Study of the global distribution of the long-lived radionuclide Tc-99 with innovative detection techniques

K. Hain, F. Gülce, M. Martschini, J. Pitters & P. Steier (Isotope Physics, University of Vienna)

G. Korschinek (TUM), G. Wallner (University of Vienna), J. Welch (TU Vienna)

The radionuclide Technetium-99 (⁹⁹Tc) has a very long half-life of 210.000 years and has been emitted into the environment by nuclear weapons tests (around 140 TBq/220 kg) and in particular by the reprocessing plants situated in Northern France and Western Great Britain and therefore should be generally present in trace concentrations on the Northern Hemisphere. Apart from the production in nuclear power production, ⁹⁹Tc also increasingly accumulates as waste from medical tracer applications. The most stable chemical form of ⁹⁹Tc (TcO₄^-) is highly water soluble and hence, shows a mobile migration behaviour in the environment so that ⁹⁹Tc is considered to be a nuclide of primary concern for the final storage of nuclear wastes. Owing to its high solubility, ⁹⁹Tc was proposed as oceanographic tracer to study ocean currents which considerably influence our climate.

However, the chemical form of Tc strongly depends on the chemical conditions e.g. on the pH-value and probably also on the emission source. Whereas the ⁹⁹Tc distribution in the North Atlantic Ocean introduced as liquid discharges by the reprocessing plants is rather well studied, there is hardly any data on the global distribution of ⁹⁹Tc in other environmental reservoirs affected by nuclear weapons fallout so that little is known on the general migration behaviour in these environments. The reason is that the detection of trace concentrations of ⁹⁹Tc by its activity requires large sample volumes (several hundred to 1000 litres of ocean water) due to its long half-life and mass-spectrometric techniques are usually limited by interfering background with the same mass. In addition, Tc is lacking a stable isotope which is usually necessary to normalize the measurement results in mass-spectrometry to arrive at a final concentration in the sample.

In the present project, the ultra-sensitive detection method Accelerator Mass Spectrometry (AMS) will be applied to obtain a first large data set on ⁹⁹Tc concentrations in the environment including depth profiles from the Pacific Ocean to study the migration behaviour of ⁹⁹Tc from nuclear weapons fallout. Samples were collected on the cruise KH-12-4 within the frame of the GEOTRACES program. In order to establish the present concentrations of ⁹⁹Tc in Austria, it was planned to analyse river water and soil samples from the Vienna metropolitan area. Recent research has demonstrated that an ombrothrophic peat bog better serves this purpose as it forms an almost closed system which is fed only by precipitation. Therefore the peat bog has been collecting fallout ⁹⁹Tc during the most intensive phase of weapons testing and can be considered as reservoir for fallout radionuclides. In addition, the reducing conditions of a peat bog represents a unique opportunity to study the existence of particle-reactive Tc. For this purpose, pore and surface water samples were collected in the Pürgschachener Moor and the Rotmoos, both situated in Styria.

In order to overcome the problem of interference from background with the same mass, two different approaches will be applied: A gas-filled magnet setup available at the Technical University of Munich, which has already shown to reach a sufficient suppression of the background, and an Ion-Laser-InterAction-Mass Spectrometry (ILIAMS) setup recently installed at the University of Vienna. The latter one is a novel technique, which enables the use of low-energy accelerator systems where beam time is readily available. This is crucial to increase the throughput of samples for ⁹⁹Tc analysis and hence, to improve the database for the ⁹⁹Tc distribution in the environment which is urgently needed to establish the general environmental level of ⁹⁹Tc. Within the frame of this project, ILIAMS is developed for the detection of ⁹⁹Tc and its ability to suppress background interfering with ⁹⁹Tc will be tested. For both methods chemical procedures for Tc extraction and purification from environmental samples are developed in cooperation with the Atominstitut of the TU Wien, which provides ^99mTc as chemical yield tracer. For the normalization of the results, a known amount of another Tc isotope will be added to the samples, which is the first time this technique is used in combination with AMS.

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