After Fukushima incident it has become a regulatory requirement to have Automatic Seismic Trip System (ASTS) in nuclear reactors on detection of earthquake. This is designed to scram the reactor upon the occurrence of a seismic event, before turbine trip or other conditions resulting from the seismic disturbance could cause a scram. The earlier scram could give a lead time between 5 to 20 seconds. This lead time could provide resulting benefits such as reduced loads during the seismic event and, therefore, fewer burdens on the plant systems. It may also reduce the likelihood of a Loss of Coolant Accident (LOCA) or severe transient after a seismic event.

The main purpose of Seismic Instrumentation for Nuclear reactors is to generate automatic trip whenever an earthquake occurs beyond the design basis. Also it is required to record the acceleration history in order to do the post evaluation so that regulatory authority can take suitable decision regarding the restart of the reactor after Seismic Trip.

Strong motion seismic instrumentation at the Nuclear Power Point (NPP) consists of Seismic Switches and Seismic recorders. Seismic Switches are used for generating automatic trip signal to the Nuclear reactor and the data from the Seismic recorder is used for post earthquake analysis. In this analysis, the data from the Seismic recorder is compared with the design model to see the validity of the analysis as well as to check the exceedence of the design basis earthquake. Based on this analysis the decision is taken on restarting of the reactor.

The Seismic switch triggers on detecting the acceleration greater than the preset PGA level and generates auto trip signal for safe shut down of NPP. The PGA level setting is normally kept as the PGA of the OBE level response spectrum of the respective NPP sites. In a reactor building each floor is provided with a local seismic recorder (LSR) (sensor and recording unit). LSR’s can record data from 3-axis MEMS acceleration sensor mounted inside when the Seismic Switches are triggered. Local
Seismic recorders are also configured for level triggering. This means an LSR is continuously monitoring its 3-axis acceleration data and records if any axis acceleration crosses a preset acceleration level. for example 10mg .

A Central Seismic Recorder (CSR) is connected to all LSR’s through a communication link. Graphical User Interface (GUI) software in the CSR can configure each LSR during installation. The recorded data from the LSR are also transferred to CSR periodically. CSR is having a large storage capacity (1 TB or more) in its external hard disc. These data are time tagged in real time with the help of a stable local clock always accurately synchronized to Universal Time Reference by latching on to an improvised GPS (Global Positioning System) receiver.

The system has been qualified to meet the stringent NPCIL standards as per PC-E-403 specifications. The following qualification tests have been performed and the system has cleared it successfully: