THERMODYNAMIC EVALUATION OF THE OXIDE SYSTEMS STABILITY AS APPLIED TO THE MATERIALS OF THE OXYGEN CONTROL SENSOR IN SODIUM

The oxygen content in liquid sodium is an important normalized parameter that must be controlled during the operation of reactor fasilities with a sodium coolant. When developing a means of monitoring oxygen in liquid sodium, a careful selection of materials from which the sensitive elements of the control device (sensor) will be made is necessary. The criteria for selecting the sensor material include: acceptable corrosion resistance in liquid sodium under operating conditions; sufficient conductivity of oxygen ions at operating temperatures to generate a useful signal; and satisfactory resistance to thermal shock. Thermodynamic analysis allows to select materials that are potentially corrosion resistant to sodium in the sensitive element of the oxygen control sensor. The article presents a thermodynamic analysis of some metal oxide systems in contact with the sodium heat carrier of a sealed non-isothermal circulation circuit and an analysis of the stability of oxide systems in liquid sodium, which are the basis of ceramic sensing elements. It is shown that under certain conditions, deoxidized sodium can partially reduce the oxides of ceramics based on zirconium and hafnium with the addition of oxides of certain lanthanides. In addition, sodium reduces silicon oxide, on the basis of which sitall is produced, which is a candidate material for sealing the sensor sensor element. The analysis is performed using diagrams in the “electromotive force - temperature” coordinates constructed from thermodynamic data.