Proceedings of ICAPP 2017 April 24-28, 2017 - Fukui and Kyoto (Japan)

STUDY ON PERFORMANCE OF OXYGEN SENSORS WITH SOLID AND LIQUID REFERENCE ELECTRODES IN LIQUID LBE WITH THE PARAMETERS OF OXYGEN POTENTIAL AND TEMPERATURE

Pribadi Mumpuni Adhia, Masatoshi Kondob, Minoru Takahashib

aDepartment of Nuclear Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology,

2-12-1-N1-18, Ookayama, Meguro-ku, Tokyo, 152-8550 Japan E-mail: adhi.p.aa@m.titech.ac.jp

bLaboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1-N1-18, Ookayama, Meguro-ku, Tokyo, 152-8550 Japan

E-mail: yamanami-yuhi@ruby.dti.ne.jp

The performance of oxygen sensor with liquid type reference electrode (RE) Bi/Bi2O3 and solid type RE Fe/Fe3O4 were investigated in the liquid lead-bismuth eutectic (LBE) with three oxygen potentials in the temperature range of 450°-600°C. PbO, Fe3O4, and Cr2O3 were mixed with the liquid LBE to make the oxygen potential in the liquid LBE equilibrium with their formation potentials. The results showed that the cell potential exhibited from both of oxygen sensors could have good agreement with the theoretical ones in the LBE with PbO and Fe3O4. However, the cell potential deviated from the theoretical one in the LBE with the low oxygen potential equilibrium with the formation potentials of Cr2O3,

I. INTRODUCTION

Lead and lead-bismuth eutectic (LBE: Pb-55.5Bi)

have been proposed as coolant candidates for fast reactors (FR) and the candidates of coolant and spallation target for the accelerator-driven system (ADS)1. On the other hand, lead-lithium alloy, Pb-17Li, has been chosen as one of the candidate tritium breeders for fusion reactors2.

However, these lead and lead alloys are corrosive to structural materials such as austenitic steels at high temperature above 450°C. The suppression of corrosion requires adequate control of oxygen concentration in the lead alloys during both of the operation and shutdown periods of the lead-alloy-cooled fast reactor (LFR)3 and ADS. The required control range in LFR is in the order of magnitude of 10-8 to 10-7 wt% that corresponds to the oxygen potential between the formation potentials of PbO and Fe3O4. The spallation products may decrease the oxygen potential in the LBE of ADS.

On the other hand, in the case of the Pb-17Li in fusion blanket, it has been made clear that the Li composition is preferentially depleted from the alloy by the oxidation4,5. Therefore, the oxygen potential in the alloy must be kept lower than that for the formation of the Li rich oxide. It is known that this oxygen potential is much lower than that for the adequate oxygen potential in the LFRs and ADSs.

Oxygen sensors will be used in the LFRs, the ADS and the fusion blanket to measure and control the oxygen potential. The oxygen sensor should be reliable in various oxygen potential and temperature ranges, that is, the sensor output should give reliable value not only between the formation potential of PbO and Fe3O4 but also in oxygen potential much lower than the formation potential of Fe3O4.

In the range of such very low oxygen concentration in molten lead alloys, the cell potential of the zirconia solid electrolyte oxygen sensor is dominated by not only ionic conductivity but also electrical conductivity of the solid electrolyte6. However, the performance of the oxygen sensor has not been investigated sufficiently in the range of such very low oxygen potential.

The purpose of this study is to investigate the electrical effect of solid electrolyte on the performance of oxygen sensors in molten LBE environment with high, medium and very low oxygen potentials corresponding to the formation potentials of PbO, Fe3O4, and Cr2O3, respectively. The reference electrode (RE) used for the oxygen sensor were liquid metal Bi/Bi2O3 and solid Fe/Fe3O4 RE. The temperature of the molten LBE was chosen in the range of 450 - 600°C II. EXPERIMENT II.A. Experimental Apparatus

Figure 1 (a) shows the schematic drawing of

experimental apparatus. The apparatus consists of the stainless steel vessel and the stainless steel SS304 crucible with the amount of LBE about 450 g placed on to it. The electric heater was surrounded on the outer wall of the vessel to control the LBE temperature and the thermocouple (TC) was immersed into the LBE to measure the LBE temperature. The solid electrolyte oxygen sensor and the lead wire made of Mo were immersed into the LBE. Gases of Ar and Ar+3% H2 were used as a cover gas.

Proceedings of ICAPP 2017April 24-28, 2017 - Fukui and Kyoto (Japan)

Fig. 1. (a) Schematic drawing of experimental apparatus for measurements of cell potential in molten LBE environment; (b) Schematic drawing of the oxygen sensor.

II.B. Structure of Oxygen Sensor Figure 1 (b) shows the schematic drawings of the

oxygen sensor used in this study. Magnesia-stabilized zirconia (MSZ) was used as solid electrolyte material. The dimensions of the solid electrolyte were 5 mm in inner diameter, 1.5 mm in thickness, and 50 mm in length. Two types of reference electrode (RE) material in the solid electrolyte tube were used, a liquid bismuth with bismuth oxide powder (Bi/Bi2O3) and a solid iron powder with a solid iron oxide powder (Fe/Fe3O4). A lead wire made of molybdenum (Mo) was inserted into the solid electrolyte tube to contact electrically with the RE materials. The details on the fabrication method of the oxygen sensor have been explained in the previous paper7.

The oxygen sensors work as the galvanic cells expressed by

Mo, LBE |MSZ| Bi/Bi2O3, Mo Mo, LBE |MSZ| Fe/Fe3O4, Mo

The cell potential E is expressed by the Nernst equation:

worO

refO

2

2ln4 P

PF

RTE (1)

where R is the gas constant (8.3143 J/mol K), T is the absolute temperature in Kelvin, F is the Faraday constant (96485 C/mol), and PO2

ref and PO2wor are the oxygen partial pressures in the reference electrode (RE) and the working electrode (WE) in LBE, respectively. Note that the oxygen partial pressures of liquid Bi/Bi2O3 RE and liquid LBE (WE) are the superficial pressures that correspond to the oxygen potentials of oxygen ions dissolved in the liquids. The oxygen potentials of oxygen ions are equal to the oxide

formation potentials in equilibrium conditions. The oxygen partial pressures of solid Fe/Fe3O4 RE is equal to the oxygen partial pressure of the gas phase in the gap of the powder that is equilibrium with Fe3O4 formation potential.

The oxygen partial pressure of RE and WE can be determined by the redox equilibrium reaction as follows:

zy2 OMz2OM

y2 (2)

The relation between the superficial oxygen partial pressure and Gibbs free energy for oxide formation ΔGo can be written as

RT

GP

z

2exp

0OM

Ozy

2 (3)

The Gibbs free energies for oxide formation are given by

TG 304.5+-1,095,6000OFe 43

for Fe3O4 (4)

TG 290.5+-585,8500OBi 32

for Bi2O3 (5)

TG 256.8+-1,132,4000OCr 32

for Cr2O3 (6)

TG 100+-219,2500PbO for PbO (7)

In a range of very low oxygen potential in LBE, the

electric conductivity of solid electrolyte material has comparable or more dominant effect on the cell potential compared with ionic conductivity. A parameter called pe’

is defined by the PO2 at which the ionic conductivity and the n-type electronic conductivity of the electrolyte are equal7. In this case, the Nernst equation in Eq (1) can be re-written as follows:

41

worO

41

41

refO

41

O

2

2

ref2O

wor2O

2

'

'ln)(ln

4Ppe

PpeF

RTPdtF

RTEP

Pion

(8)

At very low oxygen potential in the LBE, the value of PO2

wor is very small in comparison with pe’, and the maximum cell potential E that can be recorded is approximated by

pe'

P

FRTE

refO

max2ln

4 (9)

II.C. Experimental Method and Condition

The performance of oxygen sensor with Bi/Bi2O3 RE

and Fe/Fe3O4 RE were investigated by measuring the open circuit potential between two electrodes in LBE in a temperature range of 450 - 600°C. The measured cell potential of each sensor was compared with the theoretical one calculated from the Nernst equation. An electrometer

Proceedings of ICAPP 2017April 24-28, 2017 - Fukui and Kyoto (Japan)

with high input impedance (EM-05) was used to measure the potential different.

The oxygen potential in LBE was controlled to be equilibrium with the PbO, Fe3O4, and Cr2O3 formation potentials shown in Fig. 2. The oxygen potential in Pb-17Li is lower than the formation potential of lithium oxide that is much lower than the present conditions because of the effect of Li in Pb-17Li. To control the oxygen potential in LBE, mass-exchanger method was used, i. e., metal and metal oxide powders were mixed into the molten LBE. The amounts of the metal powder Fe and Cr were 4 g, and those of Fe3O4 and Cr2O3 were 15 g. The amount of metal oxide powder PbO was 4 g without any addition of Pb powder. The details of the method of the oxygen potential control have been described in the previous paper6.

The experimental conditions are presented in Table I. The equilibrium superficial oxygen partial pressures in LBE are calculated from the Gibbs free energies for oxide formation using Eqs. (3) – (7). The equilibrium oxygen concentrations in LBE are obtained from the equilibrium superficial oxygen partial pressures. The equilibrium superficial oxygen partial pressures in REs are also calculated, and then, the theoretical values of cell potential are obtained using Eq.(1). Fig. 2. Gibbs free energy for formation of some oxides and

oxygen partial pressure in liquid metal.

TABLE I. Experimental Conditions. Reference electrode

Powder for mass-exchanger method

Cover gas Temperature (°C)

Equilibrium oxygen concentration in LBE

(wt%)

Equilibrium superficial

oxygen partial pressure in LBE (atm)

Theoretical value of cell

potential obtained by Eq.

(1) (mV)

Bi/Bi2O3

Fe3O4 + Fe Ar

450 6.47 x 10-10 2.20 x 10-32 501.02 500 3.23 x 10-9 8.30 x 10-30 495.67 550 1.26 x 10-8 1.52 x 10-27 490.41 600 4.28 x 10-8 1.53 x 10-25 484.93

Cr + Cr2O3 Ar + 3%H2

450 6.78 x 10-17 2.52 x 10-46 986.18 500 1.00 x 10-15 8.50 x 10-43 989.09 550 1.06 x 10-14 1.06 x 10-39 992.00 600 8.62 x 10-14 5.93 x 10-37 994.91

Fe/Fe3O4

PbO Ar

450 1.21 x 10-4 5.31 x 10-22 -379.41 500 3.23 x 10-4 8.45 x 10-20 -383.08 550 6.87 x 10-4 4.28 x 10-18 -386.33 600 1.39 x 10-3 1.66 x 10-16 -389.73

Cr + Cr2O3 Ar +3%H2

450 6.78 x 10-17 2.52 x 10-46 501.19 500 1.00 x 10-15 8.50 x 10-43 498.74 550 1.06 x 10-14 1.06 x 10-39 496.28 600 8.62 x 10-14 5.93 x 10-37 493.83

III. RESULTS AND DISCUSSION

Figure 3 shows the comparison of the measured cell

potential with the theoretical calculation for oxygen sensor with Bi/Bi2O3 RE in LBE with Cr2O3 and Cr powders, and in LBE with Fe3O4 and Fe powders. The data shown in Fig.

3 were the average data for the last 1 hour after the steady state condition was attained.

In case of very low oxygen potential in LBE with Cr2O3 and Cr powders, the measured cell potential deviated from the theoretical result obtained from Eq. (1). Even the maximum measured cell potential was lower than

Proceedings of ICAPP 2017April 24-28, 2017 - Fukui and Kyoto (Japan)

theoretical result. The superficial oxygen partial pressure in LBE (working electrode) can be measured by using Eq. (1). The minimum superficial oxygen partial pressure values that can be measured by the oxygen sensor with Bi/Bi2O3 RE oxygen sensor are summarized in Table 2.

In case of medium oxygen potential in LBE with Fe3O4 and Fe powders, the oxygen sensor could attain steady state condition for each temperature. The measured cell potential agreed well with the theoretical result obtained from Eq. (1). The LBE could be controlled to be equal with the Fe3O4 formation potential, which means that the oxygen control using mass exchanger method was successfully demonstrated in this experiment.

Fig. 3. Cell potential (E) of oxygen sensor measured in LBE using oxygen sensor with Bi/Bi2O3 RE.

Figure 4 shows the results of performance of oxygen sensor using Fe/Fe3O4 RE oxygen sensor in LBE withCr2O3 and Cr powders, and in LBE with PbO powder.

In case of very low oxygen potential in LBE with Cr2O3 and Cr powders, the cell potential read by the oxygen sensor deviated from the theoretical calculation, possibly because the equilibrium condition with Cr2O3 formation potential could not be attained like the previous result using Bi/Bi2O3 RE oxygen sensor in Fig. 3. The maximum measured cell potential was lower than the theoretical calculation. The oxygen partial pressure in LBE (working electrode) can be measured by using Eq (1). The minimum

oxygen partial pressure values that can be measured by the oxygen sensor with Fe/Fe3O4 RE are summarized in Table II.

In case of high oxygen potential in LBE controlled by PbO powder, Good agreement between experimental data and theoretical calculation can be attained. The discrepancy was small. This is probably because the equilibrium condition with PbO formation potential can be achieved.

TABLE II. The Minimum Value of Oxygen Partial Pressure Obtained from the Experiment.

T (oC) Bi/Bi2O3 RE Fe/Fe3O4 RE

PO2, min (atm) PO2, min (atm)

450 7.94 x 10-41 2.21 x 10-41

500 4.77 x 10-38 7.85 x 10-39

550 7.93 x 10-36 2.48 x 10-36

600 6.78 x 10-34 3.75 x 10-34

Fig. 4. Cell potential (E) of oxygen sensor measured in LBE using oxygen sensor with Fe/Fe3O4 RE.

The results of the sensor with Bi/Bi2O3 RE and

Fe/Fe3O4 RE oxygen sensor were quite similar. The oxygen sensor could work well in LBE with the oxygen potential equilibrium with the formation potential of PbO and Fe3O4.

Proceedings of ICAPP 2017April 24-28, 2017 - Fukui and Kyoto (Japan)

In this condition, the oxygen sensor was a pure ionic conductor without any effect of the electrical conductor of the solid electrolyte. However, in LBE at very low oxygen potential, the cell potential deviated from the theoretical calculation. Both two sensors indicated the same order of oxygen partial pressure in LBE. It seems that the pe’ value is greater than the oxygen partial pressure of the formation potential of Cr2O3 in LBE. Therefore, the theoretical cell potential in Eq. (1) will deviate to the cell potential in Eq. (9).

The experimental data of pe’ value below the temperature of 600°C were limited. Several data were reported for the temperature higher than 600°C. Figure 5 shows the pe’ value versus temperature obtained in the present study and others for the consideration. The Nikkato data9 were obtained from the data sheet given by the manufacturer. The coulometric titration method was used. Zhuiykov10 data are based on an analytical calculation for (ZrO2)0.9(Y2O3)0.1 solid electrolyte. Etsell11 data were obtained experimentally in the range of 635°-1100°C using cell potential (EMF) measurement technique for (ZrO2)0.9(CaO)0.1 solid electrolyte. The pe’ value obtained from this study is in good agreement with the Etsell’s data extrapolated from the temperature range of 635°-1100°C. The good agreement could be obtained between the result of the present study and those of Etsell’s, where by the present experimental method are similar to the Etsell’s one. On the other hand, the analytical calculation developed by Zhuiykov has a good agreement with the Nikkato’s data.

Fig. 5. pe’ value and temperature.

IV. CONCLUSIONS

The major conclusions of the present study are as follows: 1. Both of the measured cell potentials showed good

agreement with the theoretical result obtained from the Nernst equation for LBE with high and medium oxygen potentials equilibrium with the oxide formation potentials of PbO and Fe3O4, respectively. The

electrical effect of solid electrolyte on the performance of oxygen sensor can be neglected.

2. The measured sensor cell potential deviated from the theoretical result obtained from the Nernst equation for LBE with low oxygen potential equilibrium with the formation potential of Cr2O3.The electrical effect of solid electrolyte on the performance of oxygen sensor should be taken into account.

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Proceedings of ICAPP 2017 April 24-28, 2017 - Fukui and Kyoto (Japan)

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