ML20212F211
| ML20212F211 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 09/22/1999 |
| From: | Alexion T NRC (Affiliation Not Assigned) |
| To: | Cottle W HOUSTON LIGHTING & POWER CO. |
| References | |
| TAC-MA4925, NUDOCS 9909280077 | |
| Download: ML20212F211 (6) | |
Text
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Mr. Willi m T. Cottin i.
- Przsident tnd Chiaf Ex:cutiva Offic:r September 22, 1999
. STP Nuclur Op:r: ting Company l
South Texas Project Electric h,'
Generating Station l
- P. O. Box 289 l
Wadsworth, TX. 77483
SUBJECT:
SOUTH TEXAS PROJECT, UNIT 2 - REVIEW OF STEAM GENERATOR.
INSPECTION 90-DAY REPORT (TAC NO. MA4925) l
Dear Mr. Cottle:
By letter dated January 19,1999, STP Nuclear Operating Company (STPNOC) submitted its steam generator 90-day report, " South Texas Unit-2 Cycle 7 Voltage-Based Repair Criteria Report." The report was submitted in accordance'with Generic Letter 95-05 as a result of implementing the voltage-based alternate repair criteria in the South Texas Project Unit 2 technical specifications.
The amendment approving the use of voltage-based repair criteria for this unit included a reporting threshold of 1 x 10 for the calculated conditional probability of tube burst. STPNOC estimated a conditional burst probability below this threshold using an NRC staff-approved l
methodology.- The estimates of the primary-to-secondary leak rate during a postulated main i
steam line break (MSLB) for South Texas Project Unit 2 were below the allowable MSLB leak rate of 15.4 gallons per minute using an NRC staff-approved methodology. The staff concludes
- that STPNOC implemented the voltage-based repair criteria in accordance with its licensing basis. The staff's review of the report is enclosed.
Sincerely, ORIGINAL SIGNED BY Thomas W. Alexion, Project Manager, Section 1 Project Directorate IV & Decommissioning i
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Division of Licensing Project Management i
P PDR Office of Nuclear Reactor Regulation l
. Docket No. 50-499 o
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Enclosure:
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DATE 9 /N /99 9 / M /99 h 07/29/99 9 /1 k'/99
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Mr. Cornelius F. O'Keefe Mr. J. J. Sheppard, Vice President Senior Resident inspector Engineering & Technical Services U.S. Nuclear Regulatory Commission STP Nuclear Operating Company P. O. Box 910 P. O. Box 289 Bay City, TX 77414 Wadsworth, TX 77483 A. Ramirez/C. M. Canady S. M. Head, Supervisor, Licensing City of Austin Quality & Licensing Department Electric Utility Department STP Nuclear Operating Company 721 Barton Springs Road P. O. Box 289 Austin, TX 78704 Wadsworth, TX 77483 Mr. M. T. Hardt Office of the Governor Mr. W. C. Gunst ATTN: John Howard, Director City Public Service Board Environmental and Natural P. O. Box 1771 Resources Policy San Antonio, TX 78296 P. O. Box 12428 Austin, TX 78711 Mr. G. E. Vaughn/C. A. Johnson Central Power and Light Company Jon C. Wood P. O. Box 289 Matthews & Branscomb Mail Code: N5012 One Alamo Center Wadsworth, TX 74483 106 S. St. Mary's Street, Suite 700 San Antonio, TX 78205-3692 INPO Records Center Arthur C. Tate, Director 700 Galleria Parkway Division of Compliance & Inspection Atlanta, GA 30339-3064 Bureau of Radiation Control Texas Department of Health j
Regional Administrator, Region IV 1100 West 49th Street U.S. Nuclear Regulatory Commission Austin, TX 78756 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011 Jim Calloway Public Utility Commission of Texas D. G. Tees /R. L. Balcom Electric Industry Analysis Houston Lighting & Power Co.
P. O. Box 13326 P. O. Box 1700 Austin, TX 78711 3326 Houston, TX 77251 Judge, Matagorda County Matagorda County Courthouse 1700 Seventh Street Bay City, TX 77414 Jack R. Newman, Esq.
Morgan, Lewis & Bockius 1800 M Street, N.W.
Washington, DC 20036-5869 May 1999 s
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UNITED STATES NUCLEAR REGULATORY COMMISSION o
WASHINGTON, D.C. 20666 0001
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REVIEW BY THE OFFICE OF NUCLEAR REACTOR REGULATION OF THE STEAM GENERATOR 90-DAY Rt',GRT STP NUCLEAR OPERATING COMPANY SOUTH TEXAS PROJECT. UNIT 2 DOCKET NO. 50-492 1.0
[N_TRODUCTION N
By letter dated January 19,1999, STP Nuclear Operating Company submitted for staff review a report for South Texas Project Unit 2 (STP-2), " South Texas Unit-2 Cycle 7 Voltage-Based Repa r Criteria Report." The report was submitted in accordance with Generic Letter (GL) 95-05," Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking," as a result of implementing a voltage based alternate repair criteria in the STP-2 technical specifications (TSs).
The TSs allow steam generator (SG) tubes having outside diameter stress corrosion cracking (ODSCC) that is predominately axially oriented and confined within the tube support plates to remain in service on the basis of, in part, bobbin coil voltage response. GL 95-05 guidelines for implementing the voltage-based repair criteria state that inspection results and associated tube integrity analyses shou!d be submitted within 90 days of each plant restart following an SG tube inspection. The report should include, at a minimum, the projected end-of-cycle (EOC) calculations on voltage distribution and the associated tube leakage and tube burst probability under main steam!ine break (MSLB) conditions.
2.0 GENERAL PLANT DESCRIPTION STP-2 has been in operation since 1989. STP-2 has four Westinghouse Model E4 SGs. The tubes are %-inch diameter and were fabricated from mill annealed alloy 600. The SGs have drilled hole stainless steel tube support plates (TSPs).
On September 24,1998, the staff approved a permanent change to the licensee's TSs that implemented an SG voltage-based alternate repair criteria in accordance with GL 95-05. The voltage-based tube repair criteria (1) permit tubes having indications confined to within the thickness of the TSPs with bobbin voltages less than or equal to 1 volt to remain in service; (2) permit tubes having indications confined to within the thickness of the TSPs with bobbin voltages greater than 1 volt but less than or equal to the upper voltage repair limit to remain in service if a motorized rotating pancake coil (RPC) probe or an acceptable inspection alternative does not detect degradation; and (3) require tubes having indications confined within the l
thickness of the TSPs with bobbin voltages greater than the upper voltage limit be plugged or repaired. For the first outage implementing the voltage-based repair limit, the licensee calculated the upper voltage limit for the TSPs and the flow distribution baffle plate (the first f
tube support plate) to be 3.30 volts and 2.71 volts, respectively.
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' 3.0 STAFF ASSESSMENT 3.1 Insoection Scoos and Results i The EOC-6 inspection of the STP'-2 SGs consisted of a complete,100 percent addy current (EC) bobbin probe, full-length examination of the tube bundles in all Unit 2 SGs in accordance with the TSs. A 0.610-inch diameter probe was used for all hot and cold leg TSPs where the
- voltage-based repair criteria were applied. An RPC examination was performed on all TSP
- bobbin indications greater than 1 volt. Thirty nine indications on the hot-leg side and one indication on the cold-leg side had a bobbin voltage above 1 volt. All the hot-leg indications
~ over 1 volt were RPC examined.- Thirty-four were confirmed as flaws and repaired. The one cold-leg indication was initially classified as wear and later reclassified as a potential crack-like indication with a voltage of 1.23 volts. The licensee was unable to examine the one cold-leg indication with an RPC and treated the indication as if it were an RPC confirmed indication and subsequently repaired the tube.
The licensee detected a total of 1485 ODSCC indications at TSP intersections which were compared to the voltage-based repair criteria during the EOC-6 inspection. The 34 RPC-confirmed indications over 1 voit (bobbin) were repaired. Ten more TSP indications were removed from service due to non-ODSCC causes.
Based on growth rate and size of indications, SG A is considered to be the limiting SG for Cycle 7 operation.- As discussed in Section 3.2.3 of this review, this conclusion is supported by the calculated estimate of accident tube leakage.
3.2 Corrective Actions for Future insoections The bobbin signal at the first pre heater baffle plate intersection on the cold-leg side was initially called ast wear indication with a voltage of 2.05 volts. A later reexamination of this bobbin signal indicated that it may be a potential crack-like signal and its voltage was revised to 1.23 volts. As a potential ODSCC indication, the indication would be required by the TSs to be RPC inspected. By the time the reassessment was completed, equipment needed for an RPC
' examination had been removed from the SG. The licensee plugged the tube assuming that an RPC examination would have confirmed the indication as crack like.
.The licensee ha's ' developed a corrective action plan to ensure the proper classification of cold-leg indications in future inspections. The licensee will inspect all bobbin indications in the pre-heater baffle plate intersections with an RPC probe. The licensee will use both the bobbin
~and RPC data to classify the indications as ODSCC or wear. The staff finds the licensee's corrective action plan acceptable.
. 3.3 Evaluation of Probabilistic Methodolooies for Estimatina Conditional Probability of Burst and Total Leak Rate Under Postulated Steam Line Break Conditions Acceptable tube integrity at the conclusion of Cycle 6 operation is demonstrated, in part, by a calculated conditional probability of tube burst under an MSLB for the limiting SG which is less than the TS reporting threshold and a calculated accident-induced SG tube leak rate below the
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TS reporting threshold. Three distinct probabilistic calculations are necessary to determine t-
~ these results. The following summarizes the staff's evaluation of the results reported on these calculations, j
3.3.1 Projected EOC Voltage Distribution
- The licensee's 90-day report compared the as-found distribution of voltages determined from l
- the EOC-6 refueling outage to those estimated from the predictions made following the prior
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outage inspection (EOC-5). The EOC-6 projections are based on a constant probability of detection (POD) of 0.6 and the assumption that a 1.0-volt repair criterion was applied during the EOC-5 inspection. At the time of the EOC-5 inspection, the licensee had not yet implemented voltage-based repair criteria. The actual inspection utilized a " plug-on detection" criterion i
~(indications under 1 volt and confirmed by RPC were repaired). The actual number of as-found l
indications at EOC-6 in each SG were significantly below the projections for EOC-6. As the i
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licensee's projections indicated, SG-B was found to have the largest number of indications and l
SG A was found to have the largest number of indications over 1 volt. The actual measured l
voltages for SG A include three indications above 2.5 volts that were not projected. The largest projected bobbin voltage was 2.5 volts. ~ In general the indication population above 0.5 volt is l
substantially overestimated in the projections based on a constant POD of 0.6.
in order to obtain the most conservative results with respect to the growth rate distribution used in Monte Carlo simulation, the licensee utilized SG specific growth rate distributions obtained l
from operation in Cycles 5 and 6. ~ GL 95-05 recommends that the more conservative growth i
l distribution from the last two cycles be used for projecting EOC distributions for the next operating cycle. Cycle 6 data includes three growth values over 1 volt and one value over 2.1 volts while the Cycle 5 data are all equal to or less than 1 volt. Therefore, Cycle 6 data would l
yield more conservative results for steamline break (SLB) leak rate and tube burst probability l
projections for the EOC 7 condition.
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i The Cycle 6 growth rates for SGs A and D are higher than the composite growth distribution f
and, per the recommended methodology, SG-specific growth rates are to be used for SGs A l
and D, while the composite growth method should be applied for SGs B and C. Since the i
growth data for SG-D does not include any of the top four growth values observed for Cycle 6,
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the licensee conservatively added the top three growth values to the SG-specific growth distribution applied to the EOC-7 projections for SG-D.
Some plants have experienced relatively high growth rates for indications in tubes deplugged l
and returned to service at the beginning of their last operating cycle. The licensee for STP 2 i
stated that no tubes were recovered during this first outage implementing the voltage based alternate repair criteria. Some plants also noted an increase in growth rate with an increase in
. beginning-of-cycle (BOC) voltage. Since this is the first cycle to implement the voltage-based alternate repair criteria, this observation will not be able to be confirmed until a future cycle.
Using the inspection findings in the EOC-6 inspection and the appropriate probability density
~ function of growth rates, the licensee calculated the projected EOC-7 voltage distribution for bobbin coil probe TSP indications. Based on having the. larger indications, SG A is predicted to
' be the limiting SG for the next cycle of operation. The staff independently verified the licensee's calculations by completing a Monte Carlo simulation to estimate the EOC-7 voltage distributions. The results of these calculations confirm that the predictive methodology used by L
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4 the licensee to estimate the EOC voltage distribution is consistent with the guidance provided in GL 95-05.
3.3.2 Conditional Probability of Tube Burst During an MSLB Following the EOC-5 refueling outage, the licensee estimated the EOC-6 burst probabilities for the four SGs. The burst probabilities were projected to be 1.7 x 104,9.0 x 10'5,6.0 x 10'5, and 2.4 x 104, for SGs A, B, C, and D, respectively. Using the actual inspection results as the input
- into the calculatione for tube burst, the licensee determined the as-found conditional burst 4
probabilities to be 3.8 x 10,2.5 x 10 5,5.8 x 10 5, and 1.2 x 10-5. The TSs specify a reporting threshold of 1 x 10'8 The licensee's calculated and as found burst probabilities for all four SGs are well below the reporting threshold.
Following the guidance in GL 95-05, the licensee estimated the EOC 7 conditional tube burst probabilities. The calculated probability of tube burst for SGs A, B, C, and D were determined d
4 to be 4.2 x 10,7.810~5,8.8 x 10'5, and 1.2 x 10, respectively. The licensee's projected j
values are well below the TS reporting threshold of 10-2, and therefore, the estimated tube burst probability due to ODSCC at TSPs is within acceptable limits for Cycle 7 operation. The staff ran confirmatory calculations which were in agreement with the licensee's results.
3.3.3 SLB Leak Rate Projection
- The staff evaluated the SLB leak rate reported by the licensee in a manner similar to the staff's evaluation of the reported conditional tube burst probabilities. For SGs A, B, C, and D the EOC-6 projected MSLB leak rates were previously estimated to be 1.4 x 10'8,5.5 x 10'8,4.2 x 10, and 1.3 x 10' gpm (equivalent volumetric rate at room temperature). The estimate based on actual inspection results obtained during the EOC 6 outage are 3.2 x 10'8,1.8 x 10,2.0 x 4
10', and 4.6 x 104 gpm (equivalent volumetric rate at room temperature). Although the value projected for SG A was slightly smaller than the estimates based on the EOC 6 inspection results, all values are much smaller than the STP 2 SLB leak rate reporting threshold of 15.4
.gpm. The licensee projects the EOC-7 MSLB leak rates for SGs A, B, C, and D to be 3.3 x 10',2.7 x 10~,5.2 x 10'8, and 8.2 x 10'8 gpm, respectively. The staff ran confirmatory
. calculations and is in agreement with the licensee's values. The licensee's values are much smaller than the STP-2 reporting threshold limit of 15.4 gpm (equivalent volumetric rate at room temperature) and are acceptable.
3.4
. Database for Tube Intearity Calculations i
in order to calculate the conditional tube burst probabilities and postulated SLB primary-to-secondary leak rate, the methodology accepted in GL 95-05 requires the use of burst and leak rate data obtained from model boiler tubes and tubes removed from actual SGs. The industry 4
has developed correlations relating bobbin coil voltage to the measured leak rate, probability of leakage, and burst pressure through testing of these tubes. The leak and burst correlations utilized in the licensee's analyses were based on the Addendum 2 to the SG Degradation j
Specific Management (SGDSM) database, " Steam Generator Tubing Outside Diameter Stress Corrosion Cracking at Tube Support Plates Database for Alternate Repair Limits, submitted to the NRC on June 5,1998, by the Nuclear Energy Institute (NEI). The NRC staff completed its review of the database and associated correlations for %-inch diameter tubes and concluded
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j T-i that they were acceptable in a letter to NEl dated November 20,1998. Therefore, the staff
. finds the alternate repair criteria correlations utilized by the licensee are acceptable.
3.5 '
Tube Pull Results l
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. GL 95 05 calls for periodic tube specimen removals to monitor the morphology of ODSCCL l
degradation at TSP intersections and to obtain additional data for inclusion in the correlations
. relating bobbin coil voltage amplitude to tube burst pressure, probability of leakage, arid leak rate. The licensee removed two tubes from SG-A (R18C100 and R19C83). These are the first
' tubes removed from STP-2. The removed tube sections were examined by nondestructive and destructive examination techniques. Axially oriented ODSCC and associated intergranular -
. attack (IGA) were found contained within the TSP crevice regions of the mill annealed Alloy 600 tubing. The flow distribution baffle plate (TSP-1) regions did not have corrosion. The staff has reviewed the destructive examination results of the pulled tube specimens and concluded that the ODSCC morphology exhibited in the tubes is consistent with the criteria specified in GL 95-05 and the TSs.
4 3.6.
Probe Wear Criteria The licensee used an alternative method to evaluate probe wear as opposed to the method outlined in GL 95-05 for evaluating probe wear. The method was developed by NEl and was found acceptable by the NRC staff as discussed in a letter from Brian Sheron of the NRC to Alex Marion of the NEl dated February 9,1996. STP 2 was given approval to use this alternative method in the staff's safety evaluation for the voltage-based alternate repair criteria amendment dated September 24,1998.
The bobbin probe wear guidance utilized by the licensee states that when a probe does not pass the 15 percent wear limit, the licensee is directed to reinspect the full length of all tubes with indications above 75 percent of the voltage limit inspected since the last successful probe wear check. Since the repair limit is 1 volt, all tubes measured with a worn probe containing indications above 0.75 volt were reinspected with a new probe The licensee compared the voltages measured with worn probes to the voltages measured with new acceptable probes to ensure that the voltages measured with the worn probes are comparable. No new indications were detected with the new probes. STP-2 had no occurrences for which a worn probe was less than 0.75 volt and the new probe voltage exceeded the plugging limit. The licensee's
- approach addressing probe wear is acceptable.
- 4.0
SUMMARY
The projected EOC-7 conditional probability of burst and projected leak rate under postulated MSLB conditions were less than the TS reporting thresholds. The staff has reviewed the licensee's methodology, has run confirmatory calculations, and has found that the licensee's
- methodology at this time yields acceptable results.
Principal Contributor: A. Keim-Date: September.22, 1999 l
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