Permeability measurement in composite aerogels: application to nuclear waste storage

For long-life nuclear wastes (essentially actinides) research is in progress to develop new matrices with improved chemical durability. High-silica-content glasses possess a high chemical durability and we propose a new process to prepare silica glass which will embed the nuclear wastes. A porous network (silica aerogel) is used as a host matrix for nuclear wastes. The porous aerogel is soaked in a solution containing the actinides in nitrate salt form. After drying and nitrate decomposition, the composite material is fully sintered trapping the nuclear waste. Neodymium and cerium oxides are used to simulate the actinide oxides. A new kind of aerogel (composite aerogel) is proposed owing to its improved mechanical strength and relatively large permeability. A simple analysis of impregnation data is used to calculate the permeability. Increasing the bulk density from 0.4 to 1.4 g/cm3 decreases the permeability from 200 to 20 nm2 and increases the mechanical properties by two orders of magnitude. The influence of the porous network features (mechanical properties, mean pore size) on the ability of the material to soak up the simulating salts is discussed. This process allows the preparation of materials with a simulant content close to 20 wt%.