Stabilizing Iodine in Pyrochlore: Toward New Nuclear Waste Forms.

The long-term immobilization of iodine-129, a key radionuclide in nuclear waste, remains a major challenge for sustainable nuclear energy. Pyrochlore ceramics, which can be integrated into glass-ceramic waste forms, are widely recognized for their chemical durability and their capacity to immobilize actinides and other long-lived waste elements, but have not previously been demonstrated for iodine. Here, we show iodine can be accommodated into the pyrochlore structure by partial substitution for fluorine in a known oxyfluoride pyrochlore. Structural characterization reveals a solubility limit and associated lattice distortion upon iodine incorporation. To address these constraints, we introduce a high-entropy design strategy and computationally screen over 40,000 multication pyrochlore compositions, identifying candidates with the potential for substantially increased iodine loading. Many candidates exceed 33 wt % iodine, outperforming most existing ceramics, and in principle, iodine loadings as high as 74 wt % are accessible. These results highlight high-entropy pyrochlores as a promising and tunable materials platform for advancing safe and effective immobilization of iodine radionuclides in nuclear waste management.