ESTONIAN ENVIRONMENT

External dose rate (in-situ and laboratory measurements)

 
Environmental radioactivity and radiation dose in Estonia
 
The research activities carried out in the framework of this project concentrate on the following topics:
Study of environmental radioactivity, radioecology and dose formation in the Estonian environment by using radiometric and high-resolution gamma spectrometric methods,
Development of nuclear analysis methods and quality assurance for environmental samples;
Theoretical studies and development of computer simulations for time-dependent nuclear resonant forward scattering of synchrotron radiation and its applications.

In 1998 and 1999 a major attention has been concentrated to the assessment of technogenic radionuclide pollution and of the accompanying dose impact in the vicinity of the Sillamäe waste depository, oil-shale-fired power plants, etc.; to the development of methods for the interpretation of relaxation phenomena arising in time-dependent nuclear resonant forward scattering of synchrotron radiation. Below a few descriptions of the results obtained in these fields are briefly summarized.

Correlations of naturally occurring radioactive materials, NORM, activity concentrations confirming an existence of at least two general soil types in Estonia characterized by two different concentration ratios, C(U)/C(Th), have been found. While in most of the territory the concentration of U approximately equals to that of Th, in the coastal region of N Estonia and especially of NE Estonia, significantly above-average uranium concentrations up to 350 Bq/kg are accompanied by the elevated C(U)/C(Th) ratios in  the range of 1.5 .. 5. These U-rich areas causing enhanced population doses and radon risk are rather unique places all over Europe, but are incompletely studied yet.

Oil-shale is the most important fuel in Estonia, where about 85% of it is used for energy production. According to the amount of released gas pollution, chemically toxic trace elements and radionuclides in fly-ash, the Estonian oil-shale-fired power plants are considered among the largest atmospheric pollution sources in Europe. Therefore the assessment of their environmental impact, including the radiological one, as well as a clarification of radionuclide pathways in the environment is of interest. Our first studies have confirmed that in the vicinity of these power plants (NE Estonia) in the surface soil considerably higher (than in deeper soil layers) concentrations of naturally occurring radionuclides (NORM), 40K, 226Ra and 232Th, are found. Later we have shown even higher enrichments of surface soils in 210Pb caused by fly-ash deposition. These circumstances have stimulated additional methodical work and research: participation in the international intercomparison exercises for quality assurance in high-resolution gamma spectrometry, more detailed studies by of soil radionuclide profiles, determination of radionuclide content in various oil-shale ash fractions and their balance in the combustion processes, development and application of models for the simulation of atmospheric transfer of fly-ash radionuclides.

The estimation of radionuclide concentrations in local biofuels (wood, peat and their mixtures), in their ash products, in local building materials, etc., and of their radiological impact have been started. These studies have revealed a few interesting phenomena of radioecological importance, e.g. a time-dependent behaviour of radon exhalation rate in ashes, a particle-size-dependent enrichment of specific radionuclides, etc. Detailed works on these findings are in progress.

 
In the last few years a new powerful experimental method of nuclear resonant forward scattering of synchrotron radiation has been developed. The time-resolved method being an alternative and complementary to the conventional Mössbauer spectroscopy has been successfully applied for multiple studies of iron-containing compounds and doped systems. Our important contribution to the field lays in the development of theory and of the relevant simulation software to interpret the experimental results on bioinorganic compounds with randomly oriented iron complexes, which exhibit nuclear Zeeman interaction and/or temperature-dependent relaxation behaviour. The software package SYNFOS has been composed and upgraded to meet the specific properties and features of the studied samples. In addition, the performed theoretical analysis have revealed a few important general features of the method (e.g., clear asymmetry-induced changes in time spectra in comparison to energy domain spectra).
As a result, the performed theoretical simulations for iron porphyrins have shown a rather good compliance with the measured time spectra, e.g., temperature dependent quadrupole splitting of oxy and deoxy myoglobin, distribution of hyperfine fields in the iron mineral core of bacterioferritin from streptomyces olivaceus, etc. In 1998-1999 a theoretical description of time-dependent features in time spectra connected with paramagnetic relaxation has been completed. Our treatment enables one to simulate time-dependent changes in the full range of relaxation times, including the informative intermediate region. Our software package SYNFOS has been used and is continuously in use in connection with experiments on bioinorganic complexes carried out in the HASYLAB and ESRF synchrotron radiation facilities.
137Cs in soil (Chernobyl fallout, 1986)
Ra226 in Estonian soil (1991-1993)