Progress and perspectives on hexavalent uranium waste forms for the immobilization of uranium-rich radioactive wastes

Abstract

Uranium-bearing radioactive wastes generated from the nuclear fuel cycle and the nuclear medicine sector require conditioning to form stable waste forms for long-term geological disposal. Various candidate ceramic and glass-ceramic composite waste forms designed for the incorporation of tetravalent uranium (U(IV)) have been developed largely based on the general observation that the oxidative dissolution of primary U(IV) minerals controls uranium migration in the geological environment. As such, waste forms must be processed under reducing conditions to maintain U(IV) and often with addition of metal powders to control the reducing environment, significantly increasing the processing cost. In addition, these U(IV) waste forms often require extensive chemical durability testing to see how U(IV) oxidation to U(VI) would impact U releases. Based on numerous natural analogue and laboratory studies, some U(VI) minerals and synthetic phases are found to be chemically durable, making them candidate U(VI) waste forms. This article is not intended to provide a comprehensive review rather serve as a perspective focusing on the current advances and future trend of the U(VI) waste forms for the encapsulation of uranium-bearing nuclear wastes, with some highlighted cases and analysis. Technical challenges and perspectives are provided to identify the knowledge gaps and opportunities for future research.