ML18064A441
| ML18064A441 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 10/28/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18064A440 | List: |
| References | |
| NUDOCS 9411040273 | |
| Download: ML18064A441 (6) | |
Text
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555--0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION CONTAINMENT STRUCTURAL INTEGRITY ON THE BASIS OF THE TWENTIETH YEAR TENDON SURVEILLANCE PALI SADES PLANT CONSUMERS POWER COMPANY DOCKET NO. 50-255
1.0 INTRODUCTION
By letter dated June 16, 1992, as supplemented by information contained in letters dated February 7 and September 8, 1994, the licensee submitted for staff review a report on the results for the twentieth year physical surveillance of the Palisades Plant containment building post-tensioning system.
In accordance with the plant technical specifications (TS), the licensee is required to perform periodic tendon surveillances consisting of sheathing filler material inspection, tendon lift-off force measurement, anchorage component inspection, tendon wire inspection and tensile testing, grease leakage check, and tendon retensioning and resealing.
For the twentieth year surveillance, four dome, four vertical, and six hoop tendons were randomly selected and subjected to lift-offs, with one tendon in each group being detensioned.
The surveillance was performed in accordance with the procedure contained in Regulatory Guide (RG) 1.35, Revision 3.
On the basis of the results of the twentieth year surveillance and inspection of the various elements of the system as compared to the criteria established in the TSs and in RG 1.35, Revision 3, the licensee concluded that no abnormal degradation has occurred in the post-tensioning system used in the Palisades containment.
2.0 EVALUATION The staff reviewed the results of the twentieth year surveillance of the various elements of the post-tensioning system as contained in the surveillance report and found them to be mostly in conformance with the criter~a established in the relevant plant TSs and in RG 1.35, Revision 3.
However, a close examination of the results which show some deviations and the manner in which they were considered, raised some concerns.
The staff's concerns, the licensee's responses to these concerns, and the staff's evaluation are presented as follows.
2.1 Grease Water Content The grease has absorbed water content of 15.5% in tendon number D2-23 and of 10.2% in tendon number V-20, and in addition, small quantities of free water were found iTI three anchorage grease cans. The staff's concern is that water r--94i 1040273 941020
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... in contact with the anchorage may cause stress corrosion of the anchor head or tendon wire, or hydrogen embrittlement of the anchorage.
However, *according to the licensee, the inspection of the anchorages of the 14 tendons did not show any visible corrosion or cracking of any of the anchorage components.
On the basis of this information, the staff concludes that the presence of water in the grease is limited to a few tendons, and its amount appears to be insignificant to cause corrosion or cracking of the anchorage components.
2.2 Grease Voids From the data on grease removed and grease replaced there appeared to be some voids in the grease.
Most of the grease voids are less than 5%, except in one tendon where the grease void is approximately 15%.
Large voids in sheathing may indicate that the tendon wires are not fully protected against corrosion.
Large voids may also indicate that there may be leakage of grease into concrete, thus potentially reducing concrete strength.
From the sheathing filler streaks which exist on the outside of the containment wall at various locations, it appears that there has been some leakage from the sheathing.
In response to this concern, the licensee offered the following explanations.
The licensee noted first that repair was performed on an additional 30 vertical tendon grease can gaskets for grease leakage which, the licensee believes, resulted from an ambient temperature change after hot greasing operations. The licensee further indicated that grease leakage was also found scattered along construction joints, and such a situation is not unique to Palisades. The licensee did not believe that grease voids had any impact on the integrity of the tendon and concrete as evidenced from the visual inspection of the concrete and the lift-off readings of the tendons.
As a more convincing evidence the licensee stated that during the steam generator replacement, 52 tendons were removed from the transfer opening and were examined and found to have adequate grease coverage with no evidence of corrosion.
The staff reviewed the licensee's response and found the reasons and the evidence to be convincing.
The effects of grease voids on the containment integrity appear to be insignificant.
2.3 Tendon Lifting Force The staff has reviewed the lift-off forces of four dome, four vertical and six hoop tendons and also the information on the force and elongation of the three retensioned tendons, one for each group.
With the exception of one hoop tendon (H29AE) and one vertical tendon (V218), the lift-off forces are above the lower bound values (i.e, the prescribed lower limits) established for each group.
The one hoop and one vertical tendon lift-off forces are below their respective prescribed lower limit, but above 95% of this limit. The force and elongation of each of the three retensioned tendons were analyzed.
The analysis indicates that there is some non-linearity in the force/elongation relation in hoop tendon H29AE.
According to the licensee, this was caused by a binding of the tendon and would not have any effect on the performance of the tendon.
The explanation appears to be convincing.
Subsection 7.1.6 of RG 1.35, Revision 3, implies that the trend of the pre-stress loss should be determined if the loss is larger than expected, for example, in this case, tendons H29AE and V218.
In order to determine the trend, it is necessary to perform a regression analysis of the tendon lift-off forces for all the tendons in a group, from all the surveillances conducted to date after the structural integrity test. The licensee has provided graphs showing the average lift-off force curve for each group, but it did not give any information on the trend.
Using the incomplete information (e.g., no data on the first-and third-year surveillances), the staff has performed an independent regression analysis for each of the group of tendons (see attached figs. 1-3). The results of the analysis indicate that for each of the groups the pre-stressing forces will not be less than the minimum required before the next scheduled surveillance. However, it should be noted that this is based on the regression analysis of incomplete data and therefore, the results are at best approximate.
In view of this, the licensee should perform regression analyses with the complete data to che~k the staff's approximate analysis before the forthcoming scheduled surveillance in 1997.
The licensee should keep the NRC staff informed about the results of the regression analysis. It is important that in the analysis for each group of tendons, the individual lift-off forces for each of the tendons should be plotted and average values should not be used for a group. -
2.4 Inspection and Testing of Sheathing Filler and Tendon Hardware The anchorage components (anchorhead, bushing, shims, buttonheads, and bearing plates) were inspected for corrosion and cracks by the licensee.
No cracks were found on any anchorage components.
No visible corrosion, or visible oxidation or pitting was found.
The buttonheads were inspected for their physical condition and a count of protruding and missing buttonheads was made and found to be within the acceptable limit.
One wire from each of the detensioned dome, hoop, and vertical tendons was inspected and tested.
The corrosion condition of the wire, its variation in diameter, and the elongation at failure are all within the allowable limits.
The results of the chemical tests of the sheathing filler material were found to be within the acceptable limits. All the acceptable limits are specified in RG 1.35, Revision 3, or in the plant TS.
The staff has reviewed the results and found them to be reasonable.
3.0 CONCLUSION
On the basis of the staff's review and evaluation of the information provided by the licensee, the staff concurs with the licensee's conclusion that the Palisades containment building has experienced no abnormal degradation of the post-tensioning system.
However, the licensee should use all the available data and perform an adequate regression analysis, as discussed above, both before and after the 1997 scheduled surveillance. This is required in order to assure that the tendon forces will not be below the minimum required before the next scheduled surveillance.
Principal Contributor:
C. P. Tan Date: October 28, 1994
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