ML15264A500
| ML15264A500 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 10/02/1987 |
| From: | Jabbour K Office of Nuclear Reactor Regulation |
| To: | Tucker H DUKE POWER CO. |
| References | |
| NUDOCS 8710080063 | |
| Download: ML15264A500 (10) | |
Text
Docket Nos.: 50-269, 50-270 D
and 50-287 1987 SVarga N~RPUR~
GLainas Local PDR Mr. H. B. Tucker, Vice President PD#23 Reading Nuclear Production Department MDuncan Duke Power Company HPastis 422 South Church Street OGC-Bethesda Charlotte, North Carolina 28242 ACRS (10)
JPartlow
Dear Mr. Tucker:
EJordan
Subject:
Request for Technical Specification Change on Primary to Secondary Leakage Rate Limits
Reference:
Oconee Nuclear Station, Units 1, 2 and 3 Oconee Units 2 and 3 (and Arkansas Nuclear One, Unit 1) unlike all other pressurized water reactors (PWRs) do not have specific limits on allowable primary to secondary leakage in the Technical Specifications (TSs). For Oconee Unit 1, the TS limit is 0.3 gallons per minute (gpm) for the primary to secondary leakage limit.
We request that within sixty days of receipt of this letter, you propose revisions to the TSs as noted in the enclosures to this letter. Enclosure 1 details TS language; Enclosure 2 details the "Bases" language; Enclosure 3 discusses our safety evaluation describing the supporting rationale for the proposed change; and Enclosures 4 and 5 state the value impact and backfit considerations.
We anticipate that you may consider requesting a one gpm limit for all three units. As discussed herein, the more restrictive limit on Unit 1 reflects the unique state of tube degradation at this plant. If you do propose such a TS revision to relax the present criterion for Unit 1, we would like to remind you to address the no significant hazards consider ations and give the basis and justification for raising the Oconee Unit 1 limit from 0.3 to 1.0 gpm. You should be aware that as a general princi ple the staff continues to recommend the adoption of the Standard Technical Specification limit of 500 gallons per day (i.e. 0.35 gpm).
Sincerely, 8710080063 871002 PDR ADOCK 05000269 Kahtan N. Jabbour, Acting Director P
PDR Project Directorate 11-3 Division of Reactor Projects-I/II
Enclosures:
As stated cc: See next page SEE PREVIOUS CONCURRENCE
- SEE PREVIOUS CONCURRENCE PD#HI-3/DRP-I/II DW3/DRP-I/II RSB PD#II-3/DRP-I/II
- HPastis/mac nc n
- WHodges KJabbour, Acting PD 09/24/87 09/,9
/87 09/25/87 0/ / /87 PWR#6
- GEdison 03/17/87
Mr. H. B. Tucker Oconee Nuclear Station Duke Power Company Units Nos. 1, 2 anC 3 Cc:
Mr. A. V. Carr, Esq.
Mr. Paul Guill Duke Power Company Duke Power Company P. 0. Box 33189 422 South Church Street Post Office Box 33189 a422 South Church Street Charlotte, North Carolina 28242 J. Michael McGarry, III, Esq.
Bishop, Liberman, Cook, Purcell & Reynolds 1200 Seventeenth Street, N.W.
Washington, D.C. 20036 Mr. Robert B. Borsum Babcock & Wilcox Nuclear Power Generation Division Suite 220, 7910 Woodmont Avenue Bethesda, Maryland 20814 Manager, LIS NUS Corporation 2536 Countryside Boulevard Clearwater, Florida 33515 Senior Resident Inspector U.S. Nuclear Regulatory Commission Route 2, Box 610 Seneca, South Carolina 29678 Regional Administrator, Region II U.S. Nuclear Regulatory Commission 101 Marietta Street, N.W., Suite 2900 Atlanta, Georgia 30323 Mr. Heyward G. Shealy, Chief Bureau of Radiological Health South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, South Carolina 29201 Office of Intergovernmental Relations 116 West Jones Street Raleigh, North Carolina 27603 Honorable James M. Phinney County Supervisor of Oconee County Wa4halla, South Carolina 29621
ENCLOSURE 1 PROPOSED TECHNICAL SPECIFICATION REVISIONS
- 1. Specification 3.1.6.4 is renumbered to be Specification 3.1.6.4.a.
- 2. Add new Specification Number 3.1..6.4.b as follows:
If at any time, the leakage through the Unit 2 or Unit 3 steam generator tubes equals or exceeds 1.0 gpm, a reactor shutdown shall be initiated within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and the reactor shall be in a cold condition within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. If the leakage is less than 1.0 gpm, an assessment shall be made as to whether operations may be continued safely or the plant should be shut down.
In either case, the NRC shall be notified in accordance with Section 6.6.2.1.
ENCLOSURE 2 PROPOSED REVISIONS TO "BASES" LANGUAGE Add the following new paragraph to the end of the "Bases" section which begins on page 3.1 - 14a of the Technical Specifications.
"The steam generator tube leakage limits (i.e., primary to secondary leakage limits) in Specifications 3.1.6.4.a and 3.1.6.4.b are intended to ensure timely shutdown of the plant for appropriate corrective action before rupture of the steam generator tubes occurs under normal operating or postulated accident conditions. These limits also serve to provide added assurance that the dosage contribution from tube leakage will be limited to a small fraction of 10 CFR Part 100 limits for a design basis steam generator tube rupture or main steam line break event."
c k UNITED STATES c
NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO SECONDARY LFAKAGE LIMITS PUKE POWER COMPANY OCONEE NUCLEPR STATION, UNITS NOS. ? AND 3 0OCKETS NOS. 50-270 AND 50-287
1.0 INTRODUCTION
AND
SUMMARY
Staff review of licensee responses to NRC Generic Letter 85-02 concerning steam generators indicated that Oconee Units 2 and 3 and Arkansas Nuclear One, Unit 1 (ANO-1), unlike all other domestic PWRs, do not have specific limits on allowable primary to secondary leakage in the plant Technical Specifications or license. Although the subject plants do have operating procedures which limit primary to secondary leakage to.2 gpm and.1 gpm for Oconee Units 2 and 3 and ANO-1, respectively, these limits are not included in the Technical Specifications for these plants.
As discussed herein, tight limits on primary-to-secondary leakage are essential to ensuring steam generator tube integrity and thus to ensuring public health and safety. Because of the importance of such limits, they should be incorporated as part of the Technical Specifications. Accordingly, the SER herein has been prepared to support regulatory action, as appropriate, to ensure that an acceptable limit is incorporated into the plant Technical Specifications for Oconee Units 2 and 3. Appropriate limits for ANO-1 are being addressed as a separate action by the staff.
2.0 DISCUSSION As evidenced by operating experience (i.e., numerous leaks and occasional ruptures), periodic inservice inspections of steam generator tubes have not been fully effective in identifying all steam generator tubing flaws which violate the minimum wall thickness requirements inherent in the Technical Specification plugging limits.
For this reason, tight limits on allowable primary to secondary leakage play an important role in ensuring that the plant is shutdown for appropriate repairs before gross rupture of a tube occurs.
Steam generator tube integrity was designated as an Unresolved Safety Issue (USI) in 1978 after it became apparent that steam generator tubes were subject to widespread degradation, frequent leaks, and occasional ruptures. For the interim period pending resolution of these USIs, the staff concluded that there was adequate basis to assure continued safe operation of PWRs. The staff's basis for continued operation was formally documented in NUREG-510 and NUREG-0571 and included primary to secondary
.leakage limits as an important element. This was expressed in NUREG-0571 as follows:
-2 "Primary-to-secondary leakage rate limits, and associated surveil lance requirements have been established to provide assurance that the occurrence of tube cracking during operation will be detected and that appropriate corrective action will be taken expeditiously.
Early experience with leaks in B&W steam generators indicates that if a crack occurs, it will propagate around the circumference of the tube in a short period of time (several hours) provided that there is flow of sufficient energy to drive it. However, the crack formed will produce an identifiable leak, the detection of which will result in prompt corrective action (i.e., plant shutdown)."
In citing this basis, it was implicitly assumed by the staff that all PWRs had Technical Specification limits on allowable primary to secondary leakage rate limits of 1.0 gpm or less.
The basis for this assumption was based in part on a survey conducted by the NRC staff by letter dated December 9, 1977.
Duke reported in response to the survey (Duke Power Company letter dated February 6, 1978) that it had a primary to secondary limit of 1.0 gpm. Although this limit was not identified specifically as a Technical Specification limit, the staff inferred this to be the case.
Duke Power Company updated its response by letters dated May 14, 1979, and May 8, 1981, and continued to report that it had a 1.0 gpm limit.
It is interesting to note that the staff had requested that Duke Power Company incorporate a primary to secondary leakage limit..into the Technical Specification in a letter dated March 17, 1977.
In response to this request, Duke Power Company submitted a proposed change to incorporate a 1.0 gpm limit into the Technical Specifications for Oconee Units 1, 2, and 3 by letter dated June, 1977. On October 4, 1977, the staff imposed a 0.3 gpm Technical Specification limit for Oconee Unit 1. This was in response to severe tube degradation in the Unit 1 steam generators and was consistent with limits imposed by the staff for other PWRs with severely degraded steam generators. However, the staff took no action to address Duke's proposed 1.0 gpm limit for Oconee Units 2 and 3.
Failure to incorporate a specific limit on primary to secondary leakage at Units 2 and 3 appears to have been an inadvertent oversight on the part of the staff.
As a result of the staff's USI effort (NUREG-0844), the staff issued a number of staff-recommended actions concerning steam generators in NRC Generic Letter 85-02. These recommendations included a recommendation that PWRs with less restrictive limits than the Standard Technical Specification (STS) limits on allowable primary to secondary leakage adopt the STS limits; namely, a 1.0 gpm limit on total primary to secondary leakage and a 500 gpd (i.e., a.35 gpm) limit for leakage from any one steam generator. The 1.0 gpm limit is intended to ensure that the dosage contribution from tube leakage will be limited to a small function of 10 CFR Part 100 limits in the event of an SGTR or MSLB event.
The 500 gpd limit was derived from test data for Westinghouse steam generator tubes and corresponds to the expected leakage for the maximum sized crack length which would not be expected to fail under worst case pressure differential loadings associated with a design basis MSLB or FLB
-3
(
Reference:
Westinghouse Report No. WTD-SM-77-058, dated July 1977, which was submitted by Carolina Power and Light Company in a letter to the NRC dated July 17, 1977).
Although the test data was developed for Westinghouse SG Model 51 tubes, the staff believes that this data can reasonably be applied to B&W tube geometries as a first order approximation since the radius/thickness ratios and the burst strength of the B&W tubes are within 5% of those for the Westinghouse tubes.
To the staff's know ledge, the Westinghouse data is unique in that it is based upon test specimens containing actual cracks induced by fatigue rather than simulated cracks produced with an electro discharge machining (EDM) procedure which would tend to produce less conservative results.
Approximately two-thirds of operating PWRs (including Oconee 1) have Technical Specification leak rate limits which are at least equally restrictive as the STS limits and which are therefore consistent with the staff's recommendation in Generic Letter 85-02. As a minimum, all plants with the exception of Oconee Units 2 and 3 and ANO-1 have Technical Specifications or license conditions which limit total primary to secondary leakage to 1.0 gpm. In addition to assuring that site boundary dose will be within the Part 100 guidelines during design basis accidents, the 1 gpm limit has proven effective in minimizing the potential for tube ruptures under normal operating conditions as evidenced by the fact that only four of several hundred primary to secondary leaks to date have involved a gross tube rupture. The 1 gpm limit on total primary to secondary leakage also serves to minimize periods during which tubes may be vulnerable to rupture, given a postulated accident such as a main steamline or feedline break, although the 500 gpd (
.35 pgm) limit per steam generator in the STS provides an even more effective limit in this regard. (As previously noted, the staff has recommended (but not required) that plants with less restrictive limits than the STS limits adopt the more effective STS limits.)
In the absence of specific limits regarding allowable primary to secondary leakage in the Oconee Units 2 and 3 Technical Specifications, primary to secondary leakage is limited in the Technical Specifications for these units only by the 10 gpm limit on total RCS leakage. Continued plant operation beyond 1.0 gpm to 10 gpm would represent an order of magnitude departure from industry practice to date. It would permit further crack extension beyond what the staff has long considered to be prudent from the standpoint of maintaining tube integrity.
No supporting evidence has been provided to the staff to demonstrate that the tubes have been designed to accommodate flaws leaking at the rate of 10 gpin while maintaining (1) an extremely low probability of abnormal leakage in accordance with GDC-14, and (2) sufficient margin to assure that when stressed under postulated accident conditions, the probability of rapidly propagating fracture is minimized in accordance with GDC-31.
In addition to tube integrity considerations, primary to secondary leakage rates exceeding 1.0 gpm would be in excess of what was assumed in the Oconee FSAR safety analyses, which were reviewed by the staff, for purposes of demonstrating that offsite boundary doses during design basis accidents do not exceed the 10 CFR Part 100 guideline limits.
-4 The licensee for Oconee Units 2 and 3 has stated in its response to NRC Generic Letter 85-02 (Duke letter dated July 17, 1985) that "normally, when the measured leak rate reaches.3 to.5 gpm, an orderly shutdown is initiated in order to identify which tubes(s) are leaking and effect repairs where necessary."
Actually, based on discussions with the NRC resident inspector, it is the staff's understanding that the plant operating procedures for Oconee Units 2 and 3 require that when primary to secondary leakage exceeds 0.2 gpm, the plant should reduce power in order to reduce leakage pending identification of which steam generator is leaking.
Once the leaking steam generator has been identified, the plant should commence shutdown. However, because of the importance of primary to secondary leak rate limits in ensuring steam generator tube integrity, the staff believes that appropriate limits should also be incorporated as part of the plant Technical Specifications for these units.
In phone conversations with the staff, Duke Power Company representatives have indicated a willingness to propose a change to the Technical Specifications to limit primary to secondary leakage to 1.0 gpm consistent with the limit in effect for a number of other B&W plants.
However, Duke would propose such a change only if such a limit would also be applicable to Oconee Unit 1. As previously noted, the Technical Specification limit for Oconee 1 is currently.3 gpm which is approximately equivalent to the STS and to the staff's recommendation in Generic Letter 85-02
3.0 CONCLUSION
S The staff continues to encourage all PWRs to implement the STS limits on primary to secondary leakage.
As a minimum, however, the staff concludes that a 1.0 gpm limit on total primary to secondary leakage should be incorporated into the plant Technical Specifications to continue to assure that these units will continue to be in compliance with GDC 14 and 31.
This change will bring the limits for Oconee Units 2 and 3 to within the envelope of limits which (1) have been established for other PWRs, (2) were assumed to exist for all PWRs when establishing the bases for continued safe operation pending USI resolution, and (3) were assumed in the FSAR safety analyses for design basis accidents.
Principal Contributor:
Emmett L. Murphy
ENCLOSURE 4 VALUE IMPACT ANALYSIS The proposed Technical Specification limit on primary to secondary leakage is a regulatory issue rather than a risk reduction issue. The staff believes that at least a 1.0 gpm limit (preferably a 0.35 gpm limit) will ensure that these units will continue to be in compliance with GDC 14 and 31.
This change will bring the limits for Oconee Units 2 and 3 to within the envelope of limits which (1) have been established for other PWRs, (2) were assumed to exist for all PWRs when establishing the bases for continued safe operation pending resolution of USIs A-3, A-4, and A-5, and (3) were assumed in the FSAR safety analyses for design basis accidents.
Potential risk reduction benefits associated with the staff's proposal are believed to be small, due primarily to the fact that it is already the licensee's policy to commence plant shutdown when primary to secondary leakage rate reaches 0.3 to 0.5 gpm (Duke Power Company letter dated July 17, 1985).
The staff's proposal would not increase the number of unplanned plant shutdowns since the licensee is already commencing plant shutdowns consistent with the staff's proposal and the plant operating procedures.
Therefore, plant shutdowns in accordance with the staff proposed limit would not result in increased cost.
As is typically the case for PWRs, the Technical Specifications for Oconee Units 1, 2, and 3 specify that a sample inspection of the tubes in the affected steam generator shall be performed if the specified leak rate limits are exceeded prior to shutdown. The staff's proposal for a reduced leak rate limit would therefore also reduce the leak rate threshold at which an unplanned steam generator inspection would have to be performed.
Information available to the staff indicates that the licensee will normally perform a tube inspection, regardless of whether the leak rate limit has been exceeded.
The staff notes that tube inspections were performed during shutdowns to repair leaking tubes in September 1981 at Oconee Unit 2 and in June 1980, October 1982, November 1982, December 1982, and November 1984 at Oconee Unit 3.
(The staff does not have information concerning any other steam generator related shutdowns of these units which may have taken place since 1980.)
Thus, the staff does not anticipate any net cost or Occupational Radiation Exposure impacts associated with steam generator tube inspections performed in accordance with the proposed change to the Technical Specifications.
ENCLOSURE 5 BACKFIT DETERMINATION The proposed limit on primary to secondary leakage rate is not considered a backfit as defined in 10 CFR 50.109. The proposed limit is needed to ensure that actual leakage does not exceed leakage rates assumed in the FSAR safety analyses. The FSAR assumes a 1.0 gpm leakage rate for purposes of demonstrating that the consequences of design basis accidents do not exceed 10 CFR Part 100 guidelines.
In addition, the proposed limit ensures that the Technical Specifications for Oconee Units 2 and 3 are consistent with the assumptions made by the staff when it concluded that PWRs could be operated safely for the interim period pending resolution of Unresolved Safety Issues A-3, A-4, and A-5 regarding steam generator tube integrity. The validity of these assumptions with respect to Oconee Units 2 and 3 were thought to be supported by Duke Power Company in numerous letters to the NRC staff where it was stated that each of these units has a 1.0 gpm limit on primary to secondary leakage.
The background and justification for the proposed limits is discussed in the staff's Safety Evaluation Report on this matter (i.e., Enclosure 3 of this package).