Technology :: Fukushima's fate inspires nuclear safety rethink

The crisis that unfolded at the Fukushima Daiichi nuclear plant after Japan's megaquake and tsunami is rewriting the nuclear safety guide.

The European Union, for instance, has ordered a risk assessment of all nuclear power plants in its member states. These assessments are supposed to consider each plant's ability to withstand a full range of potential hazards – from earthquakes and floods to plane crashes and terrorist attacks.

The Japanese disaster did bring some positive news. The reactors along Japan's 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, much has been made of the fact that many reactors may face quakes that exceed those they were designed to withstand. In the US, the Nuclear Regulatory Commission is giving plant operators in central and eastern parts of the country 18 months to revise their hazard assessments, following a new report reconsidering the risks posed by the region's geological faults.

Although this may result in seismic upgrades at some facilities, the performance of Japan's plants after last year's magnitude-9.0 quake suggests that existing reactors are able to survive ground shaking greater than anticipated by their designers.

Even the Onagawa plant, which sits closer to the megaquake's epicentre than Fukushima, shut down with no major damage. "Onagawa had the world's best stress test, and it seems to have passed," says Peter Yanev, a seismic risk consultant based in Orinda, California.

It was a similar story in 2007, when the Kashiwazaki-Kariwa plant in western Japan was rocked by a magnitude-6.6 quake, and last year, when a magnitude-5.8 quake hit less than 20 kilometres from the North Anna plant in Virginia. Both quakes exceeded the plants' 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's history of tsunamis, the engineers who built Fukushima Daiichi should have known that their protection was inadequate (see graphic).

Johannis Nöggerath, president of the Swiss Nuclear Society, seismologist Robert Geller of the University of Tokyo, and Viacheslav Gusiakov, who heads the Russian Academy of Sciences' 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 Scientists, vol 67, p 37).

They note that tsunamis rising up to 38 metres had already hit parts of Japan's 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 magnitude-9.5 quake across the Pacific in Chile, the plant's 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's 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't 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 Nöggerath, "I'm convinced that it would have prevented the accident".

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. "You always worry about what you haven't analysed," says Chip Lagdon, chief of nuclear safety with the US Department of Energy.