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.
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