Vibration Level Monitoring in Nuclear Power Plants

Safety industries such as Nuclear Power Plants are always monitored for structural vibrations, especially to assess the potential earthquake hazards that may have critical consequences on the ongoing fission reactions.

NPPs and SMR – Small Modular Reactors are monitored as well as key annexes buildings present on the whole nuclear site. In order to comply with strict requirements of this safety industry, the Syscom MARMOT products are highly qualified and seismically tested. They comply with all major standards and guidelines, to name a few: Reg. guide 1.12, KTA guidelines and IEC 60780, IEC 60880, Cat A & C according to IEC 61226, IEC61513, IEC 62138 main nuclear related safety standards.

The seismic monitoring system is permanently installed on critical locations around the plant and they monitor the vibrations continuously. Cat A classified systems, namely MRSK2002, are also able to TRIP the reaction automatically, typically using a voting logic of 2oo3 (two out of three) in case of OBE – Operating Basis Earthquake or SSE – Safe Shutdown Earthquakes alarm threshold exceedance.

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The main requirements of a safe monitoring system are:

INSTRUMENTATION
According to the plant typology and dimensions, a different number of measurement point is required. A complete seismic instrumentation system monitors the impact of an earthquake at critical locations and at a free field location unaffected by the buildings. The instruments must be qualified and in compliance with the international safety standards, such as IEC 61508 and IEC 61513.

CONTROL ROOM
The data coming from each vibration sensor is collected in a control room by an NCC – Network Control Center, where the operators can see if some unexpected vibration has been recorded, in order to respond quickly.

EARTHQUAKE EVALUATION
The analysis of the data recorded is made by seismologists and structural engineers, especially when the structural response exceeds specified limits. The most important thresholds are the Operating Basis Earthquake (OBE) and Safe Shutdown Earthquake (SSE) levels. It defines limits for general safety systems, especially nuclear plant facilities, while the Cumulative Absolute Velocity (CAV) calculated from data given by the free-field sensor, is usually the trigger in case of an earthquake.

AUTOMATIC SHUTDOWN
The vibration monitoring is also useful to guarantee an automatic shutdown of the critical operations of a power plant, like reactors and gas turbines. The signal is usually provided by safe relays output integrated directly into the accelerographs.