Thermodynamic assessment of the KF-ThF4, LiF-KF-ThF4 and NaF-KF-ThF4 systems

J.A Ocádiz-Flores,E Carré,J.-C Griveau,E Colineau,E Capelli,P Souček,O Beneš,R.J.M Konings,A.L Smith

doi:10.1016/j.jct.2020.106069

Abstract

A thermodynamic assessment of the KF-ThF4 binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K2ThF6 has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: Cp,mo(K2ThF6,cr,298.15K)=(193.2±3.9) J·K-1·mol-1 and Smo(K2ThF6,cr,298.15K)=(256.9±4.8) J·K-1·mol-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF4 and NaF-KF-ThF4 using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF6 are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF4 system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF5 and NaF-KThF5 pseudobinaries, with good agreement.

Highlights

The Generation IV International Forum, a group of fourteen member countries pursuing research and development for the generation of nuclear reactors, has selected six nuclear energy systems [1,2]
As fluoride salts are highly sensitive to water and oxygen, handling and preparation of samples took place inside the dry atmosphere of an argon-filled glove box, where H2O and O2 content were kept below 1 ppm. (See Table 1)
The structure could be refined with the model proposed by Ryan and Penneman [32], with distorted anti-prism for K and Th, and pentagonal bipyramid for the polyhedra of K

Summary

Introduction

The Generation IV International Forum, a group of fourteen member countries pursuing research and development for the generation of nuclear reactors, has selected six nuclear energy systems [1,2]. The Molten Salt Reactor (MSR) is, in terms of safety and performance, one of the most promising nuclear reactor designs presently being studied. Its central characteristic is that the nuclear fuel is made of a molten fluoride (or chloride) salt instead of being a solid oxide or a metal. This liquid serves both as the fuel and coolant for the reactor. Two experimental MSRs have been built in the past: the Aircraft Reactor Experiment (ARE) [3] in 1954, and the Molten Salt Reactor Experiment (MSRE), which operated successfully between 1965-1969 [4]. A comprehensive knowledge of the physico-chemical properties of the salt is needed for the safety assessment and design of modern reactors, as the irradiated salt constitutes a complex and multi-component system

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Conclusion