ML20080B906
| ML20080B906 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 08/01/1983 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20080B904 | List: |
| References | |
| NUDOCS 8308180549 | |
| Download: ML20080B906 (10) | |
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UNITED STATES
,q NUCLEAR REGULATORY COMMISSION y
.t WASHINGTON, D. C. 20555
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 86 FACILITY OPERATING LICENSE NO. DPR-53 BALTIM0RE GAS AND ELECTRIC COMPANY CALVERT CLIFFS NUCLEAR POWER PLANT UNIT N0. 1 DOCKET NO. 50-317-Introduction By application dated March 7,1983, as supplemented by letter dated May 24, 1983, Baltimore Gas and Electric Company (BG&E) requested changes to the Technical Specifications (TS) for Calvert Cliffs Unit 1.
The proposed changes to the TS would provide for revised Limiting Conditions for Operation and Surveillance Requirements for containment tendons. These requirements are cur-rently addressed in TS 3/4.6.1.6, " Containment Structural Integrity."
Background
Post-tensioning of the Unit 1 containment building was completed in 1972. The Calvert Cliffs Unit 1 integrated leak-rate and initial structural integrity tests were completed two years later in 1974 prior to plant operation.
In accordance with the surveillance' program required by TS 3/4.6.1.6, the first, third, and fifth year tendon surveillances were performed in 1975, 1977, and 1979 respectively. The intent of these periodic surveillances, as described in the bases for the TS, is to ensure that the structural integrity of the containment will be maintained comparable to the original design standards for the life of the facility.
The results of the first, third, and fifth year surveillances indicated that the containment post-tensioning system was performing adequately; however, some of the lif t-off forces that were measured during the third year (1977) sur-veillance fell below the limits established by TS 3/4.6.1.6.
Although there was no evidence that these lower measurements indicated an overall trend of degraded performance by the post-tensioning system, BG&E felt that a confirmatory reanalysis of the containment design was needed to verify conformance with the original de-sign standards. An engineering evaluation was then performed by Bechtel Power
~ Corporation which included a reanalysis of the post-tensioning system utilizing updated material properties data for the concrete used in the Calvert Cliffs Unit I containment. The results of this evaluation confirmed that the' existing post-tensioning system will continue to satisfactorily perform its design function throughout the licensed lifetime of the plant.
8308180549 830801 PDR ADOCK 05000317 p
During the engineering evaluation described above and during subsequent review, the licensee developed the following information:
1.
More realistic data were presently available for estimating time-dependent losses in the tendons than were available for use when i
the existing TS was written.
2.
The ratio of total prestress force to total design pressure force for the original design was higher than that required by the design load combinations for all three groups of tendons (hoop, vertical and dome).
3.
The TS should be revised to provide for:
Ti[ne-dependent prestress force limits.
e e Normalizing of measured lift-off forces.
e The expected scatter of tendon lift-off force measurements.
e Incorporation of acceptance criteria for the average lift-off force of each group of tendons.
e Recognize the necessity to perform an engineering evaluation of surveillance results if evidence of possible abnormal de-gradation is identified. This evaluation is required to deter-mine whether an apparent reduction in tendon performance is in-dicative of an overall degradation of tendon performance or:
l was caused by environmental conditions not properly accounted for in the test procedure;
- was the result of measurement errors introduced by a specific characteristic or deficiency inherent to the test method employed; or
- was an isolated occurrence not significantiy affecting the structural integrity of the containment.
e Revision of the action statement for surveillance inspections conducted on end anchorages, adjacent concrete surfaces, or the liner plate to recognize the fact that these inspections are conducted while the plant is shutdown.
The application dated March 7, 1983, as supplemented by letter dated May 24, 1983, addresses these items as they relate to TS 3/4.6.1.6.
_________-m.__._
COMPARIS0N OF EXISTING AND PROPOSED SPECIFICATIONS The proposed revision to TS 3/4.6.1.6 affects the action statement of the Limiting Condition for Operation (Specification 3.6.1.6), and the surveil-lance requirements for Containment Tendons, (TS 4.6.1.6.1).
The minimum required number of surveillance tendons and the required surveillance schedule are not affected.
No changes are being proposed to the require-ments for inspections of end anchorages, adjacent concrete surfaces, or the liner plate (Specifications 4.6.1.6.2 and 4.6.1.6.3).
Limiting Conditions for Operation The action statement in the existing TS is a single paragraph which requires, when the structural integrity of the containment does not conform to a level consistent with the acceptance criteria specified in the surveillance require-ments, that structural integrity be restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or that the plant be shut down.
In the proposed revision to the TS, the action statement is divided into two parts. The first part, "a", defines the action that would be applicable during tendon surveillances; the second part, "b", defines the action that would be applicable during surveillances of end anchorages, adjacent concrete surfaces, and the liner plate. This segregation of the action statements recognizes a distinction between the two basic types of surveillances governed by this speci-fication.
Inspections and tests of tendons can be performed during plant opera-tion. On the other hand, inspections of end anchorages, adjacent concrete sur-faces, and the liner plate must be performed when the plant is shutdown.
The proposed action statement relative to tendon surveillance is similar to the existing specification, except that the 24-hour limit for restoring structural integrity has been deleted and replaced with a requirement to perform an en-gineering evaluation demonstrating the continued ability of the containment structure to perform its design function.
If continued containment integrity cannbt'be assured by engineering analysis within 90 days, the plant must be shut down.
The proposed action statement relative to the surveillance of end anchorages, adjacent concrete surfaces, and the liner plate requires restoration of structural integrity or an engineering evaluation to verify structural integrity prior to returning the plant to operation.
Tendon Lift-Off Testing 9
Existing The existing TS 4.6.1.6.1 requires that individual tendon lift-off forces lie between a maximum and a minimum bound. These bounds are constant values, ex-pressed in pounds per tendon wire, which are applicable to all three groups of tendons (dome, vertical, hoop). The lower bound is approximately equal to the minimum effective prestress after 40 years based on the estimated prestress losses assumed for the original design, and is about 57% of the ultimate strength of the installed post-tensioning wires. The upper bound is approximately equal to the prestress at lift-off during the initial installation, or about 74% of.
the ultimate strength of the wires.
If the lift-off force of any tendon lies
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outside the predicted bounds, testing of the two adjacent tendons is required.
If more than one tendon out of the original sample population lie outside the predicted bounds, it is considered to be evidence of possible abnormal degra-dation of the containment structure.
The existing specification requires that lift-off testing include an unloading cycle in which each of the surveillance tendons is detensioned to determine if any wires are broken or damaged.
7 Proposed Revision Figures 4.6-1, 4.6-2 and 4.6-3 of the proposed TS define the variation in pre-stress force (in kips per tendon) with time (in years after initial prestressing) for hoop, vertical and dome tendons.
Each TS figure provides time dependent plots for expected prestress, lower limit of the expected prestress range, and lower bound prestress for individual tendon.s.
The required average prestress force is plotted as a constant value that lies between the expected prestress and lower bound prestress plots.
The figures do not include an upper bound, since the purpose of lift-off testing is to verify that the tendons can perform their intended function, and higher than expected prestress is advantageous.
The " expected" prestress plot shows the variation of prestress with time based on the "best estimate" of time dependent losses.
For the original design, shrinkage and creep losses were extrapolated from 28-day test results available at that time. When the 1977 engineering evaluation was performed, test data were available for shrinkage and creep of concrete cylinders made from the same ingredients and mix design as the containment structure and loaded at 180 days and 365 days.
This test data was correlated to the age after placement at which the dome and cylinder wall were prestressed and provided a more reliable estimate of the losses.
This same test program provided elastic moduli for the concrete for use in determining more reliable estimates of the elastic losses.
Also, test data was available from the wire manufacturer which confirmed the estimates of wire relaxation losses. The combined net effect of using the re-vised-losses is slightly higher expected prestress forces at 40 years than the corresponding value of the existing specification.
The " lower limit expected range" of prestress shown on the figures is a plot parallel to and about five percent lower than the expected prestress plot.
The lower limit was determined by considering variations in material properties and measurement tolerances. The five percent difference is consistent with the scatter between mean and minimum lift-off values experienced during past surveil-lance at Calvert Cliffs and other plants. The lower limit of expected range results in a prestress level (ratio of total prestress load to total design pres-sure load) of at least 1.20 after 40 years of service.
The " required average" prestress plot is the average prestress force that was verified to be acceptable for all of the FSAR load combinations by reanalysis of the containment structure after the 1977 surveillance. This required average prestress provides a prestress level equal to or greater than 1.20 in all cases.
It is a conservative limit established for surveillance use, and is higher than the minimum prestress level. required to meet all design criteria at the end of the life of the plant.
..The " lower bound individual" prestress shown in the figures is the lowest plot parallel to the expected prestress plot.
The difference between the lower bound individual plot and the required average prestress plot increases with time, with a maximum difference of about 10 percent at 40 years. The lower bound individual plot is provided to account for anticipated maximum l
scatter of individual test results.
The proposed TS 4.6.1.6.la requires that the normalized lift-off force of each tendon equals or exceeds the lower limit of expected range for the time of the tests.
If the normalized lift-off force of any tendon lies between the lower limit expected range and lower bound individual limit, testing of the adjacent tendons is required.
If the normalized lift-off force of any of the adjacent tendons lies between these same limits, it is considered to be evidence of possible abnormal degradation of the con-tainment structure.
Th'e proposed TS 4.6.1.6.la has two additional acceptance criteria for lift-off tests.
It also is considered to be evidence of possible abnormal degradation of the containment structure, if:
a) the normalized lift-off force of any tendon lies below the lower bound individual limit; or b) the average of the normalized lift-off forces for any group (hoop, ver-tical or dome) lies below the required average prestress limit.
c) there is more than one unacceptable (lift off force) tendon from the total sample population.
Wire and Filler Grease Visual Inspection The requirements for visual inspection of wire and filler grease are essentially the same in the existing and revised specifications.
However, the revised specification is more prescriptive in that it identifies the presence of abnormal corrosicn, cracks or other damage to a wire as evidence of possible abnormal degradation of the containment structure.
In addition, a chemical analysis of the grease is proposed in TS 4.6.1.6.lc.
This analysis had been previously per-formed under existing plant procedures.
Wire Testing The requirements for tension testing of tendon wires are the same in the existing TS and proposed TS 4.6.1.6.lb.
The existing specification identi-fies the failure of any one of the wire samples to meet the minimum tensile strength test as evidence of abnormal degradation of the containment struc-ture. The revised specification identifies such a failure as evidence of possible abnormal degradation of the containment structure.
Tendon Sample Reduction The existing TS 4.6.1.6.1 allows reduction of the tendon sample for lift-off testing, from 21 tendons to 9 tendons, based upon a finding during the first three tests (one, three, and five years) that no abnormal degrada-tion exists as indicated by the tendon lift-off test.
The proposed change
to TS 4.6,1.6.la would delete the provision for reduction of the tendon sample and include a requirement for surveillance of 9 tendons in TS 4.6.1.6.la.
Evaluation The revised action statement for the tendon TS recognizes that the acceptance criteria for the containment tendon surveillance requirements are in reality preliminary warning signals that need to be investigated.
None of the occur-rences identified as evidence of possible abnormal degradation is in itself an indication that requires emergency action.
Instead, further testing and/
or engineering analyses are indicated on a case by case basis.
The inherent safety of the containment structure design, relative to the occurrences that the revised tendon surveillance requirements consider to be possible abnormal degradation of the containment structure, is discussed herein. This inherent safety assures that reasonable periods of time are avhilable for engineering analysis supplemented by additional testing, as appropriate, following the detection of such evidence.
Moreover, no con-ditions are associated with "possible abnormal degradation" which would be more limiting during reactor start-up.
Specification 3.0.4, which normally precludes startup if an action statement is invoked, is therefore not applicable to TS 4.6.1.6.
Accordingly, the action statement for the tendon TS, radesig-nated as action statement "a", is acceptable.
With regard to the action statement associated with the surveillance of end anchorages and adjacent concrete surfaces and liner plate, the intent of the action has not changed.
The existing action statement requires restoration of structural integrity within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or a reactor shutdown is required.
Since the surveillance of end anchorages and adjacent concrete surfaces and liner plate, TS 4.6.1.6.2 and 4.6.1.6.3, is undertaken with the reactor shut-down, the existing action statement is not appropriate.
The revised TS action statement, designated as "b", requires reestablishment of structural integrity or completion of an engineering evaluation prior to increasing the reactor coolant system temperature above 200 F.
This action statement preserves the intent of the requirement that the reactor not be operated with unacceptable degradation of end anchorages and adjacent concrete surfaces and/or liner plate.
Accordingly, proposed action statement "b" is acceptable.
Tendon Lift-Off Testing The prestress limits specified in the existing and revised TS 4.6.1.6.la, ex-pressed as a percentage of the ultimate tensile strength of the tendons, are as follows:
One Year After 40 Years After Initial Prestress Initial Prestress Existing Specification Limits for Measured Lift-Off Forces Maximum Prestress 74%
74%
(Hoop, Vertical & Dome Tendons)
Minimum Prestress 57%
57%
(Hoop, Vertical & Dome Tendons)
Pro osed Specification Limits for Normalized Lift-Off Forces Expected Prestress, Hoop 63%
60%
Vertical 64%
61%
Dome 62%
58%
Lower Limit Expected Range, Hoop 60%
60%
Vertical 61%
57%
1 Dome 58%
54%
I Required Average Prestress, Hoop 51%
51%
Vertical 59%
59%
Dome 52%
52%
Lover Bound Individual, Hoop 49%
45%
Vertical 56%
53%
Dome 51%
47%
The use of normalized lift-off forces would be required by the revised TS.
Normalizing of the measured lift-off forces corrects for variations in the elastic losses, resulting from progressive shortening of the concrete as a function of the sequence in which the tendons were post-tensioned, so that the corrected value is indicative of the average prestress for the contain-ment.
The revised TS has added the requirement that the average of the normalized lift-off forces for any group equals or exceeds the specified required average prestress force. This change is justified, since the design of the containment structure is based on the average prestress force in each group of tendons. The containment design criteria requires that the containment remain in membrane compression #0r the initial structural integrity test and other working stress design load cases.
For combined factored load cases, membrane concrete cracking is pennitted. This cracking allows the tendons to be~retensioned from the effective value until force equil'ibrium is satis-fied.
In all cases, a total of three hoop, three vertical, and three doms tendons are allocated to surveillance use and are considered to be totally ineffective.
The ratio to total effective prestress force to total internal pressure force is a useful gage of membrane compression.
For the Calvert Cliffs containment design, the ratio of the minimum prestress force given in the existing TS to the design pressure force is 1.30, which is higher than the ratio required by
.the design load combinations. Reanalysis of the containment structure design performed during the 1977 engineering evaluation verified that the reductions of prestress to the " required average" prestress values, when combined with all required design loads, did not significantly affect the stres's levels in the containment structure.
Lift-off test requirements of the revised specification consider the following occurrences to be evidence of possible abnormal degradation of the containment structure:
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a) The normalized lift-off force of more than one tendon of those selec-ted for surveillance lies between the expected lower limit and the lower bound individual limit.
b)
The normalized lift-off force of any tendon lies below the lower bound individual limit.
c) The average of the normalized lift-off forces for.any group (hoop, ver-tical or dome) lies below the required average prestress limit.
There is no immediate hazard associated with continued operation of the plant, when any of the occurrences considered to be evidence of possible abnormal degradation is detected. The containment structure would continue to serve its design function if the ratio of total required average prestress force tU total design pressure force were reduced.
As an example, if the ratio were reduced to 1.10 for the hoop tendons a~nd the prestress levels of the vertical and dome groups were reduced proportionally, the post tensioning system would require a total of only 772 tendons (414 hoop, 178 vertical and 180 dome) with the specified required average prestress.
This suggests that a total of 90 tendons could have zero prestress.
Or to state this another way, the required average prestress in this example could be reduced from 51%
of the ultimate tensile strength to 46% for the hoop tendons, from 59% to 53% for the vertical tendons, and from 52% to 47% for the dome tendons with no violation of design function.
Even if the initial results of surveillance tests suggest accelerated loss of prestress below the design pressure, containment integrity would be maintained.
Pressurization of the containment from a postulated accident causes the con-tainment to grow outward and upward.
If the average prestress level is below the accident pressure such that membrane compression is lost, further expansion of the containment will cause the tendons to elongate.
This elongation results in restressing of the tendons, restoring the pressure-retaining capability of
.the containment and creating an equilibrium condition in the structure.
This phenomenon utilizes the accident pressure to increase the level of prestress and h'as'been employed to demonstrate by analysis that the ultimate capacity of prestressed containment structures exceed their design capacity by a factor of 2 or more regardless of the initial prestress level. Two such analyses are presented in References (1) and (2), which are technical papers that were pre-sented at the Workshop on Containment Integrity sponsored by the U.S. Nuclear Regulatory Comission in June 1982. Although no such analysis was conducted specifically for the Calvert Cliffs containment, the similarities in design concept with those that have undergone a detailed evaluation further enhance the conclusion that significant margins exist in the post-tensioning system beyond those identified in the design. These margins will prevent loss of containment function should lower than expected prestress levels occur.
(1)
Internal Pressure Capacity of Prestress Concrete Containment, P. Shunmugavel and T. Johnson.
(2) Analyses of Zion Containment, A. Walser.
1 j
The war' ing signals identified as "possible abnormal degradation" may require n
a significant review effort, including engineering evaluation, before resolu-tion is finalized.
Post-tensioned containments represent a sophisticated design concept requiring detailed analysis.
In addition to the complexity of the design, the number of components is substantial.
There are a total of 871 tendons (465 hoop, 202 vertical and 204 dome) installed in the containment structure. Of these, a total of 9 tendons (3 of each group) are allocated to surveillance utilization and therefore are not considered to participate in the post-tensioning function, leaving 862 tendons (462 hoop, 199 vertical and 201 dome) to contribute to the effective prestress of the structure.
Recognizing that significant margins exist in the design of the structure, it is appropriate that a period of up to 90 days be provided to allow a timely but thoroughly evaluated conclusion with regard to any early warning signals.
Based upon the above, we conclude that the revised surveillance for tendon lift-off testing ipcorporated in TS 4.6.1.la is acceptable.
Wire Inspection The revised tendon wire surveillance requirements of proposed TS 4.6.1.6.1.b.1 consider the presence of abnormal corrosion, cracks or other damage to a wire to be evidence of possible abnormal degradation of the containment structure.
There are a total of 77,533 wires (41,562 hoop,17,890 vertical and 18,081 dome) installed in the Calvert Cliffs containment structure, not counting those in the nine tendons allocated to surveillance use. Although operating experience has demonstrated that these wiras normally show surface oxide film, the detection of an abnormal condition in one of three samples in a total population of 77,533 wires does require additional investigation,* but cannot realistically be con-sidered a significant hazard.
Accordingly, we find the proposed changes to TS 4.6.1.6.1.b.1 to be acceptable.
Wire Testing The revised tendon wire tensile test requirements of proposed TS 4.6.1.6.lb.2 als6 c'onsiders the failure of any one of the wire samples to meet the minimum tensile strength test to be evidence of possible abnormal degradation of the containment structure. Again, the occurrence of one failure out of nine test specimens requires additional investigation, but does not constitute a reduction of safety margin. We find the proposed changes to TS 4.6.1.6.1.b.2 to be accep-table.
Sheath Filler Grease Surveillance The existing TS 4.6.1.6.1 had required that a sample of sheath filler grease be inspected in order to detect changes in physical appearance which might affect the anticorrosion properties of the grease.
The proposed sheath filler grease I
In this case, as in other situations involving possible abnormal degradation of the containment structure, an engineering evaluation must be completed within 90 days per TS 3.6.1.6.
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surveillance requirements of TS 4.6.1.6.lc would also require a chemical analysis of the grease to detect chemical changes. We find that the ad-dition of a Technical Specification requirement to perform a chemical a-alysis of the grease increases the likelihood that unacceptable changes to the anticorrosion properties of the grease will be detected.
Accordingly, proposed TS 4.6.1.6.lc is acceptable.
Tendon Sample Reduction l'
The licensee has conducted the one, three, and five year tendon surveillance without noting abnormal degradation of the containment structure, based upon the tendon lift-off tests.
Since the existing TS 4.6.1.6.1 allows reduction in the tendon surveillance sample from 21 to 9 tendons for subsequent 5 year surveillances, this provision may be deleted and the requirement to select 9 tendons can be incorporated in TS 4.6.1.6.la.
We find this change acceptable.
Environmental Consideration We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR Sl.5(d)(4), that an environmental impact statement or negative declaration and environ-mental impact appraisal need not be prepared in connection with the issuance of this amendment.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the conmon defense and security or to the health and safety of the public.
Date:
Principal Contributors:
D. Jaffe N. Romney
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