The Reactor unit of a power plant is usually operated for extended time periods at full power. Once a year, a shutdown of the reactor unit is scheduled for refueling, inspections and maintenance.
The energy contained in nuclear fuel is spent gradually. That is why a reactor has to be shut down, approximately every 300 days of operation. The shutdown comprises the insertion of emergency and regulation assemblies into the active zone, the increase of the concentration of boric acid in the water of the primary circuit, the relieving of pressure and the cooling down of the primary circuit and the removal of the reactor lid, including the regulation rod drives. The reactor contains nuclear fuel spent to a varying degree. During the replacement, assemblies with the least content of fission material, the ones which had been in the reactor for the longest time, are replaced with assemblies with fresh, enriched fuel. This operation is performed by means of a special apparatus — the refueling machine. Spent fuel assemblies are relocated into a storage pool just beside the reactor. In the spent fuel pool the activity and heat production of the assemblies decreases significantly. The storage period inside the nuclear power plant with a pressure water reactor varies between 3 and 10 years and depends on the storage pool design as well as on the dispositions for further management. Fuel is then transported in special containers to Interim Spent Fuel Storage facilities, where it is stored for 50 years.
The operation of a reactor unit is automatic. Correct operation of the automatic systems is constantly supervised by highly qualified operational personnel at the unit control room, which is the control center of every reactor unit.