion control procedures).
The variable AI vs. power operating band takes advantage of the increased F limits permitted at reduced power by main-q taining a roughly constant margin to design limits at all power levels (vs. an increasing margin with decreasing power in CAOC)
The major elements of the RPDC methodology are:
1.
Axial power distributions are generated with the Vepco one-dimensional NOMAD (Ref. 2) code which bound the potential AI operating band.
The NOMAD analysis produces a spectrum of xenon distributions at selected burnups via a free-oscillation technique similar to that developed by Combustion Engineering (CE) (Ref. 3).
The resulting xenon distributions are combined with rod insertions and power levels permitted by the power dependent rod insertion limit curve at the selected burnups to produce a range of power distributions (and associated AI's) at power levels between 50% and full power.
1 2.
The axial power distributions from (1) are used in a 1D/2D/3D synthesis of
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Fq (z) based on values of Fxy(z) generated by the Vepco FLAME (Ref. 4) and PDQ07 (Ref. 5) models.
The synthesis includes an axial height dependent radial xenon redistribution factor calculated by FLAME, and uncertainty factors which account for the calculational uncertainty, and manufacturing variabilities.
3.
Comparison of the resultant F 's to limits prescribed by LOCA analyses J
q defines a preliminary AI vs. power operating domain.
l 4.
The entire set of axial power distributions is also analyzed with the COBRA (Ref. 6) code relative to the 1.55 design axial power distribution for the loss of flow accident (LOFA).
This analysis defines a second AI power operating space that insures that the margin to the D'NB design basis for LOFA is maintained.
2
5.
The most restrictive AI power domain (based on LOCA and/or LOFA) defines the permissible space for normal operation (Condition I).
(For Vepco plants, the LOCA based band is usually more restrictive).
Maintenance of AI within this operrting space, coupled with adherence to control rod insertion limits, ensures that the margin to fuel centerline melt, DNB, and LOCA peak clad temperature design criteria are maintained during normal operation.
5.
Three abnormal operation (Condition II) events are also considered in the analyses supporting RPDC: uncontrolled rod withdrawal, excessive heat removal, and erroneous boration/ dilution.
The purpose of these analyses is to confirm that the over power delta-T (0PDT) and over-temperature delta-T (OTDT) trip setpoints have been conservatively calculated, and insures that required margins are maintained.
The OPDT and 0 TDT trips provide transient and steady-state protection against fuel center-line melt and DNB, respectively.
The initial conditions for the analyses of these events consist of the axial power distributions allowed by the A-I power operating domain determined in (5).
7.
The max.. m linear power density for each resulting Condition II distribution is determined by using the Fq (z) synthesis techniques (with an allowance for densification) and compared to the design basis for fuel centerline melt.
The OPDT f(AI) function is modified, if necessary, to insure that margin to the fuel center-line melt limit is maintained.
The axial power distributions from the Condition II analyses are also evaluated to confirm that the OTDT trip function and its associated f(AI) term remain valid.
In conjunction with the implementation cf the RPDC methodology, Vepco proposed to replace the current Fxy surveillance with direct monitoring of Fq (z).
In F surveillance the measured F at equilibrium conditions is augmented q
q by a factor, N(z), which accounts for the maximum potential-increase in Fq (z) during normal opertion.
The resultant augmented Fq (z) is compared to the plant LOCA Fq (z) limits to determine acceptability, or to initiate, remedial actions.
Sample Technical Specifications to be used with F surveillance are q
given.
3
While the greatest benefit of relaxed power distribution control to Vepco is the ability to return to power quickly following a trip near EOC, institution of this methodology with its wider operating band is expected to yield additional operational benefits including reduced control rod motion and coolant system boration/ dilution requirements.
SUMMARY
OF TECHNICAL EVALUATION All the analyses performed in support of RPDC employed codes which have been previously reviewed and approved by the staff (FLAME, PDQ07, NOMAD, COBRA).
The approach used for generating bounding axial power distributions is based on the free xenon oscillation technique employed for a number of years by Combustion Engineering in their axial power distribution control methodology.
(CE served as a consultant to Vepco in the implementation and application of this technique). Vepco has determined that this approach results in axial power distributions ti.at sufficiently span the Al power domain to ensure there is confidence that the most adverse conditions are available for subsequent analyses.
In addition, relevant analyses performed by CE show that the sensitivity of the results obtained employing the free xenon oscilTation methodology to variations in the impacting parameters are small, and are more than compensated for by the " bounding" nature of the approach, and the extreme distributions considered. This approach has been found acceptable for CE
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reactors for many years, and is acceptable for RPDC.
The calculation of F via a 1D/2D/3D synthesis is similar to accepted q
approaches.
Uncertainties associated with the calculation of F are based q
on comparisons to measurements.
The measurements included situations where azimuthal tilts spanning the range permitted by the technical specification limits were present. The combination of the FNU and FGR components of the uncertainty given in the report is greater than the 95/95 upper tolerance limit determined on the basis of comparisons to measurements.
The magnitude of the uncertainty assigned to the calculated value of F in the RPDC analyses is q
therefore acceptable.
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Calculations of the radial xenon redistribution factor, Xe(z), component of the F synthesis employed the FLAME code and considered a number of cycles, times q
in life and initiating conditions.
The final Xe(z) was chosen such that it bounded all observed increases in Fxy(z).
Even though this factor is now less than the previously used axially uniform value of 1.03, the analyses performed to justify the. lower values are adequate.
The LOFA analyses performed with COBRA, and the Condition II events considered are similar to those included in the Westinghouse relaxed axial offset control (RAOC) methodology.
The over-power and over-temperature AT trip functions will be evaluated on a reload basis to assure protection against fuel center-line melt and DNB design basis limits. Other accident analyses will be reevaluated on a reload basis to insure that the assumption used in the RPDC analyses remain bounding.
Monitoring of adherence to operation within the permissible AI power domain is accomplished by reliance on the ex-core detectors.
The calculated AI domain will be reduced by 3% to accommodate the maximum excore detector calibration uncertainty permitted by the Technical Specifications. In addition, Vepco plans to further reduce the AI limits for the first-time analysis. The bounding nature of the RPDC approach provides further conservatism.
The Vepco RPDC methodology contains elements similar to those included in the W (Ref. 7) and CE variable-width AI band axial power distribution control
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strategies.
Approved methods have been used in the analyses supporting RPDC and justification has been provided for the uncertainties assigned. These analyses and uncertainties are consistent with currently approved methods and practices.
In addition, the impact of cycle specific variations on the AI -
power domain, the over power and over-temperature AT trip setpoints, and other safety analyses will be evaluated on a reload basis. Based on these considerations the RPDC approach represents an acceptable methodology for use with reload cores similar to those of'the Surry and North Anna reactors.
l 5
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The proposed F surveillance is similar to the approach approved for W in q
conjunction with RAOC.
The N(z) factor by which the measured Fq (z) distribution is augmented to account for non-equilibrium normal operation is similar to the W(z) and V(z) functions used by W and Exxon, respectively, and approved for use with RA0C and PD II power distribution control strategies.
The sample Technical Specifications given in the subject report replace Fxy surveillance with F surveillance.
This is acceptable because the F q
q surveillance is more appropriate for RPDC.
The sample Technical Specifications in the report acceptably implement RPDC with the following modifications:
Specification 3/4.2, page 3/4 2-1 The asterisk at the end of the APPLICABILITY line and the footnote should be deleted.
Figure 3.2-1, 3/4 2-4 This figure should be blank and contain the legend:
"This curve is given in the Core Surveillance Report as per Specification 6.9.1.10."
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Specification 3.2.2, page 3/4 2-5 Parenthetical comments should be added to the final three lines of action a as follows:
" subsequent POWER OPERATION may preceed provided the Overpower AT Trip Setpoints (value of K ) have been reduced at least 1%
4 (in AT span) for each 1% F (z) exceeds the limit.
9 Specification 6.9.1.10 (page unnumbered)
After " initial criticality", add "unless otherwise approved by the Commission by letter", and change the end of the first paragraph to
" approved by the Commission by letter".
1 6
A complete set of these revisions will b3 approved for North Anna Unit 2, Cycle 4 and could be used as a model.
CONCLUSION We find the subject report suitable for reference as support for use of RPDC-in licensing applications.
S 7
A REFERENCES 1.
Letter from W.L. Stewart (Vepco) to H.R. Denton (USNRC), " Virginia Electric and Power Company Relaxed Power Distribution Control.('oDC)
Supplemental Information," (Oct. 21, 1985).
2.
S.M. Bowman, "The Vepco NOMAD Code and Model," VEP-NFE-1A, Virginia Electric and Power Company (May 1985).
3.
"C-E Setpoint Methodology," CENDP-199-NP Rev. 1-NP, Combustion Engi neering Inc. (March 1985).
4.
W.C. Beck, "The Vepco FLAME Model," VEP-FRD-24A, Virginia Electric and Power Co. (July 1981).
5.
M.L. Smith, "The PDQ07 Discrete Model," VEP-FRD-19A, Virginia Electric and Power Co., (July 1981).
6.
F.W. Silz, "Vepco Reactor Core Thermal-Hydraulic Analysis Using the COBRA IIIC/MIT Computer Code," Vepco-FRD-33A, Virginia Electric and Power Co. (Oct. 1983).
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7.
R.W. Miller et al., " Relaxation of Constant Axial Offset Control,"
NS-EPR-2649 Part A, Westinghouse Electric Corp. (August 1982).
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