Quantitative Analysis of Oxygen Gas Exhausted from Anode through In Situ Measurement during Electrolytic Reduction

Quantitative analysis by in situ measurement of oxygen gas evolved from an anode was employed to monitor the progress of electrolytic reduction of simulated oxide fuel in a molten Li 2 O–LiCl salt.The electrolytic reduction of 0.6 kg of simulated oxide fuel was performed in 5 kg of 1.5 wt.% Li 2 O–LiCl molten salt at 650C. Porous cylindrical pellets of simulated oxide fuel were used as the cathode by loading a stainless steel wire mesh cathode basket. A platinum plate was employed as the anode.The oxygen gas evolved from the anode was exhausted to the instrumentation for in situ measurement during electrolytic reduction. The instrumentation consisted of a mass flow controller, pump, wet gas meter, and oxygen gas sensor. The oxygen gas was successfully measured using the instrumentation in real time. The measured volume of the oxygen gas was comparable to the theoretically calculated volume generated by the charge applied to the simulated oxide fuel.


Introduction
Electrolytic reduction (also called oxide reduction or OR) of solid compounds to solid products in molten salt electrolytes has attracted widespread interest in metallurgy because it is simple, cost-effective, and environmentally friendly [1][2][3].This technique has been applied to the reduction of various metal oxides such as TiO 2 [4], SiO 2 [5] Ta 2 O 5 [6], Fe 2 O 3 [7], SnO 2 [8], Tb 4 O 7 [9], Nb 2 O 5 [10][11][12], Cr 2 O 3 [13,14], and CeO 2 [15].Additionally, it has been employed to reduce spent oxide fuel (mainly consisting of UO 2 ) to its metallic form in pyroprocessing technology for closed nuclear fuel cycles [16][17][18].Pyroprocessing is a high-temperature electrochemical fuel processing technology for recycling the spent oxide fuel from light water reactors into metal fuel for fast nuclear reactors.In the OR process for pyroprocessing, molten LiCl containing Li 2 O is generally used as the electrolyte for the reduction of the spent oxide fuel.The reduction products of the OR process are transferred to an integrated process, electrorefining, as feed materials.U, Pu, and other actinides are recovered by the electrorefining process in a LiCl-KCl eutectic salt electrolyte at 500 ∘ C [19][20][21][22][23][24][25][26].
The electrode reactions in OR processes are well known [16][17][18][27][28][29][30][31][32].The spent oxide fuels are used as cathodes by loading them into a permeable basket.The reactions at the cathode are as follows: Li where M denotes actinides such as uranium and plutonium.Simultaneously, the main reaction at the Pt anode occurs as follows: During electrolysis, the spent oxide fuel is reduced to metal and remains at the cathode.The O 2− ions produced at the cathode are then transported through the salt and discharged at the anode to form O 2 gas.In situ monitoring of the OR process enables efficient operation; its principle purpose is to monitor the performance and efficiency of a reduction cell.In particular, the  extent to which the oxide fuel is converted to metal at the cathode is key information for determining the efficiency of a reduction cell.However, in situ monitoring of the reduction extent in real time has not been realized to date.Alternatively, the cumulative electrical charge applied to the oxide fuel ( applied [C]) which is obtained from the current ( [A] = / [C/s]) and time (t [s]) during electrolysis is typically monitored.If the efficiency of the reduction cell is 100%, then  applied is equal to the theoretical amount of electrical charge ( theoretical ) required to reduce all of the metal oxide to metal.However, since not all of  applied is used to convert oxide to metal and the efficiency of the reduction cell is less than 100%, the fraction of  applied relative to  theoretical is ambiguous.
Here, we test the feasibility of quantitatively measuring O 2 gas as a means of in situ monitoring of the OR process.The advantage of this approach is that it allows monitoring not of the applied quantity but of the produced quantity from OR reactions.For this study, the OR of 0.6 kg of simulated oxide fuel (simfuel) is performed in molten Li 2 O-LiCl salt.The O 2 gas produced at the anode is exhausted and quantitatively measured by means of in situ measurement instrumentation.

Experimental Process
The OR apparatus was used in a high-purity argon (Ar) atmosphere glove box (Figure 1 The electrodes were electrically insulated from the top flange using a ceramic material at the contact area (H in Figure 2(a)).The cathode (F in Figure 2) was used by assembling the Ni conductor container and the rectangular basket (75 mm × 25 mm × 204 mm) made of three-ply layered (20-325-100, Nichidai)-STS wire meshes.Porous simfuel pellets with a tap density of 3.59 g/cm 3 (density of single pellet of 6.89 g/cm 3 , bulk density 62.86%) and a cylindrical shape (6.4 × 6.2 mm) were used by loading the STS wire mesh cathode basket.Its components and composition are listed in Table 1; it was composed of UO 2 , rare earth oxides, noble  metal oxides, and salt-soluble fission products.The simfuel was prepared by mixing of UO 2 and surrogate powders, pelletizing, and sintering.The preparation conditions and methods for simfuel have been described elsewhere [33].
Li-Pb alloy (32 mol% Li) in MgO served as the reference electrode (G in Figure 2 The anode shroud surrounds the Pt anode where O 2 gas is generated; it forms chimneys that contain the O 2 gas and form a pathway for its removal.The anode shroud used in this study consisted of a lower porous shroud for the salt phase and an upper nonporous shroud for the gas phase.Detailed descriptions of the anode and anode shroud have been reported in our previous study [34].As shown in Figure 2  measured O 2 was monitored and recorded in real time using a computer.The O 2 gas concentration in the Ar-filled glove box was carefully observed in order to determine whether O 2 gas leaked out from the OR apparatus or the connected piping.Electrolysis was performed by applying a constant voltage between the cathode and Pt anode using an external power supply (Agilent Model 6671A).The potential between the cathode and the Li-Pb reference electrode was monitored using a multimeter (Agilent, 34405A).After the OR process, the variation of Li 2 O concentration in the molten salt was checked using an autotitrator (G20, Mettler Toledo).A filter composed of a cylindrical polypropylene microfilter with a pore size of 1 m was used to protect the instrumentation from salt aerosols.

Results and Discussion
The constant-voltage electrolysis of 0.6 kg of simfuel was performed in 1.5 wt.% Li 2 O-LiCl.A set of the obtained electrolysis data is presented in Figure 3; it includes the curves of the (a) cell voltage, (b) current, and (c) cathode potential.
The constant voltage measured between the tops of the cathode and the anode was approximately 3.15 V owing to the IR drop, even though 3.35 V was applied by using the power supply (Figure 3(a)).The constant-voltage interruption at appropriate intervals was conducted to prevent excessive formation of Li metal at the cathode.The presence of Li metal at the cathode could be observed through the potential between the cathode and Li-Pb reference electrode at an open circuit potential (i.e., the Li/Li + potential of about −0.6 V).While the constant voltage was applied, the cathode potential was lowered to about −0.78 V (Figure 3(c)) because, as a cyclic voltammetry test in our previous study showed, UO 2 is reduced at potentials more negative than −0.6 V [30,31].A response current of about 40 A was passed by applying the constant voltage (Figure 3(b)).The averaged current density was 0.45 A/cm 2 for an effective anode surface area of 89.2 cm 2 immersed in the Li 2 O-LiCl salt. applied relative to 40% of  theoretical was the applied charge to the simfuel during the 6.3 h electrolysis, which was not sufficient to completely reduce the simfuel.This incomplete electrolysis (low  applied compared to  theoretical ) was inevitable to monitor the O 2 gas using the instrumentation without interruption during the daytime.The evolved gas generated from the OR anode was continuously exhausted and monitored during the electrolysis.Figure 4 shows the cumulative measured volume of O 2 gas ( measured O 2 ) compared with the theoretically calculated volume of O 2 gas ( thoretical O 2 ) generated by the cumulative applied charge,  applied . applied was obtained from the current and time shown in Figure 3(b).The change in  applied showed a step-like pattern; it repeatedly increased and then maintained a constant value because the current flowed during the voltage-on period and did not flow during the voltage-off period. thoretical O 2 was calculated as follows:  where  applied is the electrical charge applied to the simfuel during electrolysis;  is the number of electrons involved in the cathode reaction (1) (known to be four);  is Faraday constant, 96,485 C/mol e − ; and 24.5 is the molar volume of ideal gas (L/mol) at 25 ∘ C. As expected,  thoretical O 2 increased in the same manner as  applied because it was obtained using  applied in (5).The change in  measured O 2 also showed a similar pattern to that of  thoretical O 2 , which is evidence of successful production of O 2 gas by the anode reaction (4).At the beginning of the electrolysis,  measured O 2 did not show a significant increase in spite of the increase in  applied ; it increased after about 10 min.This 10 min interval between  applied and  measured O 2 was repeatedly observed during the electrolysis.This interval indicates the time needed for the O 2 gas generated in the molten Li 2 O-LiCl salt of the OR apparatus installed inside the Ar glove box to pass through the piping and to be measured in the instrument installed outside the glove box.It is notable that the difference between  thoretical O 2 and  measured O 2 gradually increased over the electrolysis period.At the end point of the electrolysis,  measured O 2 of 16 L was approximately 73% of  thoretical O 2 of 22 L that could theoretically be generated by the final value of the cumulative  applied (3.4 × 10 5 C).This result is reasonable given the fact that  applied is not all used to convert the oxide fuel to metal owing to limited current efficiency.In our previous study [34], we conducted the OR process using simfuel under experimental conditions similar to those in the present study.We obtained the reduction product containing U with the metal weight portion of 99% when the OR process with a current efficiency of 67% was conducted.This reliability was confirmed from the observation of the reduction product obtained from the OR run of this study.Some reduction products were randomly sampled from the top of the cathode basket; a cross-section of one of them is shown in the inset image of Figure 5(a).Its surface exhibits a metallic gray appearance unlike the original brown simfuel, which is also evidence of successful reduction during electrolysis.However, the unreduced region in the center of the crosssection remained because  applied (40% of  theoretical ) was not enough for complete reduction.This inhomogeneity of the reduction over the region occurs because reduction of simfuel begins at the surface and progresses toward the center [29].The concentration of Li 2 O (1.5 wt.%) in LiCl measured through titration did not show a change before and after electrolysis, which means that the O 2 gas measured using the instrumentation ( measured O 2 ) was generated from the metal oxide in the simfuel.

Figure 1 :
Figure 1: Setup for the present study: photographs of (a) Ar glove box equipped with the OR apparatus, (b) instrumentation for in situ measurement of O 2 gas generated from the Pt anode during the OR process, and (c) schematic of the flow of O 2 gas from the anode of the OR apparatus to the instrumentation.
(a)) where the concentrations of moisture and O 2 were kept at below 1 ppm and 10 ppm, respectively.Figure 2(a) shows a schematic drawing of the OR apparatus used in the present study.It consists of several components including a furnace (A in Figure 2(a)), outer STS crucible (B in Figure 2(a)), inner STS crucible (C in Figure 2(a)), top flange (D in Figure 2(a)), and electrodes (E, F, and G in Figure 2(a)).The inner STS crucible (165 mm × 165 mm × 250 mm) was used to load the electrolyte.As the OR electrolyte, 5 kg of 1.5 wt.% of Li 2 O (99.5% purity, Alfa Aesar)-LiCl (99% purity, Alfa Aesar) molten salt was used at 650 ∘ C. LiCl was thermally dehydrated before use to remove moisture.The outer STS crucible aimed to protect the furnace in case of failure of the inner STS crucible.The top flange above the crucibles provided ports to suspend the electrodes and also prevented heat loss of the electrolyte.

Figure 2 :
Figure 2: Schematic of the OR apparatus used for this study: (a) outer view (A: furnace, B: outer STS crucible, C: inner STS crucible, D: top flange, E: cathode, F: anode, G: reference electrode, H: electrical insulator, 0: thermocouple, 1: Ar inlet, and 2: bottom) and (b) flow of incoming Ar and evolved gas (Ar + O 2 ) through the top flange.
(a)).The anode module (E in Figure 2(a)) consisted of a Pt anode and a shroud.A Pt plate (40 mm × 3 mm × 200 mm) was used as the anode material.
(b), the evolved gas combination of O 2 and Ar was continuously exhausted using a pump from the upper end of the shroud to the gas outlet; the gas was piped to in situ instrumentation installed outside the Ar glove box.The setup of the O 2 gas monitoring instrumentation is presented in Figures1(b) and 1(c).The instrumentation consisted of a mass flow controller (MFC), pump, wet gas meter, and sensor for O 2 gas.The MFC (Line Tech, m3030v, Korea) was used to control the flow rate of the gas using the connected pump; it was given a set point of 1 L/min.The removal rate of O 2 gas was set to be faster than the generation rate.As shown in Figure2(b), Ar gas was allowed to flow into the top flange in order to constantly maintain the pressure inside of the OR apparatus.The wet gas meter (Shinakawa, W-NK 1B type, Japan) was employed to quantitatively measure the total volume ( tot ) of the evolved gas (a combination of O 2 and Ar gases) in real time.The volume of O 2 ( measured O 2 ) was determined by multiplying  tot by the O 2 concentration fraction ( O 2 ) measured using an oxygen sensor (Advanced Micro Instruments, M65, US).

Figure 4 :Figure 5 :
Figure 4: Comparison between the measured volume of O 2 gas ( measured O 2 ) and theoretically calculated volume of O 2 gas ( thoretical O 2 ) generated by the cumulative applied charge,  applied .

Table 1 :
Composition of the simfuel used in this study.