Thermodynamic Properties for Rare Earths and Americium in Pyropartitioning Process Solvents

Abstract

The design of a molten metal‐molten salt based chemical and electrochemical process for separation of actinides from plutonium‐uranium extraction waste requires a consistent set of thermodynamic properties for the actinides and rare earths present in nuclear waste. Standard potential data for Y, La, Ce, Pr, and Gd in molten LiCl‐KCl were obtained. Americium data obtained were standard potentials in molten LiCl‐KCl and activity coefficients for Cd and Bi. Data were obtained between 400 and 500°C. Results for the rare earth chlorides using an improved experimental technique were consistent with theory, with standard free energy of formation values somewhat more negative than those found in the literature. Special attention was given to Am in the LiCl‐KCl/Cd system because it can exist as the +2 and/or +3 ion in this system. Americium ions existed only as the +3 ion in LiCl‐KCl/Bi. Standard electrochemical potentials for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML1" overflow="scroll"> <mml:mi mathvariant="normal">Am</mml:mi> <mml:mo>/</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Am</mml:mi> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> in LiCl‐KCl eutectic at 400, 450, and 500°C were −2.893, −2.853, and −2.838 V, respectively, relative to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML2" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Cl</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Cl</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> </mml:mrow> </mml:msup> </mml:math> . Standard electrochemical potentials vs. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML3" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Cl</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Cl</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> </mml:mrow> </mml:msup> </mml:math> for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML4" overflow="scroll"> <mml:mi mathvariant="normal">Am</mml:mi> <mml:mo>/</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Am</mml:mi> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> in LiCl‐KCl eutectic were −2.83 V at 450°C and −2.78 V at 500°C. Activity coefficients for Am in molten Cd were <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML5" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML6" overflow="scroll"> <mml:mn>8</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>5</mml:mn> </mml:mrow> </mml:msup> </mml:math> at 450 and 500°C. Activity coefficients for Am in molten Bi were <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML7" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="ML8" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> </mml:msup> </mml:math> at 450 and 500°C. © 1999 The Electrochemical Society. All rights reserved.