A hydraulic accumulator is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. The external source can be a spring, a raised weight, or a compressed gas. An accumulator enables a hydraulic system to cope with extremes of demand using a less powerful pump, to respond more quickly to a temporary demand, and to smooth out pulsations. It is a type of energy storage device.
Compressed gas accumulators, also called hydro-pneumatic accumulators, are by far the most common type.
In the plant they are commonly used for the control rod drive system.
Use in Control rod system
Accumulators are used in the Control Rod Drive system (CRD) to provide the motive force for Rod insertion.
One accumulator contains water and nitrogen which is separated by a piston. A common issue is leakage past the piston. The water is accumulated and causes a control room alarm. Clearing the alarm solves the problem. If there were a lot of leakage then the alarm would start again quickly (this indicates a hole in your piston separating the water and nitrogen).
The other accumulator contains only nitrogen. A common problem with this CRD accumulator is too little nitrogen. This problem is routinely identified and correct by adding nitrogen.