Information Notice 1999-10, Degradation of Prestressing Tendon Systems in Prestresssed Concrete Containments
| ML031500244 | |
| Person / Time | |
|---|---|
| Issue date: | 04/13/1999 |
| From: | Matthews D, Newberry S 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 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
Addressees
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
0.html 03/13/2003