6/19/2011 | * IMPORTANT * Cooper Nuclear Station triggers Notification of Unusual Event due to Missouri river water levels rising above 42.5 feet

Date: 6/19/2011 12:13 PM CST
by: Jason Chan

Missouri river water levels have risen to a point (at present 43.7 feet as of 12:14 P.M. CST) which according to the NPPD (Nebraska Public Power District) flood emergency preparedness plan should have caused them to issue a NOUE (Notification of Unusual Event) for Cooper Nuclear Station.  The NOUE is the lowest of four emergency classes used within the United States.  Here is the graphic snapshot from the NPPD site which shows their flooding action plan and what water levels will trigger what event notifications to the NRC (Nuclear Regulatory Commission):

Cooper's Emergency Preparedness Plan (Flooding) View water levels at Brownville

Disclaimer: Cooper Nuclear Station measures the Missouri River's water elevation at the mean sea level. Conversions to depth of river in feet are placed in parentheses behind these levels. It is also important to note that changing river conditions, mountain snowmelt, and precipitation, all play a role in the fluctuating river levels. Procedure-required steps are noted with an asterisk.
Station management continuously tracks the river's level and anticipates precautionary measures for protecting plant equipment from flooding conditions.

If this action plan is being followed to the letter Cooper Nuclear Station should still be operational.

According to Omaha, Nebraska NBC Affiliate WOWT (Channel 6) the NOUE (Notification of Unusual Event) was indeed issued at 4:02 AM Sunday June 19, 2011:

...
As the Missouri River rises, the Nebraska Public Power District has declared a “Notification of Unusual Event” for the Cooper Nuclear Station. The notification was made at 4:02am Sunday. It is part of the safety and emergency preparedness plan that the station follows when certain flooding conditions are present.
...[1]

The AP also has an article going in to further detail:

...
Mark Becker, a spokesman for the Columbus-based utility, said the "notification of unusual event" sent to the Nuclear Regulatory Commission was expected as the river swells above record levels. The declaration is the least serious of four emergency notifications established by the federal commission.
"We knew the river was going to rise for some time," Becker said. "It was just a matter of when."
The plant was operating Sunday at full capacity, and there was no threat to plant employees or to the public, he said.
...[2]

Cooper Nuclear Station is three miles southeast of Brownville, Nebraska along the Missouri River.

Here is a description of the four emergency classifications the United States uses, starting from the least severe to the most severe taken from the NRC's (Nuclear Regulatory Commission) Backgrounder on Emergency Preparedness at Nuclear Power Plants:

...

Emergency Classification

Emergency Classification is a set of plant conditions which indicate a level of risk to the public. Nuclear power plants use the four emergency classifications listed below in order of increasing severity.

Notification of Unusual Event - Under this category, events are in process or have occurred which indicate potential degradation in the level of safety of the plant. No release of radioactive material requiring offsite response or monitoring is expected unless further degradation occurs.

Alert - If an alert is declared, events are in process or have occurred that involve an actual or potential substantial degradation in the level of safety of the plant. Any releases of radioactive material from the plant are expected to be limited to a small fraction of the Environmental Protection Agency (EPA) protective action guides (PAGs). Additional information regarding PAGs can be found on the EPA Web site at: http://www.epa.gov/radiation/rert/pags.html .  

Site Area Emergency - A site area emergency involves events in process or which have occurred that result in actual or likely major failures of plant functions needed for protection of the public. Any releases of radioactive material are not expected to exceed the EPA PAGs except near the site boundary.

General Emergency - A general emergency involves actual or imminent substantial core damage or melting of reactor fuel with the potential for loss of containment integrity. Radioactive releases during a general emergency can reasonably be expected to exceed the EPA PAGs for more than the immediate site area.
...

Note that there is also a 7 level international nuclear event scale, which is described in the following Wikipedia entry: International Nuclear Event Scale.  The reports sited are not based on the international scale.

Sources:
[1] Declaration at Cooper Nuclear Station, NBC WOWT (Channel 6) News
[2] Neb. nuke plant notifies feds of Missouri flooding, AP via ctpost.com