Sorption-barrier properties of granitic rocks and andesite-basaltic metavolcanites with respect to americium(III) and plutonium(IV)

Examinations on determination of sorption-barrier properties of granitic rocks and andesite-basaltic metamorphized volcanic rocks with respect to americium(III) and plutonium(IV) were conducted. Three samples of granitic rocks (Ieucocratic granite, medium-grained tonalite and coarse-grained granodiorite) from the Nizhnekanskiy massif (Middle Siberia) and three varieties of andesite-basaltic metavolcanites (lava/lava-breccia, tuff and tuff-lava) from the area of PA Mayak (Southern Urals) were chosen as subjects of inquiry. To evaluate some characteristic parameters of barrier properties of the studied rocks they were determined their main physical properties (density, porosity, both gas and water permeability). Low values of the porosity (1.6-10)x10-1 % vol. and gas permeability (8x10-16 - 1-10-12) cm2 were evidence of high shielding properties of the rocks' monolithic samples with respect to groundwater and fluids. Interphase distribution of americium(III) and plutonium(lV) in the systems containing simulated groundwater and monolithic (undisturbed) or crushed samples of the rocks was examined. Values of surface distribution coefficients at radionuclides' sorption on the surface of the monolithic rock samples (Ka) under various conditions of test setting were determined. The Ka values for americium are in the range of 10-37 cm, for plutonium they are 7-58 cm. The volume distribution coefficients (Kd) at sorption of these actinides on the crushed rock samples have values in the range of nx102 - nx103 cm3/g that is evidence of high sorption ability of the studied rocks with respect to radionuclides involved. It was investigated the sorption ability of main rock-forming minerals of the granitic rocks (biotite, feldspar, quartz) with respect to the actinides. By their sorption ability the minerals are graduated into the order: biotite > feldspar > quartz. Autoradiographic investigation of monolithic rock samples after the sorption of americium and plutonium onto them showed that the highest concentration of these radionuclides is connected with the biotite, microfissures and boundaries between grains of the minerals. The results of evaluation of retardation factors for forecasting estimation of the radionuclides migration rate show that the unbroken blocks of rock massifs composed of the investigated types of the rocks are practically impermeable ones with respect to the studied actinide elements