Date of Award

Fall 2022

Document Type

Open Access Dissertation

Department

Nuclear Engineering

First Advisor

Theodore M. Besmann

Abstract

Thermodynamic descriptions of high-order molten chloride salt systems are essential for the development of fast molten salt reactor (MSR) technologies. However, a complete thermodynamic assessment of the essential Na-K-Mg-U(III)-U(IV) molten chloride salt with the most prevalent CrCl2 corrosion product has yet to be provided. This is remedied in the present work through application of the CALculation of PHAse Diagrams (CALPHAD) approach to the available thermodynamic data and new measurements that include differential scanning calorimetry (DSC) observations for previously unexplored two and three component chloride salt systems. Through these efforts, a unique approach was developed for the quantification of uncertainty in phase equilibria obtained from DSC measurements, which provided a distinct advantage in determination of the experimental phase diagrams, and resulted in the apparent discovery of a previously unreported intermediate compound in the NaCl-UCl3system.

Throughout the thermodynamic assessments, the heat of mixing (&Delta mixH) for some of the needed systems were found to be unavailable. Thus, an improvement upon the Davis’ &Delta mixH estimation method was developed to allow &Delta mixH determination at the composition of maximum short-range ordering (SRO) for pseudo-binary systems. The improved approach removes the currently required necessity of arbitrarily selecting the system composition of maximum SRO and successfully reproduces the behavior of the NaCl-UCl3 and KCl-UCl3 systems, providing &Delta mixH values that agree well with reported measurements within the propagated uncertainties. In addition to determining the &Delta mixH of the NaCl-CrCl2, and KCl-CrCl2 systems, the methodology was applied to the entirety of the ACl-PuCl3 systems (A = alkali metal) and more than 27 unexplored ACl-LnCl 3 systems (Ln = lanthanide), thus opening the door to accurate thermodynamic modeling of many molten chloride salt systems.

Since the formation of corrosion species in molten salt reactor systems is driven by the salt redox condition, indicated primarily by the uranium oxidation ratio (U4+/U3+), assessment of the salt systems including UCl3-UCl4 was paramount. To date, no available thermodynamic database models the effect of trivalent and tetravalent uranium chloride on the CrCl2 corrosion potential. Therefore, the chloride salt systems in the Molten Salt Thermal Properties Database – Thermochemical (MSTDB-TC) were extended with Gibbs energy models suitable for application to chromium corrosion in fueled chloride molten salt reactor (MSR) coolants. As many thermodynamic data for the alkali systems with UCl3-UCl4 are lacking, the work demonstrates a methodology for the development of fully-constrained thermodynamic models in the modified quasi-chemical model quadruplet approximation (MQMQA) that are capable of accurate estimations of thermodynamic properties, even in systems for which very few thermodynamic data are available.

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