Anthropogenic Radionuclides
Anthropogenic radionuclides are radioactive nuclides produced from humanity’s technological use of nuclear reactions. The first major releases were from the use of nuclear fission and fusion for weapons. The majority of anthropogenic radionuclides released from this source have been dispersed in the environment by atmospheric nuclear weapon tests during the 1950s and 1960s. The second major technological application of nuclear reactions is energy generation from nuclear power plants. Here major releases have resulted from accidents, such as Chernobyl (1986) and Fukushima-Daiichi (2011), or releases from nuclear fuel reprocessing plants. Thus, humanity’s use of nuclear reactions have led to the dispersion of various irradiation products, fission products, and actinides in the environment, which have been spread globally. Obviously, monitoring these nuclides is important as they pose a potential health hazard, but also to track possible illicit nuclear activity (nuclear safeguards and forensics). However, the introduction of these nuclides into the environment has also enabled fundamental studies in different scientific disciplines like geochemistry, soil science, ecology, biology and medicine. Accelerator Mass Spectrometry is capable of determining the content of anthropogenic radionuclides down to a few atoms per gram. Similarly low detection limits are not achievable with any other analytical technique making AMS unique for ultra low-level analysis of radionuclides.
Applications
'Ivy Mike' atmospheric nuclear test, November 1952
Bomb-pulse dating. The testing of thermonuclear bombs in the atmosphere, by their epic neutron flux, have increased the 14C level in the atmosphere by up to 100% in 1963, which by today has lowered back to almost previous atmospheric levels, by uptake of the excess into the oceans and the biosphere. However, the spike provides a signal for very accurate dating (in some cases able to pin the actual year), and thus the study of many contemporary biological and ecological processes. On a global scale, the 14C bomb-pulse has also significantly improved our understanding of the carbon cycle.
Coral coring
Oceanic current tracer. Anthropogenic radionuclides provide an interesting opportunity to study the ocean currents and marine geochemistry of trace elements, as they provide a specifically marked, recently injected, signal in the world’s oceans. Radionuclides of elements behaving conservatively in the ocean can be used as ocean current tracers, while those for nutrients and readily sequestered elements can be used to study the temporal dynamics of nutrients transport and sequestration. A good example for an anthropogenic current tracer is 236U, whereas plutonium isotopes are usually readily sequestered, depending on their oxidation state. Bomb-pulse 14C is also a good current tracer, however, at the time of maximum injection and effect very little data could be collected, as AMS was not available as measurement technique at the time. Such data allow valuable insight into ocean-atmosphere gas exchange for CO2, an important factor in understanding the global carbon cycle.
Nuclear power plant Isar
Nuclear Decommissioning. During the lifetime of nuclear power plants the reactor pressure vessel and surrounding construction materials (i.e. concrete and steel) are constantly bombarded by neutrons from the reactor core. This neutron flux causes the activation of the inherent materials forming radioactive nuclei like 14C, 36Cl, 41Ca, 55Fe, 59/63Ni, 60Co, 99Tc or 152/154Eu. Despite some rather short-lived γ emitting nuclei like 60Co or 152/154Eu most of these nuclides feature relatively long half-lives making them difficult to assess by classical counting techniques. Accelerator mass spectrometry can thus be an alternative method to determine the content of these activation products and aids to set-up sufficient decommissioning strategies for large power plant components.
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