80% of the world's nuclear fleet are in fact light water reactors - like Fukushima - which require continuous cooling and a constant source of power.

Even the latest passive safety proposals like liquid fluoride thorium reactors don't conclusively solve the fundamental problem of overheating after a cooling system failure [1].

[1] http://www.americanscientist.org/issues/pub/2010/6/not-so-fa...