Fukushima Daiichi Nuclear Disaster

Let us return to the example of the Tōhoku earthquake and tsunami and subsequent Fukushima Daiichi nuclear disaster. Fukushima Daiichi and several other Japanese nuclear plants were all exposed to tsunami hazard, in the sense that they were close enough to the coast that a tsunami could affect their operations. The plants are designed to automatically shut down during earthquake and tsunami events, but the shutdown process itself requires power, which is provided by diesel generators. They are also protected by seawalls that are designed to prevent flooding by waters up to a specified height.

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Figure 10.23: Damage at Fukushimi Daiichi Nuclear Power Plant. In the background, note the seawall which was insufficient to prevent damage.
Credit: Tokyo Electric and Power Company

There were two major sources of sensitivity at Fukushima Daiichi, one of which applied to all of the other Japanese nuclear facilities, and one of which was particular to Fukushima Daiichi. If the on-site diesel generators at any plant flooded and failed, no additional failsafe mechanism was available, and a meltdown became possible. This potential for failure greatly increased sensitivity of these plants and the surrounding populated places and property. More importantly in this example, if the seawalls were too low and could therefore be overtopped by a tsunami, then flooding might disable the generators. This is exactly what happened at the Fukushima Daiichi plant. Its seawall was 19 feet high. Despite warnings in a 2008 report suggesting that the plant could be exposed to a tsunami of up to 33 feet, when the tsunami struck, the plant was still protected only by the existing 19-foot seawall. The tsunami that made landfall reached over 40 feet high, even larger than the earlier report had suggested was possible. Because the seawall was inadequately protective relative to the magnitude of potential hazard, the plant was more sensitive to a catastrophic meltdown, which in turn increased the sensitivity of nearby populations to exposure to radioactive materials and long-term contamination of property and natural resources.

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Figure 10.24: Map of First-Year Radiation Dose Estimate in the Area near Fukushima Daiichi Nuclear Power Plant.
Credit: National Nuclear Security Administration: Fukushima map

What lessons might we learn from the Fukushima nuclear disaster that could reduce sensitivity to similar future hazard events? This is a particularly tricky question in this case. Earthquakes of the magnitude of the Tōhoku earthquake, which was the initial hazard event that triggered the tsunami and subsequent nuclear disaster, are extremely rare. However, sensitivity to a hazard of this magnitude was sufficiently great that the result was a catastrophe. The extents to which countries should prepare for very rare events with potentially extreme consequences are difficult political and policy questions.

However, setting those questions aside, there are two main ways in which sensitivity could have been reduced in this situation. First, the seawall was far too short and could have been overtopped by a much smaller seismic event and tsunami. To reduce this sensitivity, seawalls protecting nuclear power plants should be built to withstand a tsunami of the highest possible levels. Second, any additional strengthening or redundancy in the electrical power system responsible for powering shutdown during a seismic event would further reduce reactor sensitivity to a tsunami.

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Devastating Events

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How should governments address low-probability hazard events with potentially devastating consequences? How would you assess how much money is too much money to spend to reduce the likelihood of a highly unlikely event?

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