Technetium was immobilized in a Mg2TiO4 matrix by atomic substitution of titanium for 20, 50 and 80mol%. Leach tests, conforming to the ASTM C-1220-92 specifications, were carried out using simulated leachants (granite water, clay pore water and saturated salt solution) in order to determine normalized leach rates (NL) and fractional releases (FR%).
FR% for technetium have been measured < 0.1% for all Tc-loadings. Typical NL are1.7 E−2 g/m2d and 1.1 E−1 g/m2d for a 50 mol% and 80 mol% Tc-loading. In comparison to published fractional releases for glass, the matrix proves to be a more suitable host.
In addition, inactive pellets were produced and leached for investigation of the behaviour of the matrix material itself and the influence of other phases due to possible incomplete reactions during fabrication. Two types of inactive pellets were produced: Mg2TiO4 and Mg2TiO4/MgTiO3/MgO mixed pellets, the latter by decreasing the temperature during fabrication to 1285°C. Release of titanium from MgTiO3 proves to be higher than for Mg2TiO4.
It was found that the leachability of the host matrix increases upon increasing the technetium loading. As a compromise between waste volume reduction and leachability, it is suggested that technetium loadings do not exceed about 50 mol%.