Novotvorene mineraly v bedrichovskem vodarenskem tunelu v Jizerskych horach – priklad uvolnovani uranu z jizerskeho granitu

Frequent occurrence of newly formed minerals was noted on the surface of the porphyritic biotite monzogranite during underground geological mapping of the Bedřichov water-supply tunnel in the Krkonoše-Jizera Composite massif. Whitish films of U-bearing opal and calcite, showing a green-yellow luminescence in the ultraviolet light, occur on the surface of joints formed by blasting and around goafs in the walls of the tunnel. The most abundant mineral schróckingerite NaCa3(U02)(C03)3(S04)F . 10 H20 often crystallizes on the surface of these films and can easily be mechanically separated. After the wall is washed with water and a brush, U-opal persists on wall and schróckingerite continues to be precipitated. Assuming an uranium content of 28.33 wt. % in schróckingerite, about 2.5 kg of this mineral is to be contained in this tunnel section. Uranyl fluor-sulphate-carbonate is formed on the tunnel walls through the reaction of "fertile" pore water enriched with C02. The equilibrium between schróckingerite crystallization and solution is controlled mainly by the variation in air humidity in the tunnel. Remobilized uranium was necessary for the formation of recent uranyl fluor-sulphate-carbonates schróckingerite crystallized in an environment of increased Eh potential by decomposition of accessory minerals containing uranium, e.g., thorite, uraninite, allanite, brockite, and monazite in the Jizera granite. Recent uranium-bearing minerals on the walls of the Bedrichov tunnel and presence of uranium in the ground water leaving the tunnel into local stream, illustrate continuous leaching of uranium and associated elements in the Jizera granite by oxidized ground water soaking through numerous fractures originated or activated during tunnel excavation almost 30 years ago. The release of uranium from granite accessory minerals and its mobilisation in the form of water soluble schrockingerite is a new information about circulation of radionuclides which might be intruded into the underground nuclear waste repository during its construction and operation in granites with high uranium content.