Numerous studies expose the potential of brannerite to become a good matrix concentrating fis-sion products and actinides. Minerals can complement the data collected from the synthetic ma-terials and offer an advantage of a long-time exposure to radiation. Natural metamict brannerite from Akchatau, Kazakhstan, and its annealed sample were studied by the EPMA, Raman spec-troscopy, TGA, DSC, XRD and HTXRD. The radioactivity of pristine and annealed samples of brannerite was measured. Brannerite from Akchatau is characterized by the absence of the sig-nificant amounts of REE and yttrium. The studied brannerite regains its structure at a temperature ~650 °C, revealed by the HTXRD and DSC. HTXRD was also performed on the annealed recrys-tallized brannerite. The thermal expansion for brannerite has been determined for the first time. Brannerite structure expands anisotropically with the temperature increase. All the thermal ex-pansion coefficients are positive except for αβ. Decreasing beta parameter indicates a “shear structural deformation“. The angle between the 1-st axis of the tensor and the crystallographic a axis decreases with the increase of the temperature. The structure expands mostly in the α11 di-rection, approaching the bisector of the β angle. Brannerite has a low CTE at room temperature - αv=16 × 10-6 °C-1, which increases up to 39.4 × 10-6 °C-1 at 1100 °C. In general, the thermal stability of brannerite is comparable to that of the other perspective oxide radioactive waste-immobilizing matrices (e.g. Ln2Zr2O7, CePO4, CaTiO3, CaZrTi2O7). The calculated thermal expansion of bran-nerite and the understanding of its underlying crystal chemical mechanisms may contribute to the behavior prediction of the material (both metamict and crystalline) at high temperatures.