Solubility of niobium(V) in cementitious systems relevant for nuclear waste disposal: Characterization of the solubility-controlling solid phases

Neutron activation of 93Nb in structural materials of nuclear reactors generates the radioactive, beta--gamma emitting 94Nb isotope (t1/2= 2.04.104 a). Waste streams containing 94Nb are disposed in repositories for low-and intermediate-level waste, which are characterized by the extensive use of cementitious materials. The chemical behavior of niobium in these systems remains ill-defined, especially with respect to the solid phases controlling its solubility. Oversaturation solubility experiments in synthetic solutions representative of cementitious pore water (10 < pH < 12.5) show the precipitation of Nb, with [Nb] < 10-7 M at equilibrium. The increase in solubility observed at pH > 12.5 is attributed to negatively charged hydrolysis species stabilized by Ca. Solids controlling Nb solubility were characterized by a multi-method approach. XRD, Raman and quantitative chemical analysis support the predominance of cubic pyrochlore Ca2Nb2O7(cr) at pH & AP;13.5, as well as in systems at pH & AP;12 and high [Ca]. The incorporation of K in the pyrochlore structure was observed in pore water systems containing potassium, whereas CaNb2O6(cr) plays a predominant role at pH & AP;10-11. This work pro-vides an improved scientific basis to assess solid phase formation and solubility behavior of Nb(V) in cementi-tious systems relevant for nuclear waste disposal.

Automated summary

This study investigates the behavior of the radioactive isotope 94Nb generated in structural materials of nuclear reactors. The research reveals that the solubility of niobium is influenced by the solid phases in the process of disposing 94Nb waste using cementitious materials. The experimental results demonstrate the impact of pH value and calcium concentration on the solubility of niobium in simulated pore water.