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Oxides are one of the most important impurities in the fuel salt of molten salt reactors (MSRs), and excessive oxide impurities pose a risk to the safe operation of MSRs. This study focused on investigating the precipitation behavior between Th 4+ , U 4+ , and Be 2+ with O 2- in the 2LiF-BeF 2 (FLiBe) eutectic salt system. The results showed that the solubility of UO 2 was 5.52 × 10 -3 mol kg -1 , and the solubility product ( K sp ) of UO 2 was 6.14 × 10 -7 mol 3 kg -3 in FLiBe salt at 650 °C. It was also found that the O 2- ion would firstly react with U 4+ to form UO 2 , and then the excessive O 2- would react with Be 2+ to generate BeO in the FLiBe system. Despite conducting the solubility experiment of ThO 2 and titration experiment of FLiBe-ThF 4 , the system failed to achieve the solubility and the K sp of ThO 2 . The main reason for this was that O 2- preferentially reacted with Be 2+ over Th 4+ to form precipitates, in other words, Be 2+ exerted a protective effect against Th 4+ . Above all, this work experimentally demonstrated that in the FLiBe system, O 2- preferentially combines with U 4+ to form a precipitate, followed by Be 2+ , while Th 4+ is relatively inert., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The low-pressure distillation of FLiBe salt containing ThF 4 was carried out at 1223 K and <10 Pa using thermogravimetric equipment. The weight loss curve indicated a rapid distillation stage at the beginning of distillation, followed by a slow stage. The composition and structure analyses showed that the rapid distillation process originated from the evaporation of LiF and BeF 2 , while the slow distillation process was mainly attributed to the evaporation of ThF 4 and LiF complexes. Precipitation-distillation coupled method was employed for the recovery of FLiBe carrier salt. XRD analysis indicated that ThO 2 was formed and remained in the residue with the addition of BeO. Our results showed that the combination of precipitation and distillation treatment was an effective way to recover carrier salt., Competing Interests: We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work., (This journal is © The Royal Society of Chemistry.)

The evaporation behaviours of various components were investigated by using a low pressure distillation method in a 2LiF-BeF 2 -ZrF 4 mixture containing irradiated ThF 4 and UF 4 . The experiment showed that BeF 2 and ZrF 4 were found to mainly condensate at the outer cover, the coolest zone, and their relative volatilities vs. LiF were 9.8 and 32.2, respectively, while for ThF 4 and UF 4, at four different temperature zones the values were almost constant, at 0.1 and 0.3. The radioactivity of various nuclides was further detected using gamma spectrometer analysis. 137 Cs was hardly observed due to long half-time decay. 233 Pa was found to co-evaporate with the carrier salt, while 239 Np mainly remained in the residual salt as 237 U. In different temperature zones, the decontamination factors of rare earth in receiver salts ranged from 10 to 10 3 . On the basis of the investigation, it was proposed that the distribution of various nuclides after distillation, may be helpful to design the feasible condensate system to recover the carried salt in a molten salt reactor., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this study, the behavior of fission product iodine released from the melting process of a mixture consisting of UF 4 irradiated with neutrons and 2LiF-BeF 2 (FLiBe) salt was studied. The experiment showed that a large amount of iodine was released immediately during melting and captured by Ni metal foils. The transient release of iodine observed in this experiment is attributed to the redox reaction between the hot atoms of the fission product iodine that cumulated due to long-time irradiation. The effect of the redox status of the molten salt on the transient release of iodine was also investigated. Based on this investigation, it was proposed that the activity ratios of 131 I to salt-seeking fission products in the fuel salt, as an effective diagnostic criterion, may be used for the surveillance of the redox potential of fuel salts in a molten salt reactor., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)