NOC-AE-02001364, Revised Request for Approval of an Alternative Approach for Containment Concrete and Tendon Surveillances (RR-ENG-37)
| ML022380251 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 08/20/2002 |
| From: | Jordan T South Texas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| G25, NOC-AE-02001364 | |
| Download: ML022380251 (11) | |
Text
Nuclear Operating Company South Trays Pm/ed Eket*c Generating Statlon PO Box289 WadMworth. TevAs77483 -AA. ,-
August 20, 2002 NOC-AE-02001364 File No.: G25 10CFR50.55a U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852 South Texas Project Units 1 and 2 Docket Nos. STN 50-498, STN 50-499 Revised Request for Approval of an Alternative Approach for Containment Concrete and Tendon Surveillances (RR-ENG-37)
Reference:
Letter, T. J. Jordan to NRC Document Control Desk, "Request for Approval of an Alternative Approach for Containment Tendon Surveillances (RR-ENG-37)
In accordance with the provisions of 10CFR50.55a(a)(3)(i), the South Texas Project requests approval of an alternative to ASME Section Xl, paragraph IWL-2421(b), which specifies the inservice inspection intervals to be applied in examination of containment concrete and unbonded post-tensioning systems, and IWL-2521, which describes the tendon selection process. This request supercedes a previous submittal (Reference 1). Change bars have been added in the margin to denote the differences between the submittals.
Implementation of alternative requirements for the Unit 1 and Unit 2 reactor containment examinations will not present an undue risk to the public health and safety, and will provide an acceptable level of quality and safety.
The South Texas Project requests Nuclear Regulatory Commission approval of this proposed schedule by January 15, 2003, to facilitate scheduling for subsequent inspections of containment concrete and unbonded post-tensioning systems. Although this request is neither exigent nor an emergency, prompt review by the Nuclear Regulatory Commission is requested.
If there are any questions, please contact either Mr. R. L. Engen at (361) 972-7363 or me at (361) 972-7902.
Vice President, Engineering & Technical Services PLW
Attachment:
Revised Request for Approval of an Alternative Approach for Containment Concrete and Tendon Surveillances (RR-ENG-37)
NOC-AE-02001364 Page 2 of 2 cc:
(paper copy) (electronic copy)
Ellis W. Merschoff A. H. Gutterman, Esquire Regional Administrator, Region IV Morgan, Lewis & Bockius LLP U. S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 400 M. T. Hardt/W. C. Gunst Arlington, Texas 76011-8064 City Public Service U. S. Nuclear Regulatory Commission Mohan C. Thadani Attention: Document Control Desk U. S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike R. L. Balcom Rockville, MD 20852 Reliant Energy, Inc.
Richard A. Ratliff A. Ramirez Bureau of Radiation Control City of Austin Texas Department of Health 1100 West 49th Street C. A. Johnson Austin, TX 78756-3189 AEP - Central Power and Light Company Cornelius F. O'Keefe Jon C. Wood U. S. Nuclear Regulatory Commission Matthews & Branscomb P. O. Box 289, Mail Code: MN116 Wadsworth, TX 77483 C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704 IC
ATTACHMENT REVISED REQUEST FOR APPROVAL OF AN ALTERNATIVE APPROACH FOR CONTAINMENT CONCRETE AND TENDON SURVEILLANCES (RR-ENG-37)
Attachment NOC-AE-02001364 File No.: G25 Page 1 of 8 SOUTH TEXAS PROJECT UNITS 1 AND 2 Revised Request for Approval of an Alternative Approach for Containment Concrete and Tendon Surveillance (RR-ENG-37)
A. Components for Which Exemption is Requested Component: Reactor Containment Concrete and Unbonded Post-Tensioning Systems Function: The components support continued containment structural integrity in the event of a loss of coolant or steam line break accident.
Class: ASME Code Class CC B. Applicable Code ASME Boiler & Pressure Vessel Code,Section XI, 1992 Edition C. Code Requirements from Which Relief is Requested The South Texas Project is a two-unit facility:
"* Both Containment structures use the same pre-stressing system and are essentially identical in design;
"* Post-tensioning operations were completed less than two years apart; and
"* Both Containment structures are similarly exposed to the outside environment.
IWL-2421(b) describes the inservice inspection schedule to be followed for examination of containment concrete and unbonded post-tensioning systems for sites with two plants:
(1) For the containment with the first Structural Integrity Test, all examinations required by IWL-2500 shall be performed at 1, 3, 10, 20, and 30 years. Only the examinations required by IWL-2524 and IWL-2525 need be performed at 5, 15, 25, and 35 years.
(2) For the containment with the second Structural Integrity Test, all examinations required by IWL-2500 shall be performed at 1, 5, 15, 25, and 35 years. Only the examinations required by IWL-2524 and IWL-2525 need be performed at 3, 10, 20, and 30 years.
Note: IWL-2500 describes requirements for examination of concrete and unbonded post-tensioning systems. IWL-2524 addresses examination of tendon anchorage areas, and IWL-2525 covers examination of corrosion protection medium and free water.
IWL-2521 describes the tendon selection process. Tendons are to be selected on a random basis from the population of all tendons not examined during earlier inspections with at least three tendons selected from each group (horizontal and inverted U).
D. Proposed Alternate Examination In accordance with 10CFR50.55a(a)(3)(i), the South Texas Project proposes an alternative approach to that specified in ASME Section Xl, Subsection IWL-2421 (b), that will provide an acceptable level of quality, and safety. The proposed alternative will apply revised inspection intervals for performing the full containment concrete and post tensioning system surveillances.
Attachment NOC-AE-02001364 File No.: G25 Page 2 of 8 The proposed inspection intervals are as follows:
" For the containment with the first Structural Integrity Test, all examinations required by IWL-2500 shall be performed at 1, 3, 10, and 30 years. Only the examinations required by IWL-2524 and IWL-2525 need be performed at 5 and 20 years. Only the examinations required by IWL-2522, 'Tendon Force Measurements," will be performed at years 15 and 35.
" For the containment with the second Structural Integrity Test, all examinations required by IWL-2500 shall be performed at 1, 5, and 20 years. Only the examinations required by IWL-2524 and IWL-2525 need be performed at 3, 10 and 30 years. Only the examinations required by IWL-2522, "Tendon Force Measurements," will be performed at year 25.
In accordance with 10CFR50.55a(a)(3)(i), the South Texas Project proposes an alternative approach to that specified in IWL-2521 that will provide an acceptable level of quality and safety. This alternative will be applied only in years 15, 25, and 35. The proposed alternative will require liftoff testing of two horizontal tendons in the affected unit, rather than three horizontal and three vertical randomly selected tendons. One tendon will be adjacent to the main steam penetration (tendon 2H039). The other will be randomly chosen from a group of five horizontal tendons anchored at buttress 2. If either of the two tendons examined does not meet the acceptance standards of IWL 3221.1, the South Texas Project will evaluate the condition in accordance with IWL 3310.
E. Basis for Relief from Code Requirements Based on the inspection results from previous cycles, there is little value to be gained by conducting a full inspection every five years. Given the expected rate of change in the material condition of the reactor containment structure, inspections at the code-specified intervals are not necessary, and result in unnecessary expense and risk to workers performing the inspections.
Inspection of the tendons selected from the population required by the code results in unnecessary expense and risk to workers performing the inspections. Buildings and structures surround buttresses 1 and 3, and access to the tendons requires a large rental crane. Buttress 2 is accessible by using an on-site crane.
F. Basis for Alternative Containment Structural Design The purpose of the containment post-tensioning system is to provide strength to resist internal pressure during postulated design basis accidents. The reinforced concrete containment structure is designed to resist loads imposed by external events such as wind, seismic activity, and tornadoes.
The South Texas Project containment structure is a post-tensioned concrete cylinder with steel liner plates, hemispherical top and flat bottom. The cylindrical portion and the hemispherical dome of the Containment are pre-stressed by a post-tensioning system consisting of horizontal and vertical tendons.
Three buttresses equally spaced around the Containment provide anchor points for the horizontal tendons. The cylinder and the lower half of the dome are pre-stressed by horizontal tendons with anchors separated 360 degrees around the structure, bypassing
Attachment NOC-AE-02001364 File No.: G25 Page 3 of 8 the intermediate buttresses. Anchorage for each successive hoop is progressively offset 120 degrees from the one beneath it.
The vertical inverted U-shaped tendons are continuous over the dome, forming a two way post-tensioning system for the dome. These tendons are anchored in a continuous gallery beneath the base slab, which provides for installation and inspection of the vertical tendons.
Margin of Safety The South Texas Project containment structure includes a substantial design margin for pressure. The design pressure for the building is 56.5 psig, but the calculated maximum pressure that could occur following a design basis accident is 41.2 psig. The resulting design margin is 37% [56.5/41.2 = 1.37]. This exceeds the 10% design margin discussed in Chapter 6 of NUREG-0800, "Standard Review Plan."
Previous Examination Results Examinations have been conducted at one, three, five, and ten years following the initial post-tensioning operations for Unit 1, and for Unit 2.
The containment concrete surface, including coated areas, has been visually examined for areas of large spalling, severe scaling, D-cracking in an area of 25 ft 2 or more, other surface deterioration or disintegration, or significant grease leakage. No damage or degradation of the concrete surfaces was identified during the examinations.
The condition of unbonded post-tensioning systems has been determined by:
"* Tendon force measurements; Test results are summarized in Table 1.
" Tendon wire and strand sample examination and testing; At surveillances that included liftoff testing, one wire was removed from each of two tendons (one horizontaland one vertical) along with the anchoragehardware I and inspected for deterioration or corrosion. The tendon wires and anchorage hardware were free of corrosion with no signs of cracking.
" Examination of tendon anchorage areas; The anchor components were inspected after end-cap removal for corrosion protection medium coverage. All were properly covered.
The concrete surface surrounding the bearing plates was visually inspected for evidence of cracks greaterthan 0.01-inch in width. The only cracks identified were minor surface shrinkage cracks, a normal characteristicof concrete.
"* Examination of corrosion protection medium and free water; Samples of the corrosion protection medium were tested for water content, reserve alkalinity, concentrations of water-soluble chlorides, nitrates, and sulfides. The values were well below the acceptable limits as specified in Table IWL-2525-1.
"* Addition of corrosion protection medium.
Grease additions have been evaluated and found acceptable. No evidence of internalgrease leakage has been found.
Attachment NOC-AE-02001364 File No.: G25 Page 4 of 8 Trend Analysis Results from previous tendon examinations show that the progression of tendon pre- I stress loss is close to the predicted behavior. The IWL-3221.1 (b) limit for acceptability is 95% of the predicted value. Using regression analysis (NRC Information Notice 99-10, "Degradation of Prestressing Tendon Systems in Prestressed Concrete Containments,"
October 7, 1999), the trend lines for the four tendon groups indicate that pre-stress loss will remain in the acceptable range for the life of the plant. The trend data are summarized in Table 2 and Figure 1. The worst-case trend is for Unit 1 horizontal tendons, which are trending toward 96.5% of predicted lift-off force at year 40 of plant life (i.e., predicted value minus 3.5%). The proposed alternative includes this worst case group within the surveillance population.
Figure 1 reflects the belief (supported both theoretically and by experience) that pre stress loss should occur linearly with the logarithm of time, but nonlinearly on a linear time scale. A consequence of the non-linearity is that the majority of the lifetime loss is expected to occur during the first ten years. Even though data points are available only for the first ten years of plant life, this period covers the majority of the expected pre stress losses. Therefore, ten-year data provides a high confidence that the projected lifetime trend lines are reasonably representative of actual lifetime behavior.
NRC Information Notice 99-10 Information Notice IEN 99-10 addresses three NRC concerns with tendons: 1) breakage of prestressing tendon wires; 2) accelerated relaxation as an effect of high temperature on the prestressing forces in tendons; and 3) trend analysis of prestressing forces. Each of these issues is addressed here.
Breakage: The STP design assumes 1-percent wire breakage. With -186 wires per tendon, and 229 tendons total, this implies an assumption of 426 broken wires per unit. To date, STP has documented fewer than 30 broken or damaged wires in each unit (0.06%), with about half of these being the result of destructive surveillance testing and all but one of the others 'occurring during initial installation.
Accelerated Relaxation: Laboratory test data used to estimate tendon tension relaxation was obtained at approximately 70 degrees Fahrenheit. However, because actual tendon temperature may be higher than that, there is potential for steel relaxation to be underestimated. The proposed alternative surveillance addresses this issue by examining a tendon that is next to the containment main steam penetration. This tendon is exposed to higher temperatures than the remaining tendons, and is a conservative indicator of relaxation of tendon tension that may have occurred. There is no need to examine vertical tendons as part of the alternate surveillance performed at years 15, 25, and 35, since any accelerated relaxation due to higher temperatures (if present) would be seen at the highest-temperature horizontal tendon. This tendon conservatively bounds all tendons (horizontal and vertical) with regard to thermal-accelerated relaxation.
Trending: Trended results of previous STP surveillances are given in Table 2 and Figure 1. The trends were calculated using regression analysis consistent with IEN 99-10. (See 'Trend Analysis" section above for additional discussion.)
Attachment NOC-AE-02001364 File No.: G25 Page 5 of 8 Probabilistic Safety Assessment The frequency of radionuclide release due to reactor containment failures at the time of an accident has been determined by a Level 2 Probabilistic Safety Assessment for the South Texas Project. Due to all analyzed accident sequences and containment failure modes, the Large Early Release Frequency (LERF) is 6.1 E-07 events/year. The major contributors to this release frequency are from containment bypass sequences involving an induced steam generator tube rupture or interfacing LOCA. The containment building failure mode is a very small contributor to LERF.
Summary Over the ten-year history of test and examination, the post-tensioning system has behaved as designed, and no damage or degradation of the concrete surfaces was identified during the examinations. All tendon groups at the South Texas Project are following a trend that is projected to remain acceptable for 40 years of plant life.
Furthermore, the design has a substantial margin of safety, such that pre-stress loss would have to be far greater than predicted to reduce the ability of the containment structure to withstand the calculated accident pressure loads. Degradation this significant is detectable with the proposed alternative surveillance. Therefore, modifying the surveillance protocol as described is appropriate.
G. Duration of Proposed Alternative This relief request is applicable to the current South Texas Project IWE/IWL inspection interval, which expires September 8, 2008.
H. Implementation Schedule The South Texas Project requests Nuclear Regulatory Commission approval by January 15, 2003, to support procurement and scheduling of the year 15 surveillance to be performed in 2003.
Attachment NOC-AE-02001364 File No.: G25 Page 6 of 8 Table 1 Tendon Force Measurement Summary:
Deviation from Predicted Values (%)
Unit 1- Unit 1- Unit 1- Unit 2- Unit 2- Unit 2- All- All- All Horizontal Vertical All Horizontal Vertical All Horizontal Vertical Low -5.9 -1.7 -5.9 -5.9 -1.0 -5.9 -5.9 -1.7 -5.9 Median 1.1 0.7 1.0 0.3 2.3 1.1 0.7 1.3 1.1 Mean 0.7 0.9 0.7 1.3 2.5 1.7 1.0 1.6 1.2
Attachment NOC-AE-02001364 File No.: G25 Page 7 of 8 Table 2 Tendon Force Projected Trends:
Projected Deviation from Predicted Values (%)
Trend Unit 1- Unit 1- Unit 1- Unit 2- Unit 2- Unit 2- All- All- All Horizontal Vertical All Horizontal Vertical All Horizontal Vertical Yr. 15 -1.91 1.71 -0.68 -0.75 5.32 0.68 -1.45 2.63 -0.21 Yr. 20 -2.38 1.87 -0.94 -1.08 5.75 0.52 -1.87 2.81 -0.44 Yr. 30 -3.03 2.09 -1.31 -1.55 6.35 0.30 -2.45 3.07 -0.77 Yr. 40 -3.50 2.25 -1.57 -1.88 6.78 0.14 -2.86 3.25 -1.01
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