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Experimental contribution to thermodynamics of the Ag-Cd-In, In-Zr and In2O3-ZrO2 systems

Kasi Gajavalli has defended his thesis on 19th March 2019 in Saint-​Paul-lez-Durance (France).

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Publication Date > 19/03/2019

Summary

In the occurrence of a severe accident with loss of coolant on a Pressurized Water Reactor, the silver indium-cadmium absorbing alloy which constitutes the control rods is likely to interact at a high temperature with their guide tubes or with the fuel rods cladding, both in Zircaloy. In this context, the thermodynamic study of the Ag-Cd-In-Zr-O multicomponent system and in particular of the liquid phase and its associated equilibria is a necessary step for a reliable estimate of the fission product release and of the corium progression. More precisely, the aim of the thesis is to bring forth an experimental contribution to the thermodynamic study of the binary Ag-Zr, In-Zr, quasi-binary In2O3-ZrO2, and ternary Ag-Cd-In subsystems for which high temperature data are still missing.

For the Ag-Cd-In system, a specific thermal analysis protocol has been developed in order to measure the liquidus temperatures for ten Cd and In rich alloys. These new results, combined with literature data, have been used to establish a more accurate CALPHAD description of the system.

The binary Ag-Zr and In-Zr systems are difficult to study experimentally because of both the high melting point of zirconium and its high reactivity, particularly at the liquid state. The determination of the formation enthalpies of intermetallic compounds in these binaries has been undertaken using drop-solution calorimetry in liquid aluminum. First, the partial drop-solution enthalpies of the Ag, In and Zr pure elements at infinite dilution in liquid aluminum have been measured at various temperatures. For Zr, which is very hard to dissolve in liquid aluminum, this enthalpy was obtained indirectly from dissolution experiments of the Al3Zr compound. Then, the drop solution enthalpy of the In2Zr compound in liquid Al at 1173K has been measured in order to obtain the formation enthalpy of the In2Zr compound. In parallel, this enthalpy has also been determined by high temperature direct synthesis reaction calorimetry at 1273K with a good agreement between the two techniques. First syntheses of a series of In-Zr alloys followed by thermal analyses and metallographic examinations have allowed to clarify the phase equilibria in this binary which is still poorly known.

For the In2O3-ZrO2 system, the melting temperatures of various In2O3-ZrO2 binary compositions have been measured by laser flash heating for the first time. The semi-transparent nature of ZrO2 and the strong vaporization of In2O3 make the study challenging for such kind of experimental technique, which remains the most relevant considering the very high temperature range involved. Raman spectroscopy and scanning electron microscopy have been used to characterize the samples after melting. The overall consistency of the new results between them and compared to literature data has been tested by means of a thermodynamic optimization following the CALPHAD method. The
final objective is to introduce these new thermodynamic data and descriptions in the NUCLEA database which is used for severe accident modelling.

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