Recent tremors in drilling areas are too small to count in nuclear risk assessments, NRC official says

A multiyear review of earthquake hazards at U.S. nuclear plants ordered after Japan's Fukushima Daiichi disaster will not include data from recent earthquakes that may have been triggered by underground wastewater injection from shale gas and oil wells, a Nuclear Regulatory Commission official said.

The quakes in question are typically too small to affect the analysis, said Yong Li, senior geophysicist in NRC's Office of Nuclear Reactor Regulation.

"Quakes below [magnitude] 5.0 would not be considered," he said in an interview. "Nuclear power plants are designed with a more rigorous standard" and can withstand much greater ground motion forces than those resulting from a quake measuring below 5.0, he said.

NRC is ready to reassess evidence linking injection to earthquakes, he added, particularly if larger quakes occur and the connection to drilling activity is shown.

Concerns about a linkage between earthquakes and deep injection of waste drilling fluids have gained intensity following a series of quakes in the past year.


U.S. Geological Survey scientists have reported that the frequency of earthquakes occurring between Alabama and Montana has been rising sharply since 2001, coinciding with a major increase in waste injections from oil and gas drilling in that region of the United States.

Bill Leith, associate coordinator of the USGS Hazards Program, said earlier this month that a large increase in earthquakes in Oklahoma is "spatially associated" with drilling injection. Deputy Interior Secretary David Hayes commented last week that while increased seismic activity in the U.S. midcontinent area appears to be man-made, "it remains to be determined if they are related to either changes in production methodologies or to the rate of oil and gas production" (EnergyWire, April 16).

A rattling New Year's Eve quake in Youngstown, Ohio, prompted state officials to shut down a Class 2 injection well in Youngstown township owned by an affiliate of D&L Energy Group. That temblor measured magnitude-4.0 and followed 10 other quakes last year that officials rated at magnitude-2.7.

"We are closely watching this kind of research and the activity" associated with drilling, Li said.

He acknowledged that some analysts say a magnitude-5.6 quake in Oklahoma last year may be linked to injection. "There are some controversial views about the earthquake in Oklahoma," he said. "The general consensus is that it was caused by natural faulting.

"But there are some views that there is a potential that it could be associated with injection, too. So we keep monitoring that kind of research, and we pay attention to any unusual activity around a nuclear power plant case by case," he said.

Reassessing risk

NRC has directed U.S. reactor owners to reassess the seismic risks to their plants, one of the major commission actions that followed the calamitous accident at the Fukushima Daiichi nuclear complex a year ago.

The 96 plants located east of the Rocky Mountains will use a new analytical model developed over four years by NRC, USGS and the industry's Electric Power Research Institute (EPRI). It includes measured and calculated data on earthquake impacts in the central and eastern United States over the past 400 years and replaces models dating from 1998 and 1999.

Each reactor owner is required to use the new computer analysis to assess the ground motion forces that would be generated by earthquakes of various magnitudes and compare those with prior analyses. Then the companies must evaluate the risks against the designs and safety features at each site, said Jeffrey Hamel, EPRI's manager on the project.

He said the analysis is likely to result in higher estimates of ground motion hazards but that won't necessarily show that seismic risks have increased.

"You have to take all that hazard information, those calculations, then transfer them to the site. Then the site-specific design and safety features have to be factored into an overall calculation to understand where it stacks up," he said in an interview.

Hamel added that a pattern of more frequent quakes that may be triggered by drilling activity also would not change the model's approach unless the frequency increased substantially.

"As long as a single point [of earthquake magnitude] isn't exceeding a design basis of the plant, there is no envisioned cumulative effect," he said, referring to the design basis threat that is based on a maximum quake that experts assume could occur at a site. "If you have a series of three every day, you may want to up the inspections." But not if quakes are occurring once every one, two or three years, he added.

Hamel said the most recent quakes nearby drilling and injection operations are not part of the new computer analysis, which extends through 2008. That is not a gap in the model, however, he said.

"We have ample data in the lower-magnitude range," he said. "When we see more quakes in the 3, 4 or 5 range -- unless it's in areas that we've never seen events in before -- it doesn't do anything" to the model. "It's when we get bigger-magnitude events -- that is what introduces new information."

An Aug. 23, 2011, earthquake measuring magnitude-5.8 centered in Virginia generated ground forces that significantly exceeded safety guidelines at Dominion Virginia Power's North Anna nuclear facility. However, it did not cause functional damage to reactor systems or the spent fuel containers at the site, inspectors for NRC and the company concluded. The two reactors 50 miles northwest of Richmond were cleared for restart in November.

NRC had requested the industry to conduct seismic inspections in the 1990s, and the commission summed up that effort in 2002, saying that 70 percent of the plants proposed improvements as a result of their seismic reviews. But the follow-through was left to the plant operators. NRC noted that "in most cases, plants have not reported completion of these improvements to the NRC."

David Lochbaum, director of the nuclear safety project for the Union of Concerned Scientists, said that some nuclear plant owners upgraded plants to withstand greater earthquake shocks, but some two dozen did not and have not yet been required to.

The new NRC-mandated seismic risks assessments will take 18 months to complete for the central and Eastern plants, and up to three years for Western nuclear reactors, which will use different hazard models. Then several more years could be required to assess the need for structural improvements and carry them out.