A pressure compensator is a technical pressure vessel with a special design that compensates for changes in the volume of water in a closed loop when it is heated. It is a design feature of two-circuit reactors with pressurized water as a coolant (including heavy water reactors) used at nuclear power plants, nuclear submarines and ships and is usually considered as part of a technological system that maintains the pressure in the primary circuit in stationary modes and pressure deviations in transient and emergency modes of the reactor plant. The pressure compensator is at the same time a system for providing the required pressure and compensating for changes in the volume of the coolant in the primary circuit, therefore it has a double name - in technical documentation and literature it can be called both a pressure compensator and a volume compensator.