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

Abstract

Neutron activation of 93Nb in structural materials of nuclear reactors generates the radioactive, β‒‒γ 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 ≈ 13.5, as well as in systems at pH ≈ 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 ≈ 10–11. This work provides an improved scientific basis to assess solid phase formation and solubility behavior of Nb(V) in cementitious systems relevant for nuclear waste disposal.