Chemical durability of some HLW glasses: effects of hydrothermal conditions and ionizing radiation

The time dependence of leaching and dissolution under hydrothermal conditions at 2500C has been investigated in an autoclave designed for continuous sampling; it is found that the rates are in accord with those obtained by the commonly used procedures. The effect of pressure by itself has been studied in an ultracentrifuge; ambient rates for Si and Na loss remain unchanged up to about 40 MPa followed by a monotonic increase to about 1.5 times the ambient rates at 160 MPa. The surface layers formed as a result of chemical attack have been examined (XRD, IR, XPS and PIXE). It is found that Zn present in the glass or in the solution is retained in the siliceous layer which in some circumstances contains a crystalline zinc-hydroxy-silicate phase. A method for enhancing rates of displacement damage by a factor of approx. 106 has been developed. Powdered borosilicate glass and glass doped with UO2 have been neutron-irradiated in a high-flux reactor. Damage is thereby introduced in the bulk as a result of 10B(n, α)7Li and (n,fission) events and subsequent slowing-down of energetic charged ions. Irradiation doses equivalent to 106 years HLW storage have been achieved. The resultant stored energy of approx. 102 kJ kg-1 is released over the range 300 to 800 K. The initial leaching/dissolution rates are found to be increased by a factor of about three by irradiation; the changes in longer-term rates are difficult to determine due to the effects of back-reactions and changing particle size geometry and size distribution, but significant enhancement is found. Rates for fission and activation products are similar to those for bulk species