04-01 FR0104314
Radiation Damage Effects in Pyrochlore and Zirconolite Ceramic Matrices for
the Immobilization of Actinide-Rich Wastes
G.R. Lumpkin, B.D. Begg, and K.L. Smith
Materials Division, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1,
Menai, NSW 2234, Australia
ABSTRACT
Actinide-doping experiments using short-lived 238Pu and 244£m n a v e demonstrated that
pyrochlore and zirconolite become fully amorphous at a dose of 0.2-0.5 x 1016 cc/mg at ambient
temperature and exhibit bulk swelling of 5-7%. Detailed studies of natural samples have included
determination of the critical amorphization dose, long-term annealing rate, microstructural changes
as a function of dose, and the thermal histories of the host rocks. Together, the laboratory based
work and studies of natural samples indicate that the critical amorphization dose will increase by
about a factor of 2-4 for samples stored at temperatures of 100-200 °C for up to 10 million years.
These studies of alpha-decay damage have been complemented by heavy ion irradiation studies
over the last ten years. Most of the irradiation work has concerned the critical amorphization dose
as a function of temperature in thin films; however, some work has been carried out on bulk
samples. The irradiation work indicates that most pyrochlore and zirconolite compositions will
have similar critical amorphization doses at low temperatures (e.g., below 300-400 °C).
Pyrochlores with Zr as the major B-site cation transform to a defect fluorite structure with
increasing ion irradiation dose, but do not become amorphous.
INTRODUCTION
In 1998 the United States Department of Energy selected a pyrochlore-rich titanate ceramic
wasteform for the disposal of surplus weapons plutonium (1). The current strategy involves storage
of the ceramic wasteform as part of a "co-disposal" waste package at the Yucca Mountain site in
southern Nevada. This decision represents the culmination of just over 20 years of intensive
research on ceramic nuclear wasteforms, many of which contained pyrochlore, zirconolite, and
other titanate phases for the incorporation of actinides and certain fission products (2-4). The
development of pyrochlore and zirconolite based wasteforms for actinide-rich wastes arose from a
number of the early studies that revealed their excellent chemical flexibility and durability in
aqueous fluids. Both structure types are particularly well suited for the incorporation of actinides
(ACTs) and rare earth elements (REEs), an important consideration for the control of nuclear
criticality.
Studies of natural samples have provided a means of testing the extrapolation of laboratory
data to the long time periods currently required by regulatory agencies for performance assessment.
In the area of radiation damage, mineralogical studies have produced quantitative data on the
crystalline-amorphous transformation and the structure of the metamict (amorphous) state (5-9).
Experimental evidence has also appeared in the last ten years from heavy ion irradiation studies (8,
9). Both of these areas of research complement the existing data from actinide doping experiments
carried out in the mid-1980s. Studies of samples from natural systems have also produced an
enormous amount of data on the durability and alteration mechanisms in aqueous fluids. However,
only limited data are currently available on the effect of radiation damage on the dissolution rates of
pyrochlore and zirconolite. In this paper, we present an overview and synthesis of the actinide
doping experiments, studies of natural samples, and heavy ion irradiation experiments.
CRYSTAL STRUCTURES OF PYROCHLORE AND ZIRCONOLITE
The structure of pyrochlore is considered to be an anion deficient derivative of the fluorite
structure type (10-12). Numerous studies have led to the general formula A2-mB2X6.wYi.n-pH2O,
where A represents cations in eight fold coordination, B represents cations in six fold coordination,
and X and Y are anions. The A-site accommodates cations having ionic radii of 0.086-0.155 nm,
whereas the B- site incorporates cations having radii of 0.054-0.083 nm. The crystal chemistry of
pyrochlore is very complicated, owing to the potential for vacancies at the A-, X-, and Y-sites (m =
0.0-1.7, w = 0.0-0.7, and n = 0.0-1.0) and the incorporation of water molecules (p = 0-2) in the
l 1
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vacant tunnel sites. However, wasteform pyrochlores are designed around the ideal, stoichiometric
formula A2B2X6Y. These pyrochlores are comprised of the end-members CaUTi2O7, CaPuTi2O7,
REE2Ti2C>7, and other minor components. Importantly, studies of natural samples indicate that
pyrochlores can contain up to 30 wt% UO2, 10 wt% TI1O2, 16 wt% REE2O3, and 10 wt% (Zr,
Hf)O2.
Zirconolite, ideally CaZrTi2O7, is also an anion deficient derivative of the fluorite structure
type and can be visualized as a condensed, layered version of pyrochlore with reduced symmetry
(12-15). In the structure of zirconolite-2M, the monoclinic aristotype, the Ca-site is eight
coordinated and is occupied by cations having ionic radii of 0.096-0.1143 nm. The Zr-site is seven
coordinated and occupied by cations with radii of 0.067-0.104 nm. Three distinct Ti-sites
accommodate cations with radii of 0.0535-0.083 nm. Natural zirconolite commonly deviates from
the ideal composition due to extensive coupled substitutions on the Ca- and Ti-sites, but the Zr-site
is subject to only limited substitution by other elements (16). However, Vance and coworkers (17-
19) have synthesized zirconolite with actinides and REEs on the Zr-site, and have confirmed many
of the substitutions found in natural samples. Natural zirconolites have the ability to incorporate up
to 24 wt% UO2, 22 wt% ThO2, and 32 wt% REE2O3 in the structure.
ACTINIDE DOPING EXPERIMENTS
Weber and coworkers (20, 21) examined Gd2Ti2C>7 (pyrochlore) and CaZrTi2O7 (zirconolite)
doped with 3 wt% 244Cm (half-life = 18 yr) and found that bulk swelling saturated at approximately
5.0-5.6% for pyrochlore and 6.0-6.5% for zirconolite. The pyrochlore and zirconolite samples
became amorphous to X-rays at a dose of 0.26 x 1016 a/mg and 0.47 x 1016 a/mg, respectively.
TEM results indicated that both materials exhibit isolated defect clusters (~ 2.5-5.0 nm in diameter)
at dose levels of 0.05-0.10 x 1016 a/mg. Overlap of damage clusters resulted in the appearance of a
diffuse ring in electron diffraction patterns at dose levels of 0.10-15 x 1016 a/mg. Strong mottled
diffraction contrast was evident in bright field images and diffuse rings were prominent in electron
diffraction patterns at doses of 0.15-0.27 x 1016 a/mg.
The dissolution behaviour of Cm-doped Gd2Ti2O7 and CaZrTi2O7 has been determined for
the amorphous samples and for splits of the amorphous material recrystallized at 1100 °C for 12
hours (21). Samples were leached in deionized water at 90 °C for 14 days. Results of the tests
indicate that the percent weight loss of both materials were higher by a factor of 2.5 in the
amorphous samples. Although, none of the samples showed detectable Ti in solution, the
amorphous pyrochlore showed an increase in the normalized mass loss of 244Cm and its daughter
product 240Pu by factors of 17 and 49, respectively. In contrast, the amorphous zirconolite showed
an increase in the normalized mass loss of Ca and 240Pu by factors of 8 and 11, respectively. In this
material, the release of 244Cm was identical for the amorphous and recrystallized samples and
lower than the release of24^Pu in the crystalline sample by a factor of 20.
Clinard et al. (22) studied monoclinic CaZrTi2O7 doped with 5 mol% 238Pu (half-life = 87
yr). After a maximum dose of 0.47 x 1016 a/mg, bulk swelling reached 5.5 vol% and neared
saturation. XRD results showed that the monoclinic phase transformed to hexagonal or
rhombohedral symmetry at a dose of-0 .11 x 1016 a/mg and became metamict at a dose of 0.3 x
1016 a/mg. Unit cell expansion determined by XRD was anisotropic, greatest along the c axis, and
the total unit cell volume expansion reached a maximum of ~ 1 vol% at the highest measurable
dose. TEM work indicated a highly crystalline structure at 0.003 x 1016 a/mg, 10-15 nm
crystallites in a highly disordered matrix at ~ 0.1 x 1016 a/mg, and some residual crystallinity at ~
0.4 x 1016 a/mg. Optical micrographs of the surface of a polished cylinder of the material showed
some microcracking during the crystalline-amorphous transformation.
Clinard and coworkers (23) also investigated samples of CaPuTi2O7 held at room temperature
302 °C, and 602 °C. The bulk swelling of a sample held at ambient temperature saturated at 5.4
vol% at a dose of 0.42 x 1016 a/mg. The material became XRD amorphous at a dose of 0.28 x 1016
a/mg. For a sample held at 302 °C, bulk swelling saturated at 4.3 vol% at a dose of- 0.66 x 1016
a/mg. A third sample held at 602 °C remained crystalline and reached a maximum swelling of only
0.4 vol% at a dose of 0.61 x 1016 a/mg. For the sample held at ambient temperature, TEM work
revealed isolated damage tracks at 0.01-0.13 x 1016 a/mg, small crystallites in an aperiodic matrix
at 0.26 x lO1^ a/mg, and some residual crystallinity at 0.31-0.66 x lO1^ a/mg. Crystallinity
persisted to higher doses in the sample held at 302 °C. In a similar study of Synroc doped with
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244Cm, Mitamura et al. (24) found that the volume expansion was reduced by a factor 2-3 for a
sample held at 200°C when compared to the room temperature data.
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NATURAL PYROCHLORE AND ZIRCONOLITE
Pyrochlore
Following the laboratory experiments described above, Lumpkin and Ewing (25) studied 51
pyrochlore samples ranging in age from 16 to 1400 Ma. Using the Th and U contents determined
by electron microprobe, the dose was calculated according to the equation:
D = 8N 38(eX238t2  - 1) + 6N
232t
232(e^  - 1) (Eq. 1)
In this expression, N238 and N232 are the present numbers of atoms per mg of U and Th, X.232 a nd
X.238 are the decay constants of 2^2Th and 2^8u, and t is the geological age. Using XRD to
determine both the beginning (Dj) of the crystalline-amorphous transformation as well as the critical
amorphization dose (Dc), it was shown that the transformation zone increased in dose as a function
of the geological age of the samples (Fig. la). Both of the dose curves shown in Figure la were
determined by fitting the data to an equation of the form:
Di,c = D0etK (Eq.2)
In Equation 2, Do is the intercept dose for Dj or Dc and K is a rate constant. These results provided
independent support for the annealing of isolated alpha-recoil collision cascades back to the
crystalline structure in natural pyrochlore. A more detailed analysis of the dose-age data yielded a
value of Do = 1.4 x 1016 a/mg for the amorphization dose curve and K = 1.7 x 10~9 y r 1 (26).
In the natural pyrochlore samples, XRD peak intensities decrease in a regular manner with
increasing dose, without transformation of the structure to the fluorite subcell, and with no major
changes in peak symmetry (25). After making a correction for long-term annealing, the intensity
ratios of the damaged samples were fitted to an equation of the form:
I/Io = e-BD (Eq. 3)
In this equation, B is a constant related to the amount of material damaged by each alpha-decay
event. Equation 3 gave an excellent fit to the data and yielded B = 2.6x 10'16 mg/a, corresponding
to an average cascade diameter of 4.6 nm in which a maximum of 2600 atoms are displaced. An
analysis of line broadening in these samples showed that crystallite dimensions decreased from
about 500 nm to 15 nm prior to complete amorphization. The magnitude of strain increased with
dose, reaching a maximum of approximately 0.003, then decreased to values below 0.0005 prior to
attainment of the fully amorphous state.
Changes in the microstructure of pyrochlore have also been investigated by using high-
resolution TEM (25). Mottled image contrast and local 1-5 nm sized areas of damage provided the
first indication of the beginning of the crystalline-amorphous transformation. With increasing dose,
local amorphous domains increased in abundance. Furthermore, some lattice misorientation
(generally less than 5°) was observed in the crystalline areas at intermediate dose levels. At higher
dose levels, the crystalline areas diminish in abundance, giving way to a microstructure dominated
by amorphous pyrochlore. Lattice fringes were not observed at the highest dose levels, indicating
that the amorphous state had been reached. Electron diffraction patterns revealed gradually
diminishing Bragg spots and increasing diffuse rings with equivalent d-spacings of 0.30 and 0.18
nm. There was no definitive evidence for conversion to the fluorite subcell, e.g., the (111) diffracted
beams and other superlattice spots were observed throughout the transformation. After a correction
is made for long-term annealing, the transformation zone ranges from about 0.1 x 1016 a/mg to 1.2
x 1016 a/mg.
Greegor and coworkers (27-30) carried out several studies of the local structure and bonding
around Ti, Nb, Ta, and U atoms in pyrochore using EXAFS-XANES. Results of these studies
demonstrated that the M-0 coordination polyhedra of metamict (amorphous) pyrochlore exhibit
reduced bond distances, reduced coordination number, and increased distortion relative to the
undamaged crystalline structure. Furthermore, there was no periodicity in evidence beyond the
second coordination sphere, with some disruption of the M-M distances. From these studies it was
realized that only a slight increase in the mean M-M distance was required in order to explain the
overall increase in volume caused by alpha-decay damage and that this could be facilitated by
increased M-O-M angles.
" I 1 1 1 I I M I 1 1
o / o
D _ —
o ~~ o
— — o
, , , , , ,1 ,1
Figure 1. Dose-age data for natural pyrochlore and zirconolite. a) Data for pyrochlore were
collected by XRD (25). b) Data for zirconolite collected using analytical TEM (36). The upper
solid curves labelled Dc represent the critical amorphization dose and the lower dashed curves
represent the onset of alpha-decay damage. The intervening regions are the crystalline-amorphous
transformation zones.
Zirconolite
Electron diffraction and high-resolution TEM studies suggest that metamict zirconolite lacks
periodicity beyond the second coordination sphere, consistent with a random network model of the
amorphous state (31, 32). EXAFS-XANES results provide more detailed information for the Ti-
and Ca-sites, demonstrating that metamict zirconolite lacks periodicity beyond the first
coordination sphere, with reduced M-0 bond lengths, reduced coordination number, and increased
distortion of the Ti-0 polyhedra (32). The reduced coordination of Ti in the metamict samples has
recently been confirmed, pointing specifically to a five-fold coordination geometry (33).
Additional results for the Zr-, Th-, and U-sites indicate nearly identical coordination numbers and
bond lengths for the metamict and annealed samples (34). However, a reduction in medium range
order (e.g., M-O-M periodicity) and a significant increase in the range of Zr-O and Th-0 distances
were observed, suggesting that a slight variation of the M-O-M angles can have a profound effect
on long-range periodicity and medium-range order.
A detailed study of highly zoned samples from Bergell (Switzerland) and Adamello (Italy)
recently provided the first detailed results on the crystalline-amorphous transformation in natural
zirconolites (35). The concentrations of ThO2 and UO2 obtained by analytical TEM on crystals
from both localities demonstrated the substantial range of alpha-decay dose in these samples. TEM
work also revealed the changes in microstructure with increasing dose. Bright field, dark field, and
high-resolution images illustrate the appearance of mottled diffraction contrast (0.08 x 1016 a/mg)
with extensive development of amorphous domains in a crystalline matrix (0.4 x 1016 a/mg). At
higher dose levels, collision cascades overlap to produce larger amorphous areas and the remaining
crystalline domains are reduced in size to less than 10 nm (0.7-0.9 x 1016 a/mg).
Preliminary results were recently presented for seven suites of zirconolite samples ranging in
age from 16 Ma to 2060 Ma and in dose from 0.008 x 1016 a/mg to 24 x 1016 a/mg (36). Using
analytical TEM methods, dose "brackets" were determined and used to construct a plot of dose
versus age, revealing a pattern of upward curvature with increasing age for both the onset dose and
critical dose (see Fig. lb). As in the previous work on pyrochlore, this upward curvature was
interpreted as evidence for long-term annealing of isolated alpha-recoil collision cascades. The
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data were fitted using Equation 2 in order to determine the intercept dose and annealing rate
constant. Curve fits gave values of Do = 0.11 x 1016 a/mg and K = 1.0 x 10~9 y r 1 for the onset
dose curve, and Do = 0.94 x 1016 a/mg and K = 0.98 x 10~9 y r 1 for the critical dose curve.
Some information is now available on the thermal histories of the natural zirconolite samples
(37). In the case of the Bergell intrusion, the thermal history is well constrained by
thermochronology, a method of time-temperature path analysis that relies on age dating techniques
with different closure temperatures (e.g., U-Th-Pb, K-Ar, Rb-Sr, and fission track dating). From
these data, an average effective temperature of 185 °C was calculated for the intrusion. Using a
simple model of conductive heat flow together with the available geological information, estimates
of the average effective temperature were derived for four other zirconolite localities. The
combined data indicate that the zirconolites have been stored in the Earth's crust at average
temperatures of 100-200°C (certain pyrochlores studied earlier, e.g., references 25 and 26, are from
some of the same localities and therefore experienced the same P-T-t history).
HEAVY ION IRRADIATION
In one of the first studies of pyrochlore or zirconolite, Ewing and Wang (38) irradiated a thin
TEM sample of recrystallized natural zirconolite using 1.5 MeV Kr ions and showed that the
zirconolite became amorphous after a dose of 4 x 1014 ions/cm2. Subsequent work on synthetic
zirconolite samples and U-doped pyrochlore revealed that Dc ranges from 3.5 x 10
14 ions/cm2 to
6.1 x 1014 ions/cm2 at room temperature, with no systematic variation as a function of composition
or polytype (39).
Wang et al. (40) recently examined the temperature dependence of amorphization of
Gd2Ti2O7 and CaZrTi2O7 thin films. Results for 1.0 MeV Kr ions revealed critical temperature
(Tc) values of 837 °C for the pyrochlore and 381 °C for the zirconolite. Tc is the critical
temperature above which amorphization cannot occur (the annealing rate is greater than the damage
production rate). Recent studies of the temperature dependence of Dc have been carried out for a
number of zirconolite compositions (41, 42) and analyzed using the simple model given below:
S ^ T c W J (Eq.4)
In Equation 4, Do is the critical dose at zero Kelvin, Ea is the activation energy for irradiation-
enhanced annealing of damage, and k is Boltzmann's constant. The data of Wang et al. (41)
indicate Tc values of 211-1 Al °C and activation energies of 0.2-0.4 eV for five different zirconolite
compositions (see Fig. 2a). In this work, the range of Tc values correlates with composition,
generally increasing with the level of Ce and Nd substitution. Similar results have been presented
by Smith et al. (42) for two CaZrTi2O7 samples prepared at 1200 °C and 1450 °C and having
different levels of crystallographic perfection (Fig. 2b). Smith et al. (42) also demonstrated that
these data are non-linear in plots of ln(l-Do/Dc) versus 1/T, indicating that the simple model of
Equation 4 relating the activation energy, critical temperature, and critical dose does not adequately
explain the results.
In A2Ti2O7 pyrochlores with A = Y, Sm, Gd and Lu, heavy ion irradiation work has shown
no significant effect of A-site ion mass or size on the temperature dependence of the critical dose
for amorphization (43). In each case the dose for amorphization at room temperature was found to
be approximately 0.5 x 1014 ions/cm2. In stark contrast, pyrochlores of Gd2(Ti2-xZrx)O7
stoichiometry display a dramatic decrease in susceptibility to radiation induced amorphization as a
function of increasing Zr content, with the end member Gd2Zr2O7 being totally resistant to
amorphization (44, 45). Dissolution experiments using a pH 2 solution at 90 °C have been
performed on some of these samples, but the results are not totally consistent. The leach rates of
Gd and Ti increased by a factor of ~10 in the amorphous sample of Gd2Ti2O7 relative to the
crystalline sample. For LU2T12O7, the leach rates of Lu and Ti increased by factors of about 5 and
9, respectively, due to amorphization. Interestingly, no significant effect of radiation damage was
observed for the leach rate of Y in Y2Ti2O7. In this sample, the initial Ti leach rate was increased
by a factor of 6 in the amorphous sample, but this decreased over the duration of the test (43).
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o -
1
1
A ;
1
/ ;
- - -/
/ o
y
_ // f_ -
/D fi-
-e M-=# •—a" * * ' ^
1 , i i , , , . . . i , , , i , ,
Figure 2. Heavy ion irradiation data for thin TEM specimens of zirconolite. a) Selected data from
Wang et al. (41). Note the large range in the Tc that varies with composition, b) Data from Smith
et al. (42) for end-member zirconolite prepared at 1200 °C (2M polytype with stacking disorder)
and 1450 °C (perfect 2M). Stacking disorder has little effect on the temperature dependence. All
data sets have a similar critical amorphization dose at temperatures below about 473 K (200 °C).
Certain pyrochlores undergo an irradiation-induced transformation to a defect fluorite
structure, although the stability of the resulting structure is highly dependent on the ionic radii ratio
of the original pyrochlore (46). Pyrochlores with an ionic radius ratio of VA^B < 1.52 (for instance
Gd2(Ti2-xZrx)O7 with x > 1.5) transform to a stable radiation resistant fluorite-structure after
irradiation with 2 MeV Au2+ to a dose of 5 x 10^ ions/cm2. However as the ionic radius ratio of
the pyrochlore increases beyond rA/re > 1-52, e.g., Gd2(Ti2-xZrx)O7 with x < 1.5, fluorite becomes
increasingly unstable with respect to the amorphous state. No fluorite was observed in the similarly
irradiated Gd2(Ti2-xZrx)O7 with x = 0.5 or Gc^T^O? samples (rA/rs > 1.66). Similar results have
been reported for the pyrochlores NaCaNb2O6F and NaCaTa2C>6F, both of which have r\/vB = 1.80
(47).
DISCUSSION
Studies of actinide doped and natural samples indicate that pyrochlores with Ti, Nb, and Ta
as the major B-site cations become amorphous as a result of the gradual accumulation of alpha-
recoil collision cascades until the material becomes aperiodic. Heavy ion irradiation studies
generally confirm these results and show that the temperature dependence of the critical
amorphization dose may vary dramatically. For the pyrochlore structure type, this may be a direct
result of the composition as expressed by the radius ratio XAI^B- In particular, pyrochlores with
rA/i"B < 1-52 transform to a defect fluorite structure that is resistant to amorphization by heavy ion
irradiation.
The dose curves for natural pyrochlore and zirconolite indicate critical dose values of
approximatelyy  1.4 x 10
16 a/mgg and 0.9 x 10
16 a/mg, respectively. A comparison with laboratory
ctiirli<=»e r\f rvurnr'Viinrp nnH virr-nnnlitp HnnfH with 238Pn onH 244*pC'rmn  csVhoinwwes  tthhaatt  ttVhiP rritir.nl Hnsftstudies of pyrochlore and zirconolite doped with e critical dose
i i i , , . ,
, . , , 1 , ,
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values of natural pyrochlore and zirconolite are higher by a factor of approximately 2-4. This result
is probably due to the higher temperatures (100-200°C) experienced by the natural samples in their
host rocks, a proposal that is consistent with the laboratory data for samples of CaPuTi2O7 and Cm-
doped zirconolite stored at elevated temperatures (23, 24). The dose-age data also provide evidence
for the long-term annealing of isolated alpha-recoil collision cascades in pyrochlore and zirconolite.
These results indicate that further increases in the critical amorphization dose due to long-term
annealing will not occur for time periods of less than approximately 10 million years. The available
data from dissolution tests of amorphous and crystalline samples indicate some increase in leach
rates due to amorphization, but the data are not entirely consistent and require confirmation.
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