Sitting three miles distant from and about 90 feet above the Columbia River – and well above the area floodplain – neither the failure of upriver dams nor extreme local precipitation would adversely affect the safety-related systems, structures and components that are necessary to place and maintain Columbia Generating Station in a safe shutdown mode. Nor would such events prevent access to the Columbia site.
Dam Breach Assessment
With help from the U.S. Army Corps of Engineers and using all available flood data and postulated flooding events, Columbia Generating Station’s designers ensured it was sited far enough inland from the Columbia River and elevated high enough to avoid any potential flood scenario associated with the Columbia River. This includes a breach of the Grand Coulee Dam and the subsequent failure of the earthen portions of all downstream dams and release of their storage pools.
Additionally, river flows analyzed for a postulated Grand Coulee Dam breach were four to five times greater than those expected from a seismic event failure of Grand Coulee.
According to a Grand Coulee official, the likelihood of dam failure, particularly due to overtopping, is highly unlikely. Grand Coulee operators have run emergency action exercises with scenarios involving the failure of upstream dams in Canada that send a high flow over the spillways – 400 to 500,000 cubic feet per second. Though it would overwhelm the capacity of downstream dams, Grand Coulee is designed to handle up to 1,545,000 cubic feet per second through the dam structure (including the power plant) and over its spillways, without overtopping.
Dams on the Columbia River (click to enlarge)
In April 2011, at the request of Energy Northwest, the Bureau of Reclamation reviewed previous river flood modeling and re-validated that flood waters from a postulated Grand Coulee Dam failure would not be high enough to reach any Columbia Generating Station safety-related structures.
In 2012, Energy Northwest performed detailed walk downs of the site and equipment related to potential flooding. These walk downs verified that site configuration and equipment are in compliance with the plant design and able to cope with design flood conditions. A detailed report of this inspection was sent to the Nuclear Regulatory Commission. Click here to view the report.
An updated flooding hazard analysis is currently being performed for Columbia, as requested by a Nuclear Regulatory Commission order issued to all nuclear energy facilities after the Fukushima earthquake and tsunami event. This analysis will provide additional insight into beyond-design-basis flooding events and may show that there are additional measures that can be taken to enhance plant safety during the unlikely occurrence of a beyond-design flood event.
Extreme Precipitation Assessment
Columbia’s designers also considered the potential for an extreme, local precipitation event and accounted for a postulated maximum precipitation event of 9.2 inches in six hours (including 2-foot wave height) in the immediate vicinity of the Columbia Generating Station site. Columbia is sited above the level of such an unlikely event. (According to the National Oceanic and Atmospheric Administration, average annual precipitation in Richland, Wash., where Columbia Generating Station is located, is approximately 7.62 inches.)
The amount of rainfall needed to generate this maximum precipitation event is beyond any ever recorded in the area. However, it is this type of event, not the failure of upriver dams, that would present the highest postulated water levels at the site selected by Columbia designers.
Again, this postulated maximum local precipitation event would not adversely affect Columbia’s safety-related systems, structures and components. Nor would such an event prevent access to the site.
Flooding Scenarios on the Columbia River (click to enlarge)
Beyond Safety Related Design Considerations
Columbia’s designers understood that some non-safety-related out-buildings could be temporarily inundated during a maximum precipitation event – even though they are well beyond the reach of a Columbia River flood. Columbia’s staff recognized this potential and took steps to ensure that any extra equipment designated for response to a beyond-design-basis event remains available during even a postulated maximum local precipitation event. Our dedicated, on-site pumper truck is an example of such equipment.
Extreme Community Flooding Event
Columbia-experienced staff also recognized that extreme river flows associated with either failure of upriver dams or a region-wide, maximum-postulated natural event (maximum snow melt combined with maximum rainfall yielding a many-times-greater than 500-year flood in the region’s rivers and tributaries) could inundate the Tri-City area to the extent that local bridges become inaccessible and local roads and highways through Richland become fully inundated and impassable.
While such an extreme flooding event would take several days to reach maximum impact, and would not at all reach safety-related equipment at Columbia Generating Station, it could temporarily delay subsequent shift staffing, resupply of non-essential materials, or arrival of emergency response personnel (who could be summoned to respond to an un-related, subsequent emergency at the site). Access to the site from the west would remain unimpeded and therefore the western access route would be used.
Columbia is designed to be self-contained, using stored on-site supplies of essential items such as a minimum 7-day diesel fuel supply for each of its three giant emergency diesel generators. These essential, on-site supplies are of sufficient quantity to allow Columbia to safely shut down should extreme flooding occur. Columbia also has signed agreements to obtain additional emergency equipment and other supplies from two industry regional response centers located in Memphis, Tenn. and Phoenix, Ariz., each with a response time of 24 hours. Additionally, Columbia has a mutual support agreement with industry peers and other nuclear reactor operations. In addition, Columbia has procedures in place to guide the process of coordinating with offsite civil incident commanders charged with prioritizing distribution of limited resources in the event of a community-wide event.