# Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis
## 3 Discussion and Applications  
Foundational studies of radiolysis in pure water on the physico-chemical timescale provide the necessary background to pursue such processes in more complex systems--including those relevant for management of legacy wastes from nuclear technologies. This includes spent nuclear fuels from reactors, and wastes generated from processing of nuclear materials. Many of these materials are oxides and oxyhydroxides of elements, such as aluminum, that were present in the nuclear systems. For the case of certain specialized spent nuclear fuels (SNF), aluminum was used as cladding. The US government manages more than 10 metric tons of these specialized SNF assemblies, and long-term storage has led to cladding corrosion and the formation of aluminum hydroxide/oxyhydroxide hydrate layers. Radiolysis of the water associated with these materials drives the formation of molecular hydrogen and other byproducts that must be quantified and considered in decisions regarding their long-term storage. In the situation of wastes from nuclear materials processing, the production of plutonium from irradiated targets has resulted in the generation of more than 300 million liters of high-level radioactive wastes stored in underground tanks that must be managed by the Department of Energy's Office of Environmental Management (EM) [57]. The Hanford site in Washington State, USA houses 54 million gallons of hazardous radioactive waste, categorized as high-level waste (HLW) or low activity waste (LAW), resulting from decades of plutonium production, and currently stored in 177 underground steel tanks. The tank waste chemistry is complex (Figure 5a) and it consists of an aqueous supernatant, salt cake solids, and a sludge layer [58]. The supernatant liquid is a caustic mixture of 3-5 molar sodium hydroxide (NaOH) and large fractions of oxyanions, such as nitrate (NO\({}_{3}^{-}\)) and nitrite (NO\({}_{2}^{-}\)), with the dominant metallic element being aluminum (Al). The salt cake consists of water-soluble oxyanion and halide salts. The sludge consists primarily of insoluble Al oxides and oxyhydroxides, such as gibbsite (Al(OH)\({}_{3}\)), or amorphous agglomerates of varying sizes and morphologies, all of which serve to complicate retrieval and processing [58; 59]. The tank waste will be retrieved and processed into a chemically durable vitrified waste form for final disposal [60]. This remediation effort hinges on understanding components present in both the solid and aqueous phase, and controlling dissolution, precipitation, and particle aggregation reactions [61]. Foundational to this is a complete understanding of radiation-driven reactions and their implications for the speciation, structure, and dynamics of solutions representative of the waste. Within the tanks, \(\gamma\)-ray and \(\beta\) emitters are the primary radiation load (\({}^{137}\)Cs, \({}^{90}\)Sr,\({}^{99}\)Tc, \({}^{129}\)I) [60; 62; 63]. Many of these radionuclides are present in the supernatant but can also be present in the tank sludge [63], potentially impacting solution speciation and structure, as well as aggregation behavior and solubility of the solid phases. Though \(\gamma\)-radiation is the primary radiation load for the chemically complex tank wastes, to better understand the impacts, different radiation sources are being used to evaluate the various timescales and processes in simplified solutions representative of the major components of the waste, such as is shown in the Na\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-H\({}_{2}\)O ternary phase diagram in Figure 5b. Future work will place greater emphasis on increasingly complex electrolyte solutions to be more representative of simulant tank wastes, and the cascade of processes across broad time and length scales that are set in motion by ionizing radiation. Gamma rays, the primary radiation load in HLW, interact with water in a Compton process in which the incident photon behaves as a particle to produce an electron of a few hundred keV.