Dynamic reference electrode for investigation of fluoride melts containing beryllium difluoride

https://doi.org/10.1016/j.jfluchem.2008.07.017Get rights and content

Abstract

Existing designs of reference electrodes for potentiometric measurements in fluoride melts do not meet basic requirements of the long exposure corrosion tests to be performed. A new diaphragm-free three-electrode meter with a nonstationary (dynamic) beryllium reference electrode for the redox potential measurements was developed. Optimum conditions of forming dynamic beryllium reference electrode were determined in the laboratory tests in isothermal cell. The device for redox potential measurement was designed and it was shown that it is highly sensitive to changes redox potential within ±5 mV. Reliability and efficiency of diaphragm-free device with dynamic reference electrode was confirmed in the thermal convection corrosion loop operating more than 1200 h with molten 15LiF–58NaF–27BeF2 mixture (mol%) at a maximum temperature of about 700 °C.

Graphical abstract

The behavior of a new dynamic beryllium reference electrode and device for measurement of a redox potential in a molten mixture Li, Na, Be/F are studied.

Introduction

Recent years have demonstrated revisiting of interest in nuclear energy systems employing technology of molten salt fluorides for

  • consumption of transuranic elements while extracting their energy;

  • electricity production in Th–U breeder cycle;

  • coolant application in solid fuel fast reactor designs;

  • pyrochemical reprocessing of advanced fuels;

  • high temperature process heat applications, e.g., hydrogen production;

  • production of isotopes for medical needs.

For the control of the composition and the purity of the molten salt streams, in-line and reliable analytical techniques will be required [1]. One of the most important parameters to monitor and control is the redox potential of the salt because it influences the chemical and, perhaps, some physical properties of the salt. Particularly it determines the solubility of various fission products and actinides in the melt and the corrosiveness of the molten salt with respect to structural materials. Changes in the redox potential provide an indicator of off-operation of molten salt clean up systems or a leak in the system with air or other fluids entering the salt.

The objective of this work was to develop reference electrode (RE) for in-line accurate measurements (with an error not more than ±5 mV) of redox potential of fluoride melt in the long exposure corrosion tests (>1000 h).

Analysis of the literature has shown that more often for such aims various modifications of classic emf method are used (Fig. 1). This method has high sensitivity to the change of redox potential of the system, simple in apparatus performance and provides good reproducibility of results [2], [3], [4].

According to the literature data, the continuous operation time of such potentiometric devices did not exceed 100 h. Moreover, the studies were performed under isothermal conditions or the stepwise temperature changes, while the RE half-element was in equilibrium with a small volume of the fluoride salt melt for the most part of the experiment. Long exposure tests of such devices in thermal convection loops have not been reported in the literature.

Section snippets

Selection of the RE design

The set of requirements imposed on the design and operation of the device for redox potential measurement (DRPM) shows that the transfer-free galvanic elements (Fig. 1) studied earlier, cannot guarantee the stable operation within required time in corrosion loop. The specific features of experiments in thermal convection corrosion loop are the following:

  • a big enough volume of the fluoride melt in system (>1 l);

  • a nonequilibrium of the system, caused by difference in molten salt temperature (up to

Conclusions

Analysis of the literature data concerning the use of various reference electrodes for potentiometric measurements in fluoride salt melts showed that the existing designs do not meet basic requirements of the prolonged corrosion tests to be performed on the thermal convection loop.

A new diaphragm-free three-electrode meter with a nonstationary (dynamic) beryllium reference electrode was developed for measurement of the redox potential of the BeF2 containing salt melt.

Preliminary tests of the

Acknowledgment

The generous financial support of this work by the ISTC # 1606 Project is gratefully acknowledged.

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