Experimental studies and thermodynamic assessment of the Ba-Mo-O system by the CALPHAD method

E Colineau,N Dupin,C Guéneau,A.L Smith,R.J.M Konings,E Epifano,J.-C Griveau,L Herrmann,M Rutten

doi:10.1016/j.jeurceramsoc.2021.01.010

Abstract

Thermodynamic measurements on BaMoO4, BaMoO3 and BaMo3O10 are reported, that served as input for the development of a thermodynamic model of the Ba-Mo-O system using the CALPHAD methodology. The valence states of molybdenum in BaMoO4 and BaMoO3 were confirmed to be VI and IV, respectively, from X-ray Absorption Near Edge Structure Spectroscopy measurements at the Mo K-edge. The heat capacity at low temperatures of these compounds was obtained from thermal-relaxation calorimetry. Phase equilibrium data in the BaMoO4-MoO3 section were also measured, and the transition enthalpy associated with the peritectic decomposition of BaMo3O10 was determined using Differential Scanning Calorimetry. The developed thermodynamic model used the compound energy formalism for intermediate compounds, and an ionic two-sublattice model for the liquid phase. The optimized Gibbs energies were assessed with respect to the known thermodynamic and phase equilibrium data. A good agreement is generally obtained, but a number of ill-defined data were also identified.

Highlights

The chemistry of fission product (FP) elements in irradiated nuclear fuel, and more of volatile and semi-volatile elements, is of paramount importance, as FPs are the main source for the radiological consequences of a severe accident (SA) with release to the environment
Barium and molybdenum are generated with a high fission yield (~11% and ~25%, respectively [1]) during irradiation of the UO2 or (U,Pu)O2 ceramic fuel used in Light Water Reactors (LWRs), and represent key elements for the evaluation of the source term
We report a literature review of the thermodynamic and phase diagram data available on the Ba-Mo-O system, the synthesis and structural characterization of BaMoO4, BaMoO3, and BaMo3O10, the measurement of the low-temperature heat capacity of the former two phases, the determination of the transition enthalpy associated with the peritectic decomposition of BaMo3O10, and phase diagram measure ments in the BaMoO4-MoO3 pseudo-binary section

Summary

Introduction

The chemistry of fission product (FP) elements in irradiated nuclear fuel, and more of volatile and semi-volatile elements, is of paramount importance, as FPs are the main source for the radiological consequences of a severe accident (SA) with release to the environment. Barium and molybdenum are generated with a high fission yield (~11% and ~25%, respectively [1]) during irradiation of the UO2 or (U,Pu)O2 ceramic fuel used in Light Water Reactors (LWRs), and represent key elements for the evaluation of the source term. They are classified as semi-volatile fission products, which implies that their release kinetics are dependent on the redox conditions of the surrounding environment, and are determined by the evaporation of the chemical compounds formed in the irradiated fuel [1]. To respond to this necessity, the TCOFF project (Thermodynamic Characterization of Fuel Debris and Fission Products Based on Scenario Analysis of Severe Accident Pro gression at Fukushima-Daiichi Nuclear Power Station) was launched in 2017 under the auspices of the OECD/NEA [3], and the experimental study of the Ba-Mo-O system as outlined in this work constitutes a contribution to that initiative

Methods

Results

Conclusion