Elements Volume 2 Issue 6 2006 (Doi 10.2113/gselements.2.6.365) Lumpkin, G. R. - Ceramic Waste Forms For Actinides
Elements Volume 2 Issue 6 2006 (Doi 10.2113/gselements.2.6.365) Lumpkin, G. R. - Ceramic Waste Forms For Actinides
Elements Volume 2 Issue 6 2006 (Doi 10.2113/gselements.2.6.365) Lumpkin, G. R. - Ceramic Waste Forms For Actinides
for Actinides
Gregory R. Lumpkin*
A natural example of
radiation damage,
mechanical behavior,
and chemical alteration.
The SEM image shows
allanite from Amelia,
Virginia, with high (green) and low (white) levels of
Th. Radiation damage in the main part of the crystal
has compressed the crystalline, low-Th zone, resulting
in brittle failure and the development of numerous
tension cracks. Late aqueous fluids have migrated
through the cracks and caused a preferential
alteration (blue areas) of the amorphous allanite. The
zoned allanite behaves as a composite material.
INTRODUCTION
Beginning in the 1970s with research on alternative waste
forms to borosilicate glass (McCarthy 1977) and followed
by the invention of Synroca synthetic rock made up of
stable titanate minerals (Ringwood et al. 1988) considerable effort has been dedicated to the development and scientific evaluation of ceramics designed for the safe disposal
of nuclear wastes. These diverse wastes range from the
reprocessed spent fuels from commercial power reactors to
high-grade plutonium derived from decommissioned
nuclear weapons. Over the past 25 years or so, titanate
ceramics have evolved from the original polyphase assemblages to specific compositions based largely on a single
phase. Advances in waste form development and testing
have been complemented by numerous mineralogical
investigations of the analogous crystalline phases in geological environments (Lumpkin 2001; Lumpkin et al. 2004a).
Information obtained from these studies has important
implications for validation of the long-term performance of
nuclear waste forms for disposal in geological repositories.
One of the major goals of research on crystalline ceramic
waste forms is to provide a host matrix capable of providing
a much higher level of chemical durability than borosilicate
glass when placed in a geological repository. Tailored
ceramics (Harker 1988), the Synroc titanate waste forms,
and their special-purpose derivatives are reasonably well
developed and have been the subject of extensive dissolution
* Australian Nuclear Science and Technology Organisation
PMB1, Menai, NSW 2234, Australia
E-mail: grl@ansto.gov.au
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FIGURE 2
Measurements of accelerated radiation damage and dissolution tests on polyphase Synroc samples (both Na-free and
Na-rich types) doped with 244Cm have been reported by
Mitamura et al. (1992, 1994). Up to the maximum dose
achieved during this study (1.3 1015 /mg), the Na-free
samples showed a consistent decrease in density with
FIGURE 1
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Pyrochlore
Cubic pyrochlore is a derivative of the fluorite structure and
corresponds to the general formula A2B2X6Y, in which A
and B are 8- and 6-coordinated cation sites and X and Y are
4-coordinated anions sites. Typically, A = Na, Ca, Y, lanthanides, and actinides, and B = Ti, Zr, Nb, Hf, Ta, Sn, and
W. Many more elements are found in natural samples, and
hundreds of different pyrochlore compositions have been
synthesized. Prototype waste form compositions are usually
based on the CaUTi2O7 end-member. Other actinides substitute directly for U in this compound. The neutron
absorber Hf substitutes directly for Ti on the B site, whereas
Gd occupies the A site and substitutes for Ca and U.
Dissolution studies on ion-irradiated titanate samples generally indicate that maximum Y and lanthanide release
rates may be anywhere between no change and 20 times
higher as a result of amorphization (Begg et al. 2001). Similar pyrochlore samples with Zr on the B site have been
studied, and these samples retained crystallinity at a dose
that renders the titanate amorphous. After a maximum
time of 28 days, the dissolution rates of the irradiated and
unirradiated zirconate samples were the same, dropping
from 0.11 g/m2/d to about 0.050.09 g/m2/d for Ca and
from 0.010.1 g/m2/d to 0.0020.006 g/m2/d for Gd. These
values are similar to those of the unirradiated titanate
pyrochlore. The variability of results in the studies noted
above probably relates to the experimental details, including temperature and redox state, nature of the apparatus
used, and quality of the samples (e.g. porosity, surface area,
tendency to crack, soluble impurity phases).
Lumpkin and Ewing (1988) have shown that natural
pyrochlores with Nb, Ta, Ti, and minor Sn and Zr on the B
site are subject to amorphization at a critical dose of about
1016 /mg. This dose is about 23 times higher than that for
CaPuTi2O7 (Clinard et al. 1984a) and is consistent with conditions encountered during storage at elevated temperatures in the Earths crust (Lumpkin 2001). This dose, however, increases with the geological age of the sample for
unmetamorphosed samples; this points to long-term
annealing via atomic diffusion. This study also provided a
systematic evaluation of strain, crystalline domain size, and
microstructural details as a function of dose. An example of
natural pyrochlore, associated with zirconolite, from Ti-rich
veins of the Adamello massif in northern Italy is shown in
FIGURE 3. These actinide-rich, radiation-damaged, and compositionally zoned crystals have coexisted for forty million
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TABLE 1
Waste form
Main phases
Application/waste loading
Synroc-C
Zirconolite, perovskite,
hollandite, rutile
Synroc-D
Zirconolite, perovskite,
spinel, nepheline
Synroc-F
Tailored
ceramics
Magnetoplumbite, zirconolite,
spinel, uraninite, nepheline
Pyrochlore
Pyrochlore, zirconolite-4M,
brannerite, rutile
Zirconolite
Zirconolite, rutile
Separated actinides,
up to about 25 wt%
Monazite
Monazite
Actinidelanthanide wastes,
up to about 25 wt% actinides
Zircon
Zircon
Glass-ceramics
Others
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Zirconolite
Backscattered SEM image (colorized) showing intergrowth of natural pyrochlore (Py, gray) and zirconolite
(Z, blue, green, yellow) from Ti-rich veins, Adamello, Italy. Pyrochlore
(containing about 30 wt% UO2) occurs as overgrowths on zirconolite, is
amorphous due to alpha-decay damage, and exhibits a darker rim due
to minor hydration. Zirconolite is chemically zoned, with up to about
25 wt% Th + U oxide, and its structure ranges from crystalline to amorphous. Note the absence of cracking from differential swelling.
FIGURE 3
Cubic Zirconia
These compounds are essentially fluoritedefect fluorite
structures based on the general formula MO2-x with M = Ca,
lanthanides, actinides, and other elements simulating
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Perovskite
Brannerite
Brannerite, ideally UTi2O6, consists of layers of Ti octahedra
connected by columns of U octahedra. It is a minor but
actinide-rich phase in some of the pyrochlore-based compositions designed for the disposal of Pu. Brannerite may
account for up to 20 percent of the U and 15 percent of the
Pu in these waste forms (Thomas and Zhang 2003). Natural
and synthetic brannerites can incorporate substantial amounts
of Ca, REE, Th, and other elements. In both cases, the incorporation of lower-valence cations on the A site may be charge
balanced by oxidation of some U4+ to U5+ and/or U6+ ions.
Synthetic samples are easily amorphized by ion irradiation.
Electron microscopy studies show that most natural brannerites with ages greater than about 10 Ma are fully amorphous due to alpha-decay damage and are commonly
altered by natural aqueous fluids. Altered natural brannerite typically loses U, and the concentration may decrease to
approximately 1 wt% UO2 in the most heavily altered areas.
The most important findings to emerge from experimental
studies (e.g. Zhang et al. 2003) are that certain species, e.g.
phthalate, will increase the solubility of titanium. In bicarbonate solutions, however, the uranium release rate is
strongly dependent on bicarbonate concentration. In acidic
solutions, the dissolution of brannerite involves a preferential release of uranium. TEM results have shown that brannerite exposed to a solution with a pH = 2 at 90C for four
weeks produces a relatively small amount of reaction product, which consists mainly of polycrystalline anatase.
When exposed to a pH 11 solution at 90C for four weeks,
a fibrous secondary phase is formed on the surface. This
phase is amorphous and contains a significant proportion
of U6+, as indicated by surface analytical studies. Thomas
and Zhang (2003) have shown that the aqueous dissolution
of brannerite in a open atmosphere can be modelled as a
function of pH using two reaction steps: oxidation of U4+ at
the surface followed by release of U6+ into solution, which
is catalyzed by protons under acidic conditions or carbonate species under alkaline conditions. This is an important
advance, as the release of uranium at 40C can be predicted
quantitatively from this model.
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FIGURE 4
Zircon
Zircon, ZrSiO4, is a common accessory mineral found in a
variety of geological environments. The major elemental
impurity in natural zircon is Hf, which substitutes for Zr.
Trace to minor amounts (generally 5000 ppm or less) of
other elements may be present, including Ca, lanthanides,
and actinides on the Zr site and P on the Si site. Higher concentrations have been reported but are exceptional. Amorphization, with a critical dose of about 4 1015 /mg in natural and actinide-doped samples, and total volume swelling
of up to 18 percent are well-known characteristics of zircon
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Monazite
Like zircon and thorite, monazite also has ABO4 stoichiometry, but the crystal structure is monoclinic and consists of
chains of alternating BO4 tetrahedra and AO9 polyhedral
sites. These chains are cross linked by edge sharing with the
AO9 polyhedra, effectively closing off open tunnels and creating a structure that is approximately 10% denser than the
zircon structure type. Natural monazite contains up to
27 wt% UO2 + ThO2 and remains crystalline in spite of high
accumulated alpha-decay doses; however, monazite can be
amorphized by heavy ion irradiation, and the different
compounds have been studied in some detail as a function
of temperature. Irradiated, fully amorphous synthetic monazite has excellent aqueous durability and is roughly equivalent to the corresponding crystalline samples.
Monazite is highly insoluble in most hydrothermal and
low-temperature fluids; however, solubility may be
enhanced in aqueous fluids with low pH, low phosphate
content, or high F concentrations, which can lead to the
formation of REE-fluoride complexes. At temperatures
below 250C, the solubility of monazite in aqueous solutions decreases with increasing temperature (see Boatner
and Sales 1988; Lumpkin et al. 2004a). Experiments recently
reported by Oelkers and Poitrasson (2002) have provided
results on the steady-state dissolution rates of monazite at
temperatures of 50230 C, pH ranging from 1.5 to 10, and
variable flow rate and surface area. Using a natural sample
as the starting material, these authors showed that the
release rates of the REEs and U are essentially congruent for
all experimental conditions.
Systematic ion irradiation studies of the orthophosphates
(Meldrum et al. 1997) have been conducted for six monazite-structure compounds (A = La, Pr, Nd, Sm, Eu, Gd) and
six zircon-structure compounds (A = Sc, Y, Tb, Tm, Yb, Lu).
The critical amorphization temperatures of these materials
were found to increase systematically with cation mass
ELEMENTS
from about 330 to 490K for the monazites and from about
470 to 580K for the zircon-structure orthophosphates (compare this with the high values found for silicate zircons).
OTHER COMPOUNDS
A number of materials based on compounds such as britholite, crichtonite, garnet, kosnarite, murataite, and titanite
(sphene) have been proposed as waste forms for actinides,
lanthanides, and other elements (e.g. radioactive Sr and I,
and a range of impurities such as transition metals). With
the possible exception of britholite and titanite, none of
these compounds have been studied to the same extent as
those described above. All have natural analogues, although
the contents of Th and U can be quite low in many natural
samples. Stefanovsky et al. (2004) summarized the work on
these phases, and they will not be dealt with further in this
article (apart from the comparison shown in TABLE 2).
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ACKNOWLEDGMENTS
FIGURE 5
TABLE 2
Chemical
flexibility
Waste
loading
Radiation
tolerance
Volume
swelling
Natural
analogues
Perovskite
(Ca,Sr)TiO3
Low
Medium
Low
Medium
High
Yes
Pyrochlore
Gd2(Ti,Hf)2O7
High
High
High
Low-high
Medium
Yes
Zirconolite
CaZrTi2O7
High
High
Medium
Low-medium
Medium
Yes
Zircon
ZrSiO4
High
Medium
Low (?)
Low
High
Yes
Monazite
LnPO4
High
Medium
High
High
Low
Yes
Zirconates
Gd2(Zr,Hf)2O7
High
Medium
Medium
High
Low
No
Zirconia
(Zr,Ln,Act)O2-x
High
Medium
Medium
High
Low
No
Medium
Medium
High
Low
Yes
High
Medium
Low (?)
Yes
High
High
Medium
Medium
Rare
High
Medium
Low
Yes*
Titanite
CaTiSiO5
Medium
Medium
Low
Low
Medium
Yes
Medium
Medium
Low
Low
Medium
Yes
Kosnarite
NaZr2(PO4)3
Medium
Medium
Medium
Low
Yes
Brannerite
UTi2O6
Crichtonite
Ca(Ti,Fe,Cr,Mg)21O38
Murataite
Zr(Ca,Mn)2(Fe,Al)4Ti3O16
Garnet
Ca3Zr2(Al,Si,Fe)3O12
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REFERENCES
Begg BD, Hess NJ, Weber WJ, Devanathan
R, Icenhower JP, Thevuthasan S, McGrail
BP (2001) Heavy-ion irradiation effects
on structures and acid dissolution of
pyrochlores. Journal of Nuclear Materials
288: 208-216
Boatner LA, Sales BC (1988) Monazite. In:
Lutze W, Ewing RC (eds) Radioactive
Waste Forms for the Future. NorthHolland, Amsterdam, pp. 495-564
Chakoumakos BC, Murakami T, Lumpkin
GR, Ewing RC (1987) Alphadecayinduced fracturing in zircon: The
transition from the crystalline to the
metamict state. Science 236: 1556-1559
Chakoumakos BC, Oliver WC, Lumpkin
GR, Ewing RC (1991) Hardness and
elastic modulus of zircon as a function of
heavy-particle irradiation dose: I. In situ
alpha-decay event damage. Radiation
Effects and Defects in Solids 118: 393-403
Clinard FW Jr, Peterson DE, Rohr DL,
Hobbs LW (1984a) Self-irradiation effects
in 238Pu-substituted zirconolite: I.
Temperature dependence of damage.
Journal of Nuclear Materials 126: 245-254
Clinard FW Jr, Rohr DL, Roof RB (1984b)
Structural damage in a self-irradiated
zirconolite-based ceramic. Nuclear
Instruments and Methods in Physics
Research B 1: 581-586
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