Boron-containing deposits with tritium activity in the primary circuit of HTR-10: experimental measurements and source analysis

The chemical environment of the primary coolant in high-temperature gas-cooled reactors (HTGRs) is complex, where the sources, chemical states, and migration behaviors of impurities affect safe reactor operation. In this study, white crystalline deposits with a mass of (881.70 ± 0.03) mg collected from the dust filter of HTR-10 were analyzed. The morphology of the deposits was analyzed by scanning electron microscopy, while the boron content was analyzed by energy-dispersive X-ray spectroscopy and inductively coupled plasma mass spectrometry, yielding a boron mass fraction of (19.55 ± 0.24)%. Using X-ray diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy, the samples were found to be composed of NH₄B₅O₈·4H₂O, (NH₄)₃[B₁₅O₂₀(OH)₈]·4H₂O, and H₃BO₃. The thermal behavior of the crystalline deposits was examined by thermal analysis. Based on liquid scintillation counting and high-purity germanium gamma-ray spectrometry, the mass-specific activities of 3 H and 137 Cs, decay-corrected to the date of filter cutting (3 December 2013), were estimated to be (791.55 ± 21.40) and (0.0506 ± 0.0055) Bq/mg, respectively. Based on the structural materials and operating conditions of HTR-10, the boron in the deposits was inferred to originate mainly from boron-containing graphite bricks in the core and boron carbide absorber spheres. The ammonium ions may originate from hydrazine or its decomposition products in the secondary circuit, which could enter the primary circuit through trace cross-wall migration or the decomposition of nitrogen-containing chemicals introduced during maintenance or cleaning. The results reveal the phase characteristics, migration, and deposition behavior of boron-containing impurities in the primary circuit and demonstrate, for the first time, their significant tritium enrichment. This study provides useful guidance for impurity source tracing, coolant chemistry monitoring, purification strategy optimization, and radiation protection during maintenance and decommissioning of HTGRs.