Physicochemical and Gamma-Spectrometric Characterization of Legacy Liquid Radioactive Waste from the BN-350 Reactor Facility

Legacy liquid radioactive waste (LRW) from the BN-350 sodium-cooled fast reactor is chemically heterogeneous and requires updated characterization for further management. This study investigated the physicochemical properties and gamma-emitting radionuclide composition of 13 LRW samples collected from four tanks (B-02/1, B-02/2, B-02/5, and B-02/6) at surface (“mirror”), middle, and lower levels. Five samples were analyzed as LRW and eight as evaporated LRW residues. Physicochemical analysis included pH, density, dry residue, alkalinity, and major inorganic components. Portable gamma spectrometry was applied to all samples, and laboratory HPGe measurements were performed for selected aqueous LRW samples. The saline LRW samples were strongly alkaline and highly mineralized, with dry residue values of 139.30–295.10 g/dm3. Tank B-02/6 contained distinct oil-containing, emulsion, and carbonate-rich alkaline aqueous phases. 137Cs was the dominant identified gamma-emitting radionuclide, with specific activities ranging from (1.4 ± 0.3) × 105 to (1.1 ± 0.2) × 108 Bq/kg in liquid samples and up to (6.5 ± 1.3) × 108 Bq/kg in evaporated residues. The results show that BN-350 LRW is not a homogeneous waste stream; therefore, future monitoring, retrieval, treatment, and conditioning should be planned according to matrix type, sampling depth, vertical phase heterogeneity, and 137Cs-dominated radiological characteristics.