ML17310B295
| ML17310B295 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 05/16/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17310B291 | List: |
| References | |
| NUDOCS 9405250244 | |
| Download: ML17310B295 (6) | |
Text
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'NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2055&4001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.
75 TO FACILITY OPERATING LICENSE NO.
NPF-41 AMENDMENT NO.
6 i TO FACILITY OPERATING LICENSE NO.
NPF-51 AND AMENDMENT NO.
47 TO FACILITY OPERATING LICENSE NO.
NPF-74 ARIZONA PUBLIC SERVICE COMPANY ET AL.
PALO VERDE NUCLEAR GENERATING STATION UNIT NOS.
1 2
AND 3 DOCKET NOS.
STN 50-528 STN 50-529 AND STN 50-530
- 1. 0 INTRODUCTION By letter dated November 13, 1990, the Arizona Public Service Company (APS or the licensee) submitted a request for changes to the Technical Specifications (TS) for the Palo Verde Nuclear Generating Station, Units 1, 2,
and 3
(Appendix A to Facility Operating License Nos.
NPF-41, NPF-51, and NPF-74, respective1y).
The Arizona Public Service Company submitted this request on behalf of itse1f, the Salt River Project Agricultural Improvement and Power District, Southern California Edison
- Company, El Paso Electric Company, Public Service Company of New Mexico, Los Angeles Department of Water and
- Power, and Southern California Public Power Authority.
The proposed changes involve increasing the pressurizer safety valve (PSV) setpoint tolerance from +/-1 percent to +3 percent and -1 percent, the main steam safety valve (HSSV) setpoint tolerance from +/-1 percent to
+/-3 percent, reducing the high pressurizer pressure trip setpoint (HPPT) response time from 1. 15 seconds to 0.5 second, and reducing the TS minimum auxiliary feedwater (AFW) pump flow requirement from 750 gallons per minute (GPM) to 650 GPH.
The TS PSV setpoint tolerance is proposed to be increased to +3 percent;
- however, the -1 percent tolerance is necessary to ensure that the reactor is tripped before the PSVs open.
The licensee also submitted.a letter dated April 29, 1991, which committed to reset the setpoint of PSVs and HSSVs found to have setpoints above or below the +/-1 percent range to within +/-1 percent of the nominal setpoint.
In response to concerns identified by the staff, the licensee also submitted letters dated Hay 27,
- 1992, and Hay 13,
- 1993, and November 12,
- 1993, which provided additional information in support of the proposed changes to the PSV and MSSV setpoint tolerances.
The proposed amendment affects TS Sections 3/4.3.1, 3/4.4.2, 3/4.7.1 and 3/4.7.1.2 for the PSV and HSSV tolerances, HPPT response
- time, and auxiliary feedwater flow.
During the review of this Technical Specification application, the staff became aware of a minor "offset" that exists between the two accepted testing methods for verifying valve setpoint.
The "trevitest" method (stem pulled mechanically) was generally lower than the live steam method of testing.
This 9405250244 94051b PDR ADOCK 05000528 PDRi
discrepancy is apparently due to an estimation of the valve's mean seat area in the equation used to calculate the appropriate setpoint.
The licensee identified this discrepancy and conservatively used the live steam method as a
basis for setting the valve setpoints.
Therefore, this issue of reconciling the test methods does not impact on the. setpoint tolerance evaluation that follows.
The staff is currently evaluating differences in the test methods and will address this issue separately.
- 2. 0 EVALUATION To support the proposed TS revisions for PSV and MSSV setpoint tolerances, HPPT response
- time, and AFW requirements, the licensee has performed an evaluation and analyses to determine the impact on the design basis transients and accidents for Palo Verde 1, 2,
and 3.
All of the transient and accident analyses documented in the UFSAR were evaluated by APSC to determine the impact of the proposed changes to the TS.
For the cases where the TS changes had an adverse impact on event consequences, a detailed evaluation or reanalysis of the event has been performed; APSC has provided detailed evaluations for the following events:
(1) loss of condenser vacuum (LOCV),
(2) main feedwater line break (MFLB), (3) steam generator tube rupture (SGTR),
(4) reactor coolant pump (RCP) shaft seizure, and (5) loss of coolant accident (LOCA).
The staff approved CESEC III code was used in performing the APSC evaluation.
The most conservative assumptions including the proposed changes to the TS were used to demonstrate that the acceptance criteria for each analyzed event (e.g.
peak system pressure, fuel performance, and offsite dose) were met.
For events where the maximum system pressure was a concern, such as LOCV and MFLB, the PSVs and MSSVs were assumed to have setpoints at the maximum tolerance value of +3 percent.
The PSVs have been assumed to fully open at these setpoints to immediately deliver full rated discharge flow, whereas in the current FSAR analyses, the valves were modelled to open only to 70 percent of the nomina]
area opening at the setpoint.
The licensee based this 'new assumption on test results which show that the valves attain full lift in 0.02 seconds after reaching the setpoint.
The licensee stated that the maximum additional accumulation in pressure for this.02 second delay in valve lifting would be about 2 psi more than the analysis result (still within the 110 percent of design pressure safety limit).
These analyses also assume the proposed HPPT response time of 0.5 seconds and the proposed AFW flow of 650 GPM.
The licensee stated that the reduced HPPT response time is supported by actual plant test results which show that the actual time is less than 0.3 seconds.
This change would result in an early reactor trip following an analyzed heatup.transient and would reduce the syst&tr peak pressure during the analyzed transient.
The licensee stated that the 650 GPM of AFW flow is supported by the plant an iyses which meet the necessary criteria for this minimum amount of flow.
In the analyses, the licensee also reduced the surge line form loss factor to 3.0 from '3.9 which is based on a calculation for the actual plant configuration.
The LOCV event is the limiting transient of moderate frequency occurrence for maximum overpressure.
The maximum allowable primary system pressure is 2750 psia (110.percent of dpsign pressure),
and the maximum reanalyzed primary pressure for this event is 2740.9 psia.
The maximum allowable secondary system pressure is 1375 psia (ll0 percent of design pressure),
and the maximum reanalyzed secondary pressure for this event is 1369.6 psia.
The licensee also provided the results of its reanalysis of the HFLB event which is the design basis event producing the maximum primary system pressure condition.
The reanalysis of the MFLB resulted in a peak RCS pressure of 2816 psia which is less than the peak pressure of 2843 psia documented in the UFSAR.
The results of the APS reanalyses of the
- SGTR, RCP shaft seizure, and LOCA which consider the proposed TS changes also demonstrate that the acceptance criteria for these events are met as provided in the Standard Review Plan.
In the review of the licensee's submittals, one of the staff concerns relates to the potential overall reduction of conservatism in order to meet the required limits for system overpressurization and other acceptance
- criteria, especially for the LOCV event.
The licensee has provided the following information regarding the conservatisms which exist in the analysis to demonstrate that the maximum allowable pressure will not be exceeded:
1.
Feedwater and steam flow actually ramp down to zero in about 18 seconds instead of the analyzed
- 0. 1 second.
2.
Safety valves are assumed to open at the +3 percent setpoint tolerance, whereas some are actually expected to open at lower pressures.
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3.
There is 30 psi of additional conservatism in the high pressurizer/pressure trip setpoint of 2540 psia.
Also, surveillance tests indicate that this trip response time is less than 0.3 second instead of the assumed 0.5 second.
4.
The analysis does not assume that the pressurizer spray valves open.
e 5.
The initial pressurizer level in the analysis is conservative compared to the level normally expected.
6.
Non-safety
- systems, such as the Reactor Power Cutback System and the Steam Bypass Control System, are assumed to not operate in the analysis.
7.
The moderator temperature coefficient is assumed most positive in the analysis.
8.
Other conservative conditions regarding the reactor physics parameters in the analysis are:
the least negative fuel temperature coefficient is
- assumed, bounding generic kinetic parameters are used, and the most limiting control rod is assumed to be stuck fu 1 'out.
The licensee stated that wii.h'the operating conditions experienced most of the time, the peak pressure for the LOCV would be only 2650.5 psia.
The licensee also states that the ASNE Code provides assurance of l'arge margin to failure and that the analysis performed to support the TS changes is adequate; The staff determined that if the analysis demonstrates the adequacy of the system overpressure protection with sufficiently conservative deterministic criteria
a'nd acceptably conservative input paramet'ers and analysis methodology, then no minimal margin beyond the acceptance criteria is required.
The staff has reviewed the licensee's analysis methodology and input assumptions and agrees that they are sufficiently conservative.
Thus the licensee has met the required limits for overpressurization and other acceptance
- criteria, and demonstrated that maximum allowable pressure would not be exceeded.
Regarding the specific analysis model of PSV performance, the staff agrees that the modelling of the PSVs to open fully at their setpoints (with the +3 percent tolerance) is acceptable based on PSV test data.
The previous method used in modelling the PSV performance involved opening the valve to only 70 percent open at the setpoint pressure.
The proposed method is more nearly a
best-estimate modelling technique (i.e. within 2 psi of actual expected performance as discussed above,)
and as such provides no additional conservatism beyond that required to meet. the acceptance criterion.
Although the previous method is more conservative than that being currently proposed, the staff agrees that the overall conservatism of the analysis assumptions taken together is adequate.
Another of the staff concerns relates to the proper actions the licensee should pursue to assure the best performance of the plant PSVs and NSSVs.
Plant operating history has shown that these valves frequently have not met the +/-I percent setpoint tolerance criterion, and some also have not met the
+/-3 percent criterion.
Since there is little additional margin in the overpressure analyses discussed
- above, additional setpoint drift beyond the
+/-3 percent tolerance on all valves could result in exceeding the allowable limits in the analyses.
The licensee has enhanced the maintenance activities for the PSVs and the NSSVs to both increase the number of valves tested and improve the testing methods over a sufficient period of time to determine the necessary actions to improve setpoint performance.
In addition, the licensee is investigating the root cause of the past setpoint performance.
The licensee plans to continue the enhanced maintenance and testing over a period of several testing cycles in order to verify improved valve performance.
The licensee's activities include better testing procedures and acceptance criteria and provide for better control of the testing parameters and the valve adjustments.
The staff agrees that the licensee's activities for identifying corrective actions for maintaining and testing the PSVs and HSSVs should improve the performance of these valves and minimize setpoint drift beyond the +/-3 percent tolerance.
Another of the staff concerns relates to the effect which the'ncreased PSV setpoint range has on the analyzed maximum pressurizer level. If the pressurizer should fill and subcooled liquid is discharged through the
- PSVs, the valves may not perform adequately.
The full-flowtests performed on full scale PSV models by the Electric Power Research Institute (EPRI) in 1981 demonstrated unreliable performance when discharging some subcooled liquid conditions.
The licensee performed an analysis to demonstrate that the pressurizer will not fill for the limiting transient (LOCV event) and that the PSVs will discharge only steam.
The analysis used initial conditions set to maximize the pressurizer liquid level and the PSV blowdown was modelled to be 20 percent, which is the maximum observed in the EPRI tests.
The staff agrees
that the licensee's analysis is sufficient to demonstrate that the pressurizer will not fill for the limiting event and that the PSVs will discharge only steam.
The staff also reviewed the licensee's assessment of increased radiological release as a result of the safety valve-set point tolerance change and the proposed reduction in auxiliary feedwater flow.
A steam generator mass balance was performed by the licensee that showed an increase in the duration of tube uncovery from about 14.8 minutes to 18.3 minutes.
The offsite radiological releases for a SGTR are strongly dependent on the partitioning of the primary to secondary leakage (the licensee's UFSAR assumes a maximum partition coefficient of less than 0. 1 with tubes covered; and 1.0, with tubes uncovered).
Since a significant portion of the offsite radiological releases occur during the period of tube uncovery, the licensee conservatively postulated that offsite doses will also increase in proportion to the increase in the duration of the tube uncovery.
For the most limiting SGTR scenario, this results in an increase in the 2-hour thyroid dose from 200 rem to less than 248 rem.
Adding the increase in dose due to the expanded PSV and HSSV setpoint tolerances (which the licensee calculated as a
5 percent dose increase),
results in a total 2-hour dose of 260 rem.
This value provides adequate margin to the 10 CFR Part 100 guideline of 300 rem.
The licensee also evaluated radiological release for the RCP shaft seizure
- event, and determined that the calculated
.5 rem increase was insignificant compared to the 246 rem dose reported in Supplement 2 to the staff's Safety Evaluation Report related to the Combustion Engineering Standard Safety Analysis Report (CESSAR) for System 80, and the 300 rem SRP acceptance criteria.
The staff has reviewed the licensee's assessment of increased dosage and finds the results, utilizing proper conservatisms, within 10 CFR guidelines, and therefore acceptable.
The licensee also changed the bases to Technical Specification 3/4.7. 1.2 to properly reflect the change to the surveillance requirement acceptance criteria of TS 4.7. 1.2.c, for minimum auxiliary feedwater flow.
The bases section and the associated TS were also clarified to properly reference steam generator pressure at the entrance of the steam generators.
These changes are considered to be of clarification in nature, and are considered acceptable.
Based on the above evaluation, the staff agrees that the analysis which the licensee has provided demonstrates the acceptability of the proposed TS changes.
The proposed increase in the setpoint tolerances of the PSVs to +3 percent and -1 percent and of the NSSVs to +/-3 percent has been shown to.be acceptable for meeting the plant design basis,,
and the licensee's actions to
-improve the maintenance and testing of these valves should minimize the occurrence of valve setpoints outside this range.-'he reduction of the HPPT response time to 0.5 seconds has been shown to be conservative based on actual plant test data.
The reduction of the required AFW flow to 650 GPM in the T/S as well as the other changes stated above are assumed in the reanalyses of the affected transients and accidents for Palo Verde Units 1, 2, and 3; and are therefore, acceptable.
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3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Arizona State official was notified of the proposed issuance of the amendment.
The State official had no'omments.
- 4. 0 ENVIRONMENTAL CONS IDERATION Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact have been prepared and published in the Federal Receister on April 12, 1994 (59 FR 17403).
Accordingly, based upon the environmental assessment, the staff has determined that the issuance of the amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed
- above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed
- manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
G.
Hammer C. Liang B. Holian Date:
May 16, 1994