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

Abstract

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 Th⁴⁺, U⁴⁺, and Be²⁺ with O²⁻ in the 2LiF–BeF₂ (FLiBe) eutectic salt system. The results showed that the solubility of UO₂ was 5.52 × 10⁻³ mol kg⁻¹, and the solubility product (K_sp) of UO₂ was 6.14 × 10⁻⁷ mol³ kg⁻³ in FLiBe salt at 650 °C. It was also found that the O²⁻ ion would firstly react with U⁴⁺ to form UO₂, and then the excessive O²⁻ would react with Be²⁺ to generate BeO in the FLiBe system. Despite conducting the solubility experiment of ThO₂ and titration experiment of FLiBe–ThF₄, the system failed to achieve the solubility and the K_sp of ThO₂. The main reason for this was that O²⁻ preferentially reacted with Be²⁺ over Th⁴⁺ to form precipitates, in other words, Be²⁺ exerted a protective effect against Th⁴⁺. Above all, this work experimentally demonstrated that in the FLiBe system, O²⁻ preferentially combines with U⁴⁺ to form a precipitate, followed by Be²⁺, while Th⁴⁺ is relatively inert.