Seismic alarm and monitoring system for Soviet-designed nuclear power plants

M. Wieland1, Lothar Griesser2, C. Kuendig3
1,2Electrowatt Engineering Ltd., Bellerivestr. 36, 8034 Zurich, Switzerland.
Tel. : +41-1-385-2375, Fax: +41-1-385-2425,
E-mail: lothar.griesser@ewe.ch
3GeoSys Ltd., Glattbrugg, Switzerland

The various buildings and plants of the Soviet designed nuclear reactors have been designed for two earthquake levels, that is the design earthquake and the maximum possible earthquake with peak ground accelerations ranging from 1.2% to 10% of the acceleration due to gravity. Based on this design, certain parts of the buildings and equipment do not comply with Western seismic standards. With regard to the Ignalina Nuclear Power Plant (INPP) seismic strengthening of the existing buildings and equipment is not feasible economically. Therefore, a reactor protection system based on an earthquake early warning system was recommended. This system essentially consists of six seismic stations encircling INPP at a radial distance of approx. 30 km and a seventh station at INPP. Each station includes three seismic substations each 500 m apart. The ground motion at each station is measured continuously by three accelerometers and one seismometer. Data is transmitted via telemetry to the control centre at INPP. Early warning alarms are generated if an acceleration threshold is exceeded. The alarm is used to stop the nuclear reaction by insertion of the control rods. In the RBMK reactors at Ignalina, only 2.5 seconds are required for the insertion of the control rods. The pre-warning time provided by the seismic alarm system for earthquakes occurring at distances greater than 30 km from the site is 4 seconds. Therefore, the nuclear reaction can be stopped before the strong earthquake motion arrives. This paper discusses the seismic alarm and monitoring system presently under construction at Ignalina and the application of these systems to other types of Soviet designed nuclear reactors, specifically to the VVER.