The crippled reactors at the Fukushima Dai-Ichi nuclear power plant in Japan have a set of safety systems designed to provide cooling in case of an accident. Unfortunately, these systems weren’t able to prevent serious damage to the reactors and fuel following the earthquake and tsunami on March 11, 2011.
This Powerpoint slideshow Anatomy of a Tragedy: Fukushima Dai-Ichi shows in detail how the Fukushima reactors worked under normal conditions, and then describes how the events of March 11 led to the currently unfolding crisis.