Read more: This article has been expanded in two online-only articles: 鈥�Fukushima鈥檚 fate inspires nuclear safety rethink鈥� and 鈥�Can diverse power backups boost nuclear plant safety?鈥�
The crisis that unfolded at the Fukushima Daiichi nuclear plant after Japan鈥檚 megaquake and tsunami is rewriting the nuclear safety guide.
There are some positives. Despite being shaken by an earthquake that exceeded the worst case assumed in their design, the reactors along Japan鈥檚 Pacific coast suffered no serious damage from the ground movement.
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Even the , which sits closer to the megaquake鈥檚 epicentre than Fukushima, shut down with no major damage. Its resilience reflects a healthy margin of error in reactor seismic engineering. 鈥淥nagawa had the world鈥檚 best stress test, and it seems to have passed,鈥� says Peter Yanev, a seismic risk consultant based in Orinda, California. That bodes well for the ability of reactors worldwide to withstand major earthquakes.
Fukushima Daiichi was doomed by a decision to plan for only a 5.7-metre-high tsunami 鈥� well short of the wave of up to 15 metres that engulfed the plant on 11 March 2011. In the light of this, regulators worldwide are reassessing whether other plants are vulnerable to catastrophic floods, caused by tsunami, swollen rivers or failed dams.
It would have been prudent to note that tsunamis rising up to 38 metres had hit parts of Japan鈥檚 Pacific coast some 200 kilometres to the north, and to plan for a similar onslaught, says a team of nuclear safety, seismology and tsunami specialists who have analysed the information that was available to the plant鈥檚 designers (). Not only was the sea wall too low, but diesel generators needed to power emergency cooling systems, and the switching gear that connects the plant to the grid and controls core cooling, were not in waterproof buildings. Once they flooded, disaster was almost inevitable.
聯It would have been prudent to note that tsunamis of up to 38 metres had hit Japan鈥檚 Pacific coast聰
Simply installing more backup generators won鈥檛 necessarily work, says in Cambridge, Massachusetts. 鈥淭he US nuclear industry is proposing to purchase , with the hope that something will be left working whatever happens,鈥� says Lyman. But these extra generators may still be in harm鈥檚 way, he adds, because every plant has its own terrain and threats 鈥� be they hurricanes, earthquakes or floods.
Far better, Lyman says, would be risk assessments to establish the safest sites for backup generators at each plant.
Existing Swiss plants set a good example for flood-prone locations. Their reactors, threatened by Alpine rivers, have backup cooling systems in waterproof bunkers. They also have filtered venting systems so that even if cooling fails and pressure builds 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 Johannis N枚ggerath, president of the Swiss Nuclear Society, 鈥淚鈥檓 convinced that it would have prevented the accident.鈥�