Wednesday, October 28, 2015

Ensuring Seismic Safety at America's Nuclear Power Plants

Timothy Rausch
The following is a guest post by Timothy Rausch, Talen Energy’s Senior Vice President and Chief Nuclear Officer

Companies that operate America’s nuclear energy facilities today have made significant progress in their evaluations of seismic safety as part of a series of actions the industry is taking to implement lessons learned from the 2011 Fukushima accident.

The Nuclear Regulatory Commission in 2012 required energy companies to reevaluate potential seismic hazards for each of America’s 99 reactors.

Nuclear energy facilities were designed and built with extra safety margin, in part to be able to withstand an earthquake even beyond the strongest ever at each site. Nonetheless, over the past decades, the industry has re-evaluated the seismic safety of its facilities. Each time new seismic information became available, plant operators have confirmed, and in many cases, enhanced the facility’s seismic protection.  The nuclear industry is in the midst of a comprehensive review based on the most recent reevaluations – several plants have completed these reviews and many more will be complete within the next year.

Most recently, the NRC in 2010 concluded that nuclear power plants have significant safety margin to protect against earthquakes, including those with greater ground motion than the earthquakes used to develop the original reactor designs.

The nuclear energy industry, working with the NRC, U.S. Department of Energy and other organizations, developed a new model to characterize the potential for strong earthquakes. The industry applied this model to develop new earthquake hazard estimates for each nuclear plant site. These estimates will be part of a more comprehensive evaluation to ensure nuclear plants continue to be protected against the strongest earthquakes predicted for that site.

The North Anna Power Station near Mineral, Va. 
There have been only a few cases where powerful temblors exceeded the design parameters for reactors worldwide, and in those cases the plants shut down safely. The North Anna nuclear power plant in central Virginia is the most recent example. A 5.8 earthquake with an epicenter 11 miles from the facility caused destruction at some local schools and damaged national monuments 80 miles away in Washington, D.C. Yet, there was no damage to North Anna’s safety systems and only minimal chipping and cracking of concrete outside the vital safety areas. That’s due in part to huge shock absorbers and supports that are installed to protect safety systems during forceful quakes.

In the past several years, the industry has developed new assessment processes and an updated ground motion database to undertake the NRC-required update of seismic protection. Using the new ground motion data, the analysis for most U.S. reactors shows a reduced risk to safety from earthquakes, compared to assessments in 1994 and 2008. 

A few have a higher risk, but well within a range that will protect the plant and residents near the facility. For those plants, the owners will undertake more sophisticated analyses to determine what additional safety measures should be taken, if any.  The NRC recently issued a letter providing its final determination of those plants that are expected to perform either seismic probabilistic risk assessment or seismic margin analysis of seismic events.  

These sophisticated analyses will provide the best understanding possible of plant risk and will ultimately be used to determine whether any safety enhancements could be made to further improve the plant’s seismic safety.  These assessments, involving experts from a variety of fields, take several years to complete and are well underway.  They are expected to be submitted to the NRC for review between 2017 and 2019. Once the assessments are complete, the NRC will decide if plants require any upgrades to equipment, systems and structures.

No comments: