The crisis that unfolded at the Fukushima Daiichi nuclear plant after Japan鈥檚 megaquake and tsunami is rewriting the nuclear safety guide.
The European Union, for instance, has ordered a of all nuclear power plants in its member states. These assessments are supposed to consider each plant鈥檚 ability to withstand a full range of potential hazards 鈥� from earthquakes and floods to plane crashes and terrorist attacks.
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The Japanese disaster did bring some positive news. The reactors along Japan鈥檚 Pacific coast suffered no serious damage from the earthquake, even though its magnitude exceeded the worst-case scenarios assumed in their designs. That bodes well for the ability of reactors worldwide to withstand major earthquakes.
But Fukushima Daiichi was doomed by a decision to plan for a maximum tsunami height of only 5.7 metres, well short of the wave of up to 15 metres that engulfed the plant on 11 March 2011. In the light of this error, regulators worldwide are reassessing whether other plants are vulnerable to catastrophic floods, caused by tsunami, swollen rivers or failed dams.
Shake-proofing
Since the Japanese megaquake, of the fact that many reactors may face quakes that exceed those they were designed to withstand. In the US, the is giving plant operators in central and eastern parts of the country 18 months to revise their hazard assessments, following reconsidering the risks posed by the region鈥檚 geological faults.
Although this may result in seismic upgrades at some facilities, the performance of Japan鈥檚 plants after last year鈥檚 magnitude-9.0 quake suggests that existing reactors are able to survive ground shaking greater than anticipated by their designers.
Even the , which sits closer to the megaquake鈥檚 epicentre than Fukushima, shut down with no major damage. 鈥淥nagawa had the world鈥檚 best stress test, and it seems to have passed,鈥� says , a seismic risk consultant based in Orinda, California.
It was a similar story in 2007, when the in western Japan was rocked by a magnitude-6.6 quake, and last year, when a hit less than 20 kilometres from the in Virginia. Both quakes exceeded the plants鈥� design specifications, yet the reactors remained intact.
This resilience reflects the caution of reactor designers, who build a margin of error into their seismic engineering.
Inadequate flood protection
By contrast, a wall built to withstand a 5.7-metre tsunami offers no protection from a 15-metre wave. And according to those who have analysed Japan鈥檚 history of tsunamis, the engineers who built Fukushima Daiichi should have known that their protection was inadequate (see graphic).
Johannis N枚ggerath, president of the Swiss Nuclear Society, seismologist of the University of Tokyo, and Viacheslav Gusiakov, who heads the Russian Academy of Sciences鈥� Tsunami Laboratory in Novosibirsk, have looked at the historical record of tsunamis that was available when Fukushima Daiichi was designed in the mid-1960s (Bulletin of the Atomic 杏吧原创s, ).
They note that tsunamis rising up to 38 metres had already hit parts of Japan鈥檚 Pacific coast some 200 kilometres to the north, and say that it would have been prudent to plan for a similar onslaught. Instead, based on waves seen at Fukushima in 1960, generated by a , the plant鈥檚 designers initially assumed that the worst-case scenario was a 3.1-metre tsunami. That figure was revised to 5.7 metres in 2002.
In a bitter irony, before construction at Fukushima Daiichi began, the site was excavated by more than 20 metres. This was done in part to allow the reactors to be built on bedrock to improve their seismic resilience, but it put the plant directly in harm鈥檚 way when the tsunami hit.
Worse, the diesel generators needed to power emergency cooling systems, and switching gear that connects the plant to the electricity grid and controls core cooling, were not in waterproof buildings. Once they flooded, a disaster was almost inevitable.
Swiss example
It needn鈥檛 have been this way. Swiss reactors, which could face flash floods from Alpine rivers, house their backup cooling systems in waterproof bunkers. They also have filtered venting systems so that even if cooling fails and pressure starts to build in the containment building, radioactive iodine and caesium can be removed from the steam before it is released. Had Fukushima Daiichi been designed to similar specifications, says N枚ggerath, 鈥淚鈥檓 convinced that it would have prevented the accident鈥�.
Operators of reactors that are vulnerable to severe floods may now decide to follow the Swiss example. But flooding is just one of many possible hazards, some of which are difficult to anticipate. 鈥淵ou always worry about what you haven鈥檛 analysed,鈥� says Chip Lagdon, chief of nuclear safety with the US Department of Energy.
