In 1993, HSK (the predecessor of ENSI) set up an automatic dose-rate monitoring network (MADUK) in the vicinity of nuclear facilities. MADUK consists of four subsidiary networks located within a radius of 6 kilometres of each Swiss nuclear power plant and the Paul Scherrer Institute; each has 12 to 17 sensors.
The sensors (Geiger-Müller counters) measure the local dose rate as a function of natural levels of radiation (up to 10 Sievert per hour). Readings are transmitted to ENSI every 10 minutes where they are automatically monitored for any increase over and above normal background radiation. The data is then stored.
If certain limits are exceeded, ENSI in conjunction with URA, the Environmental Radioactivity Section of the Swiss Federal Office of Public Health (FOPH) investigate the cause.