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:
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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 A U.SfU.S. Nuclear Regulatory

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Notices > 1999 > IN 9 UNITED STATES NUCLEAR REGULATORY

COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 April 13, 1999 NRC INFORMATION

NOTICE DEGRADATION

OF PRESTRESSING

TENDON SYSTEMS IN PRESTRESSED

99-10: CONCRETE CONTAINMENTS

Purpose

• Description

of Circumstances

o Prestressing

Tendon Wire Breakage o Effects of High Temperature

on the Prestressing

Forces in Tendons o Companoadedf

Presressiore

o ..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

to degradation

of prestressing

systems components

of prestressed

concrete containments (PCCs). The specific items addressed

are (1) prestressing

tendon wire breakage, (2) the effects of high temperature

on the prestressing

forces in tendons, and (3) trend analysis of prestressing

forces. It is expected that recipients

will review the information

for applicability

to their facilities

and consider actions, as appropriate, to avoid similar problems.

However, suggestions

contained

in this information

notice are not NRC requirements;

therefore, no specific 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 the degradation

of prestressing

tendon systems in PCCs and the ability of the containment

structure

to perform its function.

The relevant findings associated

with these concerns are discussed

below.Prestressing

Tendon Wire Breakage Recent observations

related to containment

prestressing

systems have revealed conditions

that may precipitate

tendon wire breakage.

Conditions

such as uneven shim stack heights on the anchor-heads, spalling and cracking of concrete beneath the anchor-head

base plates, free water in the bottom grease caps, poorly drained top anchorage

ledges, and the absence of filler grease in various areas can lead to corrosion

of tendons and eventually

to wire breakage.

Specific plant observations

and instances

of failure of tendons and associated

anchorages

are detailed in Attachment

1.Effects of High Temperature

on the Prestressing

Forces in Tendons http://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, and dome tendons. Investigations

and analyses have indicated

that the prestressing

tendon relaxation

losses range from 15.5 to 20 percent over 40 years at an average sustained

temperature

of 32 C (90 F) around the tendons. However, the tendon relaxation

loss values used in PCCs vary between 4 to 12 percent. These values were determined

at the presumed ambient temperature

of 20 C (68 F). The relevant plant observations

and discussions

are reported in Attachment

2.Comparison

and Trending of Prestressing

Forces The use of the provisions

of Regulatory

Guide 1.35.1 ("Determining

Prestressing

Forces for Inspection

of Prestressed

Concrete Containments")

or equivalent

methods are important

to maintaining

the safety function of the prestressing

tendon system and the concrete containment.

Moreover, proper comparison

and trending analysis is critical in determining

the future trends in prestressing

force in PCCs. Licensees

have reported losses using the average forces determined

from the liftoff testing, thereby masking the true variation

in the loss of prestressing

forces. An analysis using the individual

lift-off forces for regression

analysis gives results that are statistically

valid. Attachment

3 contains the staffs discussion

of the variation

in trend analysis of tendon prestressing

forces.Discussion

As nuclear power plants continue to age, in particular, plants with a PCC, the management

and mitigation

of effects of degradation

as a result of aging become increasingly

more important.

The containment

structure serves 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 prestressing

tendon system for containment

integrity

is based on the vital role it plays. However, other components

that make up the system also need to be examined.

The observations

detailed in the three attachments, and the observations

made during the Oconee site visit (see Attachment

1), indicate that other contributions

to the degradation

of containment

could potentially

compromise

its effectiveness.

PCC degradations, such as concrete spalling, water infiltration

into tendon galleries, and concrete cracking in the containment

and the containment

dome, all affect the containment's

ability to function properly.

It remains important

to ensure that the cumulative

effects of degradation

mechanisms

do not compromise

the safety of the containment.

The attributes

discussed

in the three attachments

will be useful in identifying

the potential

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 that they are required to consider industry-wide

operating

experience (including

NRC information

notices), where practical, when setting goals and performing

periodic evaluations

under Section 50.65, "Requirements

for Monitoring

the Effectiveness

of Maintenance

at Nuclear Power Plants," of Part 50 of Title 10 of the Code of Federal Regulations.

If you have any questions

about the information

in this notice, please contact one of the technical

contacts listed below or the appropriate

Office of Nuclear Reactor Regulation (NRR) project manager./s/'d by S. F. Newberry For David B. Matthews, Director Division of Regulatory

Improvement

Programs Office of Nuclear Reactor Regulation

Technical

contacts:

H. Ashar, NRR G. Hatchett, NRR 301-415-2851

301-415-3315 E-mail: hga@nrc.gov

E-mail: gxh@nrc.gov

Attachments:

1. Prestressing

Tendon Wire Breakage 2. Effects of High Temperature

on the Prestressing

Forces in Tendons 3. Comparison

and Trending of Prestressing

Forces 4. List of Recently Issued Information

Notices http://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

1 IN 99-10 April 13, 1999 Prestressing

Tendon Wire Breakage During the 20th-year

surveillance

of the prestressing

system of Calvert Cliffs Nuclear Power Plant, Unit 1, in June-July

1997, the licensee (Baltimore

Gas & Electric Company--BG&E)

found a low lift-off value compared to the prestressing

force for one of the three randomly selected vertical tendons. The low lift-off value was attributed

to the uneven shim stack heights on the two opposite sides of the anchor-head.

In accordance

with the plant's Technical

Specifications (TSs) requirement, the licensee tested two additional

vertical tendons adjacent to this tendon. However, during the lift-off testing of one of these tendons, noises were heard that indicated

that some of the tendon wires might have broken. A visual examination

of the tendon indicated

that three wires had broken at 12.7-17.2 centimeters

(5-7 inches) below the bottom of the button-heads.

Further examination

of the wires at the top of other vertical tendons revealed additional

wire breakage.

The licensee expanded the lift-off testing and visual examination

to 100 percent of the vertical tendons. Similar degradation

of other vertical tendons was found. As a part of its corrective

action, the licensee is planning to replace 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-Tensioned

Containment

Tendon Anchor-Head

Failure," described

prestressing

tendon anchor-head

failures at both units of the Joseph M. Farley Nuclear Plant. The root cause analysis of that event indicated

that there were several factors contributing

to it, such as, free water in the grease caps at the bottom of the vertical tendons, high hardness of the anchorage

material, and high stresses in the anchor-heads.

The failures had resulted from hydrogen embrittlement

of the anchor-head

material.

The free water in the bottom grease caps of the vertical tendons may have accumulated (through a number of years) from the poorly drained top anchorage

ledge of the vertical tendons (similar to the condition

at the Calvert Cliffs containments).

However, at Farley, wire failures did not occur.In general, American Society for Testing and Materials (ASTM) A-421 ("Uncoated

Stress-Relieved

Wire for Prestressed

Concrete")

wires (used at both Farley and Calvert Cliffs) are not susceptible

to hydrogen-induced

cracking.

However, BG&E's engineering

evaluation

indicated

brittle hydrogen-induced

cracking on a third of the broken wires. All of the brittle fractures

were preceded by severe corrosion.

The engineering

evaluation

also indicates

that some of the brittle fractures

may have occurred earlier but were not found during the periodic inspections.

To ensure that the stressing

washers (anchor-heads)

are not affected, BG&E visually examined the anchor-heads

at both ends of the vertical tendons and found no visible cracks or fractures.

The lessons 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 the licensee's

license renewal technical

report. As part of the visit, the staff performed

a walkdown inspection

of the OCN containments

and other structures.

The following

observations

are related to the prestressing

system degradations

reported by the staff:* At Tendon 12V6, the concrete beneath the 5.1-centimeter

(2-inch) thick anchor-bearing

plate had spalled along the outer edge; a cavity existed below the plate. Cracks in the concrete beneath the outer edge 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 three units at OCN.* The Unit 1 tendon access gallery showed water infiltration

and standing water at several locations.

The licensee indicated

that the Unit 2 tendon access gallery at one time held as much as 51 centimeters

(20 http://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 remove water.The licensee addressed

these and similar degradations

under the requirements

of its TSs or in accordance

with Criterion

XVI of 10 CFR Part 50, Appendix B.ATTACHMENT

2 IN 99-10 April 13, 1999 Effects of High Temperature

on the Prestressing

Forces in Tendons In 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

containment

structure.

Extensive

analysis and testing performed

by the licensee indicated

the cause of the consistently

lower prestressing

forces to be appreciably

higher (than estimated)

relaxation

of prestressing

steel as a result of the average high temperatures

around the tendons. The 1000-hour

and 10,000-hour

testing performed

at the Fritz Engineering

Laboratory

of Lehigh University

of the wires taken from some of the vertical tendons showed 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 the design was 12 percent.During the fourth surveillance

of tendon forces in February 1990 at Virgil C. Summer Nuclear Station, the licensee, South Carolina Electric & Gas Co., discovered

that the forces in the 115 vertical tendons were lower than expected.

Because the wires used in the prestressing

tendons were of the same size, type, and relaxation

property as those used in the Ginna tendons, the licensee concluded

that the reason for low prestressing

forces was the higher (than considered)

relaxation

of prestressing

wires. As in the case of the Ginna containment, the average temperature

around the tendons was determined

to be 32 C (90 F). To remedy the situation, the licensee retensioned

the vertical tendons at an average lock-off force of 0.685 of the guaranteed

ultimate tensile strength of the wires.During the performance

of 20th-year

tendon surveillance

in November-December

1992 at Turkey Point Station, Units 3 and 4, the licensee, Florida Power & Light Co., found that the measured prestressing

forces of a number of randomly selected tendons in both units were appreciably

lower than the predicted

forces. The lower tendon forces were found in hoop, vertical, and dome tendons. The licensee, with the assistance

of its consultant, investigated

the root cause and implemented

necessary

corrective

actions. The root cause investigation

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 the tendons. Analysis of the meteorological

data indicated

that the average sustained

temperatures

around the tendons could be estimated

as 32 C (90 F). The supplier of the prestressing

wire had provided 8 percent as the wire relaxation

loss at 20 C (68 F) and had indicated

higher relaxation

losses at higher temperatures.

In estimating

prestressing

forces, the utility had used 8 percent of the prestressing

force in the tendons as the loss due to relaxation.

Many of the prestressed

concrete containments

in the United States are typically

subjected

to average tendon temperatures

greater than 32 C (90 F) during hot weather or year around. Although only three plants reported 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 experience

more 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 tendon forces, or (4) a combination

of Items (1), (2), and (3).Regulatory

Guide (RG) 1.35.1, "Determining

Prestressing

Forces for Inspection

of Prestressed

Concrete Containments" (July 1990), provides a simple method of documenting

the installation

forces, potential

initial losses in the prestressing

force, and a method of incorporating

the time-dependent

losses. The basic concept recommended

in the guide is to establish

predicted

forces for all the tendons at various times since the http://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, the individual

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 the time-dependent

losses. This adjustment

defeats the purpose of making a correct comparison.

Sometimes

the measured lift-off force is computed using the effective

unbroken wires in the tendon, thus making the comparison

inappropriate.

Calculation

of the average effective

wire forces in the tendon from the measured tendon force is made only to ensure that it does not exceed 70 percent of the guaranteed

ultimate tensile strength of the wire.In the United States, the main cause for the lower than predicted

prestressing

forces has been identified

as the high relaxation

of the tendon steel. However, in France, where the prestressing

tendons are grouted and their prestressing

forces could not be directly measured, the cause for the indirectly

estimated

low prestressing

forces has been identified

as creep and shrinkage

of the containment

concrete.

The basic creep of the concrete could also be higher (than estimated)

at higher temperatures, giving rise to higher loss of the prestressing

force. These two effects on prestressing

forces could not be separated

without substantial

research.

On the basis of the results of the relaxation

tests on the prestressing

steel, it appears that the dominant contributing

factor is the higher relaxation

of the prestressing

steel. Nevertheless, the containment

integrity

has to be demonstrated

on the basis of the availability

of the minimum required prestressing

force.ATTACHMENT

3 IN 99-10 April 13, 1999 Comparison

and Trending of Prestressing

Forces In 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-1 Technical

Specifications, the licensee, General Public Utilities

Nuclear Corporation, subjected

a total of 11 tendons (5 hoop, 3 vertical, and 3 dome) to lift-off testing. On the basis of the data from this lift-off testing, in conjunction

with data from the previous surveillance

tests for each group of tendons, the licensee originally

performed

a trending analysis for each group of tendons and concluded

that none of the tendon groups would go below each group's minimum required force during the 40-year plant life. However, the licensee subsequently

performed

a linear regression

analysis using individual

lift-off forces rather than the average of the lift-off forces and found that the hoop tendons would go below the minimum required force beginning

in the 25th year.The licensee of the Oconee Nuclear Station, Duke Power Company, performed

the sixth tendon surveillance

on Oconee Unit 3 in the summer of 1995. The licensee, using the averages of the lift-off forces obtained to that date, plotted them on a graph on which the predicted

upper bound and lower bound are shown and concluded that the mean lift-off force for each group fell below the required values (i.e., the lower bound). A subsequent

trending analysis on the basis of individual

lift-off forces indicated

that the dome tendon force began to go below the minimum required force about 8 years after the structural

integrity

test (SIT). For other tendon groups in Unit 3, the tendon forces were not predicted

to go below the minimum required value until 40 years or more after the SIT. Since Oconee Units 1 and 2 are identical

to Oconee Unit 3, the licensee performed

a trend analysis for each of these units and found that the vertical tendon forces in Unit 1 and Unit 2 were predicted

to go below the minimum value at 30 years and 10 years after the SIT, respectively.

These results were caused by additional

wire breakage of other vertical tendons. The licensee expanded the lift-off testing and visual examination

to 100 percent of the vertical tendons. Similar degradation

of other vertical tendons was 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

of tendons to be tested.In 1996, the V. C. Summer licensee, South Carolina Electric & Gas Co., performed

the 15-year (fifth) tendon surveillance.

For each group of tendons, the licensee used the averages of the lift-off forces from each surveillance

and plotted the five points from the five surveillances

on a graph. The five points are joined by http://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 three groups of tendons would be acceptable

beyond the 20-year surveillance.

A subsequent

linear regression

analysis using individual

lift-off forces, instead of the averages, indicated

that the dome and hoop tendons would not go below the minimum required forces until 32 years after the SIT. The vertical tendons that had been 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 fifth tendon surveillance.

A detailed study that considered

both the average and the individual

lift-off forces was performed.

On the basis of the results of linear regression

analysis, the licensee concluded

that with the exception

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 in the three groups of tendons were indicated

to be above the minimum required forces well beyond the 40-year plant life.The simple regression

model is a mathematical

way of stating the statistical

relationship

that exists between two variables.

In this case, the tendon force (TF) is a dependent

variable that varies with time (T), the independent

variable.

The two principal

elements of a statistical

relationship

are (1) the tendency of the dependent

variable TF to vary in a systematic

way with the independent

variable T, and the scattering

of points about the "curve" that represents

the relationship

between TF and T. For a small sample size (2% of the population), using the average of the TF for each surveillance

test masks the true variation

between TF and T. Therefore, an analysis using the individual

lift-off forces for the regression

analysis gives results that could be statistically

validated.

On the basis of experience, as evidenced

from the examples presented

and the statistical

analysis, it is evident that the appropriate

method for evaluating

the adequacy of the tendon force is the regression

analysis using 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

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