The reaction between simulated high-level radioactive wastes (HLW) and CaO-stabilized zirconia with CaF
2 structure has been studied in the temperature range of 8001, 500 °C in air and in a mixture of 4%H
2+96%He. Under the heat treatment in air at temperatures between 1, 000 and 1, 400 °C, 4W/O simulated HLW was dissolved in the CaF
2 phase. The lattice parameter of the CaF
2 lattice increased with HLW content according to the equation a= 0.5134+0.00028x nm, where x is the HLW content in W/O. In the region of 420W/O HLW, two phases were present : the CaF
2 and scheelite type phases. The actinide and rare earth elements were dissolved in the CaF
2 lattice, of which lattice parameter varied with HLW content in the form a=0.5141+0.00014x nm. Under the heat treatment in a mixture of 4%H
2+96%He, three phases were formed in the sample containing the HLW. They were the CaF
2 phase, (Sr, Ca) ZrO
3 with orthorhombic structure and Mo metal. The actinide and rare earth elements were fixed in the CaF
2 lattice. The increase of the lattice parameter of the CaF
2 lattice was given by the expression a=0.5134+0.00022x nm.
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