ML20066A607
| ML20066A607 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 12/21/1990 |
| From: | Cottle W ENTERGY OPERATIONS, INC. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GNRO-90-00006, GNRO-90-6, NUDOCS 9101040012 | |
| Download: ML20066A607 (13) | |
Text
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&% Entergy RTM-"aao 'a*-
Operations
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W. T. Cottle s,u m - i n (4o 4a t
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December 21, 1990 U.S. Nuclear Regulatory Commission Mail Station P1-137 Washington, D.C.
20555 Attention:
Document Control Desk
SUBJECT:
Grand Gulf Nuclear Station Unit 1 Docket No. 50-416 i
License No. NPF-29 Groundwater Monitoring Program GNRO-90/00006 Gentlement
)
In 1985, the NRC issued a Safety Evaluation Report (SER) (MAEC-85/0284, dated August 19, 1985) which presented the results of NRC's review of several high groundwater events.
These events involved groundwater levels in excess of the design basis value of 109.0 feet Mean Sea Level (MSL) at dewatering well-8 (DW-8).
The SER had concluded ".... the l
integrity of safety related facilities was not compromised by the reported exceedance of the design basis groundwater level."
The SER also indicated, however, that due to Unit 2 construction activities, GGNS was not able to provide a modified groun1 water level that would have supported the closure of this issue.
Acevrdingly, the l
SER required the following:
.... the Staff concludes tl.at the final projection of a maximum post-construction groundwater le"el and I
resolution of exceedance to the design basis groundwater level can be i
delayed for up to 5 years, but no later than December,1990.
If a final resolution of this issue is delayed beyond 1990, licensre is requested to f;
provide, at that time, a status report and schedule for submittal of a
'j final report."
In accordance with this request, Entergy Operations, Inc.
has developed the enclosed status report.
[
As noted in the attached status report, stabilization of the Unit 2 site did not occur until 1989 when backfilling and installation of the clay cap was completed.
Prior to this, a " maximum post-construction groundwater evel" could t'ot be developed and activities initiated since 1
L that t me heve been designed to provide a better definition of the i
groundwater profile.
Data gathered to date indicates that the high grounowater events reported at GGNS are the result of leakage from onsite sources and precipitation in areas away from safety related structures.
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e GNRO-90/00006-Page 2 of 4 The groundwater level around safety related structures has consistently 1
been below the design maximum groundwater level of 109.0 feet MSL.
In an area south of the Radwaste Building as discussed-In the attached status report levels have exceeded 109.0 feet MSL.
In response to the need for a post-construction maximum groundwater level and to better define influences f rom-precipitation, Entergy Operations, Inc. As implementing various tasks designed to provide a better definition of groundwater level at Grand Gulf.
These tasks include the installation of fifteen additional monitoring wolls at areas around the cooling tower basin and on the north end of the site. Data from the wells around the cooling tower basin will allow-a better characterization of the magnitude of water addition from this structure to' the groundwater supply, while the new wells on the northern portion of the site will-enhance the amount of data available in evaluating overall groundwater flow across the s.ite.
The flow instrumentation installed on the dewatering pump dischnrges will be monitored to provide additional data that can be used to characterize overall groundwater flow -within the plant site.
As discussed in the attached-status report, data gathering activities with the new monitoring walls and-flow instrumentation will continue until mid-1991, normally a period high in precipitation.
This will help in resolving issues associated with high groundwater level south of the Radwaste Building and influence from precipitation.
A final report delineating _ the results of those activities will be provided by April 30, 1992.
If you requiro additional-Informa tion. please advise.
Yours truly, c~o ?
h-WTC/MR/:mte attachment:
cc:
(See Next page)
A9012121/SNLICFLR - 2
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Page-3 of 4 a
cc:
M r. 1). C.'llintz (w/a)
Mr. J. Mathis (w/a)
Mr. R. B. McGehoe (w/a)
Mr. N. S. Reynvids (w/a)
Mr. II. L. Thomas (w/o)-
Mr. Stewart D. Ebneter (w/a)-
Regional Administrator U.S. Nuc. lear Regulatory Commission Region II 101 Marietta-St., N.W., Suite 2900 Atlanta, Georgia 30323 Mr. L. L. Kintner,. Project Manager (w/a)
Of fice of Nuclear Reactor Regulation.
-U.S. Nuclear Regulatory Commission Mail Stop 11D21 Washington, D.C.:..
20555 6
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A9012121/SNLICFLR - 3
GNRO-90/00006 Page 4 of 4 l
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Mr. M. A. Dietrich (w/a)
Mr. W. K. Hughey (w/2)
Mr. C. R. liutchine.on (w/a)-
Ms. F. K. Mangan (w/o)-
Mr. M. J. Molsner (w/a)
Mr. G. W. Muonch (w/a)
Mr. T. E. Reaves (w/a)
Mr. J. L. Robertson (w/a)
Mr..F. W Titus (w/a)
Mr. G.
A.' Zinke (w/a) ~
Filo (LCTS) (w/a)
File '(llard Copy) (w/n)
Filo (RPTS) (w/a)
Filo (NI,) (w/a)
Filo (Contral) (w/a) ( 15.
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A9012121/SNI.ICFLR - 4 l
Attschment to GNRO-90/00006
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1 GRAND GULF NUCLEAR STATION GROUNDWATER MONITORING STATUS REPORT Datedt. Decembo. 20, 1990 A9012121/SNLICFLR - 5 l
I Attachment to e
i
.GNRO-90/00006 i-I.
.INRODUC_T_10t!
The Grand Gulf Nuclear Station has a Groundwater Monitoring and Construction Dewatering System which was used during construction and continuns to be used to monitor the groundwater level at various locations adjace 4 to the power block and in surrounding areas of the site.
This system is responsible for.
monitoring and dowatering of the-perched acquifer which underlies the power block. As reported in Section 2.4.13.5 of the UFSAR, the design maximum groundwater level =in the aquifer for the site was calculated to be 109.00 feet Mesn Sea Level (MSh).
Ilowever, in reports to the NRC, Grand Gulf has documented cases wherein the groundwater level at at least one dewatering well exceeded this design. level.
In order to both determine the cause of the high groundwater level event.s and to' assess the safety significance of the.
events, Entergy Operations, Inc. has completed several studies-which have supported the development of an enhanced monite ring -
program currently ongoing.
This program will gather da. from i
new monitoring wells and flow, instrumentation on existlaf, dowatering wells "Sich will be used to develop a-peut-construction.;wandwater profiln for the GGNS site.
The following sections provide a status report on the work complet.nd to date and a summary of the programs underway to resolve t.he iss':e of high groundwater levels.
II.
BACKGR0_ND U
II.1 Grand Gulf Site Sections 2.4 and 2.5 of the UFSAR. provide a detailed description r the site's geological formations'and groundwater tables,-as o
- well as present the design basis for the subsurface hydrostatic loading.
As presented in-the UFSAR, the power block structures are supported by the Catahoula Formation which is comprised of dense claystone.- The material overlying the Catahoula Formation consists of terrace depoaits, loess and alluvium.
Regionally, groundwater occurs within the terrace and alluvium deposits and, to a lessor extent, within relatively thin, fine sand lenses in the Catahoula Formation.
During construction, groundwater was-initially controlled by a ditch and sump system within the excavated area.
As structures were bu'1t, control of groundwater was switched to dowatering 0 11s.
Within the plant property, groundwater flows westward and discharges into the Mississippi River. Groundwater north and south of the power plant is at an elevation of approximately 75 feet Mean Sea Level (MSL) based ou contours of average water levels collected between January 1984 and July 1990.
The water table typically occurs in the permeable terrace, leess and alluvial deposits that overlie the less permeable silt and clay comprising the upper part of the Catahoula Formation.
a A9012121/SNLICFLR - (
_ = _. - _
Attachment-to Di
~90/00006 Dackfill placement around Unit 1 structures was completed in 1982.
Placement of Unit 2 backfill remained at about Elevation 110 until late 1988.
Backfilling and installation of the clay seal were completed in early 1989 and final site grading in the Unit 2 area was performed to stabilize the site in a post-construction condition.
11.2 Groundwater Level Moniloririg As noted above, scopage of groundwater into the Unit -1 and 2 excavation was initially controlled by pumping from sumps and, later, by a construction dowatering system that was installed in 1979 and 1980.
The construction dowatering system consists of eight 10-inch diameter wells (DW-1 through DW-8) as shown on Figure 1.
In addition, sovon 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.
Subsequently, five additional monitoring wells (MW-8 through MW-12) were installed south and east of DW-8, as well as a supplementary dewatering well (DW-8A).
These wells were installed in September, 1986 in accordance with recommendations from the 1983 Groundwater Level Study.
As will be discussed in Section III, fifteen additional monitcring wells are being installed to further clarify the direction of groundwater flow and the nature of recharge i
sources.
Groundwater levels have been monitored in regional and site monitoring wells at least monthly since 1973.
(Exceptions:
April, 1982 to September, 1982 and November, 1982 to January, 1983 - only Unit I wells monitored.)- At present, regional wells are read on two week intervals and the-results are submitted in I
the annual environmental operating report. The perched wells are iet.d on monthly intervals and any well-level equal to or greater than 109.0 feet HSL is reported to the NRC in accordance with their August 19, 1985 letter (HAEC-85/0284).
The results l
are also submitted in the annual environmental operating report.
11.3 High Groundwater Level Events 1
From March, 1984 through October, 1990, Grand Gulf has provided several reports documenting groundwater levels at one or more wells which exceeded the design maximum groundwater level of 109.00 feet M9L. These reports are sumnarized below, a.
January through July, 1983:
Unusually high rainfall caused l
the water level in the perched aquifer to rise l
significantly.
In the case of downtering well DW-8, the design maximum level of 109.00 feet M3L was exceeded during
(
the referenced period with the maxJmum recorded level j
reaching 110.21 feet MSL.
(Reference AECM-84/0020, dated March 9, 1984.)
A9012121/SNLICFLR - 7
Attachment to GNRO-90/00006 1
b.
February 28, 1985: Equipment failure (clogged scroons on i
the pump) coupled with above averagn precipitation appeared to ho the cause for the groundwater' level rise at DW-8 which peaked at 109.1 foot MSL.
(Reference AECM-85/0088, dated March 25, 1985.)
c.-
Lato. November through December, 1P85:
.A groundwater.
olevation of 111.3 foot MSL at DW-8 was discovered during a routino porched aquifer surveillance on November-25, 1985.
An investigation attributed the high level to a possible malfunction of the-conductivity probe and/or pump.
Both components were repinced. -During_ troubleshooting of this.
event, groundwater level' exceeding 109.00 feet MSL occurred on three other occasions.- (Reference AECH-86/0002, dated February 7, 1986.)'
d.
March, 1990:
In order to install flow monitoring
~
instrumentation on the DW-8 pump discharge, it was necessary to remove the pump from service. As a result of this work, the groundwater level at this well exceeded 109.00 foot MSL with a maximum 3vol of 109.8 feet MSL.
(Reference AECM-90/0062, dated April 16, 1990.)
e.
_ April, 1990:
A water lovel. reading at DW-8 on April 12,-1990 indicated a groundwater 'lovel of 109.4 foot MSL. The level incrtaso was apparently due to an incorrectly set level sensing probe.
(Reference AECM-90/0083, dated May 8, 1990.)
f.
September, 1990: A water level' reading taken from dowatering well DW-6 Indicated a level of 122.3 feet M3h.
The high level was apparently due to surface and roof run-of f which momentarily overflowed -DW-6's manholo.- This provided rochargo, however, the level:was not indicative of the_ groundwater level.
The DW-6 pump switch'had boon inadvertently left in the "off" position; therefore, the pump was unable to autoxatically maintain proper level.
(Reference AECH-90/0183, dated October 10,-1990.)
In addition to these reporta, Grand Gulf provided a "High Groundwater Level Study" which addressel the probable causes and consequences of the high groundwater levels reported in Item (aj above.
The groundwater study was submitted to the NRC on February 14, 1985 via AECM-85/0035.
This report is summa-ized in Sect.lon III.
As indicated in Items (a) through (o), victually all of the high groundwater level events occurred at DW-8.
This well is located in the circulating water pipeline tronch backfill which is southeast of the Radwaste Building. _This well is not near safety related buildings as illustrated in Figure 1.
l 1
1 A9012121/SNLICFLR - 8
Attachment to GNRO 90/00006 III.
GROUNDWATER STUDIES As previous discussed, Grand Gulf has completed several studies of the groundwater levels at and around power block structures.-
These studies have provided a better understanding of the groundwater levels and flow characteristics, and have identified addillonal actions which could be taken to provide a definite, post-construction profile of the site groundwater levels. The following summarizes these studies.
III.1 Groundwater Level Study - 1983 In response to the high groundwater level which occurred from January through July'of 1983 (as discussed in Section II.3.a),
Grand Gulf initiated a Groundwater Level Study in December, 1983. The results of the study were submitted to the NRC in February, 1985 with the exceedance attributed to:
Excess.tve precipitation at the site Lack of completion of Unit 2 structures Lack of completion of the clay seal t
General yard area grading not completed Increased infiltration from natural causes Recommendations from the Groundwater Level Study that were implemented were to re-establish temporary power to Unit 1 construction dewatering wells and operate them when groundwater level approaches elevation 109.0 feet MSL and the installation of five additional monitoring wells south and east of DW-8, as well as the Installation of a supplementary dewatering well.
Two of those monitoring wells (MW-11 and MW-12) were located in the Circulating Water Trench backfill and three (MW-8,9 and 10) were installed to the east of the Circulating Water Trench. The supplementary dewatering well (DW-8A) was installed adjacent to DW-8 in the tnrrace deposits.
These activities were impicmented in 1986 to provide a bet ter characterization of the direction of groundwater flow around both DW-8 and the circulation water pipe trench.
III.2 Subsequent Groundwater Studies Ongoing studies by Grand Gulf use data from the various monitoring and dowatering wells to evaluate the many factors that cru influence groundwater level. Water levels measured in the powt-block area and surrounding plant property between January 1964 and July 1990 were evaluated in order to determine average groundwatet flow pstterns and to determine potential sources of groundwater recharge and locations of groundwater discharge.
The Catahoula Formation was also evaluated to determino if variations in the elevaticti of the top of the Catahoula Formation contribute to the high water conditions.
The following provides a discussion of these various factors.
A9012121/SNhlCFLR - 9
Attachment to GNRO-90/00006 111.2.1 Catahoula " Hump" Groundwater levels in the southern portion of the plant (areas including the Circulating Water Piping Trench and Cooling Tower Basin) are at elevations greater than in surrounding areas._ A review of data from the mon 1 wring and dowatering wells suggest two factors which produce the higher groundwater level. One factor involves the addition of water to the groundwater inventory from the cooling tower and related piping.
This factor is discussed in Section III.2.2.
The second factor is a local increase in the elevation of the Catahoala Formation under the site. The top of the Catahoula Formation is above regional groundwater levels at the nuclear generating station, exceeding 70 foot MSL elevation'throughout most of the power block area and exceeding 100 feet MSL clovation between the power block area and the cooling tower.
The mounding of the Catahoula-at the generating station causes the regional roundwater flow patterns to diverge north.and o
i south of the
. clear generating station.
In addition, the low permeable silt and clay which compriso the catahoula Formation retard the south to north flow of m undwater from the cooling tower basin towards the power
- plant, This aids the groundwater in local mounding, or gathering, in areas south of the power block.
III.2.2 Additions to Groundwater 1
Wh.tle the mounding of the Catahoula Formation appears to rotard the flow of groundwater across the site, the local inventory of l
groundwater in the southern portion of the sito appears to be Increased by water from the cooling tower basin, circulating water piping, and/or associated systems.
This addition to the groundwater inventory tends to increase the local groundwater l
lovel.
III.2.3 Summary The combined effect of the mounded Catahoula Formation and tho l
addition of. water from the cooling tower basin, et al, is to l
increase the groundwater level in areas south of the plant l
1.e., in areas near and south of the area monitored by DW-8.
This is supported by groundwater level readings.
Groundwater levels from 1984 to the present at wells DW-1 and MW-7 (i.e., the northern side of the Catahoula Formation's mound) averaged approximately 90 foot MSL, whilo similar readings at DW-8 (i.e., southern sido of mound) averaged 107 I
feet MSL.
Further, the minimum groundwater elevation of 85 foot MSL was observed at monitoring well MW-7 in February of 1985 and the maximum elevation of 111 feet MSL was observed at downtoring well DW-8 in Noveruber of 1985.
A9012121/SNLICFLR - 10
Attachment to GNRO-90/00006 Water levels exceeded 109 feet MSL on 8 occasions between January 1984 and July 1990 and each of these occurred at dewatering well DW-8.
South of the power block area average water levels varied between 105 feet MSh immediately south of the power block to 118 feet MSL near the cooling tower.
III.3 Groundwater Recharge Sources Data from the monitoring and dowatering wells was also evaluated to determine if the higher than expected groundwater icvels were a result of offsite recharge sources.
It was determined that groundwater at the nuclear generating station a acquired solely from onsite sources.
This is indicated by regional groundwater flow patterns which are directed away rather than towards the site.
Sources of recharge include precipitation and leakage from onsite sources, such as the cooling tower basin. Within this divide, groundwater is derived from recharge and flows towards the existing downtering wells in the power block area.
Outside this divide, groundwater flows radially away from the plant site.
Some of the groundwater in the cooling tower area flows northward towards the dowatering wells in the power block area, however, most of the groundwater in that area flowe cast, south and west and, eventually, joins the regional groundvater flow system.
Water levels and precipitation rates could not be related on either a monthly or yearly basis.
Ilowever, water level respmnses to large variations in precipitation were observed in monitoring wells located between the power block and the cooling tower indicating that precipitation is a source of groundwater recharge at the facility.
In 1989, water levels rose during the wet summer months and declined in the fall in response to the abrupt decrease in precipitation.
IV.
CONTINUING _._ ACTIONS Presently, fifteen new monitoring wells are being installed to help further define the direction of groundwater flow.
Thirteen of those now wells will be located around the Unit 1 Cooling Tower and two will be located north of the plant.
Data mollected from these new wells over the next few months will help clarify the source of recharge and direction of groundwater flow.
In addition to the new wells, flow instrumentation was installed on the discharge of dewatering pumps in 1990 to pro-vide a quantifiable measure of groundwater from the wells.
Data from the new wells and flow instrumentation will be collected through mid-1991 to provide data f rom the Spring 4
(i.e., wet) season. This will provide a better definition of recharge influences atttibutnbic to precipitation.
A9012121/SNLICFLJ - 11
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V:
Attachment to GNRO-90/00006.
V.
CONCIfLSIONS l
The review of groundwater level deta to date indicates that the high groundwater events reported at GGNS are the result of leakage from onsite sources and precipitation in areas away from safety related structures.
The groundwater level around safety related structures has consistently been below the design maximum groundwater level of 109.0 feet MSL.
However, in response to the need for a post-construction maximum groundwater level and to better define influences from precipitation, Entergy Operations, Inc. is implementing tasks designed to provide a better definition of groundwater level at Grand Gulf.
As discussed in Section IV, these tasks include the installation of fifteen additional monitoring wells and monitoring of flow instrumentation installed on dowatering pump discharges.
Entergy Operations, Inc. will collect data from these sources throughout the Spring (i.e., wet) season and use the data to develop the post-construction naximum groundwater level and/or to identify additional measures which may be required to fully respond to the high groundwater level events addressed by the NRC's SER. The completion of these tasks will
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support the issuance of a final report by April 30, 1992.
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