Borosilicate and lead silicate glass matrix composites containing pyrochlore phases for nuclear waste encapsulation

Glass matrix composites intended for the immobilisation of plutonium bearing nuclear legacy waste have been manufactured. Two different matrices, a soda borosilicate glass and a lead silicate glass, are proposed for encapsulating lanthanum and gadolinium zirconates having pyrochlore crystalline structure. The fabrication of the glass matrix composites involves powder mixing followed by cold pressing and pressureless sintering (at temperatures <750ºC) or hot-pressing at relatively low temperatures (<620ºC). At the processing temperatures used the pyrochlore crystalline structure of the zirconates remains unaltered. A homogeneous distribution of isolated pyrochlore particles in the glass matrix and strong bonding of the particles to the matrix were achieved in all cases. The hot-pressing route is found to be the most convenient, since it leads to relatively high compaction and densification even with a substantial loading of pyrochlore phase (up to 40vol%). The absence of microcracks, due to the close matching of the thermal expansion coefficients of the composite constituents, together with the strong interfacial bonding, suggests that the composites have good mechanical properties and impact strength. The innovative introduction of gadolinium zirconate in a lead silicate glass matrix is found to represent an attractive approach, since the obtained composites reach reasonably high densities both by pressureless sintering and hot-pressing.