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Borosilicate glass specimens incorporating 10, 15, and 20 wt % simulated high-level waste (HLW) were leached at 20, 90, and 120°C in deionized water following the procedure accepted in Russia, GOST (State Standard) R 52 126-2003. At 90°C, the altered surface layer is formed already from the first days of leaching. The main interaction in the system occurs at the phase boundary between the altered surface layer and water, rather than at the phase boundary between the glass and water through the porous surface layer as it is commonly believed. The altered surface layer acts as a natural barrier preventing transfer of simulated waste components to the liquid phase. The barrier properties of the altered surface layer are manifested to the greatest extent at 20 wt % content of simulated HLW in the initial glass and leaching temperature of 120°C. [ABSTRACT FROM AUTHOR]