Background <p>As part of the Proryv project, the final stage of reprocessing spent nitride fuel involves the separation of americium and curium on a&#xa0;cation-exchange sorbent using displacement complexation chromatography. For radioecological safety of the environment, spent radioactive sorbent should be converted into a&#xa0;stable compound.</p> Aim <p>To test magnesium potassium phosphate (MPP) matrix for immobilization of spent radioactive sorbent formed during the reprocessing of spent nitride fuel, as well as to determine the quality indicators of the resulting MPP compound.</p> Materials and methods <p>The phase composition of obtained samples was determined by X‑ray diffractometry. Water resistance, mechanical compressive strength, resistance to thermal freeze-thaw cycles, as well as leaching of matrix-forming elements and ^{241, 243}Am and ²⁴⁴Cm isotopes was determined in accordance with standard tests.</p> Results and discussion <p>The obtained samples of MPP compound contain up to 20 wt % of the spent sorbent simulant and 10 wt % of wollastonite; their main crystalline phase represents an analogue of the K‑struvite natural mineral. The compressive strength of MPP compound is ~6 MPa, including upon the completion of freeze-thaw and water resistance tests after 90&#xa0;days of immersion in water. The compound is demonstrated highly resistant to leaching of matrix-forming elements and isotopes: the leaching rate of ^{241, 243}Am and ²⁴⁴Cm is approximately 5.8∙10⁻⁷ and 2.5∙10⁻⁷ g/(cm²∙day), respectively.</p> Conclusion <p>The prospects of using MPP compound for immobilization of spent radioactive sorbent formed during reprocessing of spent nitride fuel are shown. Quality indicators of obtained samples meet the requirements for solidified radioactive waste.</p>