Information Notice 1999-10, Degradation of Prestressing Tendon Systems in Prestresssed Concrete Containments

Degradation of Prestressing Tendon Systems in Prestresssed Concrete Containments

ML031500244
Person / Time
Issue date:	04/13/1999
From:	Matthews D B, Newberry S F Office of Nuclear Reactor Regulation
To:
References
IN-99-010
Download: ML031500244 (6)

Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... :.ffi> Index Site Map I FAQ I Help Glossary i Contact Us l lSearch AU.SfU.S. Nuclear Regulatory CommissionHom D eaONuclear E Nuclear Radioactive 11 PublicHome Who e AreI Wat We Do Reactors flMaterials Waste 11 InvolvementHome > Electronic Reading Room > Document Collections > General Communications > Information Notices > 1999 > IN 9UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001April 13, 1999NRC INFORMATION NOTICE DEGRADATION OF PRESTRESSING TENDON SYSTEMS IN PRESTRESSED99-10: CONCRETE CONTAINMENTS*

Description of Circumstances

o Prestressing Tendon Wire Breakageo Effects of High Temperature on the Prestressing Forces in Tendonso Companoadedf Presressioreo ..prison and_Trending f_Pestr .FogErces* Discu$sion

Addressees

All holders of operating licenses for nuclear power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees todegradation of prestressing systems components of prestressed concrete containments (PCCs). The specificitems addressed are (1) prestressing tendon wire breakage, (2) the effects of high temperature on theprestressing forces in tendons, and (3) trend analysis of prestressing forces. It is expected that recipients willreview the information for applicability to their facilities and consider actions, as appropriate, to avoid similarproblems. However, suggestions contained in this information notice are not NRC requirements; therefore, nospecific action or written response is required.

Description of Circumstances

Results of inspections of PCCs and PCC tendons have identified a number of concerns related to thedegradation of prestressing tendon systems in PCCs and the ability of the containment structure to perform itsfunction. The relevant findings associated with these concerns are discussed below.Prestressing Tendon Wire BreakageRecent observations related to containment prestressing systems have revealed conditions that mayprecipitate tendon wire breakage. Conditions such as uneven shim stack heights on the anchor-heads, spallingand cracking of concrete beneath the anchor-head base plates, free water in the bottom grease caps, poorlydrained top anchorage ledges, and the absence of filler grease in various areas can lead to corrosion oftendons and eventually to wire breakage. Specific plant observations and instances of failure of tendons andassociated anchorages are detailed in Attachment 1.Effects of High Temperature on the Prestressing Forces in Tendonshttp://www.nrc.gov/reading-rm/doc-collections/gen-comrn/info-notices/1 999/in9901O.html 03/13/2003 Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... Ucensees at a number of plants have reported lower than predicted prestressing forces for vertical, hoop, anddome tendons. Investigations and analyses have indicated that the prestressing tendon relaxation lossesrange from 15.5 to 20 percent over 40 years at an average sustained temperature of 32 C (90 F) aroundthe tendons. However, the tendon relaxation loss values used in PCCs vary between 4 to 12 percent. Thesevalues were determined at the presumed ambient temperature of 20 C (68 F). The relevant plantobservations and discussions are reported in Attachment 2.Comparison and Trending of Prestressing ForcesThe use of the provisions of Regulatory Guide 1.35.1 ("Determining Prestressing Forces for Inspection ofPrestressed Concrete Containments") or equivalent methods are important to maintaining the safety functionof the prestressing tendon system and the concrete containment. Moreover, proper comparison and trendinganalysis is critical in determining the future trends in prestressing force in PCCs. Licensees have reportedlosses using the average forces determined from the liftoff testing, thereby masking the true variation in theloss of prestressing forces. An analysis using the individual lift-off forces for regression analysis gives resultsthat are statistically valid. Attachment 3 contains the staffs discussion of the variation in trend analysis oftendon prestressing forces.DiscussionAs nuclear power plants continue to age, in particular, plants with a PCC, the management and mitigation ofeffects of degradation as a result of aging become increasingly more important. The containment structureserves as the final barrier against the release of fission products to the environment under postulated design-basis accident conditions. Therefore, it is essential that its integrity be maintained. Focus on the prestressingtendon system for containment integrity is based on the vital role it plays. However, other components thatmake up the system also need to be examined. The observations detailed in the three attachments, and theobservations made during the Oconee site visit (see Attachment 1), indicate that other contributions to thedegradation of containment could potentially compromise its effectiveness.PCC degradations, such as concrete spalling, water infiltration into tendon galleries, and concrete cracking inthe containment and the containment dome, all affect the containment's ability to function properly. Itremains important to ensure that the cumulative effects of degradation mechanisms do not compromise thesafety of the containment. The attributes discussed in the three attachments will be useful in identifying thepotential problem areas and in evaluating the results of the inservice inspections of containments.This information notice requires no specific action or written response. However, recipients are reminded thatthey are required to consider industry-wide operating experience (including NRC information notices), wherepractical, when setting goals and performing periodic evaluations under Section 50.65, "Requirements forMonitoring the Effectiveness of Maintenance at Nuclear Power Plants," of Part 50 of Title 10 of the Code ofFederal Regulations. If you have any questions about the information in this notice, please contact one of thetechnical contacts listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager./s/'d by S. F. NewberryFor David B. Matthews, DirectorDivision of Regulatory Improvement ProgramsOffice of Nuclear Reactor RegulationTechnical contacts: H. Ashar, NRR G. Hatchett, NRR301-415-2851 301-415-3315E-mail: hga@nrc.gov E-mail: gxh@nrc.govAttachments: 1. Prestressing Tendon Wire Breakage2. Effects of High Temperature on the Prestressing Forces in Tendons3. Comparison and Trending of Prestressing Forces4. List of Recently Issued Information Noticeshttp://www.nrc.gov/reading-rmldoc-collections/gen-commlinfo-notices/1999fin99010.html 03/13/2003 Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... (NUDOCS Accession Number 9904090219)ATTACHMENT 1IN 99-10April 13, 1999Prestressing Tendon Wire BreakageDuring the 20th-year surveillance of the prestressing system of Calvert Cliffs Nuclear Power Plant, Unit 1, inJune-July 1997, the licensee (Baltimore Gas & Electric Company--BG&E) found a low lift-off value compared tothe prestressing force for one of the three randomly selected vertical tendons. The low lift-off value wasattributed to the uneven shim stack heights on the two opposite sides of the anchor-head. In accordance withthe plant's Technical Specifications (TSs) requirement, the licensee tested two additional vertical tendonsadjacent to this tendon. However, during the lift-off testing of one of these tendons, noises were heard thatindicated that some of the tendon wires might have broken. A visual examination of the tendon indicated thatthree wires had broken at 12.7-17.2 centimeters (5-7 inches) below the bottom of the button-heads. Furtherexamination of the wires at the top of other vertical tendons revealed additional wire breakage. The licenseeexpanded the lift-off testing and visual examination to 100 percent of the vertical tendons. Similardegradation of other vertical tendons was found. As a part of its corrective action, the licensee is planning toreplace 63 of the 202 vertical tendons in Unit 1 and 64 of the 204 vertical tendons in Unit 2.NRC's Information Notice 85-10, dated February 1985, and its supplement of March 1985, "Post-TensionedContainment Tendon Anchor-Head Failure," described prestressing tendon anchor-head failures at both unitsof the Joseph M. Farley Nuclear Plant. The root cause analysis of that event indicated that there were severalfactors contributing to it, such as, free water in the grease caps at the bottom of the vertical tendons, highhardness of the anchorage material, and high stresses in the anchor-heads. The failures had resulted fromhydrogen embrittlement of the anchor-head material. The free water in the bottom grease caps of the verticaltendons may have accumulated (through a number of years) from the poorly drained top anchorage ledge ofthe vertical tendons (similar to the condition at the Calvert Cliffs containments). However, at Farley, wirefailures did not occur.In general, American Society for Testing and Materials (ASTM) A-421 ("Uncoated Stress-Relieved Wire forPrestressed Concrete") wires (used at both Farley and Calvert Cliffs) are not susceptible to hydrogen-inducedcracking. However, BG&E's engineering evaluation indicated brittle hydrogen-induced cracking on a third ofthe broken wires. All of the brittle fractures were preceded by severe corrosion. The engineering evaluationalso indicates that some of the brittle fractures may have occurred earlier but were not found during theperiodic inspections. To ensure that the stressing washers (anchor-heads) are not affected, BG&E visuallyexamined the anchor-heads at both ends of the vertical tendons and found no visible cracks or fractures. Thelessons learned from these two events indicate that the prestressing wires and anchor-heads of the button-headed prestressing systems are susceptible to cracking from tensile stress and hydrogen-induced corrosion.The severity and the extent of corrosion depend upon the ability of the moisture to reach unprotected areas,the duration of exposure, and the material characteristics.In April 1998, the NRC staff visited the Oconee Nuclear Station (OCN) to discuss issues related to thelicensee's license renewal technical report. As part of the visit, the staff performed a walkdown inspection ofthe OCN containments and other structures. The following observations are related to the prestressing systemdegradations reported by the staff:* At Tendon 12V6, the concrete beneath the 5.1-centimeter (2-inch) thick anchor-bearing plate hadspalled along the outer edge; a cavity existed below the plate. Cracks in the concrete beneath the outeredge of the bearing plates were observed for a number of tendons.* Tendon grease had leaked from a significant number of hoop tendons in the containments of all threeunits at OCN.* The Unit 1 tendon access gallery showed water infiltration and standing water at several locations. Thelicensee indicated that the Unit 2 tendon access gallery at one time held as much as 51 centimeters (20http://www.nrc.gov/reading-rmldoc-collections/gen-comm/info-notices/1999/in99010.html 03/13/2003 Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... inches) of water. The licensee is periodically purging the tendon galleries of all three units to removewater.The licensee addressed these and similar degradations under the requirements of its TSs or in accordance withCriterion XVI of 10 CFR Part 50, Appendix B.ATTACHMENT 2IN 99-10April 13, 1999Effects of High Temperature on the Prestressing Forces in TendonsIn 1979-1980, the licensee of the Robert E. Ginna Nuclear Power Plant, Rochester Gas & Electric Corporation,reported lower than predicted forces for several of the vertical tendons in its partially prestressed containmentstructure. Extensive analysis and testing performed by the licensee indicated the cause of the consistentlylower prestressing forces to be appreciably higher (than estimated) relaxation of prestressing steel as a resultof the average high temperatures around the tendons. The 1000-hour and 10,000-hour testing performed atthe Fritz Engineering Laboratory of Lehigh University of the wires taken from some of the vertical tendonsshowed that the 40-year relaxation could be between 15.5 and 20 percent at 32 Celsius (C) (90 Fahrenheit[°F]), an average temperature around the tendons during the summer. The wire relaxation assumed in thedesign was 12 percent.During the fourth surveillance of tendon forces in February 1990 at Virgil C. Summer Nuclear Station, thelicensee, South Carolina Electric & Gas Co., discovered that the forces in the 115 vertical tendons were lowerthan expected. Because the wires used in the prestressing tendons were of the same size, type, andrelaxation property as those used in the Ginna tendons, the licensee concluded that the reason for lowprestressing forces was the higher (than considered) relaxation of prestressing wires. As in the case of theGinna containment, the average temperature around the tendons was determined to be 32 C (90 F). Toremedy the situation, the licensee retensioned the vertical tendons at an average lock-off force of 0.685 of theguaranteed ultimate tensile strength of the wires.During the performance of 20th-year tendon surveillance in November-December 1992 at Turkey PointStation, Units 3 and 4, the licensee, Florida Power & Light Co., found that the measured prestressing forces ofa number of randomly selected tendons in both units were appreciably lower than the predicted forces. Thelower tendon forces were found in hoop, vertical, and dome tendons. The licensee, with the assistance of itsconsultant, investigated the root cause and implemented necessary corrective actions. The root causeinvestigation indicated that the most probable cause for lower prestressing forces (higher prestressing losses)was an increased tendon wire steel relaxation resulting from the sustained high temperatures around thetendons. Analysis of the meteorological data indicated that the average sustained temperatures around thetendons could be estimated as 32 C (90 F). The supplier of the prestressing wire had provided 8 percent asthe wire relaxation loss at 20 C (68 F) and had indicated higher relaxation losses at higher temperatures. Inestimating prestressing forces, the utility had used 8 percent of the prestressing force in the tendons as theloss due to relaxation.Many of the prestressed concrete containments in the United States are typically subjected to average tendontemperatures greater than 32 C (90 F) during hot weather or year around. Although only three plantsreported lower prestresssing forces (than the predicted) due to higher (than considered in the design)relaxation, this condition may exist at many other plants with PCCs. However, plants may not experiencemore than projected loss of prestressing force due to (1) conservative estimates of losses in the design, (2)frequent unsystematic retensioning of tendons, (3) improper use of a method of trending measured tendonforces, or (4) a combination of Items (1), (2), and (3).Regulatory Guide (RG) 1.35.1, "Determining Prestressing Forces for Inspection of Prestressed ConcreteContainments" (July 1990), provides a simple method of documenting the installation forces, potential initiallosses in the prestressing force, and a method of incorporating the time-dependent losses. The basic conceptrecommended in the guide is to establish predicted forces for all the tendons at various times since thehttp://www.nrc.gov/reading-rmldoc-collections/gen-comm/info-notices1 999/in9901 O.html 03/13/2003 Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... complete installation of tendons. As the initial elastic shortening losses could vary from tendon to tendon, theindividual tendon predicted forces can be tabulated for comparison with the measured lift-off forces.Sometimes the measured lift-off forces are adjusted to account for the initial elastic shortening loss or thetime-dependent losses. This adjustment defeats the purpose of making a correct comparison. Sometimes themeasured lift-off force is computed using the effective unbroken wires in the tendon, thus making thecomparison inappropriate. Calculation of the average effective wire forces in the tendon from the measuredtendon force is made only to ensure that it does not exceed 70 percent of the guaranteed ultimate tensilestrength of the wire.In the United States, the main cause for the lower than predicted prestressing forces has been identified asthe high relaxation of the tendon steel. However, in France, where the prestressing tendons are grouted andtheir prestressing forces could not be directly measured, the cause for the indirectly estimated lowprestressing forces has been identified as creep and shrinkage of the containment concrete. The basic creep ofthe concrete could also be higher (than estimated) at higher temperatures, giving rise to higher loss of theprestressing force. These two effects on prestressing forces could not be separated without substantialresearch. On the basis of the results of the relaxation tests on the prestressing steel, it appears that thedominant contributing factor is the higher relaxation of the prestressing steel. Nevertheless, the containmentintegrity has to be demonstrated on the basis of the availability of the minimum required prestressing force.ATTACHMENT 3IN 99-10April 13, 1999Comparison and Trending of Prestressing ForcesIn 1994, during the 20th-year tendon surveillance of Three Mile Island Nuclear Station, Unit 1 (TMI-1),prestressed concrete containment (conforming to Regulatory Guide [RG] 1.35 [Revision 3]) and TMI-1Technical Specifications, the licensee, General Public Utilities Nuclear Corporation, subjected a total of 11tendons (5 hoop, 3 vertical, and 3 dome) to lift-off testing. On the basis of the data from this lift-off testing, inconjunction with data from the previous surveillance tests for each group of tendons, the licensee originallyperformed a trending analysis for each group of tendons and concluded that none of the tendon groups wouldgo below each group's minimum required force during the 40-year plant life. However, the licenseesubsequently performed a linear regression analysis using individual lift-off forces rather than the average ofthe lift-off forces and found that the hoop tendons would go below the minimum required force beginning inthe 25th year.The licensee of the Oconee Nuclear Station, Duke Power Company, performed the sixth tendon surveillance onOconee Unit 3 in the summer of 1995. The licensee, using the averages of the lift-off forces obtained to thatdate, plotted them on a graph on which the predicted upper bound and lower bound are shown and concludedthat the mean lift-off force for each group fell below the required values (i.e., the lower bound). A subsequenttrending analysis on the basis of individual lift-off forces indicated that the dome tendon force began to gobelow the minimum required force about 8 years after the structural integrity test (SIT). For other tendongroups in Unit 3, the tendon forces were not predicted to go below the minimum required value until 40 yearsor more after the SIT. Since Oconee Units 1 and 2 are identical to Oconee Unit 3, the licensee performed atrend analysis for each of these units and found that the vertical tendon forces in Unit 1 and Unit 2 werepredicted to go below the minimum value at 30 years and 10 years after the SIT, respectively. These resultswere caused by additional wire breakage of other vertical tendons. The licensee expanded the lift-off testingand visual examination to 100 percent of the vertical tendons. Similar degradation of other vertical tendonswas found. As a part of the licensee's corrective action, the licensee used the same tendons for lift-off testing,thus subjecting the tendons to cyclic loading. A more appropriate methodology is the random selection oftendons to be tested.In 1996, the V. C. Summer licensee, South Carolina Electric & Gas Co., performed the 15-year (fifth) tendonsurveillance. For each group of tendons, the licensee used the averages of the lift-off forces from eachsurveillance and plotted the five points from the five surveillances on a graph. The five points are joined byhttp://www.nrc.gov/reading-rm/doc-collections/gen-conm/info-notices/1999/in990 1 0.html 03/13/200?

Information Notice No. 99-10: Degradation of Prestressing Tendon Systems in Prestresse... line segments. On the basis of this graph, the licensee concluded that the tendon force levels in the threegroups of tendons would be acceptable beyond the 20-year surveillance. A subsequent linear regressionanalysis using individual lift-off forces, instead of the averages, indicated that the dome and hoop tendonswould not go below the minimum required forces until 32 years after the SIT. The vertical tendons that hadbeen retensioned were predicted not to go below the minimum required force until 42 years after the SIT.In 1993, the licensee of the Crystal River Nuclear Plant, Unit 3, Florida Power Corporation, performed the fifthtendon surveillance. A detailed study that considered both the average and the individual lift-off forces wasperformed. On the basis of the results of linear regression analysis, the licensee concluded that with theexception of the vertical tendons' result, which gave a slightly steeper slope for the individual data points,there was no difference between the two methods for the hoop and dome tendons. The prestressing forces inthe three groups of tendons were indicated to be above the minimum required forces well beyond the 40-yearplant life.The simple regression model is a mathematical way of stating the statistical relationship that exists betweentwo variables. In this case, the tendon force (TF) is a dependent variable that varies with time (T), theindependent variable. The two principal elements of a statistical relationship are (1) the tendency of thedependent variable TF to vary in a systematic way with the independent variable T, and the scattering ofpoints about the "curve" that represents the relationship between TF and T. For a small sample size (2% ofthe population), using the average of the TF for each surveillance test masks the true variation between TFand T. Therefore, an analysis using the individual lift-off forces for the regression analysis gives results thatcould be statistically validated.On the basis of experience, as evidenced from the examples presented and the statistical analysis, it isevident that the appropriate method for evaluating the adequacy of the tendon force is the regression analysisusing the individual lift-off forces as the data for the trend analysis.http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1999/in9901 0.html 03/13/2003