(CNN) -- The reactors experiencing problems at Japan's Fukushima Daiichi nuclear plant are boiling water reactors -- the second most common kind of nuclear power reactor.
In a boiling water reactor, water heated by nuclear fission boils and turns into steam which powers a generator. The steam is then turned back into water by a condenser and used again.
The most common type of nuclear power reactor is a pressurised water reactor.
In a pressurised water reactor, hot pressurised water from the reactor's core boils water in a separate circuit, making steam that turns a turbine. Water from the reactor and water that is turned into steam are in separate pipes and never mix.