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ABSTRACT Zirconolite and pyrochlore are considered as promising host phases for high level waste (HLW). However, correct information on substitution mechanisms, forms of dopants incorporation in their structures and distortions in coordination polyhedra is presently unavailable. To clarify these points we use the electron paramagnetic resonance (EPR). Pyrochlore and three of zirconolite polytypes: zirconolite-2M, zirconolite-3T, and zirconolite-30 are considered. Pyrochlore is the "parent" structure for zirconolite since any zirconolite variety is produced by means of distortion of the initial pyrochlore structure. Space groups of pyrochlore and basic polymorphous zirconolite varieties found from XRD and TEM data, as well as interatomic distances and angles, were taken from reference data. This allows the determination of the most probable sites for impurities, substitution mechanisms, and local symmetry of coordination polyhedra (initial). Ions chosen for EPR were Gd (ItI) as an analog of trivalent rare earth and actinide elements which are also occurred in HLW and Fe (ItI) as a typical corrosion product which occurs in all HLW. For Gd (III) a strong ligand field approximation is suggested, theoretical computation using perturbation theory in this approximation has been carried out. All the non-diagonal members plus magnetic field were chosen as perturbation and formulae for transition frequencies, estimations of fine structure and g-factors parameters in the given approximation have been obtained. INTRODUCTION Zirconolite and pyrochlore are considered as the most promising host phases for actinide and rare earth elements including excess weapon plutonium. There has been much previous works [1-10] concerning features of isomorphous substitutions in natural and synthetic zirconolites and pyrochlores, the crystallographic and crystal chemical description of varieties, the evaluation of their stability, and stability under the effect of accelerated ions and ca-decay Stability, meaning long-term stability, of waste forms is usually evaluated by either simulation of conditions during long-term storage [3-5] or comparison of waste elements location in naturally occurring or synthetic single crystals of host phase, whose stability is established from geological data, and in polycrystalline waste forms [I I]. We have suggested a procedure for evaluation of crystalline host phases thermodynamic stability by determination of coordination polyhedra distortion degree using experimental electron paramagnetic resonance (EPR) data [12,13]. This procedure requires EPR investigation of single crystals, which can be unavailable. However, in some cases, analysis of coordination polyhedra symmetry may be carried out and paramagnetic impurity location in the crystalline structure may be determined even from EPR spectra of polycrystalline or powdered samples This is possible if the crystalline structure undergoes a second-order phase transition within a certain temperature range. The second-order phase transition changes space group sy