by Eli Kintisch on 13 July 2011, 5:42 PM

Redundant defenses against the most likely failures may not be enough to protect U.S. nuclear plants against a catastrophic failure like the one that occurred earlier this year in Japan. That's the conclusion of the first of two reviews of U.S. nuclear plants by the staff of the Nuclear Regulatory Commission (NRC).

"A sequence of events like the Fukushima accident is unlikely to occur in the United States," says the task force's report, released yesterday. But a "patchwork of regulatory requirements" currently in place ought to be reviewed, updated and possibly strengthened to address "low likelihood, high consequence" events like the Fukushima calamity, it says.

The cornerstone of the NRC's regulatory framework regarding accidents is known as the "defense in depth" strategy, defined as relying on "redundant layers of defense to compensate for potential human and mechanical failures" so as to rely on no single layer." But a review of those layers found that "significant reinforcements" are required for each type of preparation: protection (preventing meltdowns or other disasters), mitigation (making such accidents less harmful if they happen), and emergency preparedness (responding to accidents).

For example, the task force called for nuclear reactor vessel vents, which release pressure from boiled cooling water, to be "hardened." That step could mean making them operable without AC electrical power or even operator control.

The task force said that it is unclear whether Japanese utility officials were ever able to open vents to release air pressure during the accident. That could have caused the breach of the containment structure after the pressure skyrocketed in several reactor vessels.

The report also called for adding instruments to spent nuclear fuel pools to give operators better information during accidents and potentially new systems for the automatic refilling of cooling pools. Other recommendations included better emergency plans that incorporated the possibility of prolonged power blackouts or events that take out multiple reactors at once.

The Nuclear Energy Institute, the Washington, D.C.-based industry lobby, said the report lacked evidence to support "many of its recommendations." When ScienceInsider asked for details, it cited the recommendation on hardened vents. The institute's Steven Kerekes said that there is "supportive evidence" that current systems are vulnerable and that the Fukushima incident is a sufficient reason to change the rules.

Meanwhile, the Union of Concerned Scientists (UCS), a frequent critic of the NRC's rules for the industry, has released a report today that included 23 recommendations for strengthening the system that go well beyond the proposals in the NRC report. For example, UCS said that spent nuclear fuel in U.S. cooling pools should be moved to hardened cask storage. The task force said it was too soon to make such a recommendation based on available data from Fukushima.

The NRC task force report solicited no public input for its report. A much larger effort, due in 3 months from now, will incorporate public comment and testimony from hearings.