Crystallization of Rhenium Salts in a Simulated Low-Activity Waste
Borosilicate Glass
Brian J. Riley,*
,†
John S. McCloy,* Ashutosh Goel, Martin Liezers, Michael J. Schweiger, Juan Liu,
Carmen P. Rodriguez, and Dong-Sang Kim
Pacific Northwest National Laboratory, PO Box 999, Richland, Washington 99352
This study presents the characterization of salt phases that
formed on simulated low-activity waste glass melts during a
rhenium solubility study. This study with rhenium salts is also
applicable to real applications involving radioactive technetium
salts. In this synthesis method, oxide glass powder is mixed
with the volatile species, vacuum-sealed in a fused quartz
ampoule, and then heated in a furnace. This technique restricts
the volatile species to the headspace above the melt but still
within the sealed ampoule, thus maximizing the concentration
of these species that are in contact with the glass. Above the
previously determined solubility of Re
7+
in this glass, a molten
salt phase segregated to the top of the melt and crystallized
into a solid layer. This salt was analyzed with X-ray diffrac-
tion, scanning electron microscopy, energy dispersive spectros-
copy, as well as wavelength dispersive spectroscopy and was
found to be composed of alkali perrhenates (NaReO
4
, KReO
4
)
and alkali sulfates. Similar crystalline inclusions were found in
the bulk of some glasses as well.
I. Introduction
V
ITRIFICATION is the planned technology to immobilize
the low-activity radioactive waste (LAW) stored at the
Hanford site in Washington State. This waste is a byprod-
uct of 45 years of plutonium production and currently occu-
pies 2.1 9 10
11
m
3
in underground tanks. Within these
tanks are elements that span the periodic table, some of
which are long-lived isotopes with half-lives (t
1/2
) lasting for
hundreds of thousands to millions of years, most notably
technetium-99 (
99
Tc, t
1/2
= 2.1 9 10
5
yr) and iodine-129
(
129
I, t
1/2
= 1.6 9 10
7
yr). The Hanford site tanks contain
an estimated ~9 9 10
2
TBq (~1500 kg) of
99
Tc and ~1 TBq
(~180 kg) of
129
I.
1
Current calculations estimate that >90%
of the
99
Tc inventory and 20% of the
129
I inventory in the
tanks will be immobilized in the LAW glass.
2
Technetium is a problematic component in glass for sev-
eral reasons. Pertechnetate (TcO
4
) and TcO
2
are the pri-
mary forms of Tc present in glass and are at least somewhat
soluble in water, although TcO
2
is much less so.
3,4
Thus, if
released, the Tc will migrate easily into the ground water.
5,6
Also, Tc-species are volatile so the Tc retention during the
vitrification process tends to be low.
3,7–9
Although it is not
preferred, this lost fraction can be captured with off-gas
scrubbing and recycled back into the process loop.
Technetium volatilization can be higher when preferen-
tially segregated into a molten salt phase, if present, and
Kim et al.
9
noted that this is more probable when in the
presence of sulfates. It is well-known that sulfates are a cause
of concern during melting as are other anionic salts (e.g.,
NO
2
, NO
3
, Cl
,I
, CrO
4
2
) present in LAW.
10–14
This is
because a layer of salt, sometimes called “gall”, is formed on
the melt pool surface.
15
This gall layer is fed by molten inclu-
sions of salt that have a lower viscosity and density than the
melt and tend to float to the top of the melt where they
remain segregated or are reduced (such as SO
4
2
? SO
2
)
and leave the melt (i.e., volatilize) as a gas. This behavior is
a result of these anions being present in depolymerized
regions of the melt where they segregate out of the polymer-
ized glassy regions during cooling.
16–19
Some of the primary
limitations to retention are the Tc solubility and volatility in
the selected base glass, which will change with glass composi-
tion.
The ultimate goal of this study was to determine the solubil-
ity of Tc in a borosilicate glass for vitrification of Hanford
LAW, but rhenium (Re
7+
) was used as a Tc surrogate to
develop the technique. Rhenium is the most commonly
selected Tc surrogate for waste glass applications because it is
the most similar in chemistry, ionic size, and speciation in
glass.
3
The most common form of Tc found in the LAW simu-
lants, as well as the liquid and vapor phases during vitrifica-
tion, is Tc
7+
, e.g., KTcO
4
, Tc
2
O
7
.
3
Thus, the species selected
for this study were KReO
4
and Re
2
O
7
, both containing Re
7+
.
The host glass composition selected is based on the AN-105
low-sulfur tank waste, one of the earliest glasses planned to be
processed into LAW glass at Hanford and used in recent glass
formulation melter studies of Tc retention.
8,20
To avoid some
of the known problems with different behavior noted for
halide interaction with Re versus Tc,
21,22
fluorine and chlorine
were not included in the base glass composition for this study.
A process not common to oxide glass processing was
intentionally employed in this study to reduce the volatiliza-
tion potential of the rhenium salts from the melt during these
experiments. This approach, while being drastically different
from an actual melter scenario, was taken to avoid the exper-
imental artifacts associated with measuring the solubility of
volatile components. Here, glass powder was added to a
fused quartz ampoule that was subsequently vacuum-sealed
prior to heat treatment (Fig. 1). The ampoule provided an
enclosure to reduce the loss of volatile species from the glass
to the atmosphere during melting where the volatile vapors
were still confined in the headspace above the melt. This spe-
cial configuration was designed to determine the solubility of
rhenium in this glass, as discussed elsewhere.
23,24
Once melted, the glass was air quenched, mounted in
resin, cross-sectioned, and characterized. Several different
techniques were employed to characterize the glass structure
and the crystalline inclusions within the glass. These tech-
niques included: X-ray diffraction (XRD), scanning electron
C. Jantzen—contributing editor
Manuscript No. 32200. Received October 17, 2012; approved February 11, 2013.
Manuscript Authored by Battelle Memorial Institute Under Contract Number DE-
AC05-76RLO1830 with the US Department of Energy. The US Government retains
and the publisher, by accepting this article for publication, acknowledges that the US
Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish
or reproduce the published form of this manuscript, or allows others to do so for US
Government purposes.
*Member, The American Ceramic Society
†
Author to whom correspondence should be addressed. e-mail: brian.riley@pnnl.gov
1150
J. Am. Ceram. Soc., 96 [4] 1150–1157 (2013)
DOI: 10.1111/jace.12280
© 2013 The American Ceramic Society
J
ournal