Candidate liquid salts

Fusion reactors. Liquid salts are major candidates for cooling inertial and magnetic fusion energy systems [XXVI-11]. 

Unlike water, sodium, or helium, liquid fluoride salts are a family of coolants with similar general properties. The choice of a specific molten salt for a specific application is determined by functional requirements and costs. Many salts have been examined. Table XXVI-3 shows the properties for several different liquid salts and traditional reactor coolants under typical conditions. Table XXVI-4 lists leading candidates for various nuclear liquid salt applications and their key physical properties. The remainder of this Appendix discusses the various salts and the constraints that limit the choice of salt. 

The neutron absorption cross-sections of any liquid salt for reactor applications must be low to avoid excessive parasitic capture of neutrons. For thermal and intermediate neutron spectrum reactors, this probably eliminates chloride salts with their higher nuclear cross sections, even if the high cross section Cl is removed. Only fluoride salts are candidates. A wide variety of atoms have low cross sections however, the realistic candidates are also restricted by the requirements of thermodynamic stability to ensure viable materials of construction for the container. Table XXVI-5 shows the primary salt options and their cross sections. If either lithium or boron is used as a salt component, isotopically separated lithium and boron are required to have a salt with a low absorption cross section. 

The chemical and nuclear criteria define the allowable elements for a liquid salt coolant. Physical property requirements are used to define the candidate salts for specific applications. The requirements include ... 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

In the meantime, we believe that the best prediction of the toxicity of an ionic liquid of type [cation] [anion] can be derived from the often well known toxicity data for the salts [cation]Cl and Na[anion]. Since almost all chemistry in nature takes place in aqueous media, the ions of the ionic liquid can be assumed to be present in dissociated form. Therefore, a reliable prediction of ionic liquids HSE data should be possible from a combination of the loiown effects of the alkali metal and chloride salts. Already from these, very preliminary, studies, it is clear that HSE considerations will be an important criterion in selection and exclusion of specific ionic liquid candidates for future large-scale, technical applications. 

Preformulation testing provides a basic dossier on the compound and plays a significant role in identifying possible problems and suitable approaches to formulation. Such dossiers already exist for the common excipients. The requirement for aqueous solubility is paramount and preformulation can identify salt forms that are appropriate for further development. Stability and solubility studies wiU indicate the feasibility of various types of formulation such as parenteral liquids and their probable shelf lives. Similar information can be garnered for solid products from the solid physical properties. By performing these studies on a series of candidate compounds, the optimum compound can be identified and further biological and chemical studies guided to provide the best results. 

Several additional favorable properties of CBPCs make them an even better candidate for stabilization. The waste form is a dense matrix, generally with very good mechanical properties. Also it is nonleachable, does not degrade over time, is neutral in pH, converts even flammable waste into nonflammable waste forms, performs well within acceptable levels in radiolysis tests, and can incorporate a range of inorganic waste streams (solids, sludge, liquids, and salts). 

At an early stage during the preformulation program for the candidate, particularly for those with unfavorable physicochemical and physicomechanical properties as a free acid or free base, it is normal procedure to investigate a suitable range of potential salts. Typically 4-6 different salts are evaluated, if they can be isolated. Often the properties of the free acid (or free base) are hardly satisfactory and it is recommended that it is included in the comparative exercise using a battery of tests (see Section V). If a parenteral dosage form is intended and the properties of the free acid or free base are better than any of the salts, it is often possible to prepare a salt in situ in the liquid vehicle. 

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