Waterford Steam Electric Station, Unit 3, Revision 309 to Final Safety
Total Page：16
File Type：pdf, Size：1020Kb
WSES-FSAR-UNIT-3 2.5 GEOLOGY AND SEISMOLOGY Waterford 3 is situated along the west (right descending) bank of the Mississippi River, about 25 miles west of New Orleans, Louisiana. It is located in the southern portion of the Gulf Coastal Plain geologic province. The southern portion of the Gulf Coastal Plain is the Mississippi River deltaic plain physiographic province. The Mississippi River has dominated the development of geologic and physiographic features in the deltaic plain since the beginning of Neogene. The deltaic plain is characterized by low marshy terrain, much of which is covered by water. The higher natural ground within the deltaic plain generally occurs along the natural levees of existing and abandoned stream courses. The deltaic plain is an area where extensive exploration and exploitation of petroleum has occurred. As a result of exploration near the site, deep boring data are available that provide identification of strata characteristic of the site to depths of 12,000 ft. In addition, 74 borings were drilled within the site to depths up to 500 ft., to define the site stratigraphic and hydrostratigraphic sequences and to determine the engineering properties of subsurface materials. The regional geologic structures in the deltaic plain consist of salt structures, their overlying attendant faults, and growth faults. The growth faults represent previously unstable areas which were at the leading slope of sediment accumulation. The subsurface data demonstrate that such regional structures cannot affect the Waterford site. ¨(DRN 01-464) The site is within a region of infrequent and minor seismic activity. The maximum earthquake associated with the site region is the Donaldsonville earthquake of October 19, 1930, which has an epicentral intensity of nearly VI Modified Mercalli (MM). The nuclear plant has been designed for a maximum horizontal ground surface acceleration of 0.1g, about two times greater than the maximum acceleration appropriate for the Donaldsonville earthquake. In November 1982, the U.S. Geological Survey (USGS) in its capacity as advisor to the U.S. Nuclear Regulatory Commission (NRC) on geologic and seismic siting issues, provided clarification of its position with respect to consideration of the Charleston earthquake of 1886 in seismic design decisions. The clarification was contained in a letter from James Devine to Robert Jackson dated November 18, 1982 (90). õ(DRN 01-464) This clarification perceived uncertainty about earthquake causes in the East, the uncertain geologic characteristics of seismic source zones, and the difficulty that has been experienced in making licensing decisions that properly take account of these uncertainties within the constraints of the deterministic regulation, 10CFR100, Appendix A. This has been referred to as the "Charleston earthquake issue." ¨(DRN 01-464; 02-217) The Seismicity Owner's Group (SOG) was formed to develop and perform analyses to resolve the Charleston earthquake issue. The entire SOG methodology and implementing interpretations for application to probabilistic seismic hazard computations were documented, referenced (91) and submitted to the NRC for review in July 1986. The NRC issued an SER in September 1988 accepting the SOG's methodology and implementing interpretations. Following receipt of the NRC SER, the SOG performed site-specific seismic hazard computations at 57 sites. The EPRI site specific methodology and results for Waterford 3 were documented in Reference (92) LP&L reviewed the methodology and results of the seismic hazard computation, and the review was documented in Reference (93). The review concluded the seismic hazard at Waterford 3 is very low. The mean percentile annual probability of exceeding 0.10 g is 8.0E-6 and of õ(DRN 01-464; 02-217) 2.5-1 Revision 11-A (02/02) WSES-FSAR-UNIT-3 ¨(DRN 02-217) exceeding 0.05g, the annual probability is 3.0E-5. The SOG concluded, on the basis of the overall methodology and the site specific probabilistic seismic hazard computations, the Charleston earthquake issue has no material impact on the safety of nuclear power plants in the central and eastern U.S. The site specific seismic hazard computations for the 57 sites were provided by the SOG to the NRC in April 1989. õ(DRN 02-217) All seismic Category I systems, equipment and components are supported within a common rigid, reinforced concrete structure. The foundation is designed as a part of the structure so that the combined foundation and structure act as an integral unit. The structure and its mat foundation are referred to as the nuclear plant island structure (NPIS). The NPIS bears at elevation -47 ft. MSL. At this level the plant island is completely compensated and imparts an average stress to the underlying strata approximately equal to the pre-existing in situ effective stresses. The sandy aquifers which exist beneath the site are of sufficient density to preclude liquefaction during the postulated most severe earthquake conditions. The only natural slope in the area of the site is the right descending (west) bank of the Mississippi River, located about 960 ft. east of the NPIS. This bank slopes to the north at an angle of about seven degrees (8H:lV) and its failure could not adversely affect the plant. The geologic studies of the site and surrounding area were made by Law Engineering Testing Company. These studies include interpretations of geologic literature, geologic maps, topographic maps, remote sensing data, surface mapping, subsurface borings, geophysical refraction surveys, electric logs and laboratory tests. Subsections 2.5.1 through 2.5.5 present the conclusions drawn from these data. 2.5.1 BASIC GEOLOGIC AND SEISMIC INFORMATION 2.5.1.1 Regional Geology The following section provides basic information describing the physiography, geologic history, stratigraphy and structural geology of the region around the site. The geologic features influencing potential subsidence, collapse and man's activities are provided in Subsection 2.5.1.3; the groundwater conditions of the region are discussed in Subsection 2.4.13. 2.5.1.1.1 Physiography and Geomorphology The Waterford 3 site is located near river mile 129 Above Head of the Passes (AHP) (Figure 2.5-1). The site is located within the Gulf Coastal Plain physiographic province (Figure 2.5-2). The Gulf Coastal Plain extends about 600 miles inland from the coast along the site longitude, 90 degrees west, approximately 200 miles inland along longitude 88 degrees west and approximately 300 miles inland along longitude 94 degrees west. The northern margin of the Gulf Coastal Plain is bounded by five major physiographic provinces: Ouachita province, Ozark Plateau province, Central Lowlands province, Interior Low Plateaus province, and the Southern Appalachians province. The locations of these 2.5-2 Revision 11-A (02/02) WSES-FSAR-UNIT-3 physiographic provinces are shown on Figure 2.5-2. The topography of the adjacent physiographic provinces has been mainly developed along structural features in Paleozoic and older rocks. The Gulf Coastal Plain is bounded to the south by the Gulf of Mexico. The Mississippi enbayment section of the Gulf Coastal Plain is subdivided into three major physiographic areas (Figure 2.5-2), as is the submerged area of the Gulf (Figure 2.5-3). The description of these six physiographic areas are as follows: a) Mississippi River Valley and Adjacent Lowlands (Figure 2.5-2) The Mississippi River and its tributaries follow a broad, north-south trending lowland that begins at the head of the Mississippi embayment, near the junction with the Ohio River, and extends southwest about 600 miles to the Gulf. This lowland is composed of an alluvial plain that extends from the Ohio confluence to near the Atchafalaya River in Louisiana and a deltaic plain that continues to the Gulf of Mexico. The alluvial plain of the Mississippi River valley includes the alluvial valleys of the Mississippi River and its tributaries. Most tributary streams enter the Mississippi River from the west. These include the Arkansas, Red, and Missouri Rivers which supply the main sediment load of the Mississippi River. The major tributary that enters from the east is the Ohio River. The alluvial plain ranges from about 25 to 125 miles wide and slopes from a surface elevation of about +300 ft. MSL near the mouth of the Ohio River to about +30 ft. MSL at the northern margin of the deltaic plain. The slope is uniform, gradually decreasing downstream. The meandering Mississippi River and its tributaries are aggrading streams, with the present course defined by ridges or natural levees which are higher than most other areas of the floodplain. The combination of natural levees and two major ridges, Crowleys Ridge and Macon Ridge, have divided the alluvial valley into several shallow basins. These basins include: The St. Francis basin, White River lowland, Arkansas lowland, Yazoo basin, Boeuf basin, Ouachita lowland, Tensas basin, and Lower Tensas basin. The basins, or lowlands, are typically areas of very low relief and poor drainage, marked frequently with scars associated with abandoned stream channels. Within the alluvial valley, the only areas of significant topographic relief are the natural levees of the Mississippi River and larger tributaries, and the two ridges previously mentioned. Crowley’s Ridge extends 200 miles from southeastern Missouri to central Arkansas. It rises as much as 200 ft. above the surrounding lowlands. Macon Ridge extends about 100 miles from southeastern Arkansas into northern Louisiana. The ridges are the highest divides within the valley. (1) A narrow band of fluviatile terraces and deeply dissected loess hills occurs along the eastern bank of the Mississippi River valley from the Ohio River, to southwestern Mississippi. The deltaic plain is generally composed of broad basins separated by narrow, sinuous natural levee ridges which are associated with existing and abandoned distributary 2.5-3 WSES-FSAR-UNIT-3 courses.