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Trace analysis at the BfS
- By means of highly sensitive physical measuring systems the BfS is able to detect minute traces of airborne radioactivity.
- In this regard a clear distinction is made as to whether the radioactive traces detected are of natural or artificial origin.
- These measurements are referred to as trace analysis and are used – among others – for the verification of the worldwide ban on nuclear weapon tests.
Tasks and objectives of the trace analysis performed at the BfS are:
- detecting minute quantities of natural and artificial airborne radioactivity as well as
- investigating its origin, distribution and dispersion in the environment and
- being able to monitor short and long-term changes at the lowest level of activity.
Legal foundations for the investigations within the scope of the trace analysis are
- the Precautionary Radiation Protection Act (StrVG) with the measurement programmes in accordance with the General Administrative Regulation on the Integrated Measurement and Information System for the Monitoring of Environmental Radioactivity (AVV-IMIS),
- the European Atomic Energy Community (EURATOM) Treaty as well as
- the Comprehensive Nuclear-Test-Ban Treaty (CTBT).
The measurement results are summarized by the Coordinating Office Trace Analysis at the BfS and are reported to the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB), to the International Atomic Energy Agency (IAEA) as well as to the European Union (EU).
In the event of an incident (for example in the case of an accident at a nuclear power plant), the results are additionally provided in the system of the electronic situation display for emergency preparedness (ELAN).
At the measuring station Schauinsland and in Freiburg, airborne dust, noble gas and precipitation samples are taken, prepared and measured in the trace analysis laboratories at the Freiburg location.
The air dust and noble gas samples are collected continuously - generally in each case for a week. If necessary (eg after the accident in Fukushima), precipitation samples are taken and examined for radionuclides. In addition, noble gas samples from all over the world are analyzed in the noble gas laboratory in Freiburg.
Gamma spectrometry laboratory
- Traces of radioactive substances in airborne dust are detected using gamma spectrometry.
- The required samples are taken using so-called high-volume air samplers, the sampling period is usually one week.
Gamma spectrometry laboratory
The measurements are aimed at determining the activities and the activity concentrations of the various gamma-emitting radionuclides removed from the air by means of filters.
In order to detect radioactive traces, airborne dust samples are measured in the gamma spectrometry laboratory of the Freiburg office using so-called high-volume air samplers. These samples are taken
- at the measuring station on Mt. Schauinsland and
- on the roof of the Freiburg office.
To this end, air is drawn through large-surface aerosol filters with an airflow rate of 700 to 900 cubic meters per hour. The dust particles with the adhering radionuclides are removed by means of these filters..
The filters are pressed into tablets after the end of the sampling period (usually one week). In order to detect even minute amounts of radionuclides, the tablets are measured over several days by means of highly sensitive high-purity germanium detectors. In the process, lead shields are used to reduce background radiation which is present everywhere and can interfere with the measurement.
Typical limits of detection for the activity concentration of caesium-137 are at around 0.1 microbecquerel per cubic metre of air.
Not all radionuclides can be identified by means of gamma radiation. Initially, radionuclides such as strontium-90 or plutonium have to be separated radiochemically and processed accordingly for the particular measurement. This task is usually performed on a monthly basis for each measurement in the radiochemical laboratory of the Freiburg office.
The monitoring of radioactive traces in airborne dust is part of the measurement programmes in accordance with the General Administrative Regulation on the Integrated Measurement and Information System for the Monitoring of Environmental Radioactivity (AVV-IMIS) and of the EURATOM treaty among others.
Measurements beyond the scope of accreditation
Gaseous iodine cannot be removed by means of air dust filters. In order to detect this iodine, it is deposited on the surface of a solid (activated carbon for example). The sample produced in the process is analysed using gamma spectrometry.
If required (after Fukushima for example), precipitation samples are also taken on measuring station Schauinsland at the Freiburg office and are analysed for radionuclides. These samples contain the radionuclides that have been washed out of the air by precipitation.
Noble gas laboratory
The radioactive isotopes of the noble gases xenon (for example xenon-133) and krypton (krypton-85) play an important role
- With its laboratory, the BfS supports the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) as a "support laboratory" in setting up a quality assurance programme for the noble gas measuring network of the CTBTO, and with control measurements of samples from the noble gas stations within this measuring network.
Noble Gas Laboratory
The BfS takes weekly air samples
- in Freiburg,
- on Mt. Schauinsland and
- at Bremgarten opposite the French nuclear power plant Fessenheim.
At currently nine other sampling stations around the world, samples to be analysed at the BfS noble gas laboratory are collected in cooperation with other institutions. For this purpose, the samples are processed in a way that they can be sent to the noble gas laboratory in pressurized cans or gas containers.
In the noble gas laboratory, the air sample is analysed by means of a gas chromatographic method; this means that the gas mixture is separated into its individual chemical components.
The respective activities of the krypton and xenon fractions are determined by means of measurements of the beta radiation using proportional counters. The respective gas volumes of the analysed krypton or xenon fractions are subsequently determined chromatographically.
Limit of detection
Typical detection limits of the measurement system for the activities are about 0.03 becquerel for krypton-85 and about 0.01 becquerel for xenon-133.
Noble gas laboratory of the BfS supports CTBTO
With its laboratory, the BfS supports the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) as a “support laboratory” in setting up a quality assurance programme for the noble gas measuring network of the CTBTO, and with control measurements of samples from the noble gas stations within this measuring network.
In the scope of these tasks, the noble gas laboratory operates a nuclide-specific xenon measurement system, the accreditation of which is being planned
Airborne dust samples collected at the measuring stations Schauinsland and in Freiburg are initially analysed in the gamma spectrometry laboratory. Then, in the radiochemistry laboratory, they are processed using special methods to individually separate strontium, uranium and plutonium.
In order to achieve the lowest possible limit of detection, between four and five weekly samples are combined to monthly samples and ashed. The activity concentrations of the nuclides mentioned above are determined from the ashes of these samples.
To this end, the sample ash is mixed with acid and is processed in a specially designed microwave oven. Next, the nuclides to be determined are separated using radiochemical analytical methods and are captured on filters or stainless steel platelets.
The strontium isotopes are measured by means of a low-level alpha/beta counter. This system is used for detecting the lowest detectable activities of alpha and beta emitters.
After the electrochemical deposition on stainless steel platelets, the uranium and plutonium isotopes are measured in an alpha spectrometer.
Limits of detection
With the described method, detection limits of
- 1 mikrobecquerel per cubik metre air for strontium-89,
- 0.03 mikrobecquerel per cubik metre air for strontium-90 as well as
- 0.0005 mikrobecquerel per cubik metre air for the isotopes uranium-234, uranium-235, uranium-238, plutonium-238, plutonium-239 und plutonium-240
State of 2017.04.12