ML20003B670
| ML20003B670 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 02/20/1981 |
| From: | VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | |
| Shared Package | |
| ML20003B669 | List: |
| References | |
| NUDOCS 8102250141 | |
| Download: ML20003B670 (17) | |
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DIFFERENTIAL SETTLEMENT BETb'EEN SERVICE BUILDING AND UNIT 2 MAIN STEAM VALVE HOUSING 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.
Measurements of differential settle-ment between the Service Building and the Unit 2 Main Steam Valve Housing (MSVH) have 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 outlines plans for short-and long-term actions that might be taken.
The report describes the background of settlement monitoring, the basis of the allowable dif ferential settlement, the settlement record to date, the projection of anticipated additional settlement, and the margin in pipe stress in the service water lines.
As demonstrated in this report, there is no immediate concern for remedial action. The report outlines the course of remedial action being considered for the long term, as well as describing the current increase in frequency of settlement i
monitoring.
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Background
The western and of the Service Building (especially in the area of the four Emergency Diesel Generator Rooms) ~i.s underlain by a variable thick-ness of compressible, soil-like, decomposed rock call "saprolite."
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2 Figure 1 shows a plan of this area, together with column lines and the locations of pertinent settlement monitoring points. Under the southern wall of the Service Building (along the E Line), the saprolite may be as much as 15 f t thick, whereas it may be 20 f t or more thick under the northern wall (along the C Line, 68 ft north of the E Line).
The presence of this compressible material has resulted in settlement of the Service Building to the west of the 10 Line and especially from the 14 Line to the 17 Line.
Settlement monitoring along the C Line can be considered to have started in September 1972 when (a) most of the structural loading by the Service Building had been applied and (b) the crane rail in the Turbine Building was erected along the C Line at Elev. 342.5.
Settlement of the crane rail from its as-built elevation was measured periodically by Stone &
Webster (S&W) construction surveyors, beginning in December 1973. For a more formal monitoring of settlement of the C-Line columns, points were scribed on the columns at Elev. 307 and first surveyed in August 1975.
In May 1976, the surveying firm of Moore, Hardee & Carrouth Associates l
(MH&C), under contract to Virginia Electric and Power Company, began monitoring points installed on the C Line column pilasters at Elev. 254 Settlement along the E Line was not measured until August 1976 when S&W construction surveyors determined the elevations of numerous points on the floor slab, indicated on design drawings to have been placed at l
l Elev. 271.5.
Somewhat earlier, in May 1976, MH&C started the formal l
program of settlement monitoring of points along the E Line (one in each i
Emergency Diesel Generator Room) at Elev. 271.5 on a ragular basis.
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The surveying of the floor slab of the Service Building in August 1976 indicated apparent settlements from the as-built elevations that would have caused deflections of the four buried service water lines beneath the footings of this structure.
These lines, 24-in-diameter carbon steel pipes in a 4-f t-thick reinforced concrete encase =ent, run southerly from beneath the Service Building at the 14 Line, under the 25-f t-wide roadway, and through the northern wall of the MSVH.
The NRC Office of Inspection and Enforcement was notified on December 15, 1976, under the provisions of 10CFR50.55(e), of this settlement and its effect on the 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.
Surveying of points on the MSVH in August 1976 indicated negligible settlement of this structure.
Thus, analyses described in the response to Staff Comment 2.22 have included the conservative assumption of no l
past or future settlement of the MSVH.
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_ Allowable Differential Settlement In mid-1977, there were not sufficient settlement monitoring data along the southern wall of the Service Building to establish a time-rate of settlement for projection into the future.
Therefore, the settlement record of Column C-17 (Point 171), having similar subsurface conditions, was reviewed to provide a basis for future projection.
As is discussed hereinafter, this record suggested that (a) there had been a marked de-crease in the rate of settlement since mid-1976 and (b) the rate of F
4 settlement had become equivalent to the rate of secondary compression.
Over the 40-year life of the plant, such a rate would result in only an additional 1/4 in of settlement.
For input to the pipe stress analysis of the service water lines at that time, the future settlement from April 1977 (when the remedial action was performed) was assumed to be 3/8 in.
The assumed future settlement of 3/8 in (0.031 f t) of the mouthern wall of the Service Building was subsequently included in Technical Specifi-cations 3/4.7.12.
In the proposed Technical Specifications, sent to NRC in October 1977. Table 3.7-5 contained an allowable differential settlement (in feet) between the Service Building (Point 117) and the MSVH (Point 113) of "0.031 from 4/1/77" (the date when the remedial action was performed by cutting the lines).
(When these Technical Specifications were issued by NRC in November 1977, the allowable differential settlement was mistyped as "0.31 f rom 4/1/77.") The plant operating personnel found difficulty in verifying compliance with Technical Specifications 3/4.7.12, since MH&C had not monitored the settlement of Class 1 structures during April 1977.
Therefore, when Table 3.7-5 was revised in Amendment No. 12 l
to the Facility Operating License in June 1979, the allowable differential i
settlement between Point 117 and Point 113 was corrected and the starting date changed to read "0.03 from 7/77" (the month of the first survey by MH&C following the remedial action).
The revision also added a footnote indicating that the " critical differential settlement is downward movement
. of Point 117 with respect to Point 113."
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5 Settlement Record The record of differential settlement between Point 117 and Point 113 since July 1977 is given in Table 1.
The variability in the elevations of the two structures does not reflect tPeir individual behavior but, rather, the inaccuracies of measurements from one survey to the next.
Regardless of this variability, the dif ference between the two changes in elevation for each survey is a valid measure of the differential settlement between the two points, as long as both elevations are measured from the same position of the surveying instrument or at least from two positions separated by a minimum number of turns. The measurement in November 1980 should be considered somewhat inaccurate because seven turns were made between reading Point 117 and reading Point 113; read-ings in subsequent sur-are taken on both points from a single instrument position.
As shown in Table 1, the survey in November 1980 indicated that the differential settlement between Point 117 and Point 113 had exceeded l
75 percent of the allowable 41fferential settlement given in Table 3.7-5 i
of the Technical Specifications. However, this was not immediately t
recognized due to an incorrect calculation.of the dif ference between the two changes in elevation.
The correct dif ferential settlement measured in November 1980 was not recognized until December 22, 1980. On December 23, 1980, MH&C visited the site at the request of Virginia Electric and Power l
l Company and, in a special survey, verified that the differential settlement had exceeded 75 percent of the allowable value. However, the surveys on
-January 7 and February 9,1981, both indicated a dif ferential settlement slightly less than 75 percent of the allowable value.
6 Figure 2 shows the differential settlement since July 1977 between Point 117 and Point 113 plotted against the logarithm of elapsed time since September 1,1972, when most of the load had been applied to the underlying saprolite.
(Such a semilogarithmic plot is commonly used to identify a linear relationship that shows, in most soils, a completion of primary consolidation and a settlement that is solely due to a secondary compression.
" Rate of settlement," as used herein, is the slope of a linear relationship on a semilogarithmic plot sad not the amount of settlement per year.) Figure 2 also shows the differential settlements between Points 114,115, and 116 and Point 113 that indicate a somewhat greater settlement along the E Line to the west of Point 117.
In general, however, these four pointe currently have very similar rates of settlement.
The differential settlement between Point 117 and Point 113 is also plotted in Figure 3 together with the more recent settlement record of Column C-17 (Point 171). This figure showr. the interpretation of the monitoring by S&W construction surveyors that was mentioned earlier.
Unfortunately, access to the monitoring point on this column was lost in September 1977 (as access to Columns C-15 and C-16 had been lost several months earlier), so the apparent trend of a reduced rate of settlement could not be pursued.
Subsequent monitoring by MH&C, however, shows clearly that the rate of settlement of Column C-17 has remained constant for a period of eight years and does not compare to a rate of settlement corresponding to the coefficient of secondary compression measured in laboratory consolidation tests of the saprolite.
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7 The linearization of settlement data on the semilogarithmic presentation given in Figure 3 indicates a decrease in the amount of settlement per year with the passage of time.
The annual amount of settlement beco=es asymtotic to zero under this interpretation as shown in the arithmetic projection of the rate of settlement of Column C-17 given in Figure 4 The settlement measurements of Column C-17 by MH&C shown in Figures 3 and 4 have been purged of most of the surveying inaccuracies by subtracting from each measurement the settlement of Column C-10 measured during the same survey. The record shows that there haa never been any settlement of Column C-10, so this correction is the equivalent of using Column C-10 as a bench mark.
The records of settlement of columns between Column C-10 and Column C-17 have not been shown here because they have been flawed by loss of access or by the reestablishment of points.
Columns C-11, C-12, and C-13 u3derwent some settlement soon after the crane rail was installed, but little, if any, further settlement in recent years. Columns C-14, C-15, and C-16 have continued to settle to amounts that are, respectively, about 40, 70, and 100 percent the settlement of Col amn C-17.
Anticipated Additional Settlement The long record of settlement measured at Column C-17 shows no decrease in the rate of aattlement since September 1972.
Thus, there is no basis for anticipating a future rate of settlement of the western portion of the Service Building other than the current rate.
Since the differential
8 settlement between the Service Building and the hSVH is assumed to result solely from settlement of the Service Building, the future rate of differential settlement between Point 117 and Point 113 must be assumed to be a projection of the current rate.
As shown in Figure 3, the current rate of differential settlement may be as low as about 0.08 to as much as 0.10 f t per logarithmic cycle of elapsed time.
Over the 40-year life of the plant (approximately 1.0 logarithmic cycle of elapsed time since July 1977), an ultimate dif-f arential settlement of about 0.08 to 0.10 f t (1 to 1 1/4 in) must be anticipated. Of this total, about 0.02 ft has occurred to date, leaving a remainder of about 0.06 to 0.08 f t to occur in the future.
Further-more, Figure 3 indicates that 100 percent of the allowable differential settlement given between Point 117 and Point 113 in Table 3.7-5 of the Technical Specifications is expected to be exceeded within the next two years.
Margin in Pipe Stress in Service Water Lines As described previously, the safety-related piping affected by the differential settlement are four buried service water lines running southerly from beneath the Service Building, under the 25-f t-wide roadway, and through the northern wall of the MSVH.
These four buried lines are 24-in-diameter, carbon steel pipes encased in reinforced concrete.
The line members for this piping are identified as 24-WS-426, 428, 434, 436-151-Q3.
The stress analyses of these lines and the remedial action taken in April 1977 to improve the stress conditions are described in
9 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 revelding.
For input to the pipe stress analysis at that time, the future differential settlement from April 1977 was assumed to be 3/8 in (0.031 ft).
This additional settlement resulted in a calculated stress due to total differential settlement in the service water lines of 39,285 psi as compared to an allowable stress for this load case of 35, = 45,000 psi.
As can be seen from Table 2, a differential settlement of 9/16 in (0.047 ft) from April 1977 is required to develop a stress of 44,176 psi, which more closely approaches but does not exceed the allowable stress of 45,000 psi.
The analysis to calculate the capacity of the piping to withstand this 50 percent additional differential settlement is performed without any change to the analytical model and assumptions to the analysis that was performed to support the response to Staff Comment 2.23.
The linear extrapolation performed to more closely approach the allowable strass is valid for this analytical model.
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I Possible Remedial Actions Several different possible remedial actions are being investigated at this time.
These include (a) interim revision to the Technical Specifica-tions to allow a maximum differential settlement of 9/16 in (0.047 f t)
10 instead of the 3/8 in (0.03 f t) presently permitted, (b) a closer evaluation of the conservatism in the original stress analysis, and (c) permanent modifications to the service water lines, such as a change of the connection detail between the MSVH wall to each pipe so as not to af fect the piping by any future settlement.
The progress of these investigations of remedial actions shall be reported separately.
Current' Frequency of Settlement Monitoring In addition to the special survey of Point 117 and Point 113 on December 23, 1980, MH&C is now monitoring these two points on a monthly basis, typ-ically during the first week of each month. As shown in Table 1, the first monthly reading was made on January 7,1981, and the second on February 9, 1981. Monthly readings will be continued as long as the measured differential settlement is a large percentage of the allowable value given in the Technical Specifications.
Since the current monitoring of Point 117 and Point 113 is (a) on a monthly basis and (b) made from a single instrument position, a large
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number of relatively scatter-free measurements vill become available in j
the next several months that should permit the rate of settlement of the Service Building to be determined more accurately than is indicated in Figure 3.
a
11 Conclusions As demonstrated above, the current differential settlement between Point 117 and Point 113 does not require any immediate remed.a1 action.
The projection of future settlement is being evaluated, based on the increased frequency of settlement :nonitoring, to determine the most appropriate remedial action.
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TABl.E I DIFFERENTIAL SETTLDtENT BETWEEN SERVICE Bull. DING AND UNIT 2 MAIN STEAM VALVE IlOUSING Date Elapsed Service Building Unit 2 Main Steam Differential Settlement Time Valve llousing Since 7 JUL 77 Since (Point 117)
(Point 113)
(Point !!7 Minus Point 113) 1 SEP 72 Measured Change Since Measured Change Since Difference Percentage Elevation 7 JUL 77 Elevation 7 JUL 77 In Changes of 0.03 Ft 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 0.012 272.563
-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
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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
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TABLE 2 CALCULATED STRESSES IN SERVICE WATER LINES Location of Point:
Northern end Southern end North wall of of 4-ft-thick of 4-f t-thick Main Steam encasement encasement Valve Housing Explanation of Stress Distance South of E Line, ft:
8.0 20.5 25.0 Node in NUPIPE model:
5 30 40 Load Case I 29,504 0
0 Stress immediately af ter cutting lines Load Case II 9,781 27,053 2,486 Stress due to 3/8 in additional settlement since cutting lines Total Load Cases I and II 39,285 27,053 2,486 Total stress after 3/8 in additional settlement Load Case III 14,672 40,580 3,729 Stress due to 9/16 in additional settlement since cutting linew Total Load Cases I and III 44,176 40,580 3,729 Total stress after 9/16 in additional nettlement NOTES:
1.
All stresses are in psi.
2.
Load Cases I and 11 are described in Amendments 63 and 64 to FSAR under responses to Staf f Comments 2.22, 2.23, 2.24, 2.25, and 2.26, 3.
Load Case III is lin' ear extrapolation of Case II for additional settlement of 9/16 in and equals (9/16) (8/3) (II) = 1.5 (II).
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