Improving evaluation criteria for monitoring networks of weak radioactive plumes after nuclear emergencies

Networks of monitoring stations have been set up in many European countries to detect the passage of a radioactive cloud in the event of a large-scale nuclear emergency. The layout and spatial density of these networks differs according to the needs and criteria defined by national authorities. Germany and the Netherlands decided to set up relatively dense networks for the detection of weak radioactive plumes and, additionally, environmental radioactivity from deposited aerosols. Plausible evaluation criteria are presented here to assess important properties which determine the reliability and efficiency of sections of these networks. As a test case the existing sampling design of the Dutch and German networks with 193 sensors in an area of 200 km around the nuclear power plant near the city of Lingen (Emsland) in the German federal state of Lower Saxony has been selected. For a hypothetical accident scenario 292 radioactive plumes have been simulated which are shaped by recorded weather conditions of the year 2007. To quantify the network performance frequency distributions of the proposed evaluation parameters have been analyzed. Simulation results show that 95% of the plumes are detected within 4 h after the release. Maximal values of the γ-dose rate 1 m above the ground mostly occur near the source within a radius of 5 km. However, under certain weather conditions maximal ground values may also be found more than 50 km away from the source. Within a circle of radius 90 km 98% of the recorded maximal γ-dose rates of the plumes were found by 62% of the 193 sensors. But only in a joint network of German and Dutch sensors all simulated plumes triggered an alarm. This result encourages efforts of close international collaboration, e.g. between EU member states, in network design and operation. Test series which involved the removal of sensors either randomly or in a controlled way showed that the network configuration is fit for the intended purpose of detecting a large majority of plumes. But already a small reduction in the number of sensors would degrade the performance. Whereas the joint network triggers alarms reliably, it fails to detect the true plume maxima. The assessment criteria can be used for a revision of existing networks or for planning purposes in countries such as those applying for EU membership.