The rectors at the Fukushima Nuclear Power Plant were Boling Water rectors-WBR. A WBR produces electricity by boiling water, and spinning a turbine with that steam.
On March 11th 2011, a major disaster occurred at the Fukushima Plant, when a severe earthquake off the northwest coast of Japan triggered a tsunami.
After the earthquake the reactors at Fukushima that were running were stopped, but to cool the rectors, electricity is needed. So, the cooling could not be carried out, and a meltdown of the rectors occurred, releasing radioactive materials into the environment.