The US response to Fukushima: “Defense-in-depth” or “defense-in-shallowness”?

The US response to Fukushima: “Defense-in-depth” or “defense-in-shallowness”?

How can the nuclear establishment prevent another accident like Fukushima Daiichi—or worse—from occurring? The United States and many other countries have been struggling with this question ever since the crisis began in March 2011, but there is no simple answer. One of the chief lessons of Fukushima is that overconfidence in the ability to predict the occurrence of highly uncertain but catastrophic events can lead to disaster. This problem can be addressed through robust “defense-in-depth;” that is, employing additional layers of safety to compensate for uncertainties. Defense-in-depth is not new to the nuclear industry, but Fukushima showed that the standard defenses were not sufficiently deep. However, beyond the general dictum that nuclear plants must be prepared for the unexpected, there is a divergence of views internationally about “how prepared is prepared enough.”

The crux of the disagreement is how wide safety margins should be and how many additional layers of protection are necessary to adequately assure that nuclear plants can cope with “beyond-design-basis” events. (By definition, beyond-design-basis events, like the huge earthquake and tsunami that triggered the three Fukushima Daiichi meltdowns, are more severe than the “design basis” events reactors were engineered to withstand.) Studies have shown that most nuclear reactors in the United States could be struck by natural disasters like earthquakes or floods exceeding their current design bases. But even those new estimates have uncertainties, and that causes more complications.

For instance, if the best estimate for the height of a flood is 10 meters, should one design protection for that level or for, say, a 15-meter flood, given the uncertainties that plague attempts to predict the weather over decades? 

One obstacle to greater defense-in-depth is that some nuclear plant owners, already under great financial stress in some countries like the United States because of competition from cheap natural gas, are not inclined to spend a lot of money to protect against events that they believe are highly unlikely to occur. To save money, they are willing to gamble even with highly uncertain odds.

The approaches taken by the French nuclear regulator, Autorité de Sûrété Nucléaire (ASN), and the US Nuclear Regulatory Commission (NRC), clearly illustrate differences in philosophy. ASN is requiring nuclear plants to install a “hardened safety core” of backup equipment that is qualified to withstand beyond-design-basis events. Since this equipment will be more robust than the nuclear plants themselves, there is a greater likelihood that it will be available when it is truly needed.

In contrast, the NRC is allowing nuclear plant owners to rely on portable emergency equipment that could be less robust than the nuclear plants themselves, but that is intended to be more flexible. While greater flexibility has its advantages, this strategy, known as FLEX, simply does not provide the level of protection against beyond-design-basis accidents that is needed to prevent an American Fukushima with high confidence. 

A recent report by the Union of Concerned Scientists (UCS) assesses the state of reactor safety in the United States five years after the Fukushima accident, and examines the shortcomings of the FLEX program. The report also discusses a number of important safety recommendations that the NRC considered and rejected during this period. One key recommendation that ended up in the NRC’s wastebasket was a proposal by the NRC’s own Near Term Task Force to revise its faulty framework for regulating beyond-design-basis accidents. Despite a 2013 NRC staff assessment that “there should be no implication that the Fukushima accident and associated consequences could or would have been completely avoided assuming Japan had the same regulatory framework [as the United States] prior to the accident,” the NRC commissioners have concluded the agency’s current framework is just fine. In UCS’s view, this attitude is typical of the complacency of the global nuclear establishment that helped pave the way for Fukushima. We hope that a future NRC will finally undertake all the reforms necessary to ensure that an accident like Fukushima does not happen here.