Abstract: The invention provides a method for the identification of deposits of solid materials in process plants, the method comprising the creation of a theee-dimensional image by means of tomography, preferably passive tomography, most preferably gamma-ray tomography. Typically, the solid materials comprise gamma-ray emitting solid materials and the process plants comprise nuclear process plants. In general, the deposits of solid materials are deposited from liquid media, and the liquid media preferably comprise aqueous suspensions of these materials. The method is particularly applicable to highly active nuclear waste liquids in High Activity Storage Tank (HAST) applications, and offers significant advantages over the methods of the prior art, since it comprises a non-invasive technique which does not require the prior installation of a detector in the vessel from which data are to be obtained.