f
Retention of Neptunium
in
Uranyl Alteration
Phases
formed during
Spent Fuel Corrosion
E.
C.
Buck,
R.
J.
Finch,
P.
A.
Finn,
and
J.
K.
Bates
Chemical Technology
Division
Argonne National Laboratory
9700
South Cass
Avenue
Argonne
IL
60439-4837
W-31-109-ENG-33.
Accordin
ly,
the
U.S
Government
retains
a
nonexclusive, royag-free license
!o
publish
or
reproduce the published form of this contnbutiin, or
Submitted
for
consideration in
Scientific
Basis
for
Nuclear
Waste Management
XXI
Davos, Switzerland
August
1997
*
This
work
was
performed
under guidance
of
the Yucca Mountain Site Characterization Project
(YMF')
and
is
part
of
activity D-20-43 in the
YMP/
Lawrence Livermore National Laboratory Spent Fuel Scientific
Investigation
Plan.
Work supported
by
the
US.
Department
of
Energy under contract W-31-109-ENG-38.
DISCLAIMER
This report was prepared as an account of work sponsored by
an
agency of the
United States Government. Neither the United States Government nor
any
agency
thereof, nor
any
of their employes, makes any warranty, express or implied, or
assumes
any
legal liabiiity or responsibility for the accuracy, completeness, or use-
fulness of any information, apparatus, product,
or
process disclosed, or represents
that its use would not infringe privately owned rights. Reference herein to any
spe-
cific
commercial product, process. or service by trade name, trademark, manufac-
turer, or otherwise does not necessarily constitute or imply its endorsement,
recom-
mendation.
or
favoring
by
the United States Government or any agency thereof.
The views and opinions of authors expressed herein do not neassarily state or
reflect those of the United States Government or any agency thereof.
RETENTION OF NEPTUNIUM IN URANYL ALTERATION PHASES
FORMED
DURING SPENT FUEL CORROSION
E. C. BUCK,
R.
J. FINCH,
P.
A. FINN, AND
J.
K.
BATES
Argonne National Laboratory, Argonne,
IL
60439
ABSTRACT
Uranyl oxide hydrate phases are known to form during contact of oxide spent nuclear fuel with
water under oxidizing conditions; however, less is known about the fate of fission and neutron
capture products during this alteration. We describe, for the first time, evidence that neptunium
can become incorporated into the uranyl secondary phase, dehydrated schoepite (U0,*0.8%0).
Based on the long-term durability of natural schoepite, the retention of neptunium in
this
alteration
phase may
be
significant during spent fuel corrosion
in
an unsaturated geologic repository.
INTRODUCTION
Owing
to its long half-life and alpha activity, NpB7 is considered to
be
one of the most
important radionuclides to
be
immobilized
in
a geologic repository
[
11.
As
both carbonate and
hydroxide strongly complex Np ions
in
solution, it has been presumed that these forms of Np will
most likely control the release of Np from a geologic repository
[2].
We report evidence that Np
may become incorporated into dehydrated schoepite during the corrosion of spent nuclear oxide
fuel under some repository-relevant conditions.
when exposed to oxidizing conditions in silica-bearing solutions 131. However, under conditions
expected at the proposed geologic repository at Yucca Mountain in Nevada, spent fuel is
anticipated to be contacted only with water vapor andor small amounts of dripping water.
As
the
fuel is exposed to water vapor, a surface corrosion rind may form, consisting of uranyl oxide
hydrates and depending on the species present in the groundwater, in the presence of dripping
water, uranyl -silicates, -carbonates, and -phosphate may also eventually form
[3,4].
A
similar
paragenesis has been observed at many weathered natural uraninite
(UO,)
deposits, such
as
at
Shinkolobwe in the Congo and Peiia Blanca in Mexico [4,5].
The behavior of the fission and neutron capture products, during the anticipated corrosion
of spent nuclear fuel
in
an oxidizing environment is important for determining the long-term release
rates of radionuclides. Using a unique anion topology approach for comparing the structures of
uranyl phases,
Burns
et al. [6] have predicted mechanisms by which transuranic species could be
incorporated into the alteration products of corroded spent fuel.
As
the U-0 bond length (0.18
nm)
in the linear species UOF
is
similar to that of Np-0 in NpO,” (0.165-0.181 nm) and when
coordinated by
0,-,
OH-, or
$0,
the equatorial Np-0 bond distances are only about
0.01
nm
longer than those found in similarly coordinated uranyl polyhedra, it is reasonable to expect
isomorphic substitution of the neptonyl ion in uranyl ion sites [6]. However, owing to valence
bonding considerations, the
axial
oxygens on the linear neptonyl species may require additional
valence contributions from interlayer cations or protons in the uranyl phase.
Uranium dioxide readily alters to a series of uranyl oxide hydrates and uranyl silicates
EXPERIMENTAL,
PROCEDURE
Two types of unsaturated tests are discussed in this paper. The first involves exposing
Approved Testing Material (ATM) 103
[7]
to water vapor held at 90°C. The ATM103 is a single
pin spent nuclear fuel of moderate bum-up
(33
MWd/kgM) from the Calvert Cliffs pressurized
water reactor, which exhibits low fission
gas
release
(0.25%).
Under these conditions the fuel
pellets were exposed to a thin film of water. In the other type of tests, termed “high-drip”, about
0.75
mL
of EJ-13 water is dripped onto the ATM103 fuel package every 3.5 days [8]. The EJ-13
water is
a
tuff rock-equilibrated groundwater from the
J-
13 bore-hole near Yucca Mountain. Air is
also injected into the test vessel with the water. The waste package design consists
of
fuel pellets
placed on a Zircaloy retainer which has holes to allow the passage of water into a steel collection
vessel which is positioned below. The high-drip tests have been in continuous operation for nearly
5
years. Periodically, the collection vessel is removed, and the liquid solution contents analyzed.
Another clean collection vessel is then attached to the experimental setup, and the test is continued.
The solid samples were taken from the fuel pellets on the Zircaloy retainer. The corrosion products
that developed on these spent fuel fragments were examined with optical and scanning electron
microscopy (SEM).
Representative particles of the corroded fuel were embedded in an epoxy resin, and thin-
sectioned with an ultramicrotome. The resultant
30-50
nm electron-transparent cross-sections of
corroded fuel grains were transferred to carbon-coated copper grids and examined in a JEOL
2000
FXII
analytical transmission electron microscope (AEM) operated at
200
kV with a La, frlament.
The AEM is equipped with a Gatan 666 parallel electron energy-loss spectrometer, which has an
energy resolution of about
1.6-1.8
eV. The actinide M-edges were obtained by operating the
spectrometer in the second-difference mode
[9].
This technique removes the channel-to-channel
gain
variation that occurs with parallel detectors. The method also acts
as
a frequency filter,
enhancing
the
sharp features such
as
the “white-lines” on the absorption edges. Hence, peak shape
will effect the ability to detect an element.
A
smooth edge, for instance, will be less visible than a
sharp edge. Electron diffraction patterns were taken with a charged coupled device (CCD) camera
which permits very low intensity viewing and, therefore, is ideal for electron beam-sensitive
materials.
RESULTS
In
this section, the results from the analysis of the solid uranium-bearing phases from both
the vapor and high-drip rate tests are presented. Before discussing these data, the technique of
energy-loss spectroscopy (EELS) and how it pertains to the detection of transuranics will be
addressed.
In
EELS
the shape and intensity of an elemental absorption edge depends on that its
electron cross-section and chemical state. Hence, the technique can be used to determine the local
chemical environment. Compared to X-ray absorption spectroscopy, EELS utilizes much lower
energy absorption edges. Edges
in
the range from
50
eV up to
1900
eV are most commonly
investigated; however, in this work, the edges obtained were at an exceptionally high energy-loss
range (3500-4500 eV) for a transmission electron microscope. Longer counting times (or
integration times) were required, typically
2-5
s,
at each energy offset, higher beam intensities, and
a
large number of continuous acquisitions (typically
20-50).
For the second-derivative technique,
the derivative is calculated from three spectra taken at slightly different energy offsets (1-6 eV), and
then the derivative of these three spectra is calculated. This resulted in a total analysis time of
750
s.
These conditions necessarily cause immediate amorphization of the uranyl phases but,
believe, not vaporization. Energy-loss analysis with AEM, of course, has much better spatial
resolution than x-ray spectroscopic methods.
Low Level Detection of Transuranics
For the detection of fission products and transuranic elements present in spent fuel and its
alteration products, EELS has proven to be extremely effective. In Fig. 1, EELS from an
uncorroded sample of spent fuel are shown. The major component, uranium, is visible in the two
energy-loss ranges presented. In the lower energy range (Fig. la), the uranium
N4,5
edges at 738
eV and
780
eV,
along
with a number of rare earths, are visible. The transuranic N-edges overlap
strongly with the rare earths, and as they are weak features
in
this energy range, they can not be
detected. However, in the high energy-loss range (Fig. lb), the transuranic M-edges are clearly
visible. The sharp absorption edge features in Fig. lb, correspond to the two electronic
transitions, 34,
-+
5f12
(M,)
and
3dSn
+
5AI2
(M4), of the transuranics, that arise from spin-orbit
splitting. Owing to the interference with the large number of other elements present in the fuel and
in its corrosion products, this high energy-loss range is the only region where the transuranics can
be
detected with confidence
[
101. The higher energy transuranic L- and
K-
edges are beyond the
capabilities of the system.