Effect of oxide impurities on the dissolution behavior of Th 4+ , Be 2+ and U 4+ in fluoride salts.

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.