Behaviour of the gamma dose rate network during naturally cause increase of the ambient dose rate and consequences in case of man made releases

The Federal Office for Radiation Protection runs a network of 2150 dose rate probes that covers entirely and homogeniously Germany. It is aimed to fast and reliabely detect any increase in environmental radioactivity. The probes continuously monitor the gamma dose rate (ODL) 1m above ground. It takes 10 minutes to poll data from all 2150 probes. This renders the capability to monitor in near real-time. The data are made available to the decision support system (DSS) RODOS for data accumulation and prognosis and are used to validate the forecasts. Additionally the 2-hour average of each probe is checked and feed into the federal radioactivity information system (IMIS). The monitoring system proves in an everyday use its availability to give a fast survey of a radioactive contamination of the environment. This serves as a basis for decision making and recommendation accordingly to the ]precautioning radiation protection act in the early phase of an large scale contamination of the environment. On 07/10/2002 a unusually heavy rain went over germany. The dynamics of that event yielded data similar to what is expected from a nuclear cloud. The rain caused a wash-out effect which deposits natural radionuclides from the air to the ground. The nuclides are mainly daughters of Radon and cause a short term increase in natural radiation on the ground. The sensitivity of dose rate network is sufficient to detect these natural changes. The rainfront passed from west to east and entirely covered germany from north to south. In less than two hours more that 400 probes reported an increase of the radiation background. An animation shows the rainfront and the simultaneous reports of the probes. The results are discussed with respects to what may be expected from a man-made radioactive cloud. (orig.)