In the case of a nuclear plant, the boiler is replaced with the reactor, which is more complex than a coal boiler for a number of reasons. For one, the water not only acts as the cooling fluid, but also as the neutron moderator, which means its control is vital to the operation of the system as a whole. Additionally, the water tends to pick up radioactivity from the reactor's operation, which leads to safety concerns and maintenance overhead. Finally, steam and liquid water have different moderating qualities, so most (but not all) light water designs keep the water below the boiling point and use a steam generator to feed the turbines.