Stability of technological glass containing the wastes of radioactive isotopes Sr-90 and Cs-237 under conditions of deep burial

The stability of the glass containing the wastes of radioactive isotopes Sr -90 and Cs-137 under conditions of deep burial (T = 25-350 degrees C and P = P-sat - 500 bar) was studied on the basis of the numerical modeling of th e phase relationship in closed and open locally-equilibrium systems taking into account a limited number of experimental data. In the initial system ( Na, Cs)(2)O-SrO-Al2O3-P2O5-H2O for the values exceeding 90 degrees C and P- sat, the glass decomposes resulting in a formation of highly soluble sodium hydrophosphates containing isomorphic cesium and difficultly soluble stron tium and aluminum hydrophosphates. In the ground water channel, the system is open for component Na2O. Sodium hydrophosphates together with cesium are taken out completely. Strontium is localized in a boundary system SrO-Al2O 3-P2O5-H2O, A figurative composition of a residual glass is represented by the association of the phases: Sr-hydrophosphate (SrHPO4) + variscite (AlPO 4. 2H(2)O) in the range T = 90-186 degrees C and Sr-hydrophosphate + berlin ite (AlPO4) at T > 186 degrees C and P-sat. The solubility of Sr-hydrophosp hate is n x 10(-3)-n x 10(-4) mol/kg H2O. The system exposed to a further i nfluence of flowing water occurs to be open for the component P2O5. An outp ut of phosphorus (leaching) from the system occurs. Variscite and berlinite are replaced by diaspore while Sr-hydrophosphate is substituted by difficu ltly soluble compounds: goyazite [SrAl3(PO4)(2)(OH)(5). H2O] and Sr-hydroxy apatite [Sr-10(PO4)(6)(OH)(2)]. The association goyazite + diaspore is stab le at local equilibrium for pH(H2O) = pH(sol) and P-sat in the range T = 90 -220 degrees C. For T > 220 degrees C the association Sr-hydroxyapatite is stable. The Sr-concentration in the solution does not exceed n x 10(-6)-n x 10(-7) mol/kg H2O. Therefore, solving the problem of the deep burial of ra dioactive wastes, special barrier devices should be designed for a fixation of cesium. No such devices are needed for strontium.