Looking For Better Ways to Determine Severe Weather Hazards

Thomas Nicholson
Senior Technical Advisor
Office of Nuclear Regulatory Research

 

The NRC staff evaluates flood hazards when we review applications for new nuclear facility sites. In addition, we re-examine flooding at operating nuclear power plants — a result of what we learned from the 2011 tsunami flooding at Fukushima Dai-ichi in Japan. These evaluations cover a range of flood events including extreme storms that produce intense local rainfall. The NRC works with other federal agencies to better understand events caused by severe weather as we develop ways to better evaluate possible flooding issues at these sites.

weatherBefore the Fukushima event, the NRC staff informed the Federal Subcommittee on Hydrology of the urgent need to update the National Weather Service’s reports for estimating extreme rainfall events. We use these reports as the basis for our flood design and protection studies. As a result, the subcommittee formed a task force and later the Extreme Storm Events Work Group. The work group is looking at the best practices being used to study extreme storms, and developing estimation procedures and guidance.

The Extreme Storm Events Work Group has an impressive membership. In addition to the NRC, it includes the National Weather Service, U.S. Army Corps of Engineers, U.S. Bureau of Reclamation, Federal Energy Regulatory Commission, Natural Resources Conservation Service, Tennessee Valley Authority, and the U.S. Geological Survey. The work group meets monthly to talk about ongoing activities and products federal agencies are developing to help monitor, model and publish rainfall estimates.

Based in large part on the group’s work, we held a three-day workshop last year on probabilistic flood hazard assessment. The workshop brought together more than 250 international experts and included presentations and panel discussions on extreme rainfall events, coastal storm surge flooding, river flooding, tidal waves, flood-induced dam and levee failures, and combined flood events.

More recently, the work group held a workshop at the National Weather Service to define needed extreme storm products for the nation. These products will greatly assist the federal agencies that are moving towards a risk-informed approach for assessing flooding hazards. NRC staff members are benefiting greatly by their interactions with their federal counterparts in the work group.

Nuclear power plants are built to withstand local extreme weather, but we are always learning how safety margins can be improved even more. By working with weather experts in other federal agencies, we can build on what they’re doing and our nuclear power plants will benefit from this collaboration. We can’t stop flooding from happening, but we can make sure the facilities we regulate are prepared to deal with it safely.

River Levels on the Rise – The NRC At The Ready

Lara Uselding
Public Affairs Officer
Region IV
 

NEW UPDATE: Currently, river levels are at 1000 feet 6 inches with levels not expected to increase more than a few inches over the next 24 hours. OPPD is returning the plant to full power. NRC inspectors provided around the clock coverage through last weekend and the agency will continue closely monitoring plant operations and river conditions.

UPDATE: Fort Calhoun began decreasing power at midnight, and is currently holding at a reduced power level. The river level is now predicted to crest at 1,002.4 feet on Saturday, which is lower than previously predicted. For comparison, the 2011 flood peaked at about 1,007 feet. Three people from Region IV will begin around the clock coverage today in support of the resident inspectors.

Three years ago this month marks the anniversary of the record Missouri river floods. Now, due to heavy rains, the NRC is once again watching rising Missouri River levels impacting Nebraska’s Fort Calhoun nuclear power plant,  north of Omaha.

Cooper Nuclear Station in Brownville is not anticipating a major impact this weekend.

Fort Calhoun’s procedure requires them to declare a Notice of Unusual Event and be shut down by the time river levels at the site reach 1,004 feet mean sea level. Thursday afternoon, river levels were at 998 and rising. Normally, river levels at the site range from 980 to 990 feet mean sea level.

Over the past week, NRC’s Region IV in Arlington, Texas, has been engaged in routine calls with the United States Army Corps of Engineers, National Weather Service, Federal Emergency Management Agency, National Oceanic and Atmospheric Administration, states, and local response organizations to understand changes in the predicted river levels and assess potential impacts on the plants. 

Simultaneously, the NRC has been overseeing actions that Omaha Public Power District (Fort Calhoun) and Nebraska Public Power District (Cooper) are taking to protect the plant against impending flood waters. At this time, river levels at Cooper are not projected to be high enough to require a plant shutdown.

OPPD’s actions involve the use of sand bags, flood doors, and readying mobile pumps as river levels are projected to rise. They have also ordered equipment to protect certain buildings on site. NRC resident inspectors, who live in the area and work at the plant, have been monitoring the flood preparations.

The NRC is sending more staff to the plant to support the resident inspectors and provide around the clock coverage. During the 2011 flood, river levels at Fort Calhoun reached about 1007 feet and the plant remained in a safe shutdown condition. The plant restarted late last year only after extensive flooding improvements and other safety upgrades mandated by the NRC. Fort Calhoun remains under increased NRC regulatory oversight.

Region IV will continue monitoring the situation for both plants over the weekend.