by John Livingston

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Public Perception of Browns Ferry Safety

While the response to the April storms had earned TVA much credit from the general public, the previous (March) meltdown of three nuclear reactors in Japan (of the same general design as the Browns Ferry units) and the many scenes of destruction caused by the April storm, led several local residents to raise questions about the plant's safety provisions at two public meetings since the April storms.

A standing room only crowd attended the annual performance assessment of Browns Ferry held by the Nuclear Regulatory Commission at the plant on May 9, 2011. Area residents spent about 90 minutes questioning the safety of the plant if an EF-5 tornado directly hit it. There were questions about potential flaws in the reactor design, as well as other concerns about the ability of the roof-top spent fuel pools to withstand the forces of an EF-5 tornado.

NRC Regional Administrator Victor McCree said that he was confident the plant could withstand a direct hit from an EF-5 tornado with winds as high as 300 mph. He noted that the sheet metal structure above the pools was designed with blow-out panels to reduce the likelihood that a tornado-caused vacuum would cause a significant release of radiation [Ref. 7].

In an October 3, 2011 public meeting on the results of the first NRC inspection required by the Red finding, members of the audience (including out-of-state anti-nuclear activists) again raised questions about the ability of the power plant to withstand an EF-5 tornado.

Even more questionable is the use of 4,000 pounds for the maximum car mass. In TVA country, parking lots are full of SUVs and trucks which may weigh up to 6,000 pounds. Hopefully, the NRC will re-visit some of its analyses that support its confidence in the current plant design requirements.

This time, the NRC's deputy regional administrator for operations, Leonard Wert, addressed the concerns. He said numerous studies had confirmed that the plant could safely handle a tornado, even though the structure above the spent-fuel pools would likely suffer severe damage. He said that studies after the terrorist attacks of September 11, 2001 evaluated vulnerability not only to attack, but to natural disasters [Ref. 8].

A General Observation on 'Confidence'

While the NRC officials expressed confidence in the NRC studies, a quick look at the general approach provides a few facts. The "Design-Basis Tornado and Tornado Missiles for Nuclear Power Plants" report [Ref. 9] addresses key parameters in the tornado threat evaluation process. For Region I (most of the central United States, including the TVA region), the maximum wind speed is defined as 230 mph (not the 300 mph referenced by McCree). The current EF-5 definition is simply "greater than 200 mph," but the earlier definition had a top rating (F-5) of 261 to 318 mph. Since any EF rating is based on observed damage (no direct measurements), one could question the design margin for an EF-5 tornado wind speed. Even more questionable is the use of 4,000 pounds for the maximum car mass. In TVA country, parking lots are full of SUVs and trucks which may weigh up to 6,000 pounds. Hopefully, the NRC will re-visit some of its analyses that support its confidence in the current plant design requirements.

A Few System Safety Observations

• To ensure availability when needed, all emergency safing systems should have the capability of periodic verification of operability.
• It is important that hardware performance be continually used to evaluate the effectiveness of proposed hazard controls, whether for operational systems or those used in emergency or contingency situations.
• Changing threats or environments should also be evaluated. For example, the warning coordination meteorologist in the NWS district office in Calera, Alabama has stated; "Seven of 10 significant tornado outbreaks in the state occurred in the last 10 years," which might call for a review of probability of occurrence assumptions [Ref. 2].
• Changes in operational conditions should be evaluated for potential adverse effects on plant safety. Even though the NRC has approved the use of more dense storage configurations for spent fuel rods in the large storage pools, the presence of the additional rods increases the loads on heat dissipation systems. While the support structure may provide increased resistance to storm debris, the tolerance to debris in the pools may be reduced.
• Beware of "extenuating circumstances" — while the 1975 fire led to a new federal fire protection law for nuclear power stations, the three Browns Ferry units do not meet the law's requirements for use of multiple manual safing systems in the same fire zone (fire barriers, smoke detectors and automated suppression; or a minimum separation of 20 feet between redundant circuitry with no intervening combustible materials). While the NRC accepted the TVA's plan to use operator manual actions to accomplish a post-fire safe shut-down, manual actions (shut down or activation) would be difficult after an EF-5 tornado strike.

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