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

Abstract

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) Np2O5(cr) is 2 orders of magnitude more stable than reported previously, (2) the stability of NpO2OH(aq) is reduced, (3) NpO2(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, Np2O5 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.

Effect of Carbonate on the Solubility of Neptunium in Natural Granitic Groundwater

“…In general, a relatively small amount of neptunium (Np) is contained in HLW, including spent nuclear fuels. However, the amount of neptunium can build up over a prolonged discharge time through the α-decay of 241 Am (T1/2 = 432.7 yrs) [1]. Due to its long half-life ( 237 Np: T1/2 = 2.14 10 -6 yrs) and a high radiotoxicity, neptunium is an important actinide in the safety assessment of nuclear waste disposal [2].…”

Environmental Geochemistry of Radioactive Contamination

“…Np(IV) aqueous species may be important under reducing conditions possible at some underground nuclear waste research facilities such as the WIPP and Stripa. Kaszuba and Runde (1999) compiled thermodynamic data for neptunium relevant to Yucca Mountain. They updated the database of Lemire (1984) with recent experimental data and used the Specific Interaction Theory (S.I.T.)…”

“…Their report has an extensive reference list and list of interaction parameters. Kaszuba and Runde (1999) used the EQ3NR (Wolery, 1992b) and the Geochemist's Workbench (Bethke, 1998) codes to calculate solubility and speciation in the J-13 and UE25p#1 well waters that span the expected geochemical conditions for the proposed HLW repository at Yucca Mountain. They predicted that Np(OH) 4(aq) is the dominant aqueous complex in neutral solutions at Eh < 0 mV, while under oxidizing conditions, NpO 2 + and NpO 2 CO 3 -are predominant at pH < 8 and pH 8-13, respectively.…”

“…Although the calculations of Kaszuba and Runde (1999) Neptunium is expected to be present in the WIPP in either the IV or V oxidation state. For the WIPP CCA speciation and solubility calculations, an upper bound for the solubility of Np(IV) was estimated from that of Th(IV), using the oxidation state analogy.…”

Pu(IV) Intrinsic Colloid Stability in the Presence of Montmorillonite at 25 & 80 C

“…The large volumes of plutonium (Pu) designed for storage in high-level nuclear waste repositories are predicted to be a dominant long-term dose contributor under certain scenarios (Kaszuba and Runde, 1999; Office of Civilian Radioactive Waste Management, 2002). Once it is deposited in the environment, Pu has been shown to migrate in the subsurface associated the colloidal fraction of the groundwater (e.g., Kersting et al, 1999).…”