<p>Molten salt reactors (MSRs) are promising technology to replace traditional light and heavy water reactors in the near future. Waste management for MSRs, however, remains a pressing concern that needs to be resolved prior to realizing deployment of MSRs. Specifically, the generated salt waste needs to be safely sequestered, a task made difficult by the high solubility of the salt waste that can cause the release of radionuclides into the environment. This work demonstrates a new approach to immobilize multiple fluoride salts into a single waste form by converting highly-soluble fluorides to durable and stable oxyfluorides. Notably, we report a method of conversion of CsF, a very problematic salt, into a pyrochlore structure CsNb₂O₅F. Multiphase assemblages of oxyfluoride phases are prepared in a single step synthesis. Subsequent leach testing of the as prepared oxyfluorides indicate release rates orders of magnitude lower than standard reference glasses.</p> Graphical abstract <p></p>