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Trace analysis at the BfS

  • Using ultra-sensitive physical measurement systems the BfS is able to detect minute traces of airborne radioactive substances.
  • Using these measurements a clear distinction is possible between radioactive traces of natural or artificial origin.
  • These measurements are referred to as trace analyses and are for example used for the verification of the worldwide ban on nuclear weapons tests.

Trace analysis

Objectives

  • monitoring radioactive substances in the environment in accordance with statutory obligations
  • detecting clandestine nuclear activities

Aims and objectives of trace analysis performed at the BfS are to

  • detect minute quantities of natural and artificial airborne radioactivity as well as
  • investigate its origin, distribution and dispersion in the environment and
  • monitor short and long-term changes at the lowest activity-levels.

Legal foundations

Legal foundations for investigations within the scope of trace analysis are

  • the Radiation Protection Law (Strahlenschutzgesetz) including measurement programmes given in 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 and
  • the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

The measurement results are summarized by the Coordinating Office for Trace Analysis at the BfS and are reported to the Federal Environment Ministry (BMUV), to the International Atomic Energy Agency (IAEA) as well as to the European Union (EU).

Monitoring results are also made available through an electronic system for emergency preparedness (ELAN) to report on the situation in the event of an incident (for example in case of an accident at a nuclear power plant).

Trace analysis air-borne particle collector on the roof of the BfS office in Freiburg LuftstaubsammlerTrace analysis air-borne particle collector on the roof of the BfS office in Freiburg

Air samples

At the monitoring station Schauinsland and in Freiburg samples of airborne dust and noble gases are collected and then prepared and analysed in the trace analysis laboratories in Freiburg.

The air dust and noble gas samples are collected continuously - generally for a week. If necessary (e.g. after the accident in Fukushima), precipitation samples are also taken and examined for radionuclides. In addition, noble gas samples from all over the world are analysed in the noble gas laboratory in Freiburg.

Laboratories

The BfS uses various laboratories for trace analysis:

Noble gas laboratory

Noble Gas Laboratory for trace analysis

  • Accredited laboratory according to DIN EN ISO/IEC 17025:2018
Objectives
  • environmental monitoring according to statutory obligations
  • detecting clandestine nuclear activities

The radioactive isotopes of the noble gases xenon (for example xenon-133) and krypton (krypton-85) play an important role

  • in detecting clandestine nuclear activities such as underground nuclear weapons testing and
  • as an indicator of reprocessing of nuclear fuels, which is part of the production process of plutonium for nuclear weapons.

The BfS laboratory supports the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) as a "support laboratory" in setting up a quality assurance programme for the noble gas measuring network of the CTBTO.

The BfS takes weekly air samples

  • in Freiburg and
  • on Mt. Schauinsland.

At currently eight additional sampling stations around the world, samples are collected in cooperation with institutions and analysed at the BfS noble gas laboratory. For this purpose, the samples are processed on-site and sent to the noble gas laboratory in pressurized cans or gas containers.

Method

Preparation of a noble gas sample Noble gas sampleA noble gas sample is prepared for the activity measurement.

In the noble gas laboratory, the air sample is processed using gas chromatography; this means that the gas mixture is separated into its individual chemical components.

The activities of the krypton and xenon fractions are then determined by measuring their beta radiation using proportional counters.

Finally, the gas volumes of the analysed krypton or xenon fractions are determined chromatographically.

Limits of detection

Typical activity detection limits of the measurement systems are about 0.03 becquerel for krypton-85 and about 0.01 becquerel for xenon-133.

Noble gas laboratory of the BfS supports CTBTO

The BfS laboratory supports the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) as a "support laboratory" in setting up a quality assurance programme for the noble gas measuring network of the CTBTO.

Within this framework, the noble gas laboratory operates a nuclide-specific xenon measurement system. This system is able to measure activities and activity concentrations of the four xenon-isotopes

  • xenon-133,
  • xenon-135,
  • xenon-131m and
  • xenon-133m

by simultaneous measurement of beta- and gamma radiation. If xenon is detected in air samples, the measured isotopic composition can shed light on the potential sources of the xenon. This system has been accredited since March 2022.

Gamma-spectrometry laboratory

Gamma-spectrometry laboratory for trace analysis

  • Accredited laboratory according to DIN EN ISO/IEC 17025:2018
Task

Traces of radioactive substances in airborne dust are detected using gamma-spectrometry. The required samples are taken using high-volume air samplers, the sampling duration is usually one week. In case of an incident with enhanced emissions of radionuclides to the air daily samples can be taken.

The measurements are aimed at determining activities and activity concentrations of various gamma-emitting radionuclides collected from the air using high volume samplers.

In order to detect traces of radioactivity, dust samples are analysed in the gamma-spectrometry laboratory of the Freiburg offices. These samples are taken

  • at the monitoring station on Mount Schauinsland and
  • on the roof of the Freiburg offices.

High volume samplers draw air through large-surface aerosol filters at an airflow of 700 to 900 cubic meters per hour. The dust particles with the adhering radionuclides deposit on these filters.

Method

Aerosol filter Exposed aerosol filter after samplingExposed aerosol filter after sampling

The filters are pressed into pellets at the end of each sampling period (usually one week). In order to detect traces of radionuclides, the pellets are measured for several days using highly sensitive high-purity germanium detectors. Lead shielding is used to reduce the influence from background radiation, which is present naturally 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 their gamma-ray emissions. Radionuclides such as strontium-90 or plutonium have to be radiochemically separated and processed before measurement. This is routinely performed on monthly samples in the radiochemistry laboratory of the Freiburg office.

Pressed filters Filters pressed into pellet shapeFilters pressed into pellet shape

Monitoring for traces of radioactivity in airborne dust is part of the measurement programmes given in the General Administrative Regulation on the Integrated Measurement and Information System for the monitoring of environmental radioactivity (AVV-IMIS) and of the EURATOM treaty.

Measurements beyond the scope of accreditation

Gaseous iodine

Gaseous iodine in the air is not collected on the filters. In order to detect gaseous iodine, it is adsorbed on the surface of a solid (activated carbon for example). The sample produced in this process is analysed using gamma-spectrometry.

Precipitation samples

If required (after the Fukushima accident for example), precipitation samples are also taken at the monitoring station on Mount Schauinsland and at the Freiburg office, and are analysed for radionuclides. These samples contain radionuclides that have been washed out from the air by precipitation.

Radiochemistry laboratory

Radioanalytical laboratory

Objectives: Environmental monitoring in the context of the statutory tasks

  • Detecting radioactive elements in airborne dust samples:

    • strontium
    • uranium
    • plutonium
  • Detecting clandestine nuclear activities

Airborne dust samples collected at the monitoring station Schauinsland and in Freiburg are initially analysed in the gamma spectrometry laboratory. Then, in the radioanalytical laboratory, they are processed using radioanalytical methods to separate strontium, uranium and plutonium.

Method

In order to achieve the lowest possible limit of detection, between four and five weekly samples are combined to a monthly sample and subsequently ashed. The activity concentrations of strontium, uranium and plutonium are determined from the ashes of these samples.

The sample ash is dissolved in acid and processed in a specially designed microwave oven. Then, the nuclides to be determined are separated using radioanalytical methods and are deposited on filters or stainless steel planchets.

The strontium isotopes are measured using a low-level alpha/beta counter. This system is used for detecting the lowest detectable activities of alpha and beta emitters.

After electrochemical deposition on stainless steel planchets, the uranium and plutonium isotopes are measured in an alpha spectrometer.

Filter samples are processed in the radiochemistry laboratory Radiochemistry laboratory - Filter sampleFilter samples are processed in the radiochemistry laboratory

Limits of detection

With the described method, the following detection limits are achieved:

  • 1 microbecquerel per cubic metre of air for strontium-89,
  • 0.03 microbecquerel per cubic metre of air for strontium-90 and
  • 0.0005 microbecquerel per cubic metre of air for the isotopes uranium-234, uranium-235, uranium-238, plutonium-238, plutonium-239 und plutonium-240.

Laboratories and control centers of the BfS

State of 2023.05.04

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