OTHERWISE: Disasters Fail to Follow Scripts - Room for Debate - NYTimes.com

 Design defects

Accidents have simple origins. The shuttle's rubber O rings became brittle in the cold.  BP's cut-off valve had no back-up - unlike newer ones.  The Japanese reactors' cooling pumps were diesel engines - which the tsunami flooded when it overflowed the seawall.   What happens next is unpredictable. - gwc

David Lochbaum of Union of Concerned Scientists

The Japanese reactors were designed to cope with power failure for eight hours. Most U.S. reactors can cope with outages for only four hours.
OTHERWISE: Disasters Fail to Follow Scripts - Room for Debate - NYTimes.com