ML20086R766
| ML20086R766 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 12/17/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20086R753 | List: |
| References | |
| NUDOCS 9201020213 | |
| Download: ML20086R766 (4) | |
Text
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UNITED STATES l ' 3 m /,)
NUCLEAR REGULATORY COMMISSION p
WASHINGTON, o C. 20555
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDHENT NO. 93 TO FACILITY OPERATING LICENSE NPF-35 AND AMENDMENT NO. 87 TO FACILITY OPERATING llCENSE NPF-52 DUKE POWER COMPANY, ET AL.
CATAWBA NIICLEAR STATION, UNITS 1 AND 2 DOCKET NOS. 50 413 AND 50-414 1.0 INTPODUCTION Ey letter dated November 20, 1991, as supplemented December 5, 1991, the Duke Power Company (the licensee) submitted a request for changes to the Catawba Nuclear Station, Units 1 and 2, Technical Specifications (TS). The requested changes would change the paran,eters in TS Table 2.2-1 to compensate for potential nonconservatism in the F-Delta I (axial flux differences) portion of the Overterperature-Delte Temperature (OTdT) reactor trip function for Unit 2.
The December 5, 1991, letter provided clarifying information that did not change the initial proposed no significant hazards consideration determination.
The Reactor Trip System (RTS) keeps the reactor operating within a safe region by shutting cown the reactor whenever the limits of the region are approached.
The safe operating region is defined by several considerations such as mechanical /
hydraulic limitations on equipment and heat transfer phenomena. Therefore, the RTS keeps surveillance on process variables such as pressure, pressurizer water level, flow, reactor coolant temperatures, and others that are calculated from various process variables. One of these calculated variables is the core thermal overtemperature-delta temperature (OTdT) trip which is the subject of this safety evaluation. This trip provides protection from departure from nucleate boiling (DNB) in the core, it is a function of the core power level, coolant temperature, coolant pressure, and the core power distribution.
For the core power distribution, a compensating term is derived which accounts for core power distributions more severe with respect to DNB than the reference core power distribution. This term is a function of the difference in the axial neutron flux (Delta 1 (di))
produced in the top and bottom halves of the core. The neutron flux is measured by out-of-core ion chamber detectors mounted vertically outside the pressure vessel 90 degrees apart in plan.
Each detector is centered on the core horizontal midplane and is divided into an upper (P ) and icwer half (P ).
Delta flux (di) t b
IncreasesindibehondacertainbandresultinadecreasbiN)/(P+P).thetrkpsetpoint.
is defined as P -P and the axial offset is defined as (P -P t
t 9201020213 911217 DR ADOCK 0500 3
-2 The f(dt) function is included in the equation for the OTdT trip function in the Catawba TS Table 2.2-1 Note 1.
This evaluation concerns the correction of d nonConservative assumption in the determination of the f(di) function for the Catawba Unit 2 plant as discussed below.
2.0 EVALUAY10N The OTdT reactor trip is designed to protect the core against departure frota nucleate boiling (DNB) and subsequent cladding failure. The f(delta 1) reset portion of the OTdT trip function is designed to lower the trip setpoint when axial flux differences exceed predetermined limits.
This revision changes the slope value for the pcsitive wing of the f(delta 1) reset portion of the OTdT trip function for Unit 2.
In addition, the Unit 2 values for Total Allowance,
- 2. Sensor Error (S) and Allowable Value given in TS Table 2.2-1 would be modified. This change is only required for Unit 2; the TS values for Unit I are unchanged ar.c are retained on the common TS pages for Units 1 and 2.
Since the axial offset curve is a power reduction curve, the current Westinghouse methodology (Improved Thermal Design Procedure) uses a linear extrapolation to higher power levels than permissible. This is done so that when the power reduction is applied to the extrapolated core thermal limits for large axial cffsets, the reduced power versus core thermal limits is in the range of permissible operatior.. This approach is valic fcr certain thB correlations such as the W-3 correiation previously used in Westinghouse plants because the power reduction is essentially independent of power.
However, since the axial offset limits r4) not be independent of pcwer for the WRB-1 DNB correlation used in Ur.it 2, the extrapolated slope of the positive wing of the f(delta 1) reset function may not be as high as it would be if calculations were performed usir.g actual axial offset limit data at the higher power levels and may result in a potentially nonconservative f(delta 1).
The negative wing is not adversely affected because of the typically large negative delto I band and since the axial power shapes with negative axial offsets are typically not limiting.
Because of this potential nonconservatism, Westinghouse performed an evaluation to determine a new value for the slope.of the positive sidc of the axial offset band which would conservatively bound the operating region using their approved improved Thermal Design procedure. This new value was based on explicit calculated p;ints at the higher power levels rather than on extrapolat_
values.
Based on this, the OTdT trip setpoint will be reduced by 2.414% of its value at rated thermal power for each percent that the magnitude of indicated flux difference between top and bottom of the core is more positive than -6.5%.-
For Unit 1, this trip setpoint reduction value remains at 1.641%. This change in the posithe wing results in changes to the 2 and Allowable Value specified in TS Table 2.2-1.
Included in the 2 value is an increase in the uncertainty associated with flux map accuracy to +/- 1.37 delta-1 span from +/- 1.2%
delta-1 span. The Total Allowance, 2, and S values also change as a result of using Westinghouse methodology instead of Duhe raethodology. Total Allowance, 2, and S for Unit 2 are increased to 8.9, 7.3, and 2.7 f rom 6.08, 3.0, and 2.12, respectively. The Westinghouse methodology includes the delta-I uncertainties in the K terra of the OTdT setpoint whereas the Duke methodology lef t the uncertair,ty allowances out of K) but used them to adjust the f(delta 1) trip 3
i reset function breakpoints.
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, The NRC staff has determined that these changes are acceptably correct for a nonconservatism in the Westinghouse methodology used to calculate the f(delta
- 1) reset portion of the OTdT trip function and are adequately incorporated in the proposed Unit 2 TS. The proposed Unit 2 TS changes will conservatively ensure adequate DNB protection during plant operation.
The staff has reviewed the proposed TS changes described above and finds that they ensure adequate ONB protection for the Catawba Unit 2 reactor core during plant operation. These changes are Unit 2 specific and do not affect the values for Unit 1 since the uncertainty allowances for Unit I were determined using an approved non4'estinghouse methodology.
3.0 EX1GENCY CIRCUMSTANCES The licensee, in its November 20, 1991, application, requested that the proposed TS change be approved on an exigency bas 's.
The licensee states that their evaluation and that of the vendor, Westinghouse Electric Corporation in this matter, resulted in a determination on October 21, 1991, that the resolution to the issue would require reanalysis and assockted changes to the TS.
The pcrforuance of the ioditional analysis and proposed revision to the TS was completed by Westinghouse and provided to the licensee on hovember 13, 1991. The licensee's crganizational elements then performed their review of the proposed TS changes.
The licensee then transmitted their application to the NRC on November 20, 1991.
Catawba Unit 2 is now scheduled to start up from its present refueling outage on December 15, 1991, and would need the proposed amendment prior to December 15, 1991, in craer to pernit entry into MODE 2 and reactor startup.
The NRC staff concludes thet, upon the deterreination that the resolution to the issue would require an amendment to the TS, the supporting analyses by Westinghouse and the licensee were performed in an expeditious fashion, and that the need for reviewing the application on an exigent basis could not then be avoided. Thus, pursuant to 10 CFR 50.91(a)(6), the staff finds that an exigent situation exists which would result in a delay in the startup of Unit 2.
4.0 fit 3t NO SIGNIFICANT HA?APDS CONSIDERATION The Commission has provided standards for determining whether a sigrificant hazards consideration exists (10 CFR 50.92(c)). A proposed amendment to an operating license for a facility involves no significant hazards consideration if operation of the facility, in accordance with the proposed amendment, would not:
(1) involve a significant increase in the probability or consequences of an accident previously evaluated; or (2) create the possibility of a new or different kind of accident from an accident previously evaluated; or (3) involve a significant reduction in a margin of safety.
The following evaluation in relation to the three standards demonstrates that the propcsed amendment does not involve a significant hazards consideration.
These proposed changes to the Technical Specifications do not involve a significant increase in the probability or consequences of an accident previously evaluated.
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4 Due to a potential nonconservatism discovered in the methodology used to calculate the f(d!) reset portion of the OTdT trip function, it was determined that the positive side of the axial offset band was nonconservative for Catawha Unit ?.
The f(di) reset portion of the trip function is designed to lower the trip setpoint when axial flux differences exceed predetermined limits. Since the limiting margins to DNB occur as the result of highly skewed power distributions, a slope char.go to the positive wing on the axial offset band is necessary in order to prevent the DNB limits from being exceeded. Therefore, an evaluation was performed to determine a new value for the slope of the positive sides of the axial offset band which conservatively bounds this opeiating region. Since this change ensures that the Dh8 limits are not exceeded, the probability or consequences of an accident previously evaluated are not increased.
ThE changes to the 2 and Allowable Value reflect the change in the positive wing of the axial offset band.
As discussed in the Technical Justification, included in the Z v61ue is an increase in the. uncertainty associated with flux map accuracy.
The Total Allowance, 2, and S values also change as a result of using Westinghouse rethodology to calculate the values instead of Duke methodology which was used to calcuhte the current values.
Since these changes ensure that DNB limits are not exceeded, and systems used to mitigate an Lucident are not affected, the probability or consequences of an accident previously evaluated are not increased.
As discussed above, the proposed changes to the TS are being made to ensure that DNB limits are not exceeded.
Because this change conservatively ensures that DNO limits are not exceeded, and because the operating of other plant systems are not affected, this change does not create the possibility of a new or different hind of accident from any accident previuusly evaluated.
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As discussed in the (licensee's) Technical Justification, it has been determined that the positive sides of the axial offset band was nor. conservative for Catawba Unit 2.
This change enseres that the nonconservatism in the Westinghouse l
rethodolocy is accounted for, therefore increasing the margin of safety, Basec cn the foregolog, the NRC staff has ccncluded that the standards of 10 CFR 50.92(c) are satisfied.
Therefere, the Commission has made a final determinatior that the proposed amendments do not involve a significant hazards consideration.
5.0 STATE CONSL'lTATION In accordance with the Commission's regulations, the South Carolina State officicl was notified of the proposed issuance of the amendments. The State official had no corcents.
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i 6.0 ENVIR0Nt Et1TAL CONSIDERATION The amendments change requirements with respect to instailation or use of a facility component located within the restricted area as defined in 10 CFR Part
- 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Consnission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public consnent on such finding (56 TR 61062). Accordingly, the aniendments meet the eli categoricalexclusionsetforthin10CFR51.22(c)(9gibilitycriteriafor Cursuant to 10 CFF; 51.22(b) no environmental impact statenent or environmental assessrent need be prepared in connection with the issuance of the amendt.ents.
7.0 CONCLl!SION The Commission has concluded, basad on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will r.o; be endangered by operatico in the proposed manner, (2) such activities will be conducted in compliance with the Conraission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Frincipal Contributor:
Larry Kopp, SR)E Date: Decenter 17, 1991 i
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