Power reactors where water flows into the reactor vessel where it is heated and boiled, then spins turbines to produce power, before being cooled by a heat exchanger and repeating the process. BWR reactors are more efficient than PWR, but have less failsafe. While the half-life of the nitrogen isotope produced when water comes in contact with a nuclear reaction is realtively short (about 14 seconds, if I remember correctly), it does leave an increased possibility for spread of radioactive material. The steam turbines are also slightly radioactive as a result too.