Fukushima: beyond our imagination

Bulletin of the Atomic Scientists April 2011

• Severe accidents at nuclear reactors have occurred much more frequently than what risk-assessment models predicted.
• The probabilistic risk assessment method does a poor job of anticipating accidents in which a single event, such as a tsunami, causes failures in multiple safety systems.
• Catastrophic nuclear accidents are inevitable, because designers and risk modelers cannot envision all possible ways in which complex systems can fail.

The multiple and ongoing accidents at the Fukushima reactors come as a reminder of the hazards associated with nuclear power. As with the earlier severe accidents at Chernobyl and Three Mile Island, it will take a long time before the full extent of what happened at Fukushima becomes clear. Even now, though, Fukushima sheds light on the troublesome and important question of whether nuclear reactors can ever be operated safely.

If there is one weakness of the probabilistic risk assessment method that has been emphatically demonstrated at Fukushima, it is the difficulty of modeling common-cause or common-mode failures. From most reports it seems clear that a single event, the tsunami, resulted in a number of failures that set the stage for the accidents. These failures included the loss of offsite electrical power to the reactor complex, the loss of oil tanks and replacement fuel for diesel generators, the flooding of the electrical switchyard, and perhaps damage to the inlets that brought in cooling water from the ocean. As a result, even though there were multiple ways of removing heat from the core, all of them failed.

Fukushima also demonstrated one of the perverse impacts of using multiple systems to ensure greater levels of safety: Redundancy can sometimes make things worse. At Fukushima, as with most reactors around the world, zirconium cladding surrounded and protected the fuel. But when the cooling systems stopped working, the zirconium cladding overheated. Hot zirconium interacted with water or steam, producing hydrogen gas. When this hydrogen came into contact with air in the containment building, it caused an explosion that reportedly damaged the suppression pool beneath the reactor, another protective system. In other words, in complex systems such as nuclear reactors, redundancy may have unexpected and negative consequences for safety.

The lesson from the Fukushima, Chernobyl, and Three Mile Island accidents is simply that nuclear power comes with the inevitability of catastrophic accidents. While these may not be frequent in an absolute sense, there are good reasons to believe that they will be far more frequent than quantitative tools such as probabilistic risk assessments predict. Any discussion about the future of nuclear power ought to start with that realization.

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Alexander Cockburn New Left Review

In political terms, nuclear power has always been a war on the people, starting with the Japanese in Hiroshima and Nagasaki, going on to the Marshall Islanders, ranchers and kindred inhabitants of test sites across the West, Native Americans, poor Latinos and African Americans (the usual involuntary neighbours of waste dumps), people in the path of ‘accidents’ or deliberate secret experiments, and most recently Fukushima. Not the executives of the Tokyo Electric Power Company. They are in Tokyo or heading further south. It is ‘worker heroes’—who know perfectly well they are doomed. It is the Board of tepco that should be sent to the front lines.


Look at the false predictions, the blunders. Remember the elemental truth that Nature bats last, and that folly and greed are ineluctable parts of the human condition. Why try to pretend that we live in a world where there are no force 8–9 earthquakes, tsunamis, dud machinery, forgetful workers, corner-cutting plant owners, immensely powerful corporations, permissive regulatory agencies, politicians and presidents trolling for campaign dollars?

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