Phase analysis of the solidified KF–(LiF–NaF–UF4)–ZrF4 molten electrolytes for the electrowinning of uranium

Effect of oxide impurities on the dissolution behavior of Th4+, Be2+ and U4+ in fluoride salts

“…Previous studies have shown that UF 4 in the fuel salt readily reacts with O 2− ions to form uranium oxide precipitates, as shown in eqn (2). 20,21 The formation of uranium precipitates would reduce the concentration of U 4+ and cause the loss of fuel, what's more, the large accumulation of UO 2 precipitates on the structural metal surface of the primary circuit would create the overheated areas in the reactor, posing a potential threat to the reactor operation.H 2 O + 2F − = 2HF + O 2− UF 4 + 2O 2− = UO 2 ↓ + 4F − …”

“…This property, combined with its small neutron absorption cross-section (thermal neutron absorption cross-section of 0.18 bar), has even led several researchers to advocate ZrF 4 as an ideal oxygen binding additive to prevent the precipitation of UO 2 in the fuel salt. Korenko 21 found that the introduction of oxide in the LiF–NaF–KF–UF 4 –ZrF 4 system can generate ZrO 2 precipitation, which effectively consumed the O 2− in the molten salt and prevented the precipitation of UO 2 . Studies of the LiF–BeF 2 –ZrF 4 –UF 4 system by Peng 26 showed that ZrO 2 formed initially as an O 2− addition of less than 1 mol kg −1 , with UO 2 and ZrO 2 co-precipitating at O 2− additions greater than 1 mol kg −1 .…”

Interactions between Oxide and LiF-BeF2-ZrF4-UF4 System through Electrochemical Techniques

“…Shen 13 found that the dissolution of Cr 2 O 3 and UO 2 in LiF-NaF-KF melt could be facilitated by forming some ZrO x F y species. Korenko 14 discovered that ZrO 2 instead of UO 2 forms in KF-LiF-NaF-UF 4 -ZrF 4 , whereas Romberger 15 confirmed that UO 2 and ZrO 2 could co-exist in molten-salt solution. Hence, adding ZrF 4 into LiF-BeF 2 -UF 4 is a feasible option to avoid UO 2 precipitation.…”

Inhibition effect of ZrF4 on UO2 precipitation in the LiF–BeF2 molten salt

“…Besides,Gibilaro 18 has found that the addition of calcium oxide (CaO) into a ZrF 4 -containing lithium uoride-calcium uoride (LiF-CaF 2 ) salt leads to the formation of a close to equimolar mixture of solid ZrO 2 and ZrO 1.3 F 1.4 . Korenko et al19 and our previous studies7,8,16,20 have used X-ray diffraction (XRD) phase analysis and electrochemical techniques and discovered the ZrO 2 formation when oxide contamination occurs in the ZrF 4 -containing systems (i.e., LiF-NaF-KF-ZrF 4 and LiF-NaF-KF-UF 4 -ZrF 4 ), whereas only UO 2 is observed in the systems without ZrF 4 (i.e., LiF-NaF-KF-UF 4 and LiF-BeF 2 -UF 4 ). These studies demonstrate that O 2À in zirconium-based systems can be reduced by forming Zr(IV) oxide or oxyuorides, preventing the UO 2 precipitation.…”