ML20137G048
| ML20137G048 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 08/19/1985 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20137G033 | List: |
| References | |
| TAC-M54498, NUDOCS 8508270156 | |
| Download: ML20137G048 (10) | |
Text
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ENCLOSURE Hydrologic Engineering Safety Evaluation for High Groundwater at Grand Gulf Nuclear Station Introduction The licensee by letter of March 9, 1984 (Ref 1) reported a groundwater level of 110.2 ft ms1 at the site that exceeded the design basis level (109.0 ft msl) between January and July 1983 at dewatering well DW-8.
The staff, by memorandum of September 19, 1984 (Ref 2) requested a meeting with the licensee to further discuss the issue and method of resolution. However, since the licensee had already retained Bechtel Power Corporation to prepare a report on the high groundwater levels at Grand Gulf and the staff had concluded that the high groundwater levels had not compromised the integrity of any safety-related facilities, it was decided that further staff review would be postponed until completion of the Bechtel Report and subsequent review of it by the staff.
The Bechtel Report was transmitted to the staff by letter of February 14, 1985 (Ref 3). The licensee subsequently reported (Ref 4) the occurrence of a high groundwater level (109.1 ft msl) on
^
February 28, 1985 at dewatering well DW-8.
Background
The power block area and Stand-by Service Water (SSW) basins ~for Grand Gulf Units 1 and 2 were constructed within an open excavation that extended from yard grade at elevation 132.5 ft ms1 to the Catahoula Formation at about elevation 87 ft msl.
The excavated area was supported by a soldier pile or tieback wall as shown on Figure 1.
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. Seepage of groundwater into the excavation was initially controlled by pumping from sumps and later by a well dewatering system that was installed in 1979 and 1980. The construction dewatering system' consists of eight 10-inch diameter wells (DW-1 through DW-8) as shown on Figure 1.
Seven monitoring wells (MW-1 through MW-7) were installed around the power block in 1976 to replace the 11 construction observation wells destroyed during the start of construction in 1975. These wells are 6-inch diameter PVC pipe installed within the backfill (Figure 1).
Water levels in well DW-8, which was installed in July 1980, have typically been higher than those recorded in other dewatering and monitoring wells in the power block area.
Regional groundwater levels in the Terrace Deposits outside the vicinity of the power block are typically 10 to 20 feet lower than the groundwater levels in the power block area. The explaration for these higher ground-water levels in the powerblock area is that the groundwater is perched on the less permeabie Catahoula Formation. The Catahoula Formation in the plant vicinity is about 20 t'o 40 feet higher than in the surrounding area.
Where backfilling has been completed, a two-foot thick clay seal has been placed in the area between the tieback wall and the structures to minimize infiltration and recharge. The clay seal extends at least 8 feet out from the tieback wall except where interrupted by permanent roads, parking areas and railroad tracks. Backfilling around Unit 2 has not been completed, therefore, the clay seal has not been placed in this area.
Evaluation The precipitation records for Grand Gulf Nuclear Station for the' period November 1982 through October 1983, which includes the seven months (January through July 1983) in which the groundwater level exceeded elevation 109 feet msl, were compared by the licensee with the long-tt.m mean precipitation at Jackson, Mississippi (1909-1981) (Ref 3). The comparison is shown in Table 1.
The precipitation for November 1982 through October 24, 1983, at the site was above average by about 27 inches or about 150 percent of the long-term average at Jackson. The precipitation for the seven-month period (December 1982 to June 1983) was 69.81 inches or about 35.3 inches greater than the long-term mean at Jackson of 34.52 inches. The licensee also determined that the recurrence interval for this 7 month rainfall is about once in 200 years. The February 28, 1985, groundwater elevation of 109.1 ft ms1 was also associated with a period of excessive rainfall. The staff agrees with this assessment.
The yard area south and east'of DW-8 has no permanent structures or clay seal. Figure 2 shows a cross section through this area. The area includes the excavation for the four 10 foot diameter circulating water pipelines between the Unit 1 pump house and the Unit 1 cooling tower.
The excavation for the intake and discharge lines extends about 70 feet l
eastward of the tieback wall.
Backfill for this area consists of clean granular material compacted to 90 percent Modified Proctor. The clay l
seal only covers the portion of the excavation within the tieback wall.
The area east of the Radwaste Building and DW-8 is presently a laydown area with a gravel surface. There are also two areas south of the Radwaste Building and east of the Low Volume Waste Water Basin that are lower than surrounding roads and capable of surface water retention.
. The licensee also informed the staff informally that soil cement was used to stabilize the circulating water pipe trench durir.g const'ruction.
This would further enhance the potential for high groundwater since the only outlets would be the small pathways north and east between the building and tieback wall and the larger pathway to the south in the pipe trench.
Based on the information currently available, it is the staff's judgement that rainfall infiltration into the circulating water pipe trench is the probable cause of the high groundwater readings at DW-8.
The staff does not have the type of data that would be necessary to evaluate the effect of the Unit 2 dewatering operation on groundwater levels in the power block area. However, unless considerable ponding occurred in the Unit 2 open excavation, it is likely that the Unit 2 open excavation provided some relief for groundwater accumulation in the Unit 1 area during the January-July 19, 1983, period of high groundwater.
There is insufficient data available for the staff to evaluate the effect of the tieback wall on groundwater accumulation in the power block area. The installation of a monitoring well outside the tieback wall and adjacent to an existing monitoring well inside the tieback wall may provide data that would give some indication of the effect of the tieback wall on groundwater flow.
Conclusions We have reviewed available information relative to the high groundwater levels at the Grand Gulf Nuclear Station and conclude the following:
. 1.
Groundwater in the vicinity of dewatering well DW-8 exceeded the design basis groundwater level of 109.0 ft ms1 by up to 1.f feet.
Well DW-8 is located at the southeast corner of the power block area (Figure 1) and is not near any safety-related structures.
The highest groundwater level in the vicinity of safety-related structures was 107.8 ft msl and it occurred at monitoring well MW-6.
The integrity of safety-related facilities was not compromised.
2.
Although the data are inconclusive, they suggest that the probable causes of the high g oundwater level at well DW-8 are the lack of clay seal over part of the recharge area, the location of well DW-8 in the circulating water pipe trench that was partially sealed with soil cement during construction and above average rainfall. Sealing the bottom and sides of the trench caused the trench to function like a collector sump.
3.
There are insufficient data to evaluate the effect of the Unit 2 dewatering well operation on the groundwater levels in the power block area.
4.
There are insufficient data to determine the effect, if any, of the tieback wall on normal groundwater flow.
l 5.
The licensee should commit to retain a dewatering capability for L
all of the dewatering wells until the Unit 2 excavation is l
backfilled, the clay seal completed, and a post construction groundwater data base is obtained that will allow a reasonable projection of the maximum post construction groundwater level and resolution of the issue.
6.
The licensee has reanalyzed the stability of buildiag walls and foundations for a groundwater level of 114.5 ft ms1 and combined seismic loads and determined that the structural integrity of
Unit 1 buildings will not be impaired. Considering the licensee's commitment to retain dewatering capability until completion of Unit 2 construction and backfilling activity and the low probability of occurrence of rainfall necessary to cause groundwater to exceed i
elevation-109.0 ft ms1, it is the staff's judgement that final resolution of the high groundwater issue can be delayed for up to 5 years but no later than December 1990.
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. References 1.
Letter from L. F. Dale, MP&L, to J. P. O'Reilly, NRC Region II,
Subject:
High Groundwater Elevation - Grand Gulf Nuclear Station
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AECM-84/0020, March 9, 1984.
2.
Memorandum from R. Ballard to E. Adensam, NRC,
Subject:
High Groundwater Elevation - Grand Gulf Nuclear Station, September 19, 1984.
3.
Letter from L. F. Dale, MP&L, to H. R. Denton, NRC,
Subject:
Groundwater Level at Grand Gulf AECM-85/0035, February 14, 1985.
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4.
Letter from L. F. Dale, MP&L, to H. R. Denton, NRC,
Subject:
Groundwater Level at Grand Gulf AECM-85/0088, March 25, 1935.
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TABLE 1 PRECIPITATION MEASUREMENT GRAND GULF AND JACKSON, MISSISSIPPI OBSERVED AT SITE LONG-TERM YEAR 1982-1983(1)
MEAN(2)
PERIOD INCHES INCHES November 2.77 3.80 December 16.21 5.47 January 1.51 4.97 February 6.58 4.69 March 6.56 5.67 April 14.36 5.39 14.31 4.59 May June 10.28 3.74 July 2.10 4.59 e.
August 2.17 3.59 September 1.29 2.87 October 0.71*
2.33 TOTAL 78.85 51.70
- Up to October 24, 1983.
(1) Rainfall measurements from Met Tower at Grand Gulf.
(2)
U.S. Weather Bureau Station Jackson, Mississippi for 1909-1981.
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