[downer91]

  e.g a huge earthquake or a tsunami as 
  in Japan's case, or maybe an attack
  ...
  faith in the human operators

To enumerate some regretably normal human factors:

1. deliberate procedural over-complication in the name of job security (political hazard)

2. disgruntled operator sabotage (political, but maybe that qualifies as an attack?)

3. operator error

4. operator laziness

5. operator incompetance

6. operator experimentation (experimentation was the cause of Chernobyl, after all)