Hot Isostatically Pressed (HIPed) fluorite glass‐ceramic wasteforms for fluoride molten salt wastes

Abstract

Molten pyroprocessing salts can be used to dissolve used nuclear fuel from a reactor allowing recovery of the actinides. Previously, ANSTO have demonstrated hot isostatically pressed (HIPed) sodalite glass‐ceramic wasteforms for eutectic (Li,K)Cl salts containing fission products, but this system cannot be used for the analogous molten alkali fluoride salts (eg, FLiNaK), which have utility in the application of the next generation of nuclear reactors. In this work, a novel glass‐ceramic composite wasteform has been prepared by HIPing, as a candidate for the immobilization of fission product‐bearing FLiNaK salts. The wasteform has been tailored to immobilize the high fluoride content of the waste within fluorite, whereas the waste alkali elements are incorporated in a durable sodium aluminoborosilicate glass, with total waste loadings of ~17‐21 wt% achieved. It was also demonstrated that the speciation of Mo‐ and Sb‐simulated fission products was altered by adding Ti metal due to a controlled redox environment. The resulting candidate wasteform has been studied by X‐ray diffraction and scanning electron microscopy, including the HIP canister‐wasteform interaction zone, and its performance assessed via leaching studies using the PCT and ASTM C1220 leaching protocols. Dr Vance very much enjoyed the challenge of wasteform design for problematic nuclear wastes, for which fission product‐bearing FLiNaK salts are a clear example. His ability to hone in on a wasteform solution with viable waste loadings that meet performance requirements was testament to his nearly 40 years experience in nuclear waste immobilization. The samples discussed in this work represent the last wasteform materials that he prepared.

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

“…HIP is an attractive processing option for the consolidation of radioactive waste with the ability to process a wide range of waste streams, targeting minimal secondary waste generation and minimal volatile losses during processing 4,11 . HIPed wasteforms have been demonstrated for a variety of problematic wastes, for example, uranium‐rich radioactive wastes arising from Mo‐99 production, 12,13 radioiodine from the PUREX process, 14 and fluoride molten salt wastes 15 . These wasteforms produced by HIPing can be either full ceramic (C), GC, or glass (G) and are tailored to the waste stream chemistry.…”

“…4,11 HIPed wasteforms have been demonstrated for a variety of problematic wastes, for example, uranium-rich radioactive wastes arising from Mo-99 production, 12,13 radioiodine from the PUREX process, 14 and fluoride molten salt wastes. 15 These wasteforms produced by HIPing can be either full ceramic (C), GC, or glass (G) and are tailored to the waste stream chemistry. For the management of Pu-bearing waste material, leading candidates currently being considered are zirconolite-or pyrochlorebased full C wasteforms or GC wasteforms, fabricated by HIPing.…”

Vitrification of wastes: from unwanted to controlled crystallization, a review

“…This study [Raman, 1998] is a good example of designing the additives to produce a set of targeted crystalline phases. Recently fluorapatite phases have been created by two-step sintering of iron phosphate glass with SrF 2 , in an effort to explore waste forms for immobilizing the halides in molten salt reactors [Zhou et al, 2021]; other researchers, however, have emphasized the low waste loading of fluorine in this scheme [Gregg et al, 2020b]. GC for dental applications have been demonstrated from phosphate glasses with Sr phosphates and Sr-and Ca-fluorapatite plus xenotime and monazite [Ritzberger et al, 2013] or with chlorapatite from aluminosilicate glasses [Chen et al, 2014].…”

Integrating the Safety Evaluation for a Molten Salt Reactor Operation and Fuel Cycle Facility Application

“…One additional option for creating a stable waste form from halide salts is to segregate the halide into a crystalline halide phase embedded within a high-durability glass phase that contains FP and potentially actinide oxides. Trapping the F atoms in a stable alkali fluoride phase within an aluminoborosilicate glass is the basis for the "synroc" process developed at the Australian Nuclear Science and Technology Organization (ANSTO) [97]. The ANSTO process grinds the fuel salt into a powder and mixes it with glass, forming oxides, calcining, and then hot isostatically pressing the material.…”