ML18220A777
| ML18220A777 | |
| Person / Time | |
|---|---|
| Site: | Clinch River |
| Issue date: | 08/08/2018 |
| From: | Gerry Stirewalt NRC/NRO/DLSE/RGSB |
| To: | |
| Stirewalt G | |
| Shared Package | |
| ML18220A749 | List: |
| References | |
| Download: ML18220A777 (4) | |
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Summary from Staffs Examination of Geologic Features at TVAs Clinch River Nuclear Site during a 30-31 January 2018 NRC Management Site Visit G. Stirewalt and J. Thompson May 2018 INTRODUCTION The 30-31 January 2018 site visit was planned by the applicant (TVA) to help familiarize NRC management with the proposed Clinch River Nuclear (CRN) site, for which Early Site Permit (ESP) application materials have been submitted, including the Site Safety Analysis Report (SSAR). The site visit was also thoughtfully organized by the applicant to provide the opportunity for NRC staff geologists responsible for preparing Site Evaluation Report (SER)
Sections 2.5.1 and 2.5.3 for the proposed site to directly examine evidence documenting the presence of shear-fracture zones, Late Paleozoic thrust faults emplaced during the Alleghanian orogeny around 276 to 280 Ma ago based on radiometric ages, and karst features at the site.
Staff geologists focused on these three geologic features to assess their potential for producing tectonic and non-tectonic surface deformation and resultant natural hazards at the site.
Observations made during the site visit based on examination of these features, the details of which are summarized in the following paragraphs, provided staff geologists with a clear understanding of the detailed information about these features presented by the applicant in the SSAR. The features were also observed by NRC staff during a May 2017 site audit and are discussed in the report documenting that audit (ADAMS Accession Number ML17223A428).
SHEAR FRACTURE ZONES AND PALEOZOIC THRUST FAULTS During the January 2018 site visit, staff geologists focused on examining the shear-fracture zones and analyzing the relationship between the zones and Late Paleozoic (i.e., Alleghanian) thrust faults that occur in the CRN site area. Staff maintained this focus because it was important to understand origin and timing of development of the shear-fracture zones based on geologic characteristics of the zones, clarify the relationship between thrust faults and the shear-fracture zones, and ensure that the shear-fracture zones did not represent tectonic deformation features that could pose a potential hazard for the CRN Site.
In SSAR Section 2.5.1, the applicant analyzed stylolites (i.e., localized features that are relatively common in homogeneous carbonate rocks and develop as a result of pressure solution along surfaces generally oriented at a high angle to the direction of maximum principal compressive stress) in the shear-fracture zones to interpret history of development of the zones.
During the January 2018 site visit, staff examined shear-fracture zones in core from boreholes MP-101 at potential site Location A and MP-423 at potential site Location B, both of which contain stylolites with two distinctly different orientations - one subparallel to and another at high angles to bedding. Figure 1, reproduced from SSAR Figure 2.5.1-30, shows the locations of
these two boreholes at potential site Locations A and B in geologic cross-section K-K across the CRN Site. Figure 2 shows field relationships of stylolites observed by staff in the shear-fracture zone penetrated in borehole MP-101. Based on these relationships, staff considers that stylolites oriented subparallel to bedding in the shear-fracture zones, which are not marked by strong cataclasis (i.e., mechanical grinding and crushing that would be indicative of major shear displacement along or within the zones), can be most logically interpreted as non-tectonic diagenetic pressure solution features resulting from overburden pressures (i.e., under a near-vertical maximum principal compressive stress). Staff also considers that stylolites oriented at higher angles to bedding most likely reflect a tectonic overprint on the shear-fracture zones by post-diagenetic pressure solution related to Late Paleozoic thrust faulting (i.e., under a near-horizontal maximum principal compressive stress). No field relationships examined by staff indicate that the shear-fracture zones exhibit pressure solution effects or tectonic deformation (e.g., cataclasis resulting from fault displacement) younger than Late Paleozoic. The observed field relationships support the applicants interpretation that the abundance of pressure solution features and the paucity of evidence for mechanical grain size reduction (i.e., cataclasis) in the shear-fracture zones suggest the zones accommodated strain mainly by pressure solution resulting from both non-tectonic diagenetic and later post-diagenetic tectonic effects, but with limited cataclastic deformation of the shear-fracture zones during the tectonic event (i.e.,
Alleghanian thrust faulting). The thrust faults reflect strong cataclastic deformation as show by the presence of gouge associated with the Copper Creek fault. Gouge associated with this thrust fault was examined by staff in core from boreholes CC-B1 and CC-B2, the locations of which are shown in geologic cross-section K-K (Figure 1). As Figure 3 illustrates, the cataclastic character of the fault gouge penetrated in borehole CC-B2, produced by mechanical grinding and crushing during displacement along the fault, contrasts sharply with material properties of rock units in the shear-fracture zones that mainly show evidence of pressure solution without extensive cataclasis. Staff considers that the observed characteristics of rock units in the shear-fracture zones suggest very little bedding-parallel shear displacement has occurred in the zones.
In summary, based on results of the examination of shear-fracture zones and the Late Paleozoic Copper Creek thrust fault in core during the January 2018 site visit, staff determined that the observed field relationships support the following interpretations regarding development of shear-fracture zones and timing of development of the zones in relation to Late Paleozoic thrust faulting: (a) The shear-fracture zones contain diagenetic, bedding-parallel stylolites that most likely resulted from pressure solution due to lithostatic loading during burial of the sedimentary strata in which the stylolites occur under a near-vertical maximum principal compressive stress; (b) The shear-fracture zones also contain post-diagenetic tectonic stylolites oriented at a high angle to bedding that most likely resulted from pressure solution under a near-horizontal maximum principal stress during Alleghanian thrust faulting; (c) The shear-fracture zones do not exhibit evidence of extensive cataclasis to suggest that major shear displacement was associated with development of the zones; (d) The stylolites found in the shear-fracture zones must have formed at two different times and in two distinctly different strain regimes, reflecting both diagenetic effects and a later (i.e., Late Paleozoic) tectonic overprint; (e)
Thrust faults in the site area, by analogy with the Late Paleozoic Copper Creek thrust fault
examined in core, and shear-fracture zones at the site do not exhibit any evidence for Quaternary tectonic deformation that could pose a potential hazard for the CRN Site.
KARST FEATURES During the January 2018 site visit, staff geologists focused on understanding the potential for karst development at the CRN Site and considered the possibility that karst features in the site area might have developed due to two different dissolution processes: (1) epigenic processes related to water moving downward from surface or near-surface sources, which can occur in the vadose zone above the water table and the phreatic zone below the water table; and (2) hypogenic processes resulting from aqueous solutions moving upward from deep sources. Staff maintained this focus because carbonate rocks and associated dissolution features (e.g., caves and sinkholes) occur in the site area and pose a potential hazard for the CRN Site. Figure 4 illustrates one of the larger dissolution features in the site area examined by staff during the site visit, the Copper Ridge Cave, which is a large relict cave segment in carbonate rock of the Knox Group. The cave is located about 6 km (3.7 mi) east of the CRN Site.
In SSAR Section 2.5.1, the applicant presented a detailed assessment of karst in the site vicinity and site area and at the site location. During the January 2018 site visit, the staffs direct examination of karst features in outcrops located in the site area and in core from the site location documented that most existing evidence is consistent with dissolution by epigenic processes in the vadose and phreatic zones rather than by hypogenic processes. The evidence includes a decrease in frequency of fractures and dissolution cavities with depth in boreholes; phreatic passage geometry and morphology of known caves and solution conduits within the ORR (e.g., Copper Ridge Cave); and a lack of secondary minerals characteristic of hypogenic processes. The applicant stated in the SSAR that detailed geologic mapping and a subsurface exploration program will be implemented to characterize the excavations for safety-related engineered structures at the CRN Site in regard to the presence or absence of karst features in and below the floor of those excavations. Staff anticipate that the geologic mapping and subsurface exploration programs will also make it possible to characterize any shear-fracture zones or tectonic features discovered in the excavations for safety-related structures.
In summary, based on results of the examination of karst features during the January 2018 site visit, staff recognized that karst appears to be the primary geologic hazard at the CRN Site because karst features are characteristic of the purer carbonate rock units found in the site area. Staff noted that karst features examined in the field most likely resulted from epigenic processes operating in the vadose and phreatic zones, and that there is a lack of definitive evidence for hypogenic dissolution in the site area.
FIGURES Figure 1. Geologic cross-section K-K across the CRN Site showing locations of boreholes MP-101 and MP-423 that penetrate shear fracture zones in rock units of the Chickamauga Group (i.e., the Rockdell Formation and Eidson Member of the Lincolnshire Formation, respectively) and boreholes CC-B1 and CC-B2 that penetrate fault gouge associated with the Copper Creek thrust fault. (Figure is based on SSAR Figure 2.5.1-30.)
Figure 2. Details of stylolites oriented parallel and at high angles to bedding in the shear-fracture zone penetrated in borehole MP-101. This shear-fracture zone occurs in the Rockdell Formation of the Chickamauga Group and underlies potential site Location A as shown in Figure 1. The center portion of this core was extracted from a depth between 248.1 and 250.6 ft below the ground surface. (Image by G. Stirewalt from January 2018.)
Figure 3. Fault gouge associated with the Copper Creek fault penetrated in borehole CC-B2.
The gouge, a product of mechanical crushing and grinding (i.e., cataclasis) related to thrust displacement along the fault during the Late Paleozoic Alleghanian orogeny, has been radiometrically dated at 279.5 +/- 11.3 Ma and occurs between the base of the Cambrian Rome Formation and the top of the stratigraphically younger Ordovician Moccasin Formation of the Chickamauga Group. The Rome Formation overlies the Moccasin because it was thrust over the Moccasin Formation along the Copper Creek fault. Thickness of gouge ranges from about 1.5-2.3 m (4.3-7.4 ft). The fault, located about 1 km (0.6 mi) southeast of the CRN Site, has a reported displacement of 12-50 km (7.4-31 mi) to the northwest. Regional thrust faults comparable to the Copper Creek fault in age and extent are characteristic of the Valley and Ridge physiographic province in which the CRN Site is located. (Image by G. Stirewalt from January 2018.)
Figure 4. Entrance to Copper Ridge Cave, which occurs in carbonate rock of the Knox Group.
This cave is located about 6 km (3.7 mi) east of the CRN Site. (Image by G. Stirewalt from January 2018.)