ML19305E060
| ML19305E060 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/18/1980 |
| From: | Brown S VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Harold Denton, Parr O Office of Nuclear Reactor Regulation |
| References | |
| 249-030680, 249-30680, NUDOCS 8004220234 | |
| Download: ML19305E060 (5) | |
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Veaco VIRGINI A Ell'CTRIC AND power con 4P ANY, RICHMOND, VINGINI A 23261 W
April 18,1980 Mr. Harold R. Denton, Director Serial No. 249/030680 Office of Nuclear Reactor Regulation PSE&C/CES:mac: wang Attn: Mr. O. D. Parr, Chief Light Water Reactors Branch No. 3 Docket Nos. 50-404 Division of Project Management 50-405 U.S. Nuclear Regulatory Com:nission Washington, DC 20555 Control No. F20097H1
Dear Mr. Denton:
We have received your letter dated March 6,1980 requesting additional information concerning the design groundwater levels used in North Anna Power Station Units 3 and 4 design. The enclosure submitted with your March 6, 1980 ietter listed the specific information you are requesting to complete a review of the information contained in our letter Serial No. 657 dated August 21, 1979.
We are enclosing a response to the specific questions posed in your March 6, 1980 letter. Additional background information is provided in the enclosure to aid in your review of the subject.
Should you require additional information, please contact this office.
Very truly yours, ff
- L.
Sam 'C. Brown, Jr.
. Senior Vice President - Power Station Engineering and Construction Enclosure 4
8004g30280
PSESC ENCLOSURE Serial No. 249/030680 Pg. I of 4 MAXIMUM CROUNDWATER ELEVATIONS NORTil ANNA UN]TS 3 AND 4 As requested by your letter dated March 6,1980, subject " North Anna Units 3 and 4 Ilydrologic Engineering - Questions Regarding Groundwater," we submit the following information to aid in your review of the North Anna Units 3 and 4 groundwater levels. Where information has been previously submitted, the appropriate reference document is cited herein.
As reported in Reference 1, Appendix B, groundwater at the North Anna Power 1
Station occurs under watertable conditions. Prior to construction, the groundwater surface was generally a subdued expression of the surficial topography with groundwater movement towards areas of lower elevations.
Discharge commonly occurred as springs in low lying areas, emptying to the North Anna River at approximately elevation 200 (mean sea level). The preconstruction groundwater surface is shown in Reference 1, Appendix B, Plate IIB-2.
Filling of the reservoir (Lake Anna) raised the local base Icyc1 of ground-water discharge approximately 50 ft to clevation 250 (the level at which the reservoir is maintained), thereby reducing the hydraulic gradient across the site from approximately 7 percent to approximately 6 percent, as stated in Reference 1, Appendix B.
Final grade in the plant area will be elevation 271+ and will slope toward Lake Anna. A French drain, constructed to the south of the main plant area at elevation 271, influences groundwater flow through the plant area.
Anticipated post-construction groundwater Icvels have been presented in Reference 2, Figure 2.4.13-1.
These design basis maximum groundwater elevations are based on the conservative assumption of a water surface profile sloping uniformly from the French drain at the toe of the excavated slope south of the plant area (elevation 271) to Lake Anna (clevation 250).
Ilowever, as stated in Reference 2, Section 2.4.13.1, the groundwater levels shown in Figure 2.4.13-1 do not include the effects of structures on ground-water flow and therefore, where conditions appear capable of producing locally higher groundwater levels, (i.e. ponding of groundwater) appropriate conservative modifications have been made to the design high groundwater levels to account for this effect and to insure the adequacy of safety related structures and facilitics.
The design basis post-construction groundwater levels will have an additional,
built-in conservatism in that a series of subsurface drains will be installed below the turbine and service buildings. The subsurface drainage system will maintain groundwater Icvels at approximately clevation 245 in the turbine building area and at approximately clevation 250 in the service building area. llowever, for design purposes, the effect of the subsurface drainage system of lowering maximum groundwater levels has.bcen ignored.
In response to the specific questions posed in your March 6, 1980 letter, the following information is presented:
1.
The maximum groundwater elevations listed in our 1ctter'Scrial No. 657, dated August 21, 1979 are based on Reference 2, Figure 2.4.13-1 except. the service water pumphouse (SWP]I).
The L
PSEcc Serial No. 249/030680 Pg. 2 of 4 SWPH is being designed for a maximum water level of 315 and a flood water level of 318.3. These design basis water levels are conservatively based on corresponding maximum service water reservoir levels and not actual groundwater levels as explained in our letter dated October 4,1979, Serial Number 657A.
A model was not used to predict maximum post-construction groundwater levels. The groundwater profile presented in Figure 2.4.13-1 is based ca a water surface sloping uniformly from the toe of the excavated slope to the south of the plant area (elevation 271) to Lt.ke Anna (elevation 250). This conservative assumption was made in lieu of attempting an exact prediction of post-construction groundwater levels.
Permeability and porosity of the underlying saprolite and bedrock along with preconstruction groundwater levels, are presented in Reference 1 Appendix B.
As stated therein, the permeability of the saprolite is low, which will result in approximately 95 percent of all precipitation being carried to Lake Anna as surface run off.
It is anticipated that less than 5 percent will percolate vertically and intersect the water table.
In addition, the design maximum groundwater profile has been chosen conservatively, has a controlled discharge elevation (Lake Anna), and its maximum 1cyc1 will be controlled by final plant grade as well as the French drain.
Therefore, extreme rainfall will not affect the maximum groundwater elevations being used for design.
2.
The Units 3 and 4 plant area is presently excavated to approximately clevation 234 in the turbine building area and to varying depths
'into sound rock in the service building area (elevation varies from elevation 200 to elevation 240).
The plant area has been dewatered during construction so that groundwater 1 are being artificially depressed. Therefore the current.,undwater levels have no relationship to the assumed design basis post-construction groundwater levels.
3.
As stated in Reference 1, Section 2.3.1, the Lake Anna normal pool elevation will be maintained at elevation 250, with an expected drop to elevation 247 during periods of drought.
Therefore, there is no long term Lake Anna high water level which could effect the plant area groundwater levels.
Information on short term (flood) Icvels at Lake Anna is contained in Reference 2, Appendix J.
The maximum lake elevation during the probable maximum flood is elevation 264.2, which is below final plant grade. Due to the low permeabilit.y of the North Anna subsoils and the relatively short duration of peak water levels in Lake Anna, flood levels will have an insignificant impact on the site design groundwater levels. However, structures adjacent to Lake Anna will be designed for a high groundwater elevation of 264. The specific structures being designed for flood groundwater elevations are the intake structure and the water treatment building (nonsafety related).
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PSELC Serial No. 249/030680 Pg. 3 of 4 4.
The layout of the intake structure and the circulating water intake tunnels are such that normal groundwater flow toward Lake Anna could be impeded resulting in local ponding to the south of the intake structure. To account for this possibility, the design high groundwater elevation has been increased to elevation 259 for the south side of the intake structure as well as the inboard sides of both intake tunnels.
Elevation 259 corresponds.to the top elevation of the intake tunnels and in the maximum elevation to which groundwater could pond. These are the only structures where conditions appear capable of producing locally higher groundwater levels.
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PSE6C Serial flo. 249/030680 Pg. 4 of 4 REFERENCES 1.
Preliminary Safety Analysis Report, North Anna Power Station, Units 3 and 4, Virginia Electric and Power Company.
2.
Final Safety Analysis Report, North Anna Power Station, Units 1 and 2, Virginia Electric and Power Company.
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