The Aqueous Geochemistry of Neptunium: Dynamic Control of Soluble Concentrations with Applications to Nuclear Waste Disposal

The valence state of neptunium, one of the most important radionuclides of concern for long-term emplacement of nuclear waste, primarily defines its geochemical reactions and migration behavior. We evaluate how redox potential and solid-phase stability interact and influence neptunium solubility and aqueous speciation in natural systems. Neptunium thermodynamic data for the most important valence states for natural waters, +IV and +V, are updated to correct database inconsistencies. The most significant changes are as follows: (1) Np 2 O 5 (cr) is 2 orders of magnitude more stable than reported previously, (2) the stability of NpO 2 OH(aq) is reduced, (3) NpO 2 (OH) 2 - and mixed Np(V) hydroxo-carbonato species become important at high pH, and (4) Np(OH) 5 - is disregarded as a valid species. As a result, Np 2 O 5 and Np(OH) 4 (am) are the stable solids in aquifers of low ionic strength, neptunium solubility decreases in the pH range 10−12 and increases at pH above 12, and both redox potential and Np(OH) 4 (am) solubility product control soluble neptunium concentrations at neutral pH and Eh between −0.2 and 0.3. These relationships are important for effective nuclear waste package design, such as including cement as an engineered barrier and evaluating impacts of discharged solutions on natural waters in release scenarios at nuclear waste storage facilities.