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The in-situ quantification for the reduction of Eu(III) was performed in the LiCl-KCl eutectic at various temperatures by time-resolved laser fluorescence spectroscopy (TRLFS) and UV-Vis absorption spectroscopy. From the results of absorption measurement, Eu(III) virtually underwent complete reduction to Eu(II) under the present experimental conditions, while TRLFS revealed a relatively greater reducing power of Eu(III) in the chloride molten eutectic with decreasing its concentration and increasing temperature. However, it was convincingly identified that fluorescence measurements deliver more reliable quantification, compared with the UV-Vis absorption measurements, showing a mutually hindered overlap of absorption bands of Eu(II) and Eu(III).The molten chloride salt is one of the ionic liquid solvents used in the electrorefining and electrowinning of pyrochemical process for recycling used nuclear fuel. Over the past decade, thermodynamic and electrochemical behaviors of actinides and lanthanides in molten chloride salts have been investigated to understand fundamental knowledge and reaction mechanism. 1 An efficient separation of some lanthanides is essential due to their high neutron capture crosssection. Particularly, europium is an attractive and motivating element among lanthanides because of its presence in the multiple oxidation state. Although most lanthanides exist in the trivalent valence state in molten salts, europium can be present in the mixed valence states of Eu(II) and Eu(III) which are likely to cause circulating current in the electrolysis process. 2,3 For that reason, the efficiency of electrolysis during electrorefining and electrowinning process is undoubtedly affected by circulation current arising from alternate reduction and redox reactions at the electrode.The thermodynamic properties of europium ions in molten alkali chloride salt have been studied by electrochemical methods. 3-8 The qualitative and quantitative analysis of reduction of europium from Eu(III) to Eu(II) has been performed in molten alkali chloride salts by electrochemical method, 9,10 absorption spectroscopy, 10 and electron paramagnetic resonance (EPR) technique. 11 Nevertheless, there has been still a relatively large discrepancy of the reducing capacity of europium in chloride molten salts reported in literatures. Kim et al. 10 reported that around 20% of Eu(III) was reduced to Eu(II) at 5.4 × 10 −4 M by absorption spectroscopy and electrochemical analysis in high temperature, and Park et al. 11 observed that approximately 50% of Eu(III) was reduced to Eu(II) measured by EPR technique at room temperature after solidification of molten salt samples. This divergent result requires the reliable technique for in situ quantitative analysis of reduction of trivalent europium. Recently, several groups have measured the fluorescence spectrum of Eu(II) in LiCl-KCl eutectic at various temperatures, 11-13 whereas the fluorescence spectrum of Eu(III) have not been observed in high-temperature LiCl-KCl eutectic. The fluorescen...

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