Uranium and Neptunium Desorption from Yucca Mountain Alluvium Page: 1 of 8
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MOL. 20060726.0010
Uranium and Neptunium Desorption from Yucca Mountain Alluvium
Cynthia D. Scism, Paul W. Reimus, Mei Ding and Steve J. Chipera
Los Alamos National Laboratory: P.O. Box 1663, Los Alamos, NM, 87545, scism@lanl.gov
Abstract - Uranium and neptunium were used as reactive tracers in long-term laboratory desorption studies using
saturated alluvium collected from south of Yucca Mountain, Nevada. The objective of these long-term experiments is to make
detailed observations of the desorption behavior of uranium and neptunium to provide Yucca Mountain with technical bases
for a more realistic and potentially less conservative approach to predicting the transport of adsorbing radionuclides in the
saturated alluvium. This paper describes several long-term desorption experiments using a flow-through experimental
method and groundwater and alluvium obtained from boreholes along a potential groundwater flow path from the proposed
repository site. In the long term desorption experiments, the percentages of uranium and neptunium sorbed as a function of
time after different durations of sorption was determined. In addition, the desorbed activity as a function of time was fit
using a multi-site, multi-rate model to demonstrate that different desorption rate constants ranging over several orders of
magnitude exist for the desorption of uranium from Yucca Mountain saturated alluvium. This information will be used to
support the development of a conceptual model that ultimately results, in effective Kd values much larger than those currently
in use for predicting radionuclide transport at Yucca Mountain.1. INTRODUCTION
The saturated alluvium south of the proposed high-
level nuclear waste repository at Yucca Mountain,
Nevada represents the final feature of the Lower Natural
Barrier with characteristics and processes that can
substantially reduce radionuclide migration before
reaching the regulatory compliance boundary. The
objective of this work is to demonstrate that radionuclide
retardation in the saturated alluvium is likely to be
significantly higher than is currently assumed in Yucca
Mountain Project (YMP) models. [1] This work also
involves the development of an improved reactive
transport modeling approach that is compatible with YMP
saturated zone transport process models. [1]
Our experimental efforts have focused on the
radionuclides of uranium and neptunium because of their
high solubility, relatively weak sorption, and their high
potential contributions to offsite dose in the Yucca
Mountain models. The focus has also been on desorption
measurements rather than sorption measurements, as we
hypothesize that desorption rates likely control
radionuclide fate and transport to a much greater degree
than sorption rates.
We have developed a flow-through experimental
desorption method that provides a nearly continuous
measure of desorption rates over a long period of time. [2]
Almost all previous experiments conducted by the Yucca
Mountain Project [3,4] have focused on batch sorption
measurements or very short-duration desorption
measurements, which tend to significantly underestimate
radionuclide sorption parameters because they do notinterrogate the fraction of radionuclide mass that desorbs
very slowly. Quantitative X-ray diffraction and other
methods are being used to characterize the alluvium used
in the experiments.
To support the interpretation of the experiments and
to put the experimental results into a predictive context,
we have developed a generalized sorption residence time
distribution modeling approach to account for the
drastically different desorption rates that have been
experimentally observed. [5] This approach is consistent
with a conceptual model that involves multiple sorption
sites where rates of sorption onto the sites are similar but
rates of desorption differ dramatically. The model so far
has been able to account qualitatively for desorption
behavior observed in the flow-through desorption
experiments as well as some anomalous radionuclide
transport results in column transport experiments that did
not exhibit standard equilibrium or first-order reaction
rate behavior. Through continued experimentation, we
are developing a mechanistic basis for the model and
validating the approach.
II. MATERIALS AND METHODS
II.A. Water and Alluvium
The alluvium used in the experiments is from drill
cuttings obtained from NC-EWDP-191Ml A, NC-EWDP-
22SA and NC-EWDP-IOSA (Figure 1) at depth intervals
of 225.6-227.0 meters (740.0-745.0 feet), 169.93-170.69
meters (557.5-560 feet) and 207.26-208.79 meters (680-
685 feet) below ground surface, respectively. Alluvium
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Scism, C. D.; Reimus, P. W.; Ding, M. & Chipera, S. J. Uranium and Neptunium Desorption from Yucca Mountain Alluvium, report, March 16, 2006; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc883425/m1/1/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.