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 composites involves powder mixing followed by cold pressing and pressureless sintering or hot-pressing at relatively low temperatures (<620 °C). The hot-pressing route is found to be the most convenient, since it leads to relatively high densification even with substantial loading of pyrochlore phase (40 vol.%). The absence of microcracks, due to the close matching of thermal expansion coefficients of the composite constituents, together with the strong pyrochlore particle/glass matrix interfacial bonding, suggests that the composites have good mechanical properties. The innovative introduction of gadolinium zirconate in a lead silicate matrix represents an attractive approach, since the composites reach reasonably high densities both by pressureless sintering and hot-pressing.