ACTINIDE CHEMISTRY AND REPOSITORY SCIENCE PROGRAM

LA-UR-05-9479

Donald T. Reed, Marian Borkowski, Michael K. Richmann, and
Jean-Francois Lucchini
January, 2006

The Waste Isolation Pilot Plant, WIPP, a cornerstone of the DOE' s cleanup effort, is designed to permanently dispose of defense-generated transuranic radioactive waste from research and production activities in the DOE weapons complex. The WIPP is located in southeastern New Mexico, 26 miles east of Carlsbad. Waste disposal operations began on March 26, 1999.

WIPP Site near Carlsbad, NM

Disposal rooms are excavated in an ancient, stable salt formation 2,150 feet (almost one-half mile) underground. Transuranic waste, which consists of clothing, tools, rags, debris and other disposable items contaminated with radioactive elements, mostly plutonium, are emplaced in 55 gallon steel drums for permanent disposal.

Transuranic waste is shipped to the WIPP from many generator and storage sites such as Savannah River Site, Oak Ridge, Argonne, Rocky Flat, Idaho, Nevada Test Site, Livermore and Los Alamos. All shipments are done using specially designed TRUPAC-II containers.

TRUPACT-II Shipment Approaching the WIPP Site

The Actinide Chemistry and Repository Science Program (ACRSP) is part of a larger multi-disciplinary Los Alamos group (EES-12) working in Carlsbad.  We conduct our laboratory research programs at the Carlsbad Environmental Monitoring and Research Center, an institute that is operated by New Mexico State University.  Our most important purpose is to provide laboratory research and program support on actinide-related issues to the DOE Carlsbad Field Office to maintain the EPA certification of the WIPP.  WIPP’s opening started the clock for the periodic recertification required every five years until the repository is closed. The first Compliance Recertification Application (CRA) was submitted to the EPA in March 2004. The CRA includes an up-to-date performance assessment (PA), which considers new and changed information since the first certification. Performance assessment calculates probabilistic estimates of the long-term performance of the waste disposal system. Although our primary function is to support the WIPP project, we have the capability to conduct research programs in actinide environmental chemistry for other sponsors. 

Carlsbad Environmental Monitoring and Research Center

Actinide Solubility Studies - Experimental Approach

Key Experimental Parameters

• pH between 5 and 12
• Carbonate concentrations up to 0.01 M
• Brine composition (see table below)
• Temperature of 25oC to 30oC
• Speciation (oxidation state and complexation)
• Effects of radiolysis
• Interactions with waste components (Fe, MgO, Al, Ni, Organics)

Solubility Experiments

• Solubility is approached from both under-saturation and over-saturation under controlled environmental conditions and in the presence/absence of key waste components
• In over-saturation experiments, specific oxidation states of the actinide or analog are prepared and added sequentially to simulated WIPP brine until a steady-state concentration is achieved
• In under-saturation experiments, the expected/predicted solid phases of the actinide or analog are prepared and contacted with simulated brine under controlled conditions until a steady state concentration is achieved
• The results of both approaches, in light of analogous modeling studies and calculations, are combined to establish the likely solubility under the expected subsurface conditions.

Analytical Techniques

• Total solution concentrations are determined by a combination of ICP-MS, ion chromatography, and liquid scintillation counting methods (when applicable) as a function of solution filtration
• Where feasible, absorption spectrometry or high-sensitivity laser photoacoustic spectroscopy will be used to establish the oxidation state and complex present in solution
• Solids will be characterized by XRD, SEM and TEM.  XANES/EXAFS analysis will be performed on selected samples to confirm average oxidation state and establish the near-neighbor structure, by comparison to well-characterized references, of the environmentally relevant precipitates obtained.

GWB - Generic Weep Brine

ERDA-6 - Energy Research and Development Administration Well 6

Technique Development:

pH Measurement in Brine Systems

The extension of commonly used analytical techniques and approaches to brine systems is often problematic and not straightforward. Significant effort is spent to confirm and develop experimental protocols that work for brine systems. For example the measurement of hydrogen ion concentration (pH), which is a critical parameter for actinide solubility in geochemical systems, is made difficult by the high ionic strength and buffer capacity of the brines used. In concentrated brines, variations in activity coefficients, the formation of species such as HSO4- and H2B4O7 that can consume protons during electrode standardization procedures, and potentially large junction potentials all add to the difficulty in the pH measurement.

The Gran-type titrations shown above were done with HCl with/or without addition of NaOH. Extra precautions were required to determine pH shifts for electrolytes that react with H+ or OH- [e.g., sulfate ions and magnesium brines for the precipitation of Mg(OH)2]. These titrations, performed in all the brines investigated, were used to establish a correction factor (K) for the specific pH electrode and brine according to the following general equation:

pcH = pH read + K

Neodymium (III) Solubility in Brine

Neodymium is a redox-invariant analog for plutonium (III) and americium (III), which are key contaminants in the WIPP. Its solubility in simulated WIPP brine will confirm current assumptions in WIPP PA that were based on measurements in simplified systems. The following research question is being addressed:

• What is the long –term solubility of Nd (III) and, by analogy Am (III) and Pu (III) in WIPP brine?

Initial Results for Neodymium Solubility in WIPP Brine

Uranium (VI) Solubility/Stability in Brine

Research Goals and Questions:

The most important goal of the uranium experiments is to determine the effective solubility of U(VI) under conditions that simulate the expected environment in the WIPP. U(VI) is also an analog for Pu (VI), which is the more important actinide that can exist in the +6 oxidation state in the WIPP. These experiments are centered on two scientific issues: 

1)  What are the conditions and processes in the WIPP that lead to

     the reduction of U(VI) to form U(IV) species?

2)  What is the long-term solubility of U(VI) in WIPP brine?

Research Tasks:

Task 1:     Solubility of U(VI) in WIPP Brine

Task 2:     Redox Stability of U(VI) in WIPP Brine

Task 3:     Effect of Radiolytic Products on Uranium Speciation

Experimental Results:

Effect of pH on the solubility of U(VI) in ERDA-6 brine. 

These data show that the effect of higher pH, at least initially, is to significantly lower uranium solubility.

Concentration of U(VI), based on ICP-MS analysis, in ERDA-6 Brine as a function of time in the presence of zero-valent iron under anoxic conditions. 

The apparent solubility observed (~ 10-6 M), is consistent with literature values for U(VI) in the absence of carbonate and suggest that no reduction is occurring under the conditions of the experiment.

Plutonium Solubility and Speciation in the WIPP

Research Goals and Questions:

The speciation and potential mobility of plutonium in the WIPP remains the most important concern in WIPP PA.  Plutonium, which is a multivalent high-activity actinide with a very complex environmental chemistry, is the key contaminant in TRU waste currently being emplaced in the WIPP.

For plutonium, the most important research questions are:

1)         What is the expected speciation (i.e. oxidation state

            distribution, complexation, and degree of aggregation

            and association) of plutonium in the WIPP?

2)         Will localized oxidizing zones, perhaps generated

            radiolytically, impact the overall potential release of

            plutonium by generating and stabilizing the more

            soluble higher valent plutonium oxidation states (e.g.,

            PuO2+ and PuO22+ species)?

3)         What is the extent of conservatism in the current WIPP

            assumptions on plutonium speciation and solubility?

4)         Are there simple design changes to the WIPP that will

            lead to greater immobilization of plutonium

            at a significant gain in cost benefit?

Research Tasks:

• Resolution of literature discrepancy regarding the

            reduction of Pu(VI) by Fe powder and coupons

• Technical assessment of the current WIPP PA

            database on Pu

• Prevalence of Pu(III) in the WIPP
• Solubility of Pu(IV) in the WIPP
• Reduction of Pu(V) and Pu(VI) in the WIPP
• Speciation of Plutonium in the WIPP

These tasks outline a multi-year research project to address, at least to a limited extent, current regulatory issues centered on plutonium speciation in the WIPP

Experimental Results

The absorption spectrum of Pu(VI) in GWB brine.  There is no evidence for the Pu(VI) carbonate species (~ 812 nm peak) indicating that this absorption corresponds to a hydrolyzed plutonium species in the brine.

Plutonium (VI), once added to brine has been shown to be stable in brine under anoxic conditions and over a wide range of pH.  In the presence of iron, shown below, the Pu(VI) is reduced to form Pu(IV).

Interactions Between Shewanella Alga and Plutonium Species

Research Goals and Objectives

A concurrent experimental and modeling study centered on the interactions of Shewanella alga BrY with plutonium species and phases is being conducted. The goal of this research is to investigate the long-term stability of bio-precipitated “immobilized” plutonium phases under changing redox conditions in biologically active systems. The longevity of the subsurface immobilization of plutonium (e.g., by bio-reduction) is a key consideration in the effectiveness of remediation/containment approaches used, because it affects the design/choice of immobilization approaches, and defines issues regarding the closure of contaminated sites (e.g., natural attenuation). Plutonium is the key contaminant of concern at several DOE sites that are being addressed by the overall Natural and Accelerated Bioremediation Research (NABIR) program.

Bio-precipitation of Np, initially added as Np(V), from solution under anaerobic conditions with S. alga, a metal reducing bacteria.   Data show a correlation with precipitation and biological activity suggesting that bio-reduction is occurring. 

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