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Abstract: A study of U, Th and Sr distribution in soils and conifers was realized at the Crucea area (northern Romania). The increased concentration of U and Th in the surfaced soils is related to dispersion of those elements from wastes. The bioavailability of the metals from the soil (EDTA extraction) decrease in the following order: Th(21,91%) > U(6,20%) > Sr(3,01%).
The fir Abies alba and the spruce Picea excelsa were found to have a high uptake capacities of uranium. Analyses of the evergreen vegetation indicated that in general uranium is preferentially concentrated in the roots, followed by the twigs and leafs/needles. For the behavior of Th and Sr, we noticed that they in the first place concentrate mainly in the root that is just like in the case of U. Sr shows a different behavior. Sr is in conifers found to be more concentrated in needles and twigs than in roots and stems.
Since the plants do not need U and Th neither for their metabolism, nor for their structure, it follows that the assimilation of these elements is being done through passive processes. The passive absorption implies the diffusion of uranyl ions and organically bound Th4+ from the soils in the endodermis of the roots, do to their imperfect selectivity and increased of permeability of cell membranes.
The behaviors of Abies alba and Picea excelsa are very interesting because they can reach and accumulate U, Th and Sr over very long periods of time. Consequently, planting conifers on uranium waste slopes may decrease the U, Th and Sr migration. The evergreens trees have a high potential of diminishing the quantity of mobile natural radioactive elements through bioaccumulation.

Keyword: U-mining, conifers, U, Th, Sr, phytostabilization