Dry Deactivation of Sodium Metal in a Molten LiCl-KCl-CsCl-NaCl System

Abstract

An experimental study was performed with the objective of investigating and characterizing a dry technique for deactivation of sodium metal in a molten salt system. The study was performed in three parts. First, proof-of-principle testing of the technique was performed at bench scale. It involved loading and melting tens of grams of clean sodium metal atop a pool of LiCl-KCl-CsCl eutectic at ∼300°C and then adding ammonium chloride particles while mixing. The ammonium chloride reacted with sodium to form sodium chloride and nitrogen/hydrogen off-gases. The sodium chloride assimilated into the salt pool, forming a quaternary salt mixture of LiCl-KCl-CsCl-NaCl. The proof-of-principle testing repeatedly exhibited complete deactivation of the loaded sodium metal. Second, characterization of a LiCl-KCl-CsCl-NaCl system was performed using differential scanning calorimetry to produce a partial phase diagram of LiCl-KCl-CsCl eutectic versus NaCl, which identified the liquidus, solidus, and two-phase regions of the quaternary system. Third, the dry deactivation technique was demonstrated at kg-scale in an inert atmosphere radiological glovebox with sodium metal that was previously separated in the same glovebox from uranium metal in unirradiated blanket elements for the Fermi-1 nuclear reactor. The quaternary salt product at the end of the demonstration was sampled and showed complete deactivation of the sodium metal. In short, this study qualified a technique to completely deactivate batches of sodium metal in the absence of air or water. While this study focused on the deactivation of sodium metal, including bond sodium from unirradiated blanket elements, the technique is applicable to other sources of sodium metal, such as sodium metal from batteries. Furthermore, the technique could also be extended to other alkali metal systems, including lithium, sodium, potassium, rubidium, cesium, and mixtures thereof. Sodium metal may not be disposed of as is in a nonhazardous solid waste landfill due to its reactive nature. It requires conversion (i.e., deactivation) into a stable form prior to its disposal. Sodium metal is renowned for its vigorous and potentially violent reaction with water and air, which is a conventional means of deactivating the metal. However, sodium metal reaction with air and/or water forms a caustic sodium hydroxide solution, which is also hazardous for corrosive reasons, requiring subsequent neutralization and immobilization prior to its disposal. This study describes a novel dry technique to deactivate sodium metal into a nonhazardous solid form in a single batch operation. The technique exhibited a controlled and complete deactivation of sodium metal into a solid chloride form. It has potential application to various sources of used sodium metal, e.g., from sodium batteries, and specific application to sodium metal from radiological systems with elevated concentrations of radioactive species. Enclosures for handling radioactively contaminated sodium metal require the absence of air and water, i.e., a dry environment to which the subject deactivation technique is well-suited.