ML20196J290
| ML20196J290 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 02/24/1988 |
| From: | Dwivedy K, Thornton J VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | |
| Shared Package | |
| ML20196J260 | List: |
| References | |
| NUDOCS 8803140246 | |
| Download: ML20196J290 (15) | |
Text
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l CE TECHNICAL REPORT NO. 0005 DIFFERENTIAL SETTLEMENT BETWEEN SERVICE BUILDING MD UNIT 2 MAIN STEM VALVE HOUSE NORTH MNA POWER STATION - UNITS 1 AND 2 CIVIL ENGINEERING ENGINEERING MD CONSTRUCTION DEPARTMENT VIRGINIA ELECTRIC MD POWER COMPMY REY. 1 ;
i FEBRUARY 24, 1988 i
Prepared Dy: g.2 s .gg Reviewed By: 3M sm -23 89 Reviewed By: K W. % 'g k 2-29-F8 Approved By: M s A-ZS - 86 i
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8803140246 DR 830310 l ADOCK 05000338 l DCD l l
DIFFERENTIAL SETTLEMENT BETWEEN SERVICE BUILDING AND ijNIT 2 MAIN STEAM VALVE HOUSE VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION - UNITS 1 AND 2
_ Introduction This report was prepared in response to the requirements of North Anna Technical Specifications 3/4.7.12, "Settlement of Class 1 Structures," for the operation of Units 1 and 2. Differential settlement between the Service Building and the Unit 2 Main Steam Valve House (MSVH) as defined by survey has exceeded 75 percent of the allowable settlement given in Table 3.7-5 of the Technical Specifications. As required in the Technical Specifications, this report provides a review of the observed settlement and provides a basis for actions to be taken. The 75% threshold was previously exceeded in 1980 and a report providing details and suggested remedial action was submitted by Vepco to the U.S. Nuclear Regulatory Commis'sion on February 20, 1981. (Serial No.
118). This latest report updates the results of extensive settlement monitoring since 1980, briefly describes the present basis of differential settlement, and discusses present and future margins of pipe stress in D e service water lines.
As demonstrated by this report, the reported differential settlement poses no imediate concern to the safe operation of the service water lines and the plant.
Background Info w tion The western end of the Service Building (especially in the area of the four Emergency Diesel Generator Rooms) is underlain by a variable thickness of compressible, soil-like, decomposed rock called "saprolite." Figure 1 shows a plan of this area, together with column lines and the locations of pertinent settlement monitoring points. Under the southern wall along the E line of the 102-JST-1344B-2
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Service Building and under the northern wall (along the C Line, 68 ft. north of the E Line), the saprolite is as much as 15 to 20 ft thick, resp?ctively. The presence of this compressible material has resulted in settlement of the Service Builoing to th' +.. tt of the 10 Line, especially from the 14 line to the 17 lir.e.
As explained in the February 1981 report, settlement along the C line is considered to have started in September 1972 when most of the structural loading on the Service Building had been applied and the crane rail in the Turbine Building was erected along the C line. Settlement of the crane rail relative to its as-built elevation was measured periodically by Stone & Webster (S&W) construction surveyors beginning in December 1973. S&W later scribed points on columns along the C line in 1975 to afford a more formal systematic settlement monitoring program. These points were first surveyed in August 1975. Settlement monitoring along the E line was not begun until August 1976, at which time S&W determined the elevation of numerous points on the Service Building floor slab. l The formal program of settlement monitoring was initiated in May 1976 when the surveying firr of Moore, Hardee & Carrouth Associates (MH&C), under contract to !
Virginia Electric ana Power Company, began monitoring points on the C and E ;
lines on a regular basis.
As stated in the February 1981 repor. ., the survey of the Service Building in August 1976 indicated apparent settlements from the as-built elevations that l i
would have caused increased stresses on the four 24-in-diameter buried service l i 1 water litet beneath the footings of this structure. These lines, which are 102-JST-13443-3 I
f s ,.. V- .
encased in four (4) foot-thick reinforced concrete, run southerly from beneath the Service Building at the 14 Line, under the 25-ft-wide roadway, and through the northern wall of the MSVH. Under the provisions of 10CFR50.55(e), the NRC Office of Inspection and Enforcement was notified of this settlement and its effect on the service water lines on Occember 15, 1976. The stress analyset of these lines and the remedial action taken in April 1977 to improve the stress conditions are described in Amendment 63 to the FSAR, dated July 8, 1977, under the response to Staff Comment 2.22. The remedial action taken in April 1977 included cutting the service water lines and rewelding them to reduce the
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stresses in the lines caused by prior settlement. This activity established a 3
baseline for future settlement detenninations by assuming zero settlement at that time. '
Based upon data accurralated prior to 1977, an estimated future settlement of 3/8 inch (0.031 feet) was utilized in the pipe stress analysis. This estimated valua of future (after April 1977) settlement of the southern wall of the Service Building was subsequently included in Technical Specifications 3/4.7.12. In June 1979, Table 3.7-5 of the Technical Specification was revised -
in Amendment No. 12 to the Facility Operation License to correct a typographical error and to clarify the baseline date of monitoring. The allowable differential settlement between the Service Building (Point 117) and the MSVH (Point 113) was stated as "0.03' from 7/77" (the month of the first survey by MH&C fol?owing the remed!al action). The revision also added a footnote indicating that the "critical differential settlement is down% rd movement of Point 117 with respect to Point 113." Surveying of points on the !
l MSVH in August 1976 had indicated negligible settlement of this structure.
Thus, i' was assumed that no future settlement of the MSVH would occur.
102-JST-13448-4
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Settlement Record A redundant survey point,113R, was established in November 1979 and tied in with Point 113. The purpose of Point 113R was to provide an alternate point in the event Point 113 was inaccessible during a survey. This is consistant with the intent of the Technical Specifications. The baseline reading of 113R was corrected to July 1977, the date which was considered the initial reading for Point 113.
The record of differential settlement as indicated by survey between Point 117 and Point 113 or 113R since July 1977 is given in Table 1. The variability in the elevations of the two structures does not necessarily reflect their individual behavior but, may indicate inaccuracies within survey tolerances of measurements from one survey to the next. ;
l The primary contribution to the differential settlement between the Service l Building and the MSVH is the settlement of the Service Building and in particular Point 117. Disregarding inherent survey inaccuracies, Figure 2, which is an arithmetic plot of the movement of Point 117, clearly shows there has been negligible additional settlement since 1981. The curve drawn through the data points is not a mathematical fit, but is presented to illustrate that the rate of settlement of Point 117 has steadily decreased during the monitoring period.
Figure 3 shows that there has essentially been no settlement of the MSVH, although fluctuations or inherent survey inaccuracies may increase the apparent margin of differential settlement (i.e. a negative reading or indicated upward 102-JST-13448-5
r-
... y movement of Point 113 or 113R causes an increase in the differential settlement relative to Point 117). The apparent upward movement of the MSVH in May 1987, as defined by the survey of Points 113 and/or.113R, was sufficient to cause the differential settlement to exceed 75% of the allowable.
As evidenced by Table 1, the differential settlement between Points 117 and 113 and consequently the percent of the allowable, have fluctuated significantly since 1980. As presented in Figure 4, the percent of allowable differential i
settlement fluctuated between 40 and 80 percent from mid-1978 to the present, although there has been no apparent change in foundation loading or trend of increasing differential settlement established.
The 1981 report projected a future rate of differential settlement between the Service Building and the MSVH based on the pre-1980 monitoring data. ,
Considering this relatively limited data base, the 1981 report stated that an ultimate differential settlement of 0.08 to 0.10 ft. (1 to 1 1/4 in.) could be anticipated over the 40 year plant life. Based on this attempt at future projection, the report also stated that 100 percent of the current allowable Technical Specifications limit of 0.03 ft. could be exceeded by 1983. The l post-1980 monitoring data reveals a much slower rate than was projected in 1981. Figure 5 shows the post-1980 differential settlement as well as the rates originally proj Nted on Figure 3 of the 1981 report. The post-1980 data shows considerable scatter but no discernible trend of increasing rates of !
l differenthi settlement. The mean of the differential settlement readings l since 1980 '- 0.018 ft., which is approximately 60 percent of the current allowable Technical Specifications limit of 0.03 ft. '
i 102-JST-13448-6 I
Margin in Pipe Stress in Service Water Lines As described in the 1981 report, the safety-related piping affected by the differential settlement are the four buried service water lines. The stress analyses of these lines and the remedial action taken in April 1977 to improve the stress conditions are described in Amendment 63 to the FSAR, dated July 8, 1977, under the response to Staff Comment 2.22. The detailed stress analysis results were reported in Amendment 64 to the FSAR, dated September 28, 1977, under the response to Staff Comment 2.23. The remedial action taken in April 1977 to improve the stress conditions consisted of permanently removing a portion of the concrete encasement adjacent to the MSVH, cutting the pipes, and then rewelding. This provided flexibility to the piping to withstand additional settlement. For the purpose of pipe stress analysis at that time, the future differential settlement from April 1977 was estimated to be 3/8 in (0.031 ft). The resulting stress due to this additional differential settlement was added to the existing stress in the pipe. The total projected maximum stress in the service water lines was 39,285 psi, which is 87% of the code allowable stress ( 3S c
= 45,000 psi) for this load case.
l The 1981 report noted that a future differential settlement of 9/16 inch (0.047 i i
ft) from April,1977 would be required to develop a stress that closely '
i approtches, but does not exceed, the code allowable pipe stress of 45,000 psi.
The 9/16 inch additional differential movement results in a pipe stress of 44,176 psi (Table 2, 1981 report). The analysis to calculate the capacity of i
the piping to withstand this 50 percent additional differential settlement over
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the current Technical Specifications limit was performed without any change to the analytical model and assumptions to the analysis that was perfomed to i
102-JST-1344B-7
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support the response to Staff Coment 2.23. The computer model used to generate these stresses was based on a linear relationship between differential settlement and pipe stress. The present differential settlement as defined by the May and June 1987 surveys (0.024 ft.) is only 51% of 0.047 ft. The stress in the pipe at this displacement is 37,076 psi, which is well below the code allowable stress of 45,000 psi.
Conclusions and Reconnendationss Point 117, located on the Service Building, has settled approximately 0.02 ft.
since July, 1977. Disregarding survey variabilities, there has been a trend toward negligible settlement since 1981. As shown on Figure 2, there has been very little fluctuation of the data points around the plotted settlement curve since 1985. Point 113, located on the Main Steam Valve House, has shown no discernible trend of settlement since the initial survey. As shown on Figure 3, the data points fluctuate significantly around the zero settlement line.
The apparent settlement of Point 117 (approximately 0.02 ft, since 1977) equates to 67% of the allowable differential settlement as stated in the present Technical Specifications. It can be readily seen that minor fluctuations of Point 113 (especially indicated upward movement) could cause the apparent differential settlement to become greater than the reportable limit of 0.022 ft. (75% of 0.03 ft.).
1 In 1981, it was projected that the apparent rate of differential settlement would result in the current Technical Specifications limit of 0.03 ft. being i exceeded in 1983. The post-1980 data, however, shows a much slower rate than ,
i projected, i
102-JST-1344B-8 I
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The differential settlement limit of 0.03 feet between the Service Building and the MSVH in the Technical Specifications was based upon very conservative estimates of the effects of settlement on the, service water piping. A more realistic estimate of the effect shows that the 0.03 ft, limit can be exceeded without causing an'y significant impact on the plant operability.
It is recorreended that the limit for allowable differential settlement in the Technical Specifications be increased to 0.047 ft. This realistic limit of differer,tial settlement would cause a stress of 44,176 psi in the service water lines, which does not exceed the code allowable stress of 45,000 psi. This ;
would also mean that the 75% reportable threshold would increase to .035 ft.
Therefore, additional review need only be initiated when there is any -
significant concern. This recorrrnendation is bas > d on the observation that the current value of differential settlement is about half of the proposed limit and that adequate margin will be allowed to accommodate fluctuations due to i survey inaccuracie:. Most importantly, the proposed Technical Specifications ;
limit of 0.047 ft, will ensure that the stress in the service water lines is maintained below the code allowable stress of 45,000 psi and that the piping i
system can operate safely with no impact to its structural integrity. <
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To confirm that there is no trend of increasing differential settlement between the Service Building and the MSVH, the monitoring frequency will be accelerated from semiannually to monthly for a period of six (6) months. At that time the l l
data will be evaluated and a decision made whether or not to continue at that !
frequency.
l 1
I 102.JST-1344B-9 l L _
.s ,o Revision 1 TABLE 1 DIFFERENTIAL SETTLEMENT BETWEEN SERVICE BUILDING AND UNIT 2 MAIN STEAM VALVE HOUSE Differential Settlement Elapsed /oint 117 Point 113 Since 7 Jul 77 Time Service Buildina Unit 2 MSVH (Point 117-113)
Date Since Measured Change ~ '
Me,asured Change Difference Percentag t 1 Sep 72 Elevation Since Elevation Since in Changes of 0.03' 7 Jul 77 7 Jul 77 Days (Ft.) (Ft.) (Ft.) (Ft.) (Ft.) (Percent) 7 Jul 77 1771 271.385 0 272.557 0 0 0 14 Dec 77 1931 271.370 0.015 272.547 0.010 0.005 17 1 May 78 2059 271.380 0.005 272.563 -0.006 0.011 37 21 Nov 78 2263 271.371 0.014 272.557 0 0.014 47 7 May 79 2430 271.377 0.008 272.563 -0.006 0.014 47 14 Nov 79 2621 271.373 0.012 272.565 -0.006 0.018 60 5 May 80 2794 271.370 0.015 272.559 -0.002 0.017 57 3 Nov 80 2976 271.369 0.016 272.567 -0.010 0.026 87 23 Dec 80 3026 271.367 0.018 272.562 -0.005 0.023 77 7 Jan 81 3041 271.367 0.018 272.561 -0.004 0.022 73 9 Feb 81 3074 271.368 0.017 272.562 -0.005 0.022 73 3 Mar 81 3096 271.369 0.016 272.563 -0.006 0.022 73 6 Apr 81 3130 271.366 0.019 272.560 -0.003 0.022 73 6 May 81 3160 271.362 0.023 272.559 -0.002 0.025 83 Jun 81 -- -- -- -- -- --
7 Jul 81 3222 271.368 0.017 272.556 0.001 0.016 53 5 Aug 81 3251 271.367 0.018 272.551 0.006 0.012 40 2 Sep 81 3279 271.377 0.008 272.562 -0.005 0.013 43 '
6 Oct 81 3313 271.370 0.015 272.557 0 0.015 50 4 Nov 81 3342 271.370 0.015 272.559 -0.002 0.017 57 4 Dec 81 3372 271.373 0.012 272.564 -0.007 0.019 63 8 Jan 82 3407 271.365 0.020
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272.558 -0.001 0.021 70 2 Feb 82 3432 271.366 0.019 272.558 -0.001 0.020 67 2 Mar 82 3460 271.368 0.017 272.561 -0.004 0.021 70 l 7 Apr 82 3496 271.367 0.018 f.?2.559 -0.002 0.020 67 14 May 82 3520 271.368 0.017 272.558 -0.001 0.018 60 l 2 Jun 82 3540 271.366 0.019 272.558 -0.001 0.020 67 2 Jul 82 3570 271.365 0.020 272.553 0.004 0.016 53 2 Aug 82 3600 j 271.364 0.021 272.554 0.003 0.018 60 2 Sep 82 3630 271.368 0.017 272.557 0 0.017 57 1
5 Oct 82 3663 271.367 0.018 272.549 0.008 0.010 33 9 Dec 82 3721 271.362 0.023 272.551 0.006 0.017 57 17 May 83 3878 271.368 0.017 272.555 0.002 0.015 50 l
15 Nov. 83 4438 271.369 0.016 272.558 -0.001 0.017 57 17 May 84 4618 271.366 0.019 272.556 0.001 0.018 60 16 Oct 84 4768 271.362 0.023 272.554 0.003 0.020 67 Apr 85 4948 271.372 0.013 272.558 -0.001 0.014 47 !
Nov 85 5158 271.365 0.020 272.552 0.005 0.015 50 5 May 86 l 5338 271.365 0.020 272.550 0.007 0.013 43 l 23 Oct 86 5498 271.364 0.021 272.552 0.005 0.016 53 14 May 87 5708 271.364 0.021 -- -0.003* 0.024 80 !
29 May 87 (,722 271.364 0.021 -- -0.005* 0.026 87 i 3 Jun 87 5727 271.364 0.021 -- 0.003* 0.024 80 29 Jun 87 5753 271.364 0.021 -- -0.002* 0. 02', 77 j ;
- Based en 113R Note: Negative Numbers Indicate Apparent Upward Movement i
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