GDR Germany - Federal Office for Radiation Protection24.05.2013
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Notes on the interpretation of GDR measurement results
The human body is permanently exposed to external ionising radiation originating from the natural environment. It is comprised of a terrestrial and a cosmic component. The first component comes from natural radioactive substances, i.e. radionuclides, like uranium, thorium, and potassium (40K). The second component is produced by high energy particles from outer space which are hitting the earth's atmosphere. Most of these particles are absorbed within the atmosphere, but a small fraction still reaches the ground.
The total amount of radiation, i.e. its energy-impact per mass unit for a fixed period of time, the so-called GDR or Gamma Dose Rate, is measured in units of μSv/h (micro Sievert per hour). The natural GDR in Germany ranges between 0.05 μSv/h and 0.2 μSv/h. Additional radiation originating from the Chernobyl accident in 1986 contributes only a very small proportion to the local gamma dose rate today.
The samples discussed here cover only a small part of all the possible interferences in the monitoring network. Malfunctioning probes are rapidly changed or repaired. Since the BfS is checking all data daily in routine operation, strangely behaving probes are quickly detected and if really broken, immediately labelled ''defect'' in the data base of the BfS monitoring network. In consequence, these probes are no longer used for pre-alarm. These stations might however, despite their status, deliver data open to public access via internet. The displayed 1-hour-data are indeed raw-data which are not necessarily free from technical perturbations.
Any irregular GDR value is reviewed by a specialised BfS operator. If the irregularity cannot be explained by known natural or technical effects and particularly if the dose rate increases over a longer time period or is beyond natural effects, BfS will take further steps to clarify if the data show evidence of an undisclosed radiological event and trigger appropriate action.
THE GERMAN GDR-NETWORK
In the Federal Republic of Germany, the monitoring of environmental radioactivity was initiated in the middle of the 1950s induced by the increase of artificial environmental radioactivity resulting from the nuclear weapon tests and their possible effect on public health. Continuous measurements are carried out on land, in the air, in rivers and offshore. Moreover, periodical measurements of radioactivity in food e.g. game, fish, mushrooms, field crops as well as in the soil complete the protection against artificial radioactivity. The German Federal Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) is one institute that works for the safety and protection of man and the environment against damage due to ionising radiation. To measure an increase in the radioactivity in the environment, the BfS operates a network of about 1800 stationary gamma dose rate probes that entirely and homogeneously covers the whole country in a grid of about 15 km x 15 km. The gamma dose rate (GDR) is given in the unit micro sievert per hour (µSv/h). The network is operated by 6 monitoring network nodes of the Bundesamt für Strahlenschutz (Federal Office for Radiation Protection), BfS, which are located in Berlin, Bonn, Freiburg, Neuherberg (close to Munich), Rendsburg, and Salzgitter. Besides the automatic monitoring stations mobile detection-systems for nuclide identification via spectroscopic measurements are operated by each of the network nodes.
The Figure on the right shows an example of an installed GDR-probe in Germany on the island Vilm. Such a probe contains two Geiger-Müller counters for measuring the GDR. The data logger (Messwertsender) which is connected to the probe by cable accumulates the data into 1- and 10-minute average values and sends these values on a regular basis to server stations of the corresponding network nodes. In the case of a technical problem or a GDR increase beyond a given threshold an automatic message is sent immediately by the data logger. In order to get comparable data from probes distributed all over the country it is important to ask for certain criteria concerning the condition of locations for probe installation. An optimal location is a flat lawn or meadow without trees, higher-growing plants, sealed areas, or buildings in the vicinity of the probe.
The Chernobyl reactor accident in 1986 showed that radioactive contamination of the environment is a large scale phenomenon. Since it cannot be ruled out that such an event might happen again, the BfS GDR network is the main route for the acquisition of information on the propagation of a radioactive cloud over Germany. Even a slight increase of the gamma dose rate triggers an automatic pre-alarm, which activates the BfS team on duty. The BfS specialists check the measurements and alert the emergency services in the case of a possible artificial gamma dose rate signal. The measurements are very sensitive, and as a result pre-alarm situations caused by weather conditions occur about 50 times per year. In the case of a real radioactive emergency the network can operate in intensive mode, i.e. the data from all measuring points are collected in 10-minute intervals, in order to promptly register a possible change of the radiological situation online. The computer systems of the measuring and service centres are designed with linked redundant capacity (six parallel independent systems located in 6 different computer centres distributed over Germany). Thus it is ensured, that in the case of a computer failure the tasks from one system are taken over by another computer centre without any time delay.
Fig. 9: GDR-Probe used in the network
Fig. 10: MWS3 Data logger housing