OCRWM Science and Technology Program
Source Term Targeted Thrust
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Background
The U.S. Department of Energy’s (DOE’s)
Office of Civilian Radioactive Waste Management (OCRWM) Science and Technology
Program is sponsoring technical work to assess the ability of SNF and
other waste forms to retain radionuclides over time. Eventually, in the
presence of humid air corrosion, seepage ground waters, and ambient dust,
the waste forms will degrade and certain radionuclides are potentially
released and mobilized in the near field. Technical studies that place
bounds on these release behaviors include flow-through tests conducted
at the Pacific Northwest National Laboratory (PNNL) and the hydrologically
unsaturated flow tests conducted at Argonne National Laboratory (ANL).
The degradation model developed from these data assumes that SNF dissolves
at the maximum or “forward” dissolution rate of the solid
matrix in contact with water.
Hypotheses of mechanisms that would predict lower radionuclide
releases include: (1) the retention of certain radionuclides co-precipitated
in uranium oxide alteration phases; (2) the retention of certain radionuclides
absorbed onto corrosion products from waste package internals; (3) dissolution
of certain radionuclides from the metallic phases within SNF that is slower
than that of matrix dissolution; (4) reduced effective surface area and
water flux involved in the dissolution processes; and (5) an enhanced
understanding of the thermodynamics of surface complexation reactions
at the temperatures expected in the repository.
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The source term is the initial limit on radionuclide release in the Yucca Mountain
repository. In the Total System Performance Assessment (TSPA), the degradation
of commercial spent nuclear fuel (SNF) is modeled using conservative bounding
assumptions about the environment and material performance. This model serves
the licensing requirements well; however, compelling evidence suggests that
the current model predicts faster radionuclide release than is realistic for
repository-relevant conditions.
The goal of the Source Term Targeted Thrust is to reduce conservatisms
in future repository performance assessments, while preserving public
health and safety, through enhanced understanding of waste form degradation
and radionuclide release. The technical work for this program is guided
by a Source Term Science Plan that identifies the most critical degradation
and release mechanisms and defines the approaches to understanding and
modeling them. The program is also fostering collaborations with foreign
researchers for cooperative intellectual exchange, including data, modeling
concepts, and expertise.
The Source Term Targeted Thrust will focus primarily on the mechanisms
for release of key radionuclides from the source term, specifically, Tc,
I, Se, Np, Pu, Am and U.
Chemical and Coordination Structure of Radionuclides: Understanding
Release Pathways
(Argonne National Laboratory, Jeff Fortner, PI)
This project is exploring how solid state chemistry affects the release
kinetics of key radionuclides (Tc, Np, and Pu) from SNF. Experiments are
providing definitive information about the partitioning of the key radionuclides
between grain boundary, fuel/cladding gap, and intragranular disposition
and to determine the impact of these phenomena on release behavior during
corrosion. Synchrotron x-ray spectroscopy is used to explore the oxidation
states and near-neighbor coordination environments by x-ray absorption
near edge spectroscopy (XANES) and by extended x-ray absorption fine structure
(EXAFS) methods, respectively.
Implications of Deliquescence and Decay Heat on Source Term Degradation
(Argonne National Laboratory, Jim Jerden, PI)
Enhanced understanding of the deliquescence of hygroscopic phases is important
to the development of mechanistic-based models of SNF degradation. If
an SNF mineral assemblage is not deliquescent under repository conditions,
the onset of aqueous corrosion will be delayed by a “self-drying”
process involving radioactive decay heat. If, on the other hand, the SNF
mineral assemblage is deliquescent, the amount and composition of the
aqueous film contacting the fuel will be controlled by the deliquescence
properties of the salts at the fuel surface. The deliquescent process
could maintain corrosive aqueous brine on the fuel surface. Early results,
however, suggest that this corrosive process may be self-limiting in that
U(VI) minerals that form will sequester deliquescent components. Applied
research is establishing the deliquescence relative humidity threshold
for the primary SNF assemblage at relevant temperatures and is leading
to a predictive understanding of how SNF corrodes in the brines that may
form from deliquescence.
Spent Fuel Dissolution Mechanisms and Rates
(Pacific Northwest National Laboratory, Brady Hansen, PI)
Currently in the TSPA, the waste form degradation and release are modeled
conservatively assuming that the waste form dissolves at the same rate
as if exposed to fully saturated conditions. However, more favorable,
hydrologically unsaturated conditions are expected to persist over thousands
of years. Applied research is being conducted to quantify critical parameters
for SNF dissolution under low-water and/or humid air conditions, including
water-film thickness, effective surface area, and radiolysis. Experimental,
analytical, and theoretical efforts are underway to develop defensible
models for thin-film reactivity and vapor-phase corrosion in future analyses.
Actinide Thermodynamics at Elevated Temperatures
(Pacific Northwest National Laboratory, Judah Friese, PI)
The dissolution of radionuclides in SNF is governed by surface complexation
mechanisms. The current model for these processes relies conservatively
on thermodynamic data collected, with few exceptions, at 25°C, and
extrapolated to the higher temperatures anticipated at the fuel surface.
This project will use potentiometry, solvent extraction, and spectrophotometric
and nuclear magnetic resonance measurements to collect high-temperature
thermodynamic data on uranium, neptunium, plutonium, and americium complexes
with selected inorganic ligands (e.g., carbonate, fluoride, sulfate, silicate,
and chloride). Data from this project may be used to justify reduced conservatism
in future analyses, while preserving public health and safety.
In-Package Sequestration of Radionuclides at Yucca Mountain
(Sandia National Laboratories, Pat Brady, PI)
As the waste package internals corrode, they will form metal oxides that
may sorb many radionuclides irreversibly. These same oxides have been
proposed as backfill radionuclide absorbers “getters” at low-level
radioactive waste repositories; some are presently being used to clean
up radioactive wastes in near-surface environments. Although expected
to occur, the reduction and sorption of Tc onto metallic iron in corroding
waste packages and the irreversible uptake of Np and Pu on iron oxides
are conservatively not credited in current safety analyses. In this multi-laboratory
project, the investigators are developing models to establish the nature
and extent of reductive Tc sorption that can be expected on the corrosion
products and the permanence of Tc sequestration. Parallel experiments
are being conducted to develop a model that accurately predicts the irreversible
uptake of Np and Pu by corrosion products. Evidence from natural analogues
will be used to corroborate these models over the long time spans relevant
to the repository.
Impact of Uranyl Alteration Phases of Spent Fuel on Mobility
of Np and Pu
(Notre Dame University, Peter Burns, PI)
Previous studies have demonstrated that uranyl phases formed during the
alteration of SNF can incorporate various radionuclides, thereby having
a potentially profound impact upon their mobility, perhaps improving modeled
repository performance compared to current analysis methods. Studies of
natural analogues have clearly established that these uranyl phases may
persist for hundreds of thousands of years. Applied research for this
project is exploring the uptake and retention of Np by uranyl alteration
phases as a function of crystal structure, pH, temperature, time, and
counter-ions present in solution. The stabilities, structures, and chemistries
of uranyl peroxides and the extent to which these phases may incorporate
Np under repository relevant conditions are also being investigated, and
thermodynamic properties of relevant uranyl phases are being measured.
Corrosion of Spent Nuclear Fuel: The Long-Term Assessment
(University of Michigan, Rod Ewing, PI)
This research program is a broad based effort, including natural analogue
studies, to understand the long-term behavior of SNF and its alteration
products in a geologic repository. SNF corrosion is being examined on
the basis of the interactions of molecular water with UO2 surfaces. The
radiation effects of alteration phase evolution are being probed. New
techniques, exploiting high resolution transmission electron microscopy,
are being developed to characterize, on the nano-scale, low concentrations
of radionuclides in the fuel matrix and alteration phases. These techniques
and detailed mineralogical, chemical, and isotopic analyses are being
applied to samples from the Oklo and Okelobondo natural fission reactors
to provide additional data on the migration of radionuclides.
U.S. Department of Energy
Office of Civilian Radioactive Waste Management
Office of Science and Technology and International
1551 Hillshire Drive
Las Vegas, NV 89134
1-800-225-6972
http://www.ocrwm.doe.gov
March 2005