Seismic monitoring and automatic scram systems

The following issues should govern the decision about whether to have an automatic scram system or operator action supported by seismic monitoring in the event of an earthquake  

The lower trigger level (alert) should be close to SL-1 (usually associated with operational limits), at which significant damage to safety related items is not expected. If the overall seismic capacity of the plant is lower than SL-1 (e.g. during the seismic re-evaluation), the lower trigger level should be referred to the actual seismic capacity of the plant. 

For an automatic system, the highest threshold and trigger level for a reactor scram should be defined with reference to SL-2 and to the fact that usually for earthquakes at such levels major destruction would be expected in the vicinity of the site, accompanied by the possible loss of off-site power and disruption of the water supply necessary for the removal of residual heat. All the emergency procedures and operator actions should be consistent with such a scenario. 

The sensors should be located preferably at the free field and at the locations of safety related equipment in the plant. The trigger levels should be adapted to the locations of the sensors in the plant, in accordance with the seismic dynamic analysis. For multi-unit sites, the scram logic should be coordinated among the different units. 

The control panel of the system should be located in the control room for easy access by the operator. 

---

The decision to install monitoring instrumentation and to safety classify it is usually taken on the basis of the relevance of the external event hazard for system design and, in general, on the basis of the instrumentation s significance for the plant s emergency procedures. Seismic monitoring and automatic scram systems, when installed, need to be properly classified for safety and adequate redundancy according to their objectives. 

---