A can-in-canister waste package design has been proposed for disposal of pyrochlore rich ceramics containing excess weapons plutonium. The can-in-canister configuration consists of a high-level waste (HLW) canister fitted with a rack that holds minicanisters containing the ceramic. The HLW canister is then filled with glass. The pressurized unsaturated flow (PUF) technique was used to investigate waste form/waste form interactions that may occur when water penetrates the waste containers and contacts the waste forms. Volumetric water content was observed to increase steadily from accumulation of water mass as waters of hydration associated with alteration phases formed on the glass surface. Periodic excursions in effluent electrical conductivity and pH were monitored and correlated with secondary phases formed during the test. Plutonium exited the PUF system primarily as filterable particulates. However, effluent Pu and Gd concentrations were found to decrease with time and remained at near detection limits after approximately 250 days, except during transient pH excursions. These results indicate that both Pu and Gd will be retained in the can-in-canister waste package to a very high degree.