Modeling Gamma Radiolysis of Nitrate Aqueous Solutions and Analyzing Mechanisms of Hydrogen and Hydrogen Peroxide Formation

The radiolytic behavior of high-concentration nitrate ions ( \textNO_\text3^- ) in high-level liquid waste (HLLW) storage tanks is a critical issue in nuclear waste management, as it directly relates to chemical safety during long-term storage. Radiolysis of aqueous nitrate solutions can lead to the formation of gaseous hydrogen (H₂), posing explosion hazards, and hydrogen peroxide (H₂O₂), which may induce corrosion and affect waste form stability. To systematically investigate the influence of nitrate concentration and absorbed radiation dose on the formation and evolution of H₂ and H₂O₂, a comprehensive kinetic model for the radiolysis of nitrate-containing aqueous systems was developed in this study. The model integrates primary radiolytic yields and a detailed network of radical and molecular reactions, enabling a mechanistic understanding of key species’ behaviors under continuous irradiation conditions. Simulation results indicate that increasing nitrate concentration exerts a strong inhibitory effect on H₂ generation. After 100 h of reaction, the accumulated H₂ concentration in the 1 mol/L \textNO_\text3^- system is only 28.2% of that observed in the 0.1 mol/L \textNO_\text3^- system. Moreover, in the 5 mol/L \textNO_\text3^- system, the H₂ yield further decreases to just 13.3% of that in the 1 mol/L system. This concentration-dependent suppression is attributed primarily to the efficient scavenging of the hydrated electron ( \texte_\textaq^- ) by \textNO_\text3^- , which serves as a key precursor in H₂ formation pathways. In contrast, H₂O₂ concentrations produced from radiolysis exhibit a notably different trend. Across varying nitrate concentrations, H₂O₂ accumulates gradually over time, eventually reaching a dynamic equilibrium at approximately 1.3×10⁻⁴ mol/L, irrespective of the initial \textNO_\text3^- content within the studied range. This suggests that while H₂ generation is highly sensitive to nitrate concentration, H₂O₂ accumulation is governed by a balance between its production and consumption, largely independent of nitrate concentration under these conditions. Further analysis of reaction pathways and rate constants reveals the dual role of \textNO_\text3^- in radiolysis. First, \textNO_\text3^- effectively captures \texte_\textaq^- , thereby inhibiting pathways leading to H₂ formation. Second, \textNO_\text3^- and its radiolytic derivatives, such as ·NO₂ and \textNO_\text2^- , participate in reactions that compete with H₂O₂ for consumption by key radicals, including \texte_\textaq^- , hydrogen atoms (·H), and hydroxyl radicals (·OH). By intercepting these species, \textNO_\text3^- and its reaction network suppress the decomposition of H₂O₂ and promote its net accumulation. This study provides mechanistic insights into the radiolytic behavior of nitrate-rich wastes, highlighting that high nitrate levels can significantly mitigate hydrogen explosion risks while potentially sustaining steady-state concentrations of oxidizing species like H₂O₂. The findings offer theoretical support for safety assessments of HLLW storage, informing strategies for concentration control and corrosion management in nuclear waste repositories.