What does the second law of thermodynamics say?
That one can never convert heat completely back into other forms of energy, some of it is always irrecoverable. [Optional: The fraction of heat energy which can be converted to other forms depends on the temperature at which the heat is provided. The unrecovered heat is changed to a lower temperature, and the fraction we recover depends on these two temperatures–the one at which the heat is received, and the one at which the remainder can be dumped. A power station needs not only a supply of hot steam, but also a way of dumping the heat left at the end of the cycle. Steam locomotives dumped their spent steam into the air, and therefore needed a great amount of of water, carried in their tenders. Electric power stations (of any kind) recycle their steam and cool it with air in cooling towers, like those of 3-mile island which (for some reason) became a symbol of nuclear energy.
