Beyond Design Basis Events
As Fukushima showed, external threats — such as earthquakes, tsunamis, fires, flooding, tornadoes and terrorist attacks — are some of the greatest risk factors for a serious nuclear accident. Yet, nuclear plant operators have normally considered these accident sequences (called 'beyond design basis' events) so unlikely that they have not built in complete safeguards.
Forecasting the location of the next earthquake or the size of the next tsunami is an imperfect art. Nuclear plants situated outside known geological danger zones "could pose greater accident threats in the event of an earthquake than those inside, as the former could have weaker protection built in". The Fukushima I plant, for example, was "located in an area designated, on Japan's seismic risk map, as having a relatively low chance of a large earthquake and tsunami; when the 2011 tsunami arrived, it was in excess of anything its engineers had planned for".
Read more about this topic: Nuclear Safety
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