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Abstract

A study of the migration of critical radionuclide in the area of the former uranium mine of
Žirovski vrh in Slovenia was performed. Critical natural radionuclides are defined as the
long-lived natural radionuclides from the uranium-radium decay chain, which are 238U,
234U, 230Th, 226Ra, 210Pb and 210Po. In the area of the former uranium mine at Žirovski vrh,
two waste piles with uranium mining and milling wastes exist. The study area of this
doctoral work was focused on one of these two piles, the Boršt waste pile, where uranium
mill tailings are deposited. Uranium mill tailings contain elevated levels of natural
radionuclides from the uranium decay chain. Different processes in nature can cause
migration of natural radionuclides from the waste pile into the surroundings which can
represent a hazard for the nearby local population.
Therefore, soil, grass and tree samples were collected at the Boršt waste pile. After
pretreatment of the samples, radiochemical separations of the long-lived radionuclides
238U, 234U, 230Th, 226Ra, 210Pb and 210Po were performed. 238U, 234U, 230Th, 226Ra, and
210Po were measured by alpha-particle spectrometry and 210Pb a with gas-flow
proportional counter. In addition, two sequential extraction protocols, which allow
determination of natural radionuclide concentrations in different fractions (water soluble,
organic, carbonate, Fe, Mn oxides and residual fraction), were also performed. The results
of the two protocols were statistically compared. The results for the natural radionuclide
activity concentrations in grass were used for calculation of soil-to-plant transfer factors.
The possibility of using grass for tracing the migration of natural radionuclides in soil was
evaluated. Results of the natural radionuclide concentrations in different tree
compartments (wood, shoots and foliage) were used for the calculation of concentration
ratios.
Analysis of 238U, 234U, 230Th, 226Ra, 210Pb and 210Po in soil, silage, hay and milk
samples from the surrounding area of the former uranium mine were performed. Activity
concentrations of natural radionuclides in milk were compared with a reference location
and with powdered milk samples bought from shops. Yearly effective ingestion doses for
adults and infants up to one year old for milk samples from the surrounding area of the
former uranium mine were calculated and compared with the milk sample from the
reference location and with the commercial powdered milk samples. Concentration ratios
between silage, hay and milk samples were also calculated.
The results showed that the two sequential extraction protocols give statistically
different results, with the exception of the organic fraction and residue in the case of 238U.
However, both protocols revealed that 238U, 230Th and 210Pb are retained in a swampy area
below the Boršt waste pile. According to the activity concentrations in uranium mill
tailings on the Boršt waste pile, 238U is the most mobile, followed by 226Ra, 210Pb, 230Th
and 210Po. Soil-to-grass transfer factors were comparable with literature values. In the
case of 238U and 226Ra, grass shows potential for use as an indicator for radionuclide
migration in soil. The highest activity concentration in the case of tree samples were
found in one years old needles and leaves, which means that trees concentrate
radionuclides in older needles and leaves. Activity concentrations of the radionuclides
analysed in milk samples from the area surrounding the former uranium mine at Žirovski
vrh are in general not higher than in other samples. The highest combined yearly effective
ingestion dose was found in the powdered milk sample from the Pomurske mlekarne
diary of 648 μSv/year for infants. In addition, more than 90 % of the combined annual
effective ingestion doses in all cases was due to 210Po and 210Pb.