Washington (Platts)--15Mar2013/333 pm EDT/1933 GMT

The Tennessee Valley Authority failed to adequately protect two nuclear plant sites from the potential for the failure of earthen dams upstream, a failing that may have had substantial safety significance, the US Nuclear Regulatory Commission said in letters made public Friday.

NRC cited TVA for six apparent violations of its regulations, four at its 1,210-MW Watts Bar plant and two at the two-unit 2,367-MW Sequoyah station.

The violations were associated with "yellow" findings, meaning those with substantial safety significance, the agency said in a preliminary assessment. NRC ranks safety findings as green, white, yellow and red, in increasing order of significance.

If upheld following a formal TVA response, the violations could result in additional inspections from NRC as well as fines, agency spokesman Roger Hannah said Friday.

There is no immediate safety concern, because TVA has implemented interim measures to protect the stations against dam failure. The violations applied to the period before 2009, when TVA took the temporary measures, including installing barriers to increase the height of four upstream dams.

A plan to protect the two sites from probable maximum flooding was inadequate because it did not consider protection from the failure of the earthen dams. Sequoyah is located in Soddy-Daisy, Tennessee, and Watts Bar is at nearby Spring City. Both draw cooling water from Chickamauga Lake, a reservoir along the Tennessee River.

Failure of the dams identified by NRC in heavy rains would have flooded the emergency diesel generators that provide critical back-up power for cooling at both sites, NRC said.

At Watts Bar, NRC also said operators would have been unable to carry out some flooding procedures because of equipment problems and a lack of training and familiarity with the plans.

Also at Watts Bar, a procedure to protect booster pumps from flooding was poorly written and would likely have failed, threatening the function of reactor coolant pumps, NRC said.

The probable maximum flood scenario used in NRC's analysis was based on "a level of rainfall never experienced in the recorded history of the Tennessee Valley," TVA spokesman Michael Bradley said in an email Friday.

TVA has modified its procedures based on the NRC findings, and has enhanced barriers, emergency procedures and physical protection for plant equipment, Bradley said.

"We will continue to work cooperatively with the NRC in developing plans to be prepared for these highly unlikely events," he said.

TVA can request a conference on the apparent violations and may contest the significance of the findings, potentially reducing the penalties, NRC said.

TVA agreed in June 2012 to improve flooding protection at the two plants and upgrade four upstream dams to resolve other NRC regulatory issues.

--William Freebairn, william_freebairn@platts.com
--Edited by Keiron Greenhalgh, keiron_greenhalgh@platts.com

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