The long-lived nuclide ⁷⁹Se plays a key role in safety assessments for underground radioactive waste repositories. In general, Se is assumed to diffuse out of the fuel grains and to migrate toward the periphery of fuel pellets due to the high thermal gradient during LWR reactor operation, similarly to the volatile elements I and Cs. According to this model, a significant part of the ⁷⁹Se inventory in spent fuel would be readily accessible to leaching after water ingress in the repository. However, contrary to these expectations, leach experiments did not show measurable Se release after exposing spent UO₂ fuel samples to aqueous solutions during up to one year. In order to explain this result, X-ray absorption near edge structure (XANES) spectra were measured on microsamples of high-burnup UO₂ spent fuel from two light water reactors. The results indicate that Se occurs in the fuel as sparingly soluble, almost immobile Se(-II) ion (selenide). The occurrence of soluble oxidized forms of Se could be ruled out. Theoretical XANES calculations proved to be consistent with Se occupying oxygen sites in the UO₂ crystal structure. From these results we conclude that the release of ⁷⁹Se from UO₂ spent fuel in an underground repository will be controlled by the slow dissolution of the fuel matrix and not by early short-term release. Our spectroscopic data thus explain why dissolved Se was not detected in the leach experiments. Moreover, they are essential to reliably define critical source term parameters, specifically the "Instant Release Fraction" (IRF), in performance assessment calculations.