ML15238A316
| ML15238A316 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 11/07/1996 |
| From: | Labarge D NRC (Affiliation Not Assigned) |
| To: | Hampton J DUKE POWER CO. |
| References | |
| TAC-M93942, NUDOCS 9611130341 | |
| Download: ML15238A316 (9) | |
Text
STATS 6
-2 92 7 0('It UNITED STATES 297 NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 o"
November 7, 1996 Mr. J. W. Hampton Vice President, Oconee Site Duke Power Company P. 0. Box 1439 Seneca, SC 29679
SUBJECT:
REACTOR BUILDING POST-TENSIONING SYSTEM SIXTH SURVEILLANCE OCONEE NUCLEAR STATION, UNIT 3 (TAC NO. M93942)
Dear Mr. Hampton:
By letter dated October 11, 1995, Duke Power Company (DPC) submitted the Oconee Nuclear Station, Unit 3, Reactor Building Tendon Surveillance Report.
It was submitted to fulfill the requirements of Technical Specification 4.4.2.2 regarding tendon inservice inspections.
Supplemental information was supplied by letters dated March 14, July 30, and October 3, 1996.
The staff has reviewed the surveillance report and our safety assessment is enclosed. During our review, the staff expressed the concern that the tendon surveillance of each of the three units at Oconee Nuclear Station has been repeatedly performed on the same nine preselected tendons, three for each group of tendons (dome, axial, and hoop) in each unit. These were used as a sample study of the population in each group. As a result, DPC performed a re-analysis, and determined that the projected dome tendon force of Unit 3 would be below the minimum required value (MRV) at 8 years after the structural integrity test (SIT) of the containment structure. As requested by the NRC staff, DPC also performed a re-analysis for Units 1 and 2, and found the vertical tendon forces in Unit 1 will be below the MRV at year 30 after the SIT and that Unit 2 vertical tendon forces will be below the MRV about 10 years after the SIT.
DPC has indicated that the existing tendon forces in terms of the respective MRVs are 96 percent for the Unit 3 dome tendons, and 99 percent for the Unit 2 vertical tendons. DPC judged that a re-analysis using state-of-the-art methods would demonstrate the ability of the containment structure to satisfy all design licensing basis requirements with prestressing forces (i.e., the new MRVs) lower than original design MRVs, throughout the life of the plant, citing the Turkey Point Units 3 and 4 resolution of low tendon forces as a precedent. Thus, DPC provided assurance that the integrity of each of the containments of the three units is adequate to support each unit's continued operation.
DPC has committed to the following corrective actions as a long-term resolution of our concerns:
- 1. Identify and evaluate potential causes of pre-stress loss larger than predicted -
to be completed by March 1, 1997.
9611130341 961107 PDR ADOCK 05000269 PD ro WYRC
Mr. J. W. Hampton
-2
- 2. Perform a refined structural analysis of the reactor building to check the MRVs - to be completed by January 1, 1997.
- 3. Change the technical specifications to use randomly selected tendons instead of repeated use of the same tendons for tendon surveillances
- an amendment has been submitted and is being processed.
On the basis of the results of our evaluation of the submitted information and commitments made by DPC, the staff is satisfied that the integrity of the Oconee Units 1, 2, and 3 containments is adequate to support continued operation and that public health and safety will not be impacted as a result.
This action closes TAC No. M93942.
Sincerely, David E. LaBarge, Senior Project Manager Project Directorate 11-2 Division of Reactor Projects -
I/Il Office of Nuclear Reactor Regulation Docket Nos. 50-269, 50-270, and 50-287 cc:
See next page
Mr. J.
November 7, 1996
- 3. Change the technical specifications to use randomly selected tendons instead of repeated use of the same tendons for tendon surveillances
- an amendment has been submitted and is being processed.
On the basis of the results of our evaluation of the submitted information and commitments made by DPC, the staff is satisfied that the integrity of the Oconee Units 1, 2, and 3 containments is adequate to support continued operation and that public health and safety will not be impacted as a result.
This action closes TAC No. M93942.
Sincerely, (original signed by)
David.E. LaBarge, Senior Project Manager Project Directorate 11-2 Division of Reactor Projects -
I/II Office of Nuclear Reactor Regulation Docket Nos. 50-269, 50-270, and 50-287 cc:
See next page Distribution:
PUBLI PD 11-2 Rdg.
SVarga JZwolinski HBerkow LBerry DLaBarge GBagchi OGC ACRS EMerschoff, RH RCrlenjak, RH To receive a copy of this document, indicate in the box:
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/96 OFFICIAL,RtCORD CORY
Mr. J. W. Hampton Duke Power Company Oconee Nuclear Station cc:
Mr. Paul R. Newton Mr. Ed Burchfield Legal Department (PB05E)
Compliance Duke Power Company Duke Power Company 422 South Church Street Oconee Nuclear Site Charlotte, North Carolina 28242-0001 P. 0. Box 1439 J. Michael McGarry, III, Esquire Seneca, South Carolina 29679 Winston and Strawn Ms. Karen E. Long 1400 L Street, NW.
Assistant Attorney General Washington, DC 20005 North Carolina Department of Justice Mr. Robert B. Borsum P. 0. Box 629 Framatome Technologies Raleigh, North Carolina 27602 Suite 525 1700 Rockville Pike Mr. G. A. Copp Rockville, Maryland 20852-1631 Licensing -
ECO50 Duke Power Company Manager, LIS 526 South Church Street NUS Corporation 2650 McCormick Drive, 3rd Floor Clearwater, Florida 34619-1035 Mr. Dayne H. Brown, Director Division of Radiation Protection Senior Resident Inspector North Carolina Department of U. S. Nuclear Regulatory Commission Environment, Health and Route 2, Box 610 Natural Resources Seneca, South Carolina 29678 P. 0. Box 27687 Regional Administrator, Region II U. S. Nuclear Regulatory Commission 101 Marietta Street, NW. Suite 2900 Atlanta, Georgia 30323 Max Batavia, Chief Bureau of Radiological Health South Carolina Department of Health and Environmental Control 2600 Bull Street Columbia, South Carolina 29201 County Supervisor of Oconee County Walhalla, South Carolina 29621
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY ASSESSMENT REPORT REVIEW OF SIXTH TENDON SURVEILLANCE REPORT OCONEE NUCLEAR STATION. UNIT 3 DUKE POWER COMPANY
1.0 INTRODUCTION
In accordance with 10 CFR Part 50, Appendix A, General Design Criterion 53 (2) a periodic surveillance program, among other requirements, is stipulated for the containment structure. By letter dated October 11, 1995, Duke Power Company, licensee for the Oconee Nuclear Station, submitted a report on the Oconee Unit 3 Reactor Building (i.e., containment structure) describing the Post-Tensioning System Sixth Surveillance. The surveillance of the tendon system was performed in accordance with Oconee Nuclear Station Technical Specification 4.4.2, which specifies periodic tendon system surveillance requirements consisting of sheathing filler material inspection, tendon lift off force measurement, anchorage component inspection, tendon wire inspection and tensile testing, grease leakage check, tendon de-tensioning and re tensioning and grease refilling and resealing. The surveillance involved the same nine preselected tendons, three from each group of tendons (dome, vertical, and horizontal).
For the six surveillances performed to date, each of the same preselected tendons has been subjected to two de-tensionings and re-tensionings and four lift-off tests.
The staff reviewed the report to assure that the performance of the tendon system is as designed. On the basis of the results of surveillance, the licensee concluded that the post-tensioning system for Oconee Unit 3 is in satisfactory condition. However, after a careful review of the report, the staff had reservations about the licensee's test procedure and method of analyzing the tendon lift-off force data. On January 19, 1996, the staff issued a request for additional information (RAI) to the licensee. In the RAI, the licensee was required to re-analyze the tendon force data by (1) taking into consideration the discrepancy between the "as found" and "as left" values to modify the tendon lift-off forces, and (2) using the individual tendon forces instead of the average value for a group of tendons for each surveillance for the trend analysis for that group of tendons. Duke performed the re-analysis in accordance with staff's requirements. On March 14, 1996, Duke Power Company submitted the response to the staff's RAI.
The result of the re-analysis indicated that the dome tendon force went below the minimum required value (MRV) about 8 years after the structural integrity test (SIT).
-2 In view of this finding, the licensee also re-analyzed the tendon surveillance data for Oconee Nuclear Station Units 1 and 2, in accordance with the two requirements indicated above. The results of these re-analyses, as reported in a July 30, 1996, submittal, indicated that the vertical tendon forces in Unit 1 will be below MRV about 30 years after the SIT and that those of Unit 2 were below the MRV about 10 years after the SIT. These findings prompted the licensee to commit to three corrective actions each with a definite date of completion. The staff finds these corrective actions to be responsive and appropriate.
2.0 EVALUATION The staff found the results of the sixth surveillance of the various elements of the post-tensioning system as contained in the surveillance report to be generally in conformance with the criteria established in the relevant technical specifications and in an early version of Regulatory Guide 1.35.
However, after a close examination of the licensee's tendon tests and its analysis, the staff had some reservations about the licensee's test procedure and analysis method and the conclusion as indicated below.
2.1 Tendon Lift-Off Force The staff questioned the licensee's practice of having "as-left" force greater than "as found" force; i.e., for tendons de-tensioned and re-tensioned, the re-tensioned tendons have a higher force level than they originally had, thus, reducing the pre-stress losses from the lift-off forces to be obtained during a subsequent surveillance. Furthermore, the licensee used the average values instead of the individual lift-off forces to determine the trend of the tendon forces, and there was no indication of the minimum required value (MRV) of tendon force for each group of tendons. These concerns were conveyed to the licensee.
In response, the licensee performed a regression analysis by making corrections for the difference between "as left" and "as found" forces, using the individual lift-off forces instead of the average for the analysis and providing the MRV for each group of tendons. From the results of the regression analysis of the lift-off forces available to date, it is found that the average dome tendon force for Unit 3 started to go below the MRV about 8 years after the SIT. At the sixth surveillance (1995), the dome tendon force is about 96 percent of the MRV. For the other tendon groups, the average tendon forces will not go below the MRVs until 40 or more years after the SIT.
Obviously, the results of the re-analysis of tendon forces are different from those of the analysis as contained in the original report.
In view of this finding, the staff had concerns about the tendon forces of Units 1 and 2, which are identical to Unit 3. The staff's concerns were conveyed to the licensee. In response to the staff's concerns, the licensee re-analyzed, in accordance with the regression analysis procedure as indicated above, the trend of the tendon lift-off forces obtained, so far, on Units 1 and 2, and found that the loss of pre-stress in the vertical tendons of both
-3 units is higher than the predicted values obtained earlier. The vertical tendon forces in Unit 1 will start to go below the MRV about 30 years after the SIT and those of Unit 2 started to go below MRV about 10 years after the SIT. From the regression analysis performed, it is determined that existing tendon forces in the vertical tendons of Unit 2 are about 99 percent of their MRVs. In spite of these findings, the licensee is convinced that by using state-of-the-art analytical methods, a re-analysis of the Oconee containment structures would demonstrate their ability to satisfy all design licensing basis requirements under prestressing forces lower than original design MRVs.
The licensee cited Florida Power and Light resolution of low tendon force through re-analysis for Turkey Point Units 3 and 4 as the precedent.
The licensee has given various reasons for the low tendon forces, among which are the accuracy of measurement and larger tendon steel relaxation due to higher temperatures than expected. The licensee has committed to take corrective actions which can be summarized as follows: (1) identify and evaluate potential causes of pre-stress losses larger than predicted, (2) perform a refined structural analysis of the reactor building to check the MRVs, and (3) change the technical specifications to use the method that will require randomly selected tendons instead of the same tendons at each surveillance by following Regulatory Guide 1.35, Revision 3. For the forthcoming tendon surveillance of Unit 1 the licensee will use randomly selected tendons and it is expected that more representative tendon forces will be obtained. The planned completion dates for corrective actions (1) and (2) are March 1, 1997, January 1, 1997, respectively. The submittal related to action (3) has been submitted and is being processed.
A potential source of the low lift-off force may be the repeated use of the same tendons for surveillance. These tendons have each undergone two complete cycles of loading and unloading, and four partial cycles of lifting and seating. Tendons subjected to such loading cycles most likely do not behave as other tendons in the group which have not been subjected to such inspections. Most probably there has been more pre-stress loss due to cyclic loading. In view of this observation, the use of randomly selected surveillance tendons is highly recommended and the licensee plans to do so.
The corrective actions and their completion dates as outlined by the licensee appear to be responsive and appropriate.
The licensee provided for each of the four de-tensioned and re-tensioned tendons only one measured elongation along with the corresponding applied force. The information is of little value for determining the force displacement relationship. The simultaneous measurement of elongation and jacking force during re-tensioning should be made at a minimum of three, approximately equally spaced, levels of force between zero and the lock-off force, not just at one force and one elongation. If the forces and the corresponding elongations are plotted on a graph, a straight line should result. This would indicate that the linear elastic relationship between force and elongation and can be used to check the accuracy of the
-4 measurements. It may also be used to help differentiate between the loading cycled tendon and the undisturbed tendons. This is mentioned here to emphasize the importance of obtaining such information while performing tendon surveillances.
2.2 Grease Voids In the report, grease in amounts of as much as 15 and 18 gallons were added to two vertical tendon sheathings. This can be interpreted as an appreciable loss of grease, which gives rise to two concerns: (1) the tendons may not be
-totally protected against corrosion, and (2) the grease may have leaked into the concrete, thus, potentially reducing the strength of the concrete. In response to these staff concerns, the licensee attributed the loss mainly to the effect of gravity on the vertical column of grease leading to the outflow of grease from the lower end when the grease caps and the shims were removed for de-tensioning and re-tensioning and for wire removal.
From the examination and testing of the tendon wires removed, no indication of corrosion was found and the tensile testing of the wires indicated the mechanical properties of the steel material exceed the minimum required.
As to the potential effect of grease leakage into concrete, the licensee stated that to date, there exists no evidence that supports the assumption that tendon protection grease has any detrimental effect on concrete. The staff reviewed the licensee's response and found it acceptable.
2.3 Anchorage Assembly and Tendon Wire The licensee inspected the button heads of the wires and, with the exception of one improperly formed button head, no other abnormality was found. Stress washers, shims, and end bearing plates were also visually inspected for cracking and corrosion and only some minor surface corrosion on their edges and faces were found. According to the licensee, this condition existed at the time of installation and no further evidence of deterioration has been found. The licensee visually inspected the wires removed for tensile testing for corrosion and pitting and found the general condition of the wires to be equivalent to that at the time of initial installation. Samples of the wires taken in accordance with technical specification criteria were subjected to tensile testing in the licensee's metallurgy laboratory. The tests revealed no significant changes in ultimate strength or elongation as compared to the results obtained from initial acceptance tests. Unfortunately the licensee did not provide a graph showing the load-elongation curves for the loading cycled wire and the wire used in the initial acceptance tests. On the basis of the information provided, the staff concludes that the anchorage components and the tendon wire without considering the time-dependent effect are in reasonably good condition.
-5
3.0 CONCLUSION
As a result of the staff's review and the licensee's detailed re-analysis, the pre-stress losses in the dome tendons of Unit 3 and vertical tendons of Units I and 2 has been found to be greater than predicted and may reduce or have reduced the tendon forces below the minimum required value within the licensed 40-year plant life. Otherwise, the overall condition of the containment tendon system in the three Oconee units is satisfactory. Because the containment is the last line of defense in the protection of health and safety of the public, the discovery of low lift-off forces in the surveillance tendons of all three units is a concern. The corrective actions with their planned completion dates as provided by the licensee are appropriate and are expected to resolve, in a timely manner, the staff's concern. The corrective actions and their corresponding approximate completion dates are:
a) Identify and evaluate potential causes of pre-stress losses larger than predicted. This action will be completed by March 1, 1997; b) Perform a refined structural analysis of the reactor building to check the MRV. This action will be completed by January 1, 1997; and c) Change the tec hical specifications to use randomly selected tendons instead of the same tendons for tendon surveillances. This amendment has been submitted and is being processed.
The licensee indicated that a re-analysis of the Oconee containment structures, using state-of-the-art methods would demonstrate their ability to satisfy all design licensing basis requirements under pre-stressing forces lower than the original design MRVs throughout the life of the plant. This has been demonstrated at Turkey Point Units 3 and 4, which are of nearly the same vintage as the Oconee units.
On the basis of the results of its evaluation of the licensee's letter dated October 3, 1996, the staff has been assured that the integrity of the Oconee Units 1, 2, and 3 containments is adequate to support continued operation and public health and safety will not be impacted as a result. On the basis of our evaluation of the information provided by the licensee, the staff concurs with the licensee's judgement.
Principal Contributor: Chen P. Tan Date: November 7, 1996