Salt Phases in Calcined Materials and their Hydration Properties

Calcined plutonium oxide salt mixtures are packaged accordingto U.S. Department of Energy (DOE) Standards for long-termstorage in austenitic stainless steel containers. The behavior ofthe salts during packaging and storage can be critical to the integrityof the package because deliquescence of the salt can providean electrolyte that supports stress-corrosion cracking of thestainless steel. Pure alkaline earth chlorides such as MgCl2 andCaCl2 are known to form crystalline hydrates and to deliquesceat low relative humidities. When the alkaline earth chlorides arecalcined with other salts such as KCl and NaCl, new salt phasessuch as KCaCl3 and KMgCl3 can form. Little is known aboutthe hydration and deliquescent properties of these multicomponentsalts. The interactions of water vapor with the pure phasesalts and calcined mixtures were studied using X-ray diffraction,a moisture sorption analyzer, and environmental scanningelectron microscopy. Pure phase double and triple salts such asKCaCl3, K2MgCl4, and K3NaMgCl6 were synthesized. For calcinedMgCl2/NaCl/KCl mixtures, a variety of phases includingK3NaMgCl6 are formed. The anhydrous salt K3NaMgCl6 decomposesto KMgCl3?6H2O (carnallite) and the respective alkali halidesupon exposure to water vapor. Carnallite then controls themutual deliquescence relative humidity at ~57 percent relativehumidity (RH). For calcined CaCl2/KCl/NaCl mixtures, KCaCl3(chlorocalcite) is formed. Chlorocalcite does not form crystallinehydrates before it deliquesces at ~16 percent RH.