A new type of a promising glassy-crystalline host material was prepared by ultradisperse synthesis in model experiments. The starting product is the nitric acid fraction of fission products (Cs, Sr, Ba), obtained radiochemically from irradiated nuclear fuel of a WWER-1000 reactor (1000-MW_el water-cooled water-moderated energy reactor). Equivalent amounts of La(NO₃)₃, H₃PO₄, and Fe₂O₃ are added, and water is removed on heating. After complete removal of moisture, the residue is calcined at 600°С. The phosphated calcinate is dispersed in acetone to obtain slurry and, after removing acetone, converted to the glassy form at 950°С by self-igniting high-temperature synthesis. The whole process of preparing the composite is performed in one container crucible, which is then sealed. The major constituent of the host material is iron (pyro)phosphate glass (~55%); the finely grained crystalline phase, according to X-ray diffraction data, consists mainly of monazite of the composition La₂(Sr,Ba)₃(PO₄)₄. The monazite content is about 40%. Monazite forms with the glass via covalent bonding a composite exhibiting high corrosion resistance. The steady-state rate of Cs leaching form the monazite glass is 0.1 μg cm^–2 day^–1 (90°С, 100 days).