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Multiscale speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats

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The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray Absorption Fine Structure (XAFS) spectroscopy and X-Ray Fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occured because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their intial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.
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Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los
Alamos, McGuire AFB, Mayak, and Rocky Flats
Olga N. Batuk,
Steven D. Conradson,*
,,
Olga N. Aleksandrova,
§
Hakim Boukhalfa,
Boris E. Burakov,
David L. Clark,
Ken R. Czerwinski,
Andrew R. Felmy,
#
Juan S. Lezama-Pacheco,
Stepan N. Kalmykov,
,
Dean A. Moore,
#
Boris F. Myasoedov,
Donald T. Reed,
Dallas D. Reilly,
#
Robert C. Roback,
Irina E. Vlasova,
Samuel M. Webb,
and Marianne P. Wilkerson
Synchrotron-SOLEIL, LOrme des Merisiers, Saint-Aubin - BP48, 91192, France
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
§
Ural Federal University, Mira Street 19, Ekaterinburg 620002 Russia
V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy Ave., St. Petersburg 194021, Russia
University of Nevada, MSM 245, 4505 S. Maryland Pkwy, Las Vegas, Nevada 89154, United States
#
Pacic Northwest National Laboratory, PO Box 999 MSIN: K8-96, Richland, Washington 99352, United States
Environmental Earth System Sciences Department, 473 Via Ortega, Stanford University, Stanford California 94305-4216, United
States
Radiochemistry Division, Chemistry Department, Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia
Frumkin Institute of Physical Chemistry and Electrochemistry of RAS, Leninsky av. 31, Moscow 119071, Russia
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
*
SSupporting Information
ABSTRACT: The speciation of U and Pu in soil and concrete from
Rocky Flats and in particles from soils from Chernobyl, Hanford,
Los Alamos, and McGuire Air Force Base and bottom sediments
from Mayak was determined by a combination of X-ray absorption
ne structure (XAFS) spectroscopy and X-ray uorescence (XRF)
element maps. These experiments identify four types of speciation
that sometimes may and other times do not exhibit an association
with the source terms and histories of these samples: relatively well
ordered PuO2+xand UO2+xthat had equilibrated with O2and H2O
under both ambient conditions and in res or explosions; instances
of small, isolated particles of U as UO2+x,U
3O8, and U(VI) species
coexisting in close proximity after decades in the environment;
alteration phases of uranyl with other elements including ones that
would not have come from soils; and mononuclear PuO species
and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to
extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly
demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their initial
formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short
geological but signicant sociological times. They also show that incorporating the actual disposal and site conditions and
resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and
U in the environment.
INTRODUCTION
The chemical speciation of potentially hazardous subsurface
contaminants is a crucial parameter in environmental risk
assessment and remediation because it determines their
transport, toxicology, and ultimate disposition. This tenet
especially applies to uranium and plutonium contamination that
arguably constitute the most intractable environmental
restoration problems at legacy sites from nuclear weapons
production and testing as well as weapons and reactor
accidents, a hazard that is amplied by the public perception
of radioactive elements in general and Pu in particular. The
Received: December 18, 2014
Revised: March 25, 2015
Accepted: March 27, 2015
Article
pubs.acs.org/est
© XXXX American Chemical Society ADOI: 10.1021/es506145b
Environ. Sci. Technol. XXXX, XXX, XXXXXX
paragenesis of at least U after its release is, however, an
exceedingly complicated problem, both thermodynamically and
kinetically,
16
with less being known about Pu because there
are no natural systems to study. In laboratory systems, under
oxic conditions Pu can transform to PuO2+x(actually
PuO2+xy(OH)2y·z(H2O) compounds
79
) and, often at ele-
vated temperature, uranium to U(VI) oxyhydroxides
10
at rates
dependent on the particle morphology and other reaction
conditions. In the environment, weathering induced oxidation
of U-containing minerals and U and Pu contaminants to,
respectively, VI and IV on the decades time scale has been
observed
5,11,12
on at least some occasions. This rapid
conversion of the material in a variety of original forms to
the thermodynamically favored species identied in the
laboratory was the basis for the successful restoration of the
Rocky Flats site.
12,13
Demonstrating that Pu was present in the
expected PuO2+xform and that the transport had occurred as
colloids validated models that calculated safe residual levels,
thereby reducing the total amount of contaminated material
deemed hazardous enough to require removal and disposal to
manageable quantities. Analogously, in identifying their history,
origin, and intent for nuclear forensics, assigning a useful time
to their release also presumes that these weathering reactions
will begin almost immediately and proceed to completion in a
regular way over weeks to at most a few years. This
predictability of Pu/U-O2-H2O in the laboratory contrasts
with the natural systems for U where the presence of other
elements that can react to form alteration products that inhibit
oxidation and other surface reactions by impeding diusion, the
radiation eld, and the simple mechanics of the two-phase
solidliquid system combine to give almost overwhelming
complexity to the species whose releases have caused them to
become environmental contaminants.
14
This plethora of
possible species must then also be incorporated in evaluations
of actual and potential nuclear accidents.
15
However, even while including the possibility of substitutions
and other disorder, models are still constrained to describe
these species in the context of known, crystalline compounds.
After examining U and Pu in soil samples from six dierent
locations in the U.S. and Russia, some in bulk but most as
single particles using synchrotron microprobes, we herein
present results indicating that this type of description is
incomplete. These eld-derived samples give us insight as to
the extent and rate of interaction in the environment, atomic
scale correlations with other atoms, and allow us to examine the
assumptions currently being used to predict their short and
long-term fate and transport. Over these geologically
inconsequential but societally important time periods of
decades we therefore observe both minimal and substantial
momentum in the formation of the equilibrium species as well
as nding novel species that would have resulted from
unpredicted interactions of Pu and U species with other
components of their waste streams and surroundings. The
objectives of this study were to exploit our access to samples
from so many sites and sources to explore the range of species
and correlations displayed by U and Pu contamination
including the identication of novel types; determine the
extent to which these data signied their origins and history;
and to elucidate whether and under what conditions these
species might have been transformed by chemical reactions
after their release. We will therefore not speculate on detailed
mechanisms of their formation, which would be dicult in any
event because for many sites the necessary information on, for
example, Manhattan Project era waste dumps, is not available
and also because many of the samples were obtained via
remediation and not research projects. The variety of original
sources that subsequently became soil or concrete contami-
nants includes: (1) metal turnings in lathe coolant and leaks
from a HNO3-based purication line at Rocky Flats; (2) a
nuclear armed missile explosion and re at McGuire AFB; (3)
samples around 1 km away from the reactor at Chernobyl; (4)
a sample from Reservoir 17 at Mayak; (5) a hot spot that may
have been waste from R&D operations during and shortly after
the Manhattan project from the Los Alamos TA-21 disposal
area; and (6) samples from the acidic Z-9 and neutral Z-12
disposal crib areas at Hanford. A more detailed description of
the sample provenance can be found in the Supporting
Information.
MATERIALS AND METHODS
Brief Site Background and Sampling History. The
Rocky Flats Environmental Technology Site (RFETS)
fabricated components of ssile material for U.S. nuclear
weapons.
12,13
The data shown here came from soils underneath
or adjacent to an asphalt pad designated 903 that was used for
storing drums of Pu-contaminated lathe coolant, collected
around 1998, and from the concrete oor of a building where
separations were performed involving solutions with high
concentration HNO3, obtained around 2002, with measure-
ments performed shortly after the samples were obtained. This
site was a US Department of Energy (DOE) remediation
project. These were bulk measurements with a beam of mm
dimension that could nevertheless be placed on dierent
portions of the samples.
The releases of material from McGuire AFB
1618
in 1960
and Chernobyl
1921
in 1986 involved high-temperatures,
melting, res, and explosions that consumed, respectively, a
missile with a nuclear warhead and a nuclear reactor core.
Surface soil samples from Chernobyl containing the particles
shown here were collected in summer, 1986, 0.51.5 km west
to northwest from the reactor in the Red Forest and in 1990 in
the Western Plume area, following which they were stored in
the laboratory. All the particles were separated from their host
soils in 1990. We analyzed a total of 19 U, Zr, and mixed U/Zr
particles. The re and explosion fragmented these bulk solids
and liquid melt into particles of varying sizes that in some cases
combined the various constituents before condensing and
solidifying. Particles from McGuire were collected from soil
cores in 2007. Here we show results from particles obtained
from 2 of these.
During the time plutonium was produced at Mayak a large
number of water reservoirs at this south Ural site
2225
were
contaminated with irradiated fuel processing wastes. The U-
containing particles we analyzed were found in bottom
sediment samples collected in 2010 from one of these ponds,
Reservoir 17, which at least for several years prior had been oxic
(450 mV). We report XRF mapping results for 14 of these and
EXAFS for 3 with a number of others giving XAFS spectra of
varying quality.
The Los Alamos TA-21 waste site
26,27
was created in the
years following the Manhattan project. Record keeping was
incomplete, an unmapped intact truck as well as other artifacts
were found during its excavation. We found 2 disk-like
uranium-containing particles and 13 plutonium particles with
a variety of sizes and compositions in soil samples obtained
from the excavation in a single small volume of soil that had
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B
shown high activity during routine surveys and was rumored to
be contaminated with laboratory R&D waste.
Hanford crib soil samples contained wastes from Pu
separations and recovery processes that were intentionally
disposed in the cribs into the beginning of the 1960s. The Z-9
crib was excavated in 19741976 because the amount of Pu
was so large that criticality events had become a concern. These
samples came from the surface of the Z-9 crib, collected and
archived prior to its mining operation, and from the surface of
the Z-12 crib that was studied in 2004 and 2006.
2832
Z-9
samples were described as acidic, high salt, and acid rich with
signicant amounts of CCl4, tributyl phosphate and derivatives,
lard oil, and NO3as cocontaminants, with a possible
correlation of the Pu with P.
32,33
In addition to XAFS on a
bulk sample, μ-XRF images from Z-9 soils show large numbers
of highly regular squares with sides 20 μm containing small
amounts of Pu but no other elements within the range of the
detector system (Z > K). Some of these squares had much
more substantial PuO2domains on their surfaces of varying
thickness and extent. We found 12 such squares with Pu
Figure 1. χ(R) EXAFS from particles and bulk samples for U, Pu, and Fe from the indicated locations: Fourier transform moduli (left) and real
components (right). The data are solid lines and the curve-ts are dashed. (a, b) Representative U L3edge χ(R) spectra from two of the Chernobyl
particles. (c, d, e) Representative U L3edge χ(R) spectra from three of the Mayak particles showing the dierent U oxide species. (f) A
representative Pu L3edge χ(R) spectrum from the composite McGuire particle. (g) A representative PuO2+x-like Pu L3edge χ(R) spectrum from one
of the Hanford Z-9 crib particles. (h, i) Bulk Pu L2edge χ(R) spectra from samples from the Z-9 (h, PuO2+x-like) and Z-12 (i, non-PuO2+x) Hanford
cribs. (j) Bulk PuO2+x-like Pu L2edge χ(R) spectrum from a Rocky Flats soil sample. (k) A representative non-PuO2+x-type Pu L3edge χ(R)
spectrum from a Pu particle in a Los Alamos soil sample. (l) A representative Pu L3edge χ(R) spectrum from the mixed Los Alamos Pu/Fe particle,
PuO2+x-like structure plus Fe shell at R= 3.35 Å. (m) A representative Fe K edge χ(R) spectrum from the same Los Alamos mixed Pu/Fe particle.
The transform is taken over k=3
1for all spectra to allow direct comparisons of both position and amplitude in this gure although the actual
range of the curve-t analyses of these spectra shown in the Supporting Information varies depending on the quality of the data.
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sucient to record EXAFS. The Z-12 crib material was
described as low salt and near neutral, with NO3and Flisted
as the only cocontaminants.
33
The XAFS of Z-12 was recorded
on a bulk sample.
XRF, XAFS and XRD Data Collection and Data
Analysis. All the XRF, XAFS and XRD measurements were
performed on beamlines 23 (microprobe) and 112 (bulk) at
the Stanford Synchrotron Radiation Lightsource. The X-ray
optics at beamline 23 focuses the beam to a 2 μm fwhm spot.
Details of the data acquisition and analysis and the results of the
curve-tting as χ-functions and Fourier transforms are
presented in the Supporting Information accompanying this
report in Figures S1S15 and the metrical parameters are
summarized in Table S1.
Calculation of the Correlations. The spatial correlations
for the μ-XRF element maps were calculated using the Pearson
function that measures the strength of association between a
pair of variables. For the correlations between experimental
data and the tting results we used the root-mean-square
deviation. This function is a frequently used measure of the
dierences between values predicted by a model or an
estimator and the values actually observed from the data
being modeled or estimated.
Quantitative Electron Probe Microanalysis. Quantita-
tive electron probe microanalysis of U and Zr in Chernobyl
samples was carried out at the V.G. Khlopin Radium Institute
using an SEM-microprobe CamScan-4DV, at 20 kW, current
10 nA. Synthetic ZrSiO4and UO2standards were used for the
evaluation of the recorded data. The standard software package
ZAF-4/FLS was used for the analysis.
RESULTS AND DISCUSSION
Based on the objectives of this report, this discussion begins
with the two samples of Pu from Rocky Flats and two from
McGuire AFB that were known to have originally been in
metallic form and the U that was combined with the Pu in the
re that caused the release at McGuire. This is followed by a
description of the U contamination that was also the product of
a high temperature eventthe explosion and re at Chernobyl
that also showed only UO2+xas the highest U valence. Next are
the numerous U oxide particles from a single sample from a
pond at Mayak that exhibit a wider range of valences. Finally, U
and Pu from the Z-9 cribs at Hanford, the LANL TA-21 site,
and a purication line at Rocky Flats, are presented, all known
or suspected to have been released in acidic solutions that
would have reacted with the components of the soil or
concrete. The Rocky Flats results
12
and some earlier bulk Pu
data from Hanford
32
are included here because of their value in
comparisons with information from other sites. For some of the
sites the results from some or even most of the particles
evaluated are not shown, including ones where the element
correlations or even certain aspects of the speciation are
dierent. Reiterating, our purpose is to show the range of
behavior and not an exhaustive list of all of the variations.
Figure 1 showing the EXAFS data displays a dierent order.
Insofar as many of these species exhibit the Pu/UO2+x
disordered uorite structure, scanning down the gure through
the spectra facilitates the comparison of the R= 1.8 and 3.7
peaks indicative of AnO2,rst for the U and then for the Pu
among the dierent samples, with the nal Fe EXAFS from a
particle containing both Pu and Fe in somewhat separate
domains. This comparison dictated that the Fourier trans-
formation range of the spectra included in the gure conform
to the shortest range spectrum from these samples, longer
ranges were used when possible to give the results in the
Supporting Information. UO2+x,U
3O8, and U(VI) oxyhydr-
oxides give easily distinguished EXAFS, whereas the distinction
between UO2and UO2+xis more subtle
34
in that, as with
PuO2+x,
8
it involves disorder in the UU pair and a reduction
in the EXAFS amplitude, the introduction and growth of a new
peak that is the uranyl-type oxo, or U-oxo contribution that can
overlap with the principal UO shell around 2.36 Å, and nally
the formation of other peaks from O neighbors with distances
between 2.7 and 3.4 Å. The absence of a UUorPuPu
contribution at R= 3.54.5 Å demonstrates that the U or Pu is
monomeric instead of in oxide or oxyhydroxide form. Although
these trends can be dicult to observe and compare in spectra
measured over this limited energy range, an estimate of the
relative value of xis obtained from the numbers of oxo and U
near neighbor atoms obtained from curve-ts (Supporting
Information Table S1). This table and 15 full spectra from
these and additional samples are presented in the Supporting
Information. X-ray diraction measurements were attempted
with a number of samples, with the only successes (Figure 2)
being the >200 μm particles from McGuire AFB that had been
annealed in the re. This nding of widespread crystalline
disorder in particles up to 50100 μm in size is perturbing
since it suggests that diraction patterns from bulk samples may
be sampling only a fraction of the material. This disorder is also
evident in that the low index reections are complete circles
whereas the particles could have been expected to be single
grains or at least highly textured over the limited volume
sampled by the 2 μm beam.
Pu and U Metal or Alloy Origin. Real space Fourier
transforms of their EXAFS spectra indicate (Figure 1j,
Supporting Information Figure S11, and
12
) that the Pu
speciation in Rocky Flats soil contaminated with machining
uid used for cutting Pu and concrete contaminated from a Pu,
re is PuO2+x. Pu(U)O2+xand UO2+xare also found by both
EXAFS (Figure 1f and S7) and XRD (Figure 2) that shows the
Fm3m diraction pattern with lattice constants within the error
range for these compounds in particles from McGuire AFB that
were released when a re consumed a nuclear armed missile.
Two μm resolution XRF element maps show (Figure 3) that
one particle is best described as a conglomerate of a Pu-rich and
a U-rich particle that melded without signicant mixing plus
some small Fe and Ga spots (Supporting Information Figure
S16), whereas in a second particle that was presumably
subjected to higher temperatures for a longer time the Pu and
U are intermingled to give a more homogeneous mixture
(Figure 3).
UZr Oxides at Chernobyl. Like these samples from
McGuire and Rocky Flats, materials from Chernobyl were also
exposed to high temperatures that promoted reactions and
mixing between the UO2and the Zr, air, water, graphite, and
other materials. The U particles collected from Chernobyl
exhibit homogeneous (U, Zr)Oxsolid solutions (1618 wt %
Zr) on the μm length scales of these analyses according to the
XRF mapping and quantitative electron probe microanalysis.
This would result from the high temperature promoting the
combining of the fuel and cladding materials to give (U, Zr)Ox
with varying U:Zr (up to 52 wt % Zr) ratios with some stainless
steel inclusions. On the Å length scale of the EXAFS a dierent
result is obtained. The Zr and U EXAFS can be t with,
respectively only Zr neighbors at 3.67 Å and only U neighbors
at 3.87 Å, implying signicant inhomogeneity. This clustering
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of the cations on the nanometer scale is typical of mixed UZr
oxide.
35,36
There are also particles of almost pure Zr-oxide
(Figure 4). The separate components of the missile and reactor
as well as variations in the temperature attained by individual
particles are therefore manifested in the distributions of the
elements composing the particles. The U in the mixed U/Zr
particles shows few or no oxo neighbors, consistent with UO2,
whereas particles where U is the only element with Z Ca
show more oxo and are assigned to UO2+x(Figure 1a and b and
Supporting Information Table S1).
34
It is important to note
that none of the spectral signatures of U(VI) are observed,
which for EXAFS typically constrains the amount that could be
present to less than 10%. Insofar as the initial step in U
paragenesis is oxidation so that subsequent alteration phases are
uranyl compounds, the absence of U(VI) may perhaps have
occurred originally because of the high temperature and
subsequently because further oxidation from weathering may
have been inhibited by the Zr or a surface modication
involving, for example, carbon.
37,38
U Oxides at Mayak. The 14 U-containing particles
analyzed from the single soil sample obtained from Mayak
ranged in size from 5upto50μm (Figure S4 Supporting
Information), with high U counts typically found for those 8
μm. The uorite-structured UO2+xwith varying x displayed at
Chernobyl is also exhibited by many of the particles from the
single sample of PA MayakReservoir 17 bottom sediment.
Some Mayak particles, however, contained U in higher
valences. Representative EXAFS spectra (Figure 1ce) and
their curve-ts (Supporting Information Table S1) unambigu-
ously show the three UO2+x,U
3O8, and U(VI)-oxyhdroxide
species, with no indications of mixed structures that would
indicate that the former are becoming the latter even after
decades in the pond, as has been observed at some sites.
10,11,39
This demonstration of inhibited or retarded oxidation of the U,
perhaps similar to that at Chernobyl, indicates that it is likely
that these U species in these materials are the same as when
they were originally deposited from various waste streams at
Mayak, in contrast to, for example, the U at the Fernald
processing site that showed increasing amounts of U(VI) with
longer exposure times.
11
This variation in U speciation extends
to the correlations with other elements. Although these three
particles had correlations with Ca and Fe 0.13, others were as
high as >0.7, with varying degrees of correlation between these
other two elements (Supporting Information Figures S17
S19).
The incomplete documentation for the site and Reservoir 17
in particular preclude developing a more detailed correlation
between the particle composition and the original disposal and
subsequent site history. There were, however, occasions when
treatments rendered the pond both highly acidic and alkaline,
which might have promoted surface modication of the UO2
particles that could aect subsequent oxidation. However,
whether the inhomogeneity of the speciation in this small
volume of the sample is because these U-oxides were deposited
from a number of dierent sources that were mixed, as we
postulate, or from reactions after deposition that somehow
discriminated between these particles despite their identical
conditions is uncertain. It is nevertheless of interest to compare
these results with some of the extensive literature on UO2
paragenesis.
26,40
The combination of relatively pure U oxides
of varying valence and degrees of correlation with other
elements signifying alteration phases is consistent with these
prior reports that described the decomposition of UO2by
oxidation of the surface that promoted the formation of the
uranyl alteration compounds via crystallization or precipitation
with other cations and inward along grain boundaries that
fractured the bulk UO2to release small particles. What diers
here then is that there is no source of bulk UO2. It is highly
unlikely that the radiation eld was sucient to eect
reduction.
40
The 137Cs γrays emitted by this sample conrm
Figure 2. μ-X-ray diraction patterns taken from the McGuire
particles shown in Figure 3. (a) Diraction from Pu-enriched area of
the inhomogeneously mixed particle. (b) Diraction from U-enriched
particle area for the same particle. (c) Diraction from the
homogeneously mixed McGuire particle.
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the origin of these materials from spent fuel processing,
indicating that the U source term would have been relatively
homogeneous high valence U oxides created during the
probable oxidative dissolution of the original UO2. This poses
the question of the subsequent formation of the UO2in these
particles, especially insofar as the pond has been oxidizing for
many years. A second question vis-à
-vis the studies of minerals
and laboratory systems is the homogeneity of the U speciation
in the individual particles. The range of UFeCa correlations
as well as U species (Supporting Information Figures S17
S19) implies that chemical processes were proceeding at a
signicant rate on the decades time scale. In laboratory systems
and minerals, oxidation is retarded after an initial burst of U
release because, on the scale up to 10 μm or greater, diusion is
inhibited by the accumulation of uranyl alteration compounds
and the UO2itself is masked as the higher oxides adhere to and
cover the surface instead of dissolving.
26
Given the probability
that the original U was largely homogeneous and in the form of
dispersed particles 1050 μm in size like these and that the
environmental conditions were identical for all of these
particles because of their proximity, there is again the issue of
what relatively subtle eect, perhaps limited to the surface of
the particles, caused this substantial dierence in behavior.
Pu and U from Acidic Solution. In contrast to these other
materials, disposal of Pu and U in waste streams suciently
acidic to react with concrete from a purication process line at
Rocky Flats or soil minerals in the Hanford cribs or a disposal
site at Los Alamos appears to have promoted both unusual
speciation and morphology. The Pu particles found in the XRF
maps of a smear of soil on tape from a few grams collected from
one hot spot at the Los Alamos TA-21 site that most likely
originated as R&D waste exhibit association with Fe in all three
possible ways; typical surface complexes or precipitates on Fe-
based mineral phases, isolated Pu particles with proximity but
no contact, and single particles composed of FePu mixtures
(Figure 5). We interpret these results to suggest that the
variation in particle composition is due to Pu deposition onto
the chemically diverse soil materials when the putatively acidic
solution was neutralized by reacting with the soil components,
releasing or activating some that could then possibly combine
with the Pu that was also originally dissolved. The U particles
from the Los Alamos site also exhibit substantial alteration.
Two 100 μm diameter disks are not simple U-oxyhydroxides
but instead are homogeneous 0.3:0.7 U:Zn mixtures with a
trace of Cu (Figure 6a), similar to particles reported at
Hanford.
41
Uranyl (Figure 6b) alteration products can
therefore involve additional elements from the laboratory
perhaps even galvanization from pipesas well as from soils,
Figure 3. μ-XRF maps of the PuU composite (upper) and homogenized (lower) particles from McGuire and their PuU spatial correlations.
Figure 4. (a) Tricolor μ-XRF maps for three particles collected at
Chernobyl, and (b) corresponding ZrU correlations showing μm-
scale homogeneous mixing in a particle with comparable amounts of U
and Zr, a Zr-rich particle that also contains a U- and an Fe-rich
domain, and a particle that is almost pure Zr.
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implying that they can form quickly and are subsequently
stable.
A sample showing Pu-containing particles from the Hanford
Z-9 crib (Figure 7a) indicates an even more complicated and
specic process. Although the spectrum of a bulk Z-9 sample
(Figure 1h) gives PuO2+x, a particle from a separate sample
gives a dierent result (Figure 1g). It contains many regular,
20 μm square particles composed of elements lighter than Ca,
most likely a silicate or a phosphate that could have formed
from the breakdown of the tributyl phosphate from the PUREX
process. These particles show barely detectable amounts of Pu
that are uniformly distributed either within or, less likely, on the
surface of the particle. A few of them, however, show large Pu
deposits. This nding of two populations of Pu that are distinct
in both the correlation of their distribution with respect to the
particles and their quantity imply that the low concentration Pu
was incorporated during the formation of the particles whereas
the high concentration surface precipitates formed after the
square substrates and in a way that favored growth over
nucleation, that is by a slow process involving Pu diusion or
transport. Since the maps from all of these samples never show
any signal in the areas between the larger particles, colloidal or
otherwise diuse Pu must be negligible. That the preponder-
ance of the Pu occurs in μm-scale particles that were most likely
not its original form implies that Pu was mobilized and
subsequently reacted, nucleating on the crystal surfaces as a
surface complex followed by the accumulation of additional Pu
on this surface complex to give the bulk species observed here.
Figure 5. Two-color μ-XRF maps (Pu = red, Fe = green) of Los Alamos TA-21 Pu particles. (a), a mixed PuFe particle, one of two found in the
measured samples. (b) Pu particle on an Fe-based mineral grain, one of six similar particles. (c) separated, noncorrelated Pu and Fe particles, one of
three similar particles.
Figure 6. (a) CuU and ZnU spatial correlations for a U-containing
Los Alamos particle (excitation energy is 18 100 eV), the Pearson
function was applied to calculate Rfor these correlations. (b) XANES
of the same particle compared with that of schoepite as a prototype
uranyl oxyhydroxide compound. Figure 7. (a) Pu map of a Hanford Z-9 crib sample (sensitivity to only
Pu results from subtracting the counts measured at 18000 eV that is
below the Pu L3absorption edge from those at 18100 eV that is above
it). The scale shows the dierential Pu count rate. (b) the PuP and
PuO curve-ts to the Fourier ltered, non-PuO2wave in the data,
from which the much higher quality of the PuPtit is under the
red line of the PuPt at lower kcan be discerned.
Environmental Science & Technology Article
DOI: 10.1021/es506145b
Environ. Sci. Technol. XXXX, XXX, XXXXXX
G
Molecular scale speciation in addition to this μm-scale
morphology further supports the idea that a highly reactive
source or subsequent term can promote chemistry that can
often be distinct and unexpected. In addition to the Zn:U(Cu)
particles, an entirely novel Pu species occurs in what is
apparently a Pu-enriched particle obtained from concrete
exposed to HNO3-Pu solutions from a purication line at
Rocky Flats because the XANES measured at a particular
location for the large beam gave a much higher count rate and a
spectrum unique in both its shape and high energy compared to
other locations (Figure 8). The Pu in the Hanford Z-12 crib
sample, in contrast to the Z-9 crib, is well t with only a single
shell, an O/F at a short 2.22 Å PuO/F distance (Supporting
Information Table S1) with no indication of Pu neighbor as in
PuO2+x.
8,32
Similarly, the EXAFS of the Los Alamos non-Fe-
associated Pu particles (Figure 5c) show only a single O shell at
2.31 Å (Figure 1k and Supporting Information Table S1). The
Pu species in these samples are therefore mononuclear and not
PuO2+x-like compounds, analogous to the non-UO2U(IV)
species that have been reported and therefore posing the
question of whether this analogy extends to the biogenic origin
of the latter.
42
The Pu in the FePu particle from TA-21 is PuO2-like with
PuO/Pu distances of 2.31/3.75 Å (Figure 1 and Supporting
Information Table S1). The large extra peak in χ(R)atR= 3.1
Å that does not occur in PuO2+xis t over 40% better with an
Fe shell at 3.35 Å (72% of its spectral weight) than with O
(only 51%) (Figure 9). This distance is within the range
expected for an Fe within the uorite lattice. This result, that Fe
is on occasion incorporated into PuO2+x, is corroborated by a
reanalysis of the EXAFS of PuO2precipitated by reduction with
Fe that with this new structure motif now also shows a similar
Fe neighbor shell.
43
The Fe in this particle is goethite-like, a
typical soil mineral, without any evidence of a Pu neighbor
incorporated into its structure. These results imply a crystallo-
graphically heterogeneous mixed FePu oxide with Fe > Pu
consistent with the relative count ratesand with composition
uctuations giving Pu-enriched domains containing sucient
Pu to adopt the uorite structure.
The Pu EXAFS from Hanford Z-9 crib particles display the
overall shape and PuO/Pu distances of PuO2+x(F)-type
compounds (Figure 1g).
9
However, closer examination shows a
non-PuO2feature around R= 3 Å that gives a 99% correlation
with the data when t with a P (cannot be distinguished from
Si, another possibility) shell but only 90% with O (Figure 7b),
indicating its origin in a P(Si) second shell neighbor. The
3.453.54 Å distances show that the P is O-bridged to the Pu
within the PuO2lattice, giving substitutional disorder analogous
to other oxides but unprecedented if perhaps not unexpected
for Pu. The P would be incorporated in concert with the
mobilization of PuO2+xthat subsequently recrystallized into the
large domains on the particle faces, implying that the substrate
controls this process via, for example, the high concentration of
phosphate at the surface of the crystal, and is more than merely
a passive template for the sorption of PuO2+x. The process
would be similar to that already reported, except in the eld
rather than the lab, and therefore coupled to local conditions
such as an acidic waste stream that could also have reacted with
soil minerals.
44,45
Neutralization would terminate the reaction,
but with the P retained in the PuO2as a signature of not only
the substrate but also the history of the material.
Environmental Implications. These results clearly identify
both expected and novel aspects of the environmental
chemistry of U and Pu beyond those previously reported
from laboratory and mineral studies. One important nding is
that the samples from none of these sites showed uorescence
from U and Pu above the detection limit between the particles;
these elements were always concentrated in particles without
any sign of a more diuse population. Of course, free U and Pu
are required for the chemical reactions in the soil that we have
postulated based on these results, but their concentrations
would be low, corresponding with the decades time scales of
these reactions. Alternatively, the transformations could occur
rapidly during rare occasions of extreme conditions. On the Å-
scale of speciation, in some samples, particles that were within
mm of each other for decades that most likely began as
homogeneous species or originated in the same process exhibit
dierent speciation with no indication of convergence to their
presumed thermodynamic minima, implying that the observed
species are the source terms that were, for example, inhibited
from oxidation. In other samples they are present in unusual,
non-PuO2+xforms, or as these expected oxyhydroxides but with
additional elements whose incorporation would have resulted
from transitory local conditions and chemical activation
resulting from the waste stream or perhaps in combination
with chemistry enhanced by surface phenomena. Most
Figure 8. Bulk Pu L2XANES measured on a Rocky Flats concrete
sample contaminated via leaks from an acid purication line compared
with those of Pu(IV) and Pu(VI) standards. The two experimental
spectra giving dierent results were measured at two dierent locations
in the same Rocky Flat bulk concrete sample, indicating that at least
the Pu from the rst run had occurred as a Pu-enriched particle instead
of being diused through the material.
Figure 9. Pu L3EXAFS on mixed FePu TA-21 particle: analysis of
the anomalous peak in the spectrum by comparing the curve-ts with
O and Fe and the corresponding residuals.
Environmental Science & Technology Article
DOI: 10.1021/es506145b
Environ. Sci. Technol. XXXX, XXX, XXXXXX
H
importantly, they not only provide the molecular scale chemical
speciation information that was incomplete in prior reports of
complexity in their environmental chemistry but also suggest
possible mechanisms for it that appear to recapitulate the
source term and subsequent history after release.
4649
Although
these time periods are virtually instantaneous on the geological
scale, they are signicant on the regulatory one. These results
demonstrate that duplicating and predicting the behavior of U
and Pu at a specic site are likely to require experiments that
duplicate both the site conditions and the chemical form of the
contaminant when rst released to promote the formation of
unexpected or even unknown species that may be present. In
addition to Fthat we have already reported,
32
a number of
these samples either directly show or point to the presence of
additional cationic elements as second neighbors in the PuO2
and UO2lattices, a substitutional pattern previously unknown
for Pu. This is not necessarily detrimental to environmental
regulation and control; the observed or implied stability and
diminished solubility of such ternary oxides
44,45
could actually
be advantageous in many remediation, restoration, and
licensing eorts (see Table 1).
ASSOCIATED CONTENT
*
SSupporting Information
Complete gures of the EXAFS data and t results for 15
samples including Zr for Chernobyl and Fe for the Los Alamos
TA-21 PuFe particle, XRF maps of additional elements for
the McGuire AFB particles, element correlations and XRF
maps of some Mayak particles, and a table of the curve-tting
results for the 15 EXAFS spectra as described in the text. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: +33 1 69 35 91 80; e-mail: steven.conradson@
synchrotron-soleil.fr.
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
We acknowledge Pavel M. Stukalov (1959-2010) the former
head of the Environmental Control Laboratory of PA Mayak
for his assistance with Reservoir 17 sediments and Kaiser-Hill
LLC for contributing plutonium-contaminated soils and
concretes. Los Alamos National Laboratory is operated by
Los Alamos National Security, LLC, for the National Nuclear
Security Administration of U.S. Department of Energy under
Contract DEAC52-06NA25396. Financial support was pro-
vided by the Los Alamos LDRD program, the U.S. Department
of EnergysOce of Biological and Environmental Research
(BER), as part of BERs Subsurface Biogeochemistry Research
Program (SBR) which originates from the SBR Scientic Focus
Area (SFA) at the Pacic Northwest National Laboratory
(PNNL). Advanced interpretation of the data was supported by
the Heavy Element Chemistry Program, Chemical Sciences,
Biosciences, and Geosciences Division, Oce of Basic Energy
Sciences. Other nancial support includes the Russian Basic
Research Foundation (project 10-03-01029-a) and Ministry of
Education and Science of Russian Federation (projects
02.740.11.0853 and 11.519.11.5011). Portions of this research
were carried out at the Stanford Synchrotron Radiation
Lightsource, a Directorate of SLAC National Accelerator
Laboratory and an Oce of Science User Facility operated
for the U.S. Department of Energy Oce of Science by
Stanford University.
Table 1. Signicant Properties and Results
location samples source principal species distinguishing characteristics
Chernobyl U: with Zr reactor explosion UO2no oxidation past UO2
pure U UO2+xless oxidation with Zr
Hanford Pu: Z-9 bulk and
particle acidic high salt PuO2+xincorporated P into PuO2+x
Z12 crib near neutral low salt Pu(IV)
mononuclear mononuclear species
Los Alamos Pu: isolated r&d waste Pu(IV)
mononuclear multiple species at same location
Fe surface mononuclear Pu
Fe particle PuO2+xincorporated Fe into PuO2+xin mixed Pu:Fe particle:
U: Uranyl with Zn U:Zn alteration phase
McGuire Pu and U missile re PuO2+xmixed PuUO2+x
UO2+xdiraction quality
Mayak U separations UO2+xmultiple valences and alteration products at same location in homogeneous
particles
U3O8
U(VI) oxy-
hydroxide
Rocky Flats Pu: 903B pad disposal PuO2+x
purication purication Unique (XANES) novel Pu(VI) species
Environmental Science & Technology Article
DOI: 10.1021/es506145b
Environ. Sci. Technol. XXXX, XXX, XXXXXX
I
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K
... 11,14,22,57,58 For example, in groundwater at the Mayak site (Russia), colloidal amorphous iron oxides with associated Pu were found up to 4 km away from the contamination source. 59 In contaminated soils at Los Alamos National Laboratory, Batuk et al. 60 identified unusual Pu−Fe particles. The authors suggest that the formation of these particles could have resulted from transitory local chemical conditions from the original waste stream. ...
... The authors suggest that the formation of these particles could have resulted from transitory local chemical conditions from the original waste stream. 60 Finally, Lukashenko et al. (2020) 61 identified particles of Pu containing Fe−Mn oxides in bottom sediments of streams flowing from the tunnels in the Degelen Mountain, a site of the USSR nuclear weapon testing program. 61 Analysis of the particles indicates that these materials may be formed as secondary minerals, suggesting that their formation is most likely related to sorption and coprecipitation of transuranic elements with oxides and hydroxides of iron and manganese. ...
... Interestingly, σ 2 in our fits suggests elevated levels of local structural disorder around the Pu atom. Other possible explanations for the local Pu structure observed are the formation of a surface complex on the goethite surface; however, the shortest modeled Pu−Fe distance at 3.19(2) Å is significantly shorter than the range of An−Fe distances previously reported (3.35−3.7 Å). 48,69−73 Finally, the EXAFS data are not inconsistent with the formation of a distinct poorly ordered Pu−Fe solid phase as has been hinted at in the forensics literature, 60 although the formation of a distinct Pu−Fe phase was not supported by TEM imaging. Overall, the TEM and EXAFS data demonstrate that PuO 2 exists at the highest concentration Pu coprecipitation samples (3000 ppm) but not the lower concentrations (1000 and 400 ppm). ...
Article
Understanding the interactions between plutonium and iron (oxy)hydroxide minerals is necessary to gain a predictive understanding of plutonium environmental mobility and to evaluate long-term performance of nuclear waste repositories. We investigated the fate of plutonium during the formation of ferrihydrite and its subsequent transformation into goethite. Ferrihydrite was synthesized with varying quantities of Pu(IV) following either a sorption or coprecipitation process; the ferrihydrite was then aged hydrothermally to yield goethite. The synthesized materials were characterized via extended X-ray absorption fine structure spectroscopy, transmission electron microscopy, and acid leaching to elucidate the nature of plutonium association with ferrihydrite and goethite. In samples prepared following the sorption method, plutonium was identified in two different forms: a PuO 2 precipitate and a surface-sorbed plutonium complex. For the samples prepared via coprecipitation, no PuO 2 formation occurs in the ferrihydrite precursor and in the goethite experiments where plutonium concentration is ≤1000 ppm (mg kg −1). In these coprecipitation products, plutonium is strongly bound to the minerals either via formation of an inner sphere complex, or via an incorporation process. In the coprecipitation experiments, PuO 2 formation only occurs at the highest plutonium concentration (3000 ppm), suggesting that during ferrihydrite transformation into goethite, part of the plutonium can be remobilized to form PuO 2 nanoparticles. Collectively, our results demonstrate that the nature of plutonium associated with the precursor ferrihydrite (adsorbed vs coprecipitated) will have a direct impact on the association of plutonium with its alteration product (goethite). Furthermore, the data illustrate that some properties of plutonium association with the precursor ferrihydrite are retained through the transformation into goethite. These findings show that plutonium strongly associates with iron (oxy)hydroxides formed through coprecipitation processes and in these materials, plutonium can be strongly retained by the iron minerals.
... Several reviews have summarized the different migration and accumulation pathways associated with heterogeneous (at the sub-micron scale) uranium and plutonium field samples using complementary imaging and spectroscopic micro-and nano-probes. [9,39,40] Brown et al. showed at Hanford Site (Washington State, USA) that contaminants are pluming in the zone beneath various Hanford Tank Farms. In this example, uranium is pluming at hundreds of meter scale from the Tank. ...
Article
Since the discovery of nuclear fission, atomic energy has become for mankind a source of energy, but it has also become a source of consternation. This Perspective presents and discusses the methodological evolution of the work performed in the radiochemistry laboratory that is part of the Institut de Chimie de Nice (France). Most studies in radioecology and environmental radiochemistry have intended to assess the impact and inventory of very low levels of radionuclides in specific environmental compartments. But chemical mechanisms at the molecular level remain a mystery because it is technically impossible (due to large dilution factors) to assess speciation in those systems. Ultra‐trace levels of contamination and heterogeneity often preclude the use of spectroscopic techniques and the determination of direct speciation data, thus forming the bottleneck of speciation studies. The work performed in the Nice radiochemistry laboratory underlines this effort to input speciation data (using spectroscopic techniques like X ray Absorption Spectroscopy) in environmental and radioecological metrics. Environmental radiochemistry and radioecology have intended to assess the impact and inventory of very low levels of radionuclide in specific ecosystems. But ultra‐trace environmental levels of metallic radionuclides on the one hand, and heterogeneity on the other have up until now formed the bottleneck in our efforts to input speciation data in environmental and radioecological metrics.
... In particular, little is known about the effects of acid-driven mineral transformation on the evolution of radionuclide speciation over prolonged weathering times. A few studies have investigated these combined effects on model minerals and in batch sediment systems pertinent to the Hanford site (Kanematsu et al., 2014;Batuk et al., 2015;Dublet et al., 2017;Wang et al., 2017b;Perdrial et al., 2018) and showed a direct coupling between U solid speciation and acid-driven mineral dissolution. When uncontaminated Hanford soils were reacted in suspension with acidic U-bearing solutions, initial neutralization of solution pH was driven by carbonate mineral dissolution. ...
Article
Radioactive acidic liquid waste is a common byproduct of uranium (U) and plutonium (Pu) enrichment and recycling processes whose accidental and planned release has led to a significant input of U into soils and sediments across the world, including at the U.S. DOE's Hanford site (WA, USA). Because of the particularly hazardous nature of U, it is important to predict its speciation when introduced into soils and sediments by acidic waste fluids. Of fundamental importance are the coupled effects of acid-driven mineral transformation and reactive transport on U speciation. To evaluate the effect of waste-fluid residence time and co-associated dissolved phosphate concentrations on U speciation in impacted soils and sediments, uncontaminated surface materials (from the Hanford Site) were reacted with U-containing synthetic acidic waste fluids (pH 2) amended with dissolved phosphate concentrations in both batch (no flow) and flow-through column systems for 7-365 days. By comparing dissolved U behavior and solid phase speciation as a function of flow regimen, we found that the availability of proton-promoted dissolution products (such as Si) to sequester U into uranyl silicates was dependent on waste fluid-sediment contact time as uranyl silicates were not detected in short contact time flow-through systems but were detected in no-flow, long contact time, reactors. Moreover, the dominance of uranyl phosphate as neoprecipitate U scavenger (principally in the form of meta-ankoleite) in phosphate amended systems confirmed the importance of phosphate amendments for an efficient sequestration of U in the soils and sediments. Overall, our experiments suggest that the formation of uranyl silicates in soils impacted by acidic waste fluids is likely to be limited unless reaction products are allowed to accumulate in soil pores, highlighting the importance of investigating soil U speciation in flow-through, transport-driven systems as opposed to no-flow, batch systems. This study provides insights into uranium speciation and its potential changes under acidic conditions for better prediction of risks and subsequent development of efficient remediation strategies.
... Such fundamental knowledge is a key step towards solving the extreme complexity of the chemistry problems with radionuclides, 17-26 safe disposal of nuclear wastes and prediction of the radionuclide behaviour in the environment. 17,[27][28][29][30][31] What we know A brief introduction to HERFD-XANES, RIXS and XES is given in the next section. More information about these techniques can be found in the cited literature. ...
Article
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In recent years, scientists have progressively recognized the role of electronic structures in the characterization of chemical properties for actinide containing materials. High-energy resolution X-ray spectroscopy at the actinide M4,5 edges emerged as a promising direction because this method can probe actinide properties at the atomic level through the possibility of reducing the experimental spectral width below the natural core-hole lifetime broadening. Parallel to the technical developments of the X-ray method and experimental discoveries, theoretical models, describing the observed electronic structure phenomena, have also advanced. In this feature article, we describe the latest progress in the field of high-energy resolution X-ray spectroscopy at the actinide M4,5 and ligand K edges and we show that the methods are able to (a) provide fingerprint information on the actinide oxidation state and ground state characters (b) probe 5f occupancy, non-stoichiometry, defects, and ligand/metal ratio and (c) investigate the local symmetry and effects of the crystal field. We discuss the chemical aspects of the electronic structure in terms familiar to chemists and materials scientists and conclude with a brief description of new opportunities and approaches to improve the experimental methodology and theoretical analysis for f-electron systems.
... To attain a reliable interpretation, it is desirable to supplement of XAFS data with independent analytical methods such as X-ray diffraction (XRD), electron microscopy, time-resolved laser-induced fluorescence spectroscopy, UV-Vis spectroscopy, IR spectroscopy, Raman spectroscopy, etc. However, only a few works have studied U speciation in actual contaminated samples by advanced spectroscopic or microscopic methods (Batuk et al., 2015;Gartman et al., 2015;Kaplan et al., 2016;Lind et al., 2005Lind et al., , 2007Novikov et al., 2006;Tayal et al., 2019), as the usually low concentration of U and the presence of multiple U species therein complicate the interpretation of the XAFS data and even data acquisition. Radionuclide mobility in samples collected at contaminated sites could be characterized by sequential extraction tests that provide semi-quantitative but useful information on the partitioning of contaminants with various geochemical fractions. ...
Article
Sequential extraction tests were used to study partitioning of U in the bottom sediments of two reservoirs that have been used for the temporary storage of nuclear waste at the “Mining and Chemical Combine” (Zheleznogorsk, Krasnoyarsk region, Russia). Various sequential extraction protocols were applied to the bottom sediment samples and the results compared with those obtained for laboratory-prepared simulated samples with different speciation and partitioning, e.g., U(VI) sorbed onto various inorganic minerals and organic matter, as well as uranium oxides. The distributions of uranium in fractions extracted from simulated and actual contaminated samples were compared to shed light on the speciation of U in the bottom sediments. X-ray absorption spectroscopy, X-ray diffraction, and scanning electron microscopy were also used to analyze the partitioning of U in contaminated sediments. We also compared the results obtained using the spectroscopic and microscopic techniques, as well as sequential extraction.
Article
Small-particle analysis is a highly promising emerging forensic tool for analysis of interdicted special nuclear materials. Integration of microstructural, morphological, compositional, and molecular impurity signatures could provide significant advancements in forensic capabilities. We have applied rapid, high-sensitivity, hard X-ray synchrotron chemical imaging to analyze impurity signatures in two differently fabricated fuel pellets from the 5th Collaborative Materials Exercise (CMX5) of the IAEA Nuclear Forensics International Working Group. The spatial distributions, chemical compositions, and morphological and molecular characteristics of impurities were evaluated using X-ray absorption near-edge structure (XANES) and X-ray fluorescence chemical imaging to discover principal impurities, their granularity, particle sizes, modes of occurrence (distinct grains vs incorporation in the UO2 lattice), and sources and mechanisms of incorporation. Differences in UO2+x stoichiometry were detected at the microscale in nominally identical UO2 ceramics (CMX5-A and CMX5-B), implying the presence of multiple UO2 host phases with characteristic microstructures and feedstock compositions. Al, Fe, Ni, W, and Zr impurities and integrated impurity signature analysis identified distinctly different pellet synthesis and processing methods. For example, two different Al, W, and Zr populations in the CMX5-B sample indicated a more complex processing history than the CMX5-A sample. K-edge XANES measurements reveal both metallic and oxide forms of Fe and Ni but with different proportions between each sample. Altogether, these observations suggest multiple sources of impurities, including fabrication (e.g., force-sieving) and feedstock (mineral oxides). This study demonstrates the potential of synchrotron techniques to integrate different signatures across length scales (angstrom to micrometer) to detect and differentiate between contrasting UO2 fuel fabrication techniques.
Article
Beginning in 1943, process wastes containing approximately 1.85 × 10¹⁵ Bq (200 kg) of plutonium (Pu) were released into unlined cribs, trenches, and field tiles at the Hanford Site, Washington, USA. The 216-Z-9 (Z-9) unlined trench received over 4 × 10⁶ liters of mixed Pu processing waste from the Plutonium Finishing Plant (PFP) consisting of high ionic strength (∼ 5 M nitrate, ∼ 0.6 M Al), acidic (pH ∼ 2.5) solutions, which also contained the organic solvents: CCl4, TBP, DBBP, and lard oil. A small fraction of Pu migrated deep into the subsurface vadose zone to depths of 37 m, but the mechanisms controlling Pu migration beneath the trench are unknown. In this study, we determined Pu partitioning behavior in a series of binary and ternary batch experiments containing aqueous, organic, and solid phases representative of the waste constituents and natural sediments of the Z-9 trench in order to develop a conceptual model for the transport of Pu in the subsurface. Our results show that Pu at equilibrium with low pH high nitrate waste and in the presence of a TBP/organic phase, can migrate as a Pu-TBP-nitrate complex in the organic phase as long as the low pH and high nitrate concentrations are maintained. Reducing the nitrate concentrations or increasing the pH will lead to Pu partitioning into the aqueous phase and subsequent sorption to native Hanford sediments. The results of this work suggest that Pu migration in the subsurface is likely driven by weak sorption of aqueous Pu under low pH conditions as well as the formation of Pu-TBP-nitrate complexes in the organic phase. Long-term Pu migration will be limited by the transient nature of the low pH conditions and the dispersion of the nitrate plume.
Article
To identify the role of waste composition and sediment interactions in controlling Pu and Am mobility in contaminated sediments at Hanford, a legacy nuclear site, Pu and Am concentrations in solutions equilibrated with contaminated sediments from beneath the 216-Z-9 (Z-9) Trench were compared to the solubilities of PuO2 materials, representative of the disposed wastes, in the absence of sediments. This work shows that the solubilities of PuO2 materials synthesized by different methods, and with varying particle sizes, agree with PuO2(am,hyd), although dissolution kinetics were differed between materials. According to saturation index (SI) calculations, PuO2(am,hyd) is likely also controlling Pu release from sediments under conditions where phosphate concentrations are low. However, both Pu-phosphate and Am-phosphate phases, identified in SI calculations and by high resolution transmission electron microscopy, play roles in controlling release in low pH, high phosphate shallow sediments at the top of the Z-9 Trench. The elevated phosphate is likely due to decomposition of tributyl phosphate over time. Sediments from deeper in the subsurface beneath the Z-9 Trench are less acidic and contain less phosphate, with Pu solubility likely controlled by PuO2(am,hyd) that precipitated following neutralization of the acidic waste stream. Controls on Am concentrations in deeper sediments are more complex and potentially involve sediment adsorption and/or release from Pu(1-x)AmxO2 following Am in-growth. The concentrations of both Pu and Am were elevated in the colloidal fraction associated with shallow sediments, but not in PuO2 experiments, suggesting the presence of Pu/Am pseudocolloids (e.g., Pu/Am associated with mineral colloids). However, Pu and Am association with the colloidal size fraction was not observed in deeper sediments, suggesting transport of Pu and Am to these depths beneath the Z-9 Trench was not due to colloidal transport.
Article
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The high-energy release of plutonium (Pu) and uranium (U) during the Maralinga nuclear trials (1955–1963) in Australia, designed to simulate high temperature, non-critical nuclear accidents, resulted in wide dispersion µm-sized, radioactive, Pu–U-bearing ‘hot’ particles that persist in soils. By combining non-destructive, multi-technique synchrotron-based micro-characterization with the first nano-scale imagining of the composition and textures of six Maralinga particles, we find that all particles display intricate physical and chemical make-ups consistent with formation via condensation and cooling of polymetallic melts (immiscible Fe–Al–Pu–U; and Pb ± Pu–U) within the detonation plumes. Plutonium and U are present predominantly in micro- to nano-particulate forms, and most hot particles contain low valence Pu–U–C compounds; these chemically reactive phases are protected by their inclusion in metallic alloys. Plutonium reworking was observed within an oxidised rim in a Pb-rich particle; however overall Pu remained immobile in the studied particles, while small-scale oxidation and mobility of U is widespread. It is notoriously difficult to predict the long-term environmental behaviour of hot particles. Nano-scale characterization of the hot particles suggests that long-term, slow release of Pu from the hot particles may take place via a range of chemical and physical processes, likely contributing to on-going Pu uptake by wildlife at Maralinga.
Article
Plutonium (Pu) has been released to the environment worldwide, including approximately 1.85 × 1015 Bq (200 kg) of Pu from process waste solutions to unconfined soil structures at the Hanford Site in Washington State. The subsurface mobility of Pu is influenced by complex interactions with sediments, groundwater, and any co-contaminants within the waste stream. Previous investigations at Hanford have shown that Pu exists as discrete PuO2 particles forming before or after disposal, as secondary solid phases formed from waste interactions with sediments as adsorbed/incorporated species, and/or as dissolved species. In this research, new evidence is presented for the existence of PuO2, PuO2-Bi2O3 composites, and particles from burnt Pu metal in near-surface sediments where Pu-laden acidic process waste was disposed to sediments. Pu and americium (Am) L3 X-ray absorption spectroscopy and density functional theory suggest that, in larger, more crystalline PuO2 particles, Am formed from radioactive decay is retained in the PuIVO2 structure as AmIV. The Pu and Am that were disposed of in an acidic waste stream have since migrated deeper into the subsurface with detection to at least 37 meters below ground surface. In contrast, Pu deposited near the ground surface from neutral pH waste is found to be homogeneously distributed and relatively immobile. Groundwater extractions performed on contaminated sediments indicate that both Pu and Am are recalcitrant, with Am being fractionally less extractable than Pu on a molar basis. These results suggest that the more mobile fraction of Am has migrated from the near-surface and may be present in the deeper sediments as a different phase than Pu. From these results, it is suggested that Pu and Am deposited from acidic wastes were initially mobile and became significantly less mobile as wastes were neutralized within the soil profile.
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X-ray-absorption fine structure measurements have been used to elucidate the local atomic structure of quaternary Zr, Y, Er, Ce/U cubic stabilized zirconia. These compounds display more complicated local environments than those reported for simpler binary systems. While the shortest cation-O distances are similar to those found in the binary cubic stabilized compounds, responding to the different sizes of the cations, we have identified large distortions in the first-shell oxygen distribution involving long, 2.8-3.2 Å cation-O distances that are similar to those found in the amorphous phase of zirconium. The cation-cation distributions are also found to be quite complicated (non-Gaussian) and element specific. The U-near neighbor distances are expanded relative to the Ce ions for which it substitutes, consistent with the larger size of the actinide, and the U-cation distribution is also more complicated. In terms of the effects of this substitution on the other cation sites, the local environment around Y is altered while the Zr and Er local environments remain unchanged. These results point out the importance of collective and correlated interactions between the different pairs of cations and the host lattice that are mediated by the local strain fields generated by the different cations. The presence of pair-specific couplings has not been commonly included in previous analyses and may have implications for the stabilization mechanisms of cubic zirconia.
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Oxidation–hydration weathering of uraninite, the most common U-bearing mineral in nature, comprises various physical and chemical processes that lead to the destruction of the fluorite-type structure of uraninite where U is present as tetravalent. This results in replacement of uraninite by weathering products containing U in hexavalent form, i.e. as uranyl ion, UO22+. The final assemblage of the weathering products, uranyl minerals, and their compositions depend on the various factors, namely the composition of the primary minerals and percolating oxidizing fluids that cause the alteration. The knowledge of such processes and stabilities of the uranium minerals is of the great interest namely due to demand for U as the energy source. During the past decade there has been substantial progress in understanding the mineralogy, crystallography and thermodynamics of uranyl minerals and thus a substantially improved understanding of the weathering processes themselves. This review aims to summarize the state-of-art of the current knowledge on uranium-related topics as well and identify some of the important questions that remain unanswered.
Article
Pb(II) sorption on goethite and hematite powders was studied at room temperature as a function of pH (6–8), sorption density (2–10 μmoles/m2), and [Pb]eq (0.2 μM – 1.2 mM) in 0.1 M NaNO3 electrolyte using XAFS spectroscopy. Pb(II) ions were found to be hydrolyzed and adsorbed as mononuclear bidentate complexes to edges of FeO6 octahedra on both goethite and hematite under all conditions. Hydrolysis of Pb(II) appears to be a primary source of proton release associated with surface complexation of Pb(II). A bond-valence model was used to relate the relative stabilities of iron-oxide surface functional groups and Pb(II) adsorption complexes to their structures and compositions. This combined approach suggests that Pb(II) adsorption occurs primarily at unprotonated [] sites and at [] sites. Several adsorption reactions are proposed. Comparison to EXAFS results from Pb(II) adsorption on aluminum oxides suggests that the edge lengths of surface AlO6 or FeO6 octahedra partially determine the reactivities and densities of available surface sites. The results of this study provide a basis for constructing chemically realistic descriptions of Pb(II) surface complexation reactions on Fe (hydr)oxides.
Article
Until recently, the reduction of U(VI) to U(IV) during bioremediation was assumed to produce solely the sparingly soluble mineral uraninite, UO2(s). However, results from several laboratories reveal other species of U(IV) characterized by the absence of an EXAFS U–U pair correlation (referred to here as noncrystalline U(IV)). Because it lacks the crystalline structure of uraninite, this species is likely to be more labile and susceptible to reoxidation. In the case of single species cultures, analyses of U extended X-ray fine structure (EXAFS) spectra have previously suggested U(IV) coordination to carboxyl, phosphoryl or carbonate groups. In spite of this evidence, little is understood about the species that make up noncrystalline U(IV), their structural chemistry and the nature of the U(IV)–ligand interactions. Here, we use infrared spectroscopy (IR), uranium LIII-edge X-ray absorption spectroscopy (XAS), and phosphorus K-edge XAS analyses to constrain the binding environments of phosphate and uranium associated with Shewanella oneidensis MR-1 bacterial cells. Systems tested as a function of pH included: cells under metal-reducing conditions without uranium, cells under reducing conditions that produced primarily uraninite, and cells under reducing conditions that produced primarily biomass-associated noncrystalline U(IV). P X-ray absorption near-edge structure (XANES) results provided clear and direct evidence of U(IV) coordination to phosphate. Infrared (IR) spectroscopy revealed a pronounced perturbation of phosphate functional groups in the presence of uranium. Analysis of these data provides evidence that U(IV) is coordinated to a range of phosphate species, including monomers and polymerized networks. U EXAFS analyses and a chemical extraction measurements support these conclusions. The results of this study provide new insights into the binding mechanisms of biomass-associated U(IV) species which in turn sheds light on the mechanisms of biological U(VI) reduction.
Article
The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO 2+x (0.00⩽ x⩽0.20) and PuO 2+x/PuO 2+x-y(OH) 2y· zH 2O have been determined by U/Pu L III XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75 Å) U-O distances consistent with U(VI) concomitant with a large range of U displacements that reduce the apparent number of U neighbors and (2) that the UO 2 fraction remains intact implying that these O defects interact to form clusters and give the heterogeneous structure consistent with the diffraction patterns. The PuO 2+x system, which does not show a separate phase at its x=0.25 endpoint, also displays (1) oxo groups at longer 1.9 Å distances consistent with Pu(V+ δ), (2) a multisite Pu-O distribution even when x is close to zero indicative of the formation of stable species with H 2O and its hydrolysis products with O 2-, and (3) a highly disordered, spectroscopically invisible Pu-Pu component. The structure and bonding in AnO 2+x are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements.
Article
The storage of spent nuclear fuels for long periods of time in geological repositories is one proposed solution to the stewardship of legacy, current and future nuclear waste. Recent studies have shown that UO2, the major component of spent nuclear fuel, can oxidise under repository conditions to a number of secondary phases. The weathering of naturally occurring uranium minerals can give an insight into the behaviour of SNF and this review highlights the structural and spectroscopic characterisation of a number of relevant minerals. Conversely, reducing conditions could also be possible and these phase alterations are also discussed. Furthermore, the interaction of these minerals with the common fission products caesium, strontium, technetium, iodine, selenium and the transuranic elements (Np, Pu, Am, Cm) is reviewed, as these minerals may provide a mechanism for the retardation of the mobility of these radioisotopes.
Article
The physical, chemical, and radiological characteristics of material released to the environment from accidents involving nuclear weapon components are dependent upon many factors, especially the manner in which the material is released and delivered to the environment. These characteristics will also be influenced by physical and chemical effects associated with weathering if the material remains exposed to the environment for a long period of time. This study evaluates the morphological characteristics of particles released to the environment as a result of the 1960 BOMARC incident and compares these characteristics to those described following similar incidents at Thule, Greenland (1968) and Palomares, Spain (1966). Each of these incidents involved unique circumstances and conditions that distributed actinide-rich particles to the environment with a range of distinctive morphological characteristics. Morphological and surface elemental analyses were conducted on a set of discrete particles isolated from samples of post-remediated soil collected at McGuire Air Force Base, the site of the BOMARC incident. Scanning electron microscopy and complimentary energy dispersive X-ray spectroscopy were used to perform the analyses. Non-destructive analysis of uranium and plutonium contained in each particle was measured using high-resolution gamma spectrometry. Unique characteristics of the BOMARC samples include some particles exhibiting a smooth, crystalline texture and varying elemental surface distribution of uranium and plutonium, dependent on the particle’s morphology.
Article
We have characterized the adsorption of Co(II) on the (0001) and (102) surfaces of α-Al2O3 single crystals under ambient conditions using polarization-dependent grazing-incidence X-ray absorption fine structure spectroscopy, in combination with bond valence modeling. Co(II) ions were found to be adsorbed on both surfaces in an inner-sphere fashion. Adions were found to adsorb dominantly in a tridentate fashion (i.e., bonded to three surface oxygens) on the (0001) surface and dominantly in a tetradentate fashion on the (102) surface. Based on EXAFS results and bond valence analysis, plausible surface complexation reactions for Co(II) sorption on these two surfaces can be written as represent surface water molecules, hydroxyl groups, and oxygens bonded to one, two, and three Al cations, respectively.