ML20012E721
| ML20012E721 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 03/28/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20012E719 | List: |
| References | |
| NUDOCS 9004060173 | |
| Download: ML20012E721 (4) | |
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....** SAFETY EVALUATION BY'THE OFFICE OF NUCLEAR REACTOR REGULATION h.E,L,A,1,E,0,,T,0,,T,H,E, DE,T,Ef M 1 N A,T,lp,N, p,F,,Cp,N,T A,1 NM E NT S,T,R,UCTURAL INTEGRITY ON THE BAS 15 0F THE FIFTEENTH YEAR TENDON SURVElLLANCE.
i ARKANSAS POWER AND LIGHT COMPANY ARKANSAS,,N,U,C,LE,A,R 0,NE,,, UNIT NO,.J DOCKET NO,.,50-313 1
1.0 INTRODUCTION
By letter dated July 20, 1988, as supplemented by letter dated September 13, i
1989, the licensee subinitted for staff review a report on the results of the fifteenth year physical surveillance of Arkansas Nuclear One, Unit I reactor building post-tensioning system.
In accordance with the plant technical specifications, the licensee is required to perform periodic tendon surveillances t
consisting of sheathing filler material inspection, anchor tendon lift-off force measurement, tendon wire continuity testing, tendon wire inspection and tensile testing,.and tendon retensioning and resealing.
For each of the first three surveillances (conducted at the 1, 3 and 5 year intervals), a minimum of 21 tendons (10 boop, 5 vertical and 6 dome tendons) shell be selected. For et.ch'of the subsequent surveillances (conducted every 5 years thereafter), a total of 9 tendons (3 boop 3 vertical and 3 dome tendons) shall be selected.
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The tendons shall be selected on a randon, but representative basis.
On the basis of the results of the fifteenth year surveillance and inspection or various elements of the system as corpared to the criteria established in the technical specifications, the licensee concluded that no abnormal degradation has occurred in the post-tensioning system.
2.0 EVALUATION The staff reviewed the results of the fifteenth year surveillance of various elements of the post-tensioning system as contained in the surveillance report, and found them rnostly to be in confornance with the criteria established in the relevant plant technical specifications.
However a close examination of the results and the manner in which they were considered raised some concerns. The l
following are the staff's concerns, licensee's responses to these concerns and staff's evaluation.
2.1 Grease Water Content l
The grease in the one tendon has a maximum absorbed water content of 10.37 and l
in addition small quantities of free water were found in the grease can or in the area surrounding sorre of the anchorages. Water in contact with the anchorage o
l can cause stress corrosion or hydrogen embrittlerrent.
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i In response to this concern the licensee offered the following discussion.
'Thelicenseerealizedthatlargequantitiesoffreestandingwaterandhigh percutages of water emulsified in the grease increase the chances of hydrogen i
erbrittlement of the anchorages and stress-corrosion of the anchor head or tendon wires. However, this last surveillance did not find the existence of i
high levels of free water and emulsified water in the grease. There was no evidence of significant water entry into the system. As a consequence, the potential for stress corrosion or hydrogen embrittlerent is considered to be i
low.
The staff reviewed the licensee's response and found the licensee to be well aware of the effect of the absorbed water and free water on the system especially the anchor head. Theexplanationbythelicenseeonthedifference of the two kinds of water and the effects of the amount of each kind of water on the system alleviated the staff's concerns.
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E 2.2 Grease Voids The grease voids in.sorre of the tendon sheathings were in excess of 5% and in one case in excess of 45%. 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 the concrete, thus potentially reducing concrete strength.
From the sheathing filler streaks which exist on the outside. of the containrrert wall at one location, it appears that there has been serie leakage from the sheathing.
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In response to this concern, the licensee offered the following discussion.
The licensee enlained that cf all 12 tendons, only two tendons have voids abcve 20% and tiree above 57. Most of the large voids were found in the done tendons. Reasons were given for the existence of the voids, including a void of 45.47 in one dome tendon which appeared to be an isolated case. The lift-off force cf this tendon was found to be above the minimum required tendon force, which indicated no abnornal degredation of the tendon. The licensee has conciitted to have this tendon completely detensioned during the next surveillance and a wire extracted for inspection for any evidence of corrosion.
It was noted that before installation, the tendons had been treated with corrosion protection material. Tendon surveillance procedures have been revised to t
include methods to more accurately monitor tendon grease loss and to assess its impact.
Regarding the existence of sheathing filler streaks on the outside of the containment wall, it appears that the streaks are a result of leaks from the horizontal tendon drains. The drains are plugged with screw-on caps which may not have been screwed on tight initially. The sheathing filler is present on limited areas and should be of no concern regarding concrete integrity or shear capacity.
The staff reviewed the licensee's response and found the reasons given for the existence of large tendon grease voids in one dome tendon, the appearance of grease streaks on a portion of the outside of the cor.tainnient wall, and the licensee's discussion regarding the insignificance of such occurrences to be convincing. The licensee's consiitment to detensioning the dome tendon and
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- i taking wire'sampics in the next surveillance to confirm the tendon integrity 1
is reassuring. However, an inspection of the concrete around the tendon.with large grease voids to ascertain no grease leakage into concrete to further j
assure containment integrity is highly advisable.
g 2.3 Tendon Lift-Off Force I
The surveillance showed that all the tendon lift-off forces are above the minimum required prestressing forces. Howeverforthosehorizontal(hoop) tendons which were surveyed in more than one surveillance a trend in the measuredlift-offforcesindicatedthattheprestressingforcesinthese i
tendons will be below the minimun design required value before the end of the design service life or in some cases before the next scheduled surveillance.
l In response to this concern, the licensee presented the results of a detailed study of all tendons which have been inspected more than once indicating that j
the tendon force loss was greater in hoe) tendons than in done and vertical tendons. This is attributed mainly to tie effect of friction and curvature.
The prestressing loss experienced due to surveying a tendon more than one tirae is, approximately, linterly proporticnal to the amount of curvature. Although overstressing the tendons to 80% of their specified ultimate strength during retensionits is intended to compensate for most of the losses due to friction and curvature, apparently some residual back friction is slowly released with-time.
It appears that the detensioning and retensioning operation has resulted i
in additional losses.
In view cf these findings, it was deterrained that the prestressing forces in tendons inspected more then once are not representative of the forces as a group. The prestressing forces of all tent *cns inspected should be taken into consideration to establish the group tendon force trend.
A statistical regression analysis was performed on the basis of all the data obtained for the horizontal tendons from all the surveillances performed to date together with their initial stressing forces. The results of the. analysis indicate that the trend of the tendon forces is such that thcy will rueet the minimum design requirement for the 40 year service life.
The staff reviewed the licensee's response and found that the licensee's detailed analysis of each group of tendons, with differentiation between tendons inspected once and those inspected more than once, appears to l
demonstrate convincingly that reductions in tendon forces larger than norrnal in hoop tendons which have been inspected more than once are due to the slow release of curvature friction and the effect of detensioning and retensioning on tendon behavior. Therefore, it appears rational that the prestressing forces in these tendons cannot be solely taken to represent the tendon forces of the tendon group.
3.0 CONCL_US),0N On the basis of our review end evaluation of the information provided by the licensee on the fifteenth year tendon surveillance, the staff concurs with the licensee's conclusion that no abnormal degradation has occurred in the post-tensioning system uscd in the concrete containnent of Arkansas Nuclear One, Unit 1.
It is expected that the results of the twentieth year tendon
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surveillance should confirm the licensee's findings. The staff suggests however, that to further ensure the s.aterial condition and integrity of the post-tensioning system, the licensee should consider acceptance criteria for 1
free water content and grease void, and establish actions to be taken when the criteria are not met. An example of an action would be to perform an internal
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engineering evaluation of the containnient and verify that containment integrity is maintained within 30 days of discovery that the acceptance 4
criteria are not met.
i F-Dated: March 28, 1990 Principal Contributors:
C. P. Tan, ESGB T. Alexion, PDIV
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