Geologic Faults and the North Anna Power Plant

faults (brown lines) cut through the Piedmont bedrock at the North Anna Power Station and nearby
faults (brown lines) cut through the Piedmont bedrock at the North Anna Power Station and nearby
Source: ESRI, ArcGIS Online

Geologic faults are a zone of weakness within bedrock. Mapping the location of faults accelerated in the 1960's and 1970's when sites were being chosen for constructing nuclear power plants.

Faults increase risks; bedrock which moved in the past might move again in an earthquake, potentially cracking the containment vessel and releasing radioactivity. Identifying locations of "solid" bedrock without a fault reduced risk of a nuclear incident.

In the United States, only one nuclear power plant was built on top of a fault. The Virginia Electric and Power Company (VEPCO), now Dominion Energy, concealed the discovery of a fault when getting a license for Units 1 and 2 of the North Anna Power Plant.

The Atomic Energy Commission granted the Federal licenses in 1973, but a local geologist shared his knowledge with the North Anna Environmental Coalition. He knew the fault was there, and that the utility was aware of it, from a site visit in 1970 after a construction excavation wall for Reactor Unit 1 collapsed. VEPCO had purposely deleted a required reference to the geologic fault before submitting licensing information to the Atomic Energy Commission.

That organization publicized the existence of the fault, and enlisted the help of the Virginia Attorney General's office. Two years later, the US Nuclear Regulatory Commission (successor to the Atomic Energy Commission in 1975) fined the utility company for making a "material false statement" during the license process.1

Only civil penalties were imposed, because the staff of the Atomic Energy Commission/Nuclear Regulatory Commission had also helped to delay revelation of the fault.

The fault was unknown when the site was chosen and construction started on Unit 1. In 1970 a chlorite seam was exposed during excavation. The construction company's chief geologist did not classify the chlorite seam as a fault, concluding that there was no evidence of geologic offset. The construction company decided the seam was caused by weathering.

Three independent geologists who visited the excavation judged the seam to be evidence of earth movement. It was not until 1973 that further excavation for a planned Unit 3 revealed there had been six-ten feet of displacement at the chlorite seam.

Even then, an outside consultant hired by VEPCO disagreed with the construction company's geologists who had concluded that the seam marked the location of a fault. The outside consultant:2

...hypothesized that they were viewing a coincidental appearance of two pegmatite dikes rather than a single dike which had been offset by movement along a fault.

Other geologists for the outside consultant determined that they had discovered a fault, and rejected proposals to refer to it as a "shear" or other terms that would not trigger the same close Federal review once a "fault" was identified. However, the paperwork submitted for the Atomic Energy Commission to authorize construction of Units 1 and 2 did not mention the discovery of a fault at the site.

It appears the utility officials were concerned that opponents of the nuclear reactors would delay award of the Federal licenses. The Virginia Electric and Power Company (VEPCO) obscured the discovery in order to get enough time to document that the fault was not "capable."

"Capable" was a regulatory term of great significance. Faults at or near the ground surface were classified as capable of new movement, and thus a threat to a nuclear power plant, if the ground had moved at least once within the past 35,000 years or if movement of a recurring nature had occurred within the past 500,000 years. A fault could also be defined as capable if it had not moved within 500,000 years, but was associated with other capable faults.

A fault would be judged "not capable" and not considered to be a threat, if the land had been stable for a long period of time:3

...structural features which are geologically old (at least pre-Quaternary) such as many of those found in the Eastern region of the United States shall, in the absence of conflicting evidence, demonstrate that the fault is not a capable fault within this definition.

The bedrock at the North Anna Power Station is part of the Ta River Metamorphic Suite, dating back to the Cambrian. That bedrock was accreted to Virginia as part of the Chopawamsic Terrane, an Ordovician volcanic-plutonic arc complex added to the edge of Virginia during the Taconic orogeny. If the last displacement at the chlorite seam dated back to the formation of Pangea or to its breakup, then the fault was too old to be classified as "capable."4

the North Anna Power Station (red circle) is located on the Ta River Metamorphic Suite, within the Chopawamsic Terrane
the North Anna Power Station (red circle) is located on the Ta River Metamorphic Suite, within the Chopawamsic Terrane
Source: Virginia Department of Energy, Geology Mineral Resources

Units 1 and 2 went online in 1979 and 1980. In 2001, the epicenter of a 5.8 magnitude earthquake was 11 miles to the southwest near the town of Mineral. Both units shut down, and the Nuclear Regulatory Commission allowed a restart 80 days later. The reactor containment buildings were built to withstand shaking equal to 12% of the force of gravity. The intensity of the earthquake exceeded the plant's design, but for only about three seconds. Only minor damage occurred at the North Anna Power Station.5

The most significant impact was the shifting of spent fuel casks. They were designed to stay in place during the maximum forces predicted during an earthquake at the North Anna Power Station, in order to maintain the required separation between containers of hot radioactive waste. Sensors at the site documented that those forces were exceeded briefly in 2011.

The older model TN-32 casks slid on the concrete pad during the Mineral earthquake, but the shift in position did not create a safety risk. The Nuclear Regulatory Commission reported:6

Twenty five of the twenty seven TN-32 casks slid up to 4.5 inches on the concrete pad during the quake. Six cask sets (12 casks) were closer than the 16 foot separation distance specified in the FSAR...

...Thermal performance measurements for all loaded casks found no abnormal temperature differences.

After the 2011 earthquake, the Nuclear Regulatory Commission considered the seismic risks for a proposed Unit 3:7

Staff experts, including geologists, also evaluated the behavior of a previously discovered fault, commonly referred to as fault "a", near the North Anna site. This is a geologically old structure, i.e., at least one million years old, and the staff determined it was not a potential seismic source. There was no evidence of rupture or deformation of fault "a" as a result of the Mineral earthquake.

Faults in Virginia

North Anna Power Station

Virginia Earthquakes

Links

References

1. "Nuclear Critics Decry Vepco Safety Conditions," Washington Post, May 1, 1979, https://www.washingtonpost.com/archive/local/1979/05/01/nuclear-critics-decry-vepco-safety-conditions/e0819293-1ceb-41d9-b1d3-08f869dfcf1f/; "Geologic Faulting Under The North Anna Nuclear Power Plant - The History Of Dominion-Virginia Power's Seismic Cover-Up," Blue Ridge Environmental Defense Fund, October 8, 2005, https://www.bredl.org/pdf3/051008_factsheet_Allegation2005A0014.pdf; Peter Galuszka, "June Allen's foresight on North Anna," Washington Post, September 2, 2011, https://www.washingtonpost.com/blogs/all-opinions-are-local/post/june-allens-foresight-on-north-; Bradford F. Whitman, "Memo: Recommending Against Prosecution of Virginia Electric Power Company For Failure to Disclose a Geologic Fault (18 U.S.C. 1001)," U.S. Department of Justice, May 11, 1977, http://pogoarchives.org/m/nss/doj-north-anna-memo-19770511.pdf (last checked December 23, 2021)
2. Bradford F. Whitman, "Memo: Recommending Against Prosecution of Virginia Electric Power Company For Failure to Disclose a Geologic Fault (18 U.S.C. 1001)," U.S. Department of Justice, May 11, 1977, http://pogoarchives.org/m/nss/doj-north-anna-memo-19770511.pdf (last checked December 23, 2021)
3. Bradford F. Whitman, "Memo: Recommending Against Prosecution of Virginia Electric Power Company For Failure to Disclose a Geologic Fault (18 U.S.C. 1001)," U.S. Department of Justice, May 11, 1977, http://pogoarchives.org/m/nss/doj-north-anna-memo-19770511.pdf; "Appendix A to Part 100 - Seismic and Geologic Siting Criteria for Nuclear Power Plants," regulations of the US Nuclear Regulatory Commission, https://www.nrc.gov/reading-rm/doc-collections/cfr/part100/part100-appa.html (last checked December 23, 2021)
4. David B. Spears, Brent E. Owens, and Christopher M. Bailey, "The Goochland-Chopawamsic Terrane Boundary, Central Virginia Piedmont," Chapter 7 in "Geology of the National Capital Region - Field Trip Guidebook," Circular 1264, US Geological Survey, 2004, https://pubs.usgs.gov/circ/2004/1264/html/trip7/ (last checked December 23, 2021)
5. "Dominion's North Anna Station Sets New Standard for Earthquake Response," Power, November 1, 2012, https://www.powermag.com/dominions-north-anna-station-sets-new-standard-for-earthquake-response/; "Inspectors check Virginia nuclear plant for earthquake damage," Washington Post, August 30, 2011, https://www.washingtonpost.com/national/health-science/inspectors-check-virginia-nuclear-plant-for-earthquake-damage/2011/08/30/gIQA8SU3rJ_story.html (last checked December 23, 2021)
6. "North Anna Independent Spent Fuel Storage Installation Response to Earthquake," US Nuclear Regulatory Commission, y, September 1, 2011, https://www.nrc.gov/docs/ML1200/ML12005A011.pdf (last checked December 23, 2021)
7. "Report on the Safety Aspects of Dominion Virginia Power Combined License Application For North Anna Unit 3," Nuclear Regulatory Commission, November 15, 2016, https://www.nrc.gov/docs/ML1631/ML16312A412.pdf (last checked December 24, 2021)

pegmatite dike offset by about three feet due to movement along a fault near the North Anna Power Station
pegmatite dike offset by about three feet due to movement along a fault near the North Anna Power Station
Source: US Geological Survey (USGS), Geologic Framework for Seismic Hazards in Central Virginia


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