DISPOSAL OF HIGH LEVEL NUCLEAR WASTE IN RUSSIA: STUDY OF DURABILITY OF NA-AL-P GLASS AND PRODUCT OF ITS CRYSTALLIZATION IN A HOT WATER

The first Russian mined underground repository for high-level radioactive waste (HLW) will be built at depth 500 m in granite-gneisses, Siberia. 4500 m³ of HLW in the form of Na-Al-P glass (3.7x10⁷ TBq of total activity) will be disposed there by 2047. Over time properties of the matrix including its retardation ability in respect of radionuclides will be changed due to crystallization of the glass. This is confirmed by results of our study on interaction of Na-Al-P glass containing imitators of the waste components (Ni, Fe, Cs, Sr, Ce, Nd, U) and product of its devitrification with water (90^oC). Normalized leaching rate of the elements from the glass were determined as, g m^-2 day^-1: 0.02 (Cs), 0.01 (Sr), 0.008 (U), 0.0009 (Ce), 0.0006 (Nd). The concentration of Cs in the solution in contact with the crystallized sample increased by 3 orders of magnitude compared the test with glass. Unlike Cs for Sr, Ce, Nd, and U the difference did not exceed an order of magnitude due to their incorporation into monazite-structure phase with a very low solubility in the water. According to change of solutions composition after passage through filters of various diameters (450, 200, 100, and 20 nm) the conclusion was made that Cs exists only in dissolved form in the experiments with glass and crystalline sample also. At the same time Sr, U, and lanthanides in the experiment with glass are in dissolved state and as colloid particles, but after contact of water with crystallized sample they mainly exist as colloidal particles. These results should be considered in the safety assessment of disposal of the vitreous HLW and at development of system of engineering barriers in order to prevent escape of the radionuclides from the repository into environment.