The reactors at the fukushima nuclear power plant were boiling water reactors – BWR. A BWR produces electricity by boiling water, and spinning a turbine with that steam.
On march 11, 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 wear running wear stopped, but to cool the rectors, electricity is needed. So, the cooling could not be carried out, and a meltdown of the reactors occurred, releasing radioactive materials into the environment.