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OCR for page 3432
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3432-3439, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human Welfare, "
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Nuclear waste forms for actinides
RODNEY C. EWING*
Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109-2iO4
ABSTRACT The disposition of actinides, most recently
239Pu from dismantled nuclear weapons, requires effective con-
tainment of waste generated by the nuclear fuel cycle. Because
actinides berg., 239PU and 237Np' are long-lived, they have a major
impact on risk assessments of geologic repositories. Thus, de-
monstrable, long-term chemical and mechanical durability are
essential properties of waste forms for the immobilization of
actinides. Mineralogic and geologic studies provide excellent
candidate phases for immobilization and a unique database that
cannot be duplicated by a purely materials science approach. The
"mineralogic approach" is illustrated by a discussion of zircon
as a phase for the immobilization of excess weapons plutonium.
The disposition of "waste" generated by the nuclear fuel cycle is
one of the most pressing, and potentially costly, environmental
problems for the 21st century, a heritage from the atomic age of
the 20th century. Proposed strategies are complicated, not only by
the large volumes and activities of waste, but by the political and
public policy issues associated with the long times considered for
containment and disposal (104-106 years). Furthermore, the waste
includes fissile material, e.g., 239Pu, of high energy content. Three
primary sources of actinide-bearing waste in the United States are
as follows.
High-level waste (HLWJ resulting from reprocessing to reclaim
Missile materials for weapons production. Approximately 380,000
m3 (100 million gallons) of HEW have a total radioactivity of 960
million curies (1 Ci = 37 GBq) (1~. The greatest volumes (340,000
m3) are stored in tanks at Hanford, WA, and Savannah River, SC.
Over 99 percent of the present activity is from nonactinide
radionuclides with half-lives <50 years (reprocessing has re-
moved much of the actinide content); however, after 500 years,
the total activity will be substantially reduced, and the primary
radionuclides will be 238Pu, i3iSm, and 24~Am. After 50,000 years,
most of the activity will be associated with longer-lived radionu-
clides, such as 239Pu and 240Pu. Also related to reprocessing are
much lower activity waste contaminated with transuranic ele-
ments, TRU waste. These are defined as containing 100 nanocu-
ries of a-emitting transuranic isotopes, with half-lives >20 years,
per gram of waste. Over 60,000 m3 are stored retrievably at
Department of Energy sites, destined for disposal at the Waste
Isolation Pilot Plant in New Mexico (24. The estimated cost of
remediation and restoration actives in the Department of Energy
complex during the next decades is in the order of 200 billion
dollars (34.
Used or spent nuclear fuel resulting from commercial power
generation. Just over 20 percent of the electricity generated in the
United States is produced by nuclear power reactors. In 1995,
32,200 metric tons of spent fuel with a total activity of 3O,200 MCi
were stored by the electric utilities at 70 sites (either in pools or
in dry storage systems) (2, 4~. By 202O, the projected inventory
will be 77,100 metric tons of heavy metal (MTHM) with a total
activity of 34,600 MCi. Although the volume of the spent fuel is
only a few percent of the volume of HEW, >95% of the total
PNAS is available online at www.pnas.org.
activity (defense-related plus commercially generated waste) is
associated with the commercially generated spent nuclear fuel
(3~. At present, none of the spent fuel will be reprocessed, and all
is destined for direct disposal in a geologic repository.
The dismantlement of nuclear weapons. Under the first and
second Strategic Arms Reduction treaties, as well as unilateral
pledges made by both the United States and Russia, several
thousand nuclear weapons will be dismantled. Initially, this will
result in an estimated 100 metric tons of weapons plutonium that
will require long-term disposition. The disposition strategy should
not only protect the public and the environment but must also
ensure that the plutonium is not readily recoverable for use in
weapons (5~. Present U.S. strategy calls for "burning" the Pu as
a mixed-oxide fuel in existing or modified reactors followed by
direct disposal with commercially generated spent fuel in a
geologic repository (6, 74. A smaller portion of the Pu (tens of
metric tons) is destined for immobilization into a durable solid
followed by geologic disposal. The present program has an
anticipated cost of two billion dollars.
World-wide, since the first creation of milligram quantities of
plutonium by Glenn Seaborg in 1941, the global inventory of
plutonium has reached 1,350 tons and continues to increase by
~70 tons/year (Table 1~. This commercially generated plutonium
is in two forms: (i) incorporated in spent fuel destined for direct
geologic disposal (>600 metric tons of plutonium is in the spent
fuel in the U.S.); and (ii) plutonium separated by reprocessing of
commercial fuel, which is estimated to reach 300 tons by the year
2000. This is greater than the amount of plutonium presently in
nuclear weapons (9~. Considering that the bare critical nia.~s for
weapons grade plutonium is 15 kg of metal (this number is
substantially reduced in the presence of a neutron reflector),
safe-guarding this plutonium is essential. In fact, the need for
safeguards to protect against the diversion of separated pluto-
nium applies equally to all grades of plutonium (104. The peaceful
use of nuclear energy will inevitably require a strategy for the
disposition and disposal of actinides.
1
Why Are Actinides Important?
Although there are a number of fission product radionuclides of
high activity (137ts and 90Sr) and long half-life (99Tc, 200,000
years; 1291, 1.6 x 107 years) in spent nuclear fuel, actinides and
their daughter products account for most of the radiotoxicity of
nuclear waste after the first 500 years of disposal (Fig. 1~. After
several hundred years, radiotoxicity is dominated by 239Pu (half-
life = 24, 100 years) and 237Np (half-life = 2,000,000 years). Thus,
a major part of the long-term risk is directly related to the fate of
these two actinides in the geosphere (natural, crust al concentra-
tions of Pu are on the order of 10-11 ppm for 239Pu).
Plutonium has several important and unique properties: (i)
239Pu is fissile; (`ii) 239Pu with a half-life of 24,100 years decays to
Abbreviations: HEW, high-level waste; Ma, million years ago, MeV,
million electronvolts; dpa, displacements per atom; REE, rare-earth
element.
*To whom reprint requests should be addressed. e-mail: rodewing@
umich.edu.
3432
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Representative terms from entire chapter:
spent fuel
Colloquium Paper: Ewing
Table 1. Estimated global plutonium inventory (metric tons) at
the end of 1996 (8)
-
In spent fuel
USA
Japan
France
Germany
650
90
70
50
860
80
150
Total
In operating reactors
Separated by civilian reprocessing
Military inventories
Former USSR
USA
France
China
Israel, India, Pakistan
Total
Estimated world inventory, +70
metric tons/year production 1,350
140
100
6
252
23sU7 another fissile radionuclide with a half-life of 700 million
years (the bare critical mass of weapons grade uranium is ~50
kg); and (`iii) Pu has four oxidation states (3+, 4+, 5+, and 6+~
in natural water-rock systems. Although crystalline PuO2 has a
low solubility, Pu may exist as PuO2+ or PuO22+ aqueous species,
with the former predominating in oxidized natural waters. Ad-
ditionally, in the 3+, 5+, and 6+ oxidation states, Pu forms strong
carbonate complexes (pH >5) (12~. Actual plutonium concen-
trations in solution are further complicated by the possibility of
disproportionation among oxidation states, or-radiolysis of water
to produce oxidants, such as H2O2, c-decay-induced amorphiza-
tion of the solid that increases leach rates, and the formation of
intrinsic actinide or actinide-bearing colloids that can increase
actinide concentrations in ground waters by several orders of
magnitude. Thus, the geochemistry of Pu has the full array of
dissolution, transport, and precipitation mechanisms that are
typical in geologic systems, in addition to radiation effects; as with
other multivalent elements (e.g., insoluble U4+ VS. mobile U6+),
this can lead to either dispersal or concentration in the geosphere.
Thus, it is essential to evaluate the long-term behavior of Pu
either as it exists in spent nuclear fuel or is immobilized in solid
waste forms. The purpose of this paper is to illustrate the unique
contributions that mineralogy and geochemistry can make in the
design and selection of durable waste forms for the long term
disposal of plutonium.
t- 104
o
._
-
~5 103
-
° 102
·C)
. _
X
o 10
._ :
~' 10°
._
CE: 1 0-11 ~1 o1
\ , Fission and \
Actinides and
3434 Colloquium Paper: Ewing
Proc. Natl. Acad. Sci. USA 96 (1999
Table 2. Actinide-bearing phases that are candidates presently under consideration for plutonium immobilization
Mineral Ideal formula GeoRef citations, 1998 Durable heavy mineral*
Pyrochlore:
Zirconolite
Apatite
Zircon
Monazite
Baddelyitet _ . .
*Heavy minerals identified and discussed in monograph on heavy mineral occurrences (23~. Heavy minerals that were noted,
but which are not included in this table, are gadolinite, allanite, thorite, titanite, and xenotime. Thorite (ThSiO4) and xenotime
(YP04) are isostructural with zircon; gadolinite, allanite, and titanite are either relatively rare or less durable than the
minerals listed in the table.
"Presently receiving the greatest attention within the U.S. program for the disposition of fissile materials.
tCubic and tetragonal polymorphs of ZrO2 have been considered as waste form phases; however, naturally occurring ZrO2
is most commonly monoclinic baddelyite. Although not cited in ref. 23, it is included in this list because of its known durability
and because it is considered both as a waste form and as inert matr~x fuel for burning plutonium (9~.
(Ca,REE)Ti207
CaZrTi207
Ca4 -xREE6+x(Sio4~6-y(Po4Jy~o'F)2
ZrSiO4
CePO4
ZrO'
494
62
5,098
8,055
1,711
744
Yes
Yes
Yes
minerals by Hutton (23) provides a detailed list of heavy minerals
(Table 2~. Note the absence of potentially important actinide-
bearing phases: pyrochlore, zirconolite, and baddeleyite, because
of either their relative rarity or lower durability. The second issue
is how much geologic and mineralogic data exist for these phases.
Table 2 gives the number of citations for these phases as taken
from GeoRef (American Geological Institute). The result is not
surprising; those phases that are important to geochronology
(zircon, apatite, and monazite) account for a major portion (95%)
of the published literature. These studies are of two types: (i)
laboratory studies to determine the ability of the minerals to
retain isotopic signatures as a result of their physical and chemical
durability in a variety of geologic environments; and (ii) age-
dating studies that essentially confirm the results of the laboratory
studies in actual, long-term geologic environments. Both types of
studies provide the essential data required for waste form design
and selection.
Present research on waste forms for Pu-immobilization in-
cludes a relatively short list of phases (Table 2~. In the U.S., most
of the effort within the Materials Disposition program of the
Department of Energy focuses on immobilization in a ceramic
(24), particularly cubic pyrochlore and its monoclinic derivative,
zirconolite, because there are considerable data on these mate-
rials as waste form phases (22~. However, the Department of
Energy evaluation and selection process (24) is very different
from the mineralogic approach presented in this paper.
To illustrate the mineralogic approach, I review relevant work
on zircon, drawn mainly from the mineralogic and geochemical
literature, to demonstrate the utility of the mineralogic approach,
as well as the extensive amounts of data that are already available
in the literature. Although the structures of zircon (25) and
actinide orthosilicates (26, 27) have been known for 30 years, I
focus on studies applicable to the analysis of zircon as an actinide
host phase.
Zircon as an Actinide Waste Form
Zircon (I4~/amd; Z = 4) occurs in nature with uranium and
thorium concentrations typically up to 5,000 ppm, but reaching 10
wt %. Zircon is an extremely durable mineral (28), often found
as a heavy mineral in stream sediments, that, after transport over
great distances, shows limited chemical alteration or physical
abrasion (23, 29~. I~he widespread distribution of zircon in the
continental crust, its tendency to concentrate trace elements
(lanthanides and actinides), its use in age dating, and its resistance
to chemical and physical degradation (30-33) have made zircon
probably the most useful accessory mineral in geologic studies.
Zircon has been identified as an actinide-bearing phase in
polyphase ceramic waste forms (34~. Zircon also occurs in the
Chernobyl lavas as an important actinide-bearing phase (6-12
atomic percent uranium) (35~. The propensity to incorporate
actinides and its durability have lead to the suggestion that zircon
be used to immobilize actinides (36-38~. Based on the ability of
natural zircon to reta~n Pb, Gentry et al. (39) suggested that
materials, like zircon, could effectively retain radioactive waste.
This early suggestion was prescient but did not evaluate the extent
of solid-solution of actinides ~n zircon and did not consider the
much greater radiation damage that would occur in such a
radioactive waste form (404.
Structure. The zircon structure consists of triaDgular dodeca-
hedral ZrOs groups that form edge-shar~ng cha~ns parallel to the
a axis and SiO4 tetrahedral monomers that form edge-sharing
chains with alternating ZrOs groups parallel to the c axis (25, 26~.
U and Th replace the Zr in low concentrations; however, com-
positions of ASiO4, in which A4+ = Zr, ~, Th, Pa, U, Np, Pu, and
Am, have been synthesized (27~. The regular increase in the unit
cell volume with the increasing ionic radius of the A-site cation
confirms the homologous topologies of these structures (Fig. 24.
Four of these compositions, hafnon (HfSiO4), zircon, coffinite
(USiO4), and thorite (ThSiO4), occur naturally. Structure refine-
ments (27) and structural analyses (41, 42) suggest complete
miscibility between ZrSiO4 and HfSiO4, but there are miscibility
gaps on the ZrSiO4-USiO4-l~hSiO4 jo~ns (43~. Zircon with 9.2
atom percent plutonium (8.1% Pu238; 1.1% Pu239) substituting for
Zr has been synthes~zed (44~. This is equal to a waste loading of
10 wt % Pu, but the max~mum extent of the solubility of Pu in
z~rcon has not been determined. That a pure, endmember
composition, PuSiO4, has been synthesized (27) suggests exten-
sive substitution of Pu for Zr is possible (42~.
The zircon structure is part of a larger class of ABO4 structure
types (silicates and phosphates) and is closely related to the
0.34 ~
n25
0.33 ~
0.32 t
0.31
-
a'
0.30 _
° 0.29
a)
() 0.28
0.27
0.26
0.25 _
Zircon Structure
~/~/
Hf I l I
0.080 0.085 0.090 0.095 0.100 0.105 0.110
ionic Radius (nary)
jPu
Pa - / "Th
FIG. 2. Variation in unit cell volume of actinide silicates, ASiO4,
with the zircon structure as a function of ionic radius (VIIIA4+). Data
are from Keller (27) after Speer (41).
Colloquium Paper: Ewing
structure of monazite, CePO4, (another durable phase commonly
used in geologic age-dating). Numerous A-site compositions (La,
Pr, Nd, Sm. Eu, Gd, Tb, Tm, Yb, Lu, Sc, and Y) with the silicate
and phosphate zircon and monazite structure-types have been
synthesized, and their structures have been refined. These syn-
thesized materials have provided the basis for detailed optical-
absorption spectroscopy and electronic paramagnetic resonance
studies of U. Pu, Cm, Np, and Gd incorporation into the
zircon-structure type (45, 46~.
Geochronology. Recent progress in the utilization of zircon in
age dating has come from the use of the sensitive high-resolution
ion microprobe (SHRIMP), a method that allows the measure-
ment of isotopic ratios on areas as small as 20-30 ,um, thus
providing age dates on separate zones within single crystals of
zircon. Detrital zircons in a quartzite at Mount Narryer, Western
Australia have been dated at 4,100-4,300 million years ago (Ma),
the oldest terrestrial minerals yet found (47~. In Western Aus-
tralia, the Jack Hills contain slightly younger (3,900-4,270 Ma)
detrital zircons (48~. The zircons in Australia are individual,
recycled grains in a younger (3,500 Ma) sequence of metamor-
phosed sedimentary rocks, as are similarly dated zircons from the
Sino-Korean craton in northeast China (~3,800 Ma) (49~. The
oldest so-called intact crust is found in the early Archean (3,800-
3,960 Ma) granitoids in northwestern Canada (50) and western
Greenland (51~. The zircons formed at the same time as the first
crustal rocks on Earth. The oldest zircons in the solar system are
found as rare inclusions in meteorites and were dated at 4,560 Ma
(52~. There are many hundreds of papers that can be cited to
illustrate the use of zircon in dating very old rocks, but of greater
importance are that these field studies establish the effects of
geologic processes on the durability of zircon either under surface
conditions of sedimentary transport (29) or at much higher
temperatures, e.g., granulite facies metamorphism (53), ex-
tremely high metamorphic pressures (54-57), or meteorite im-
pact (58~.
The most recent and dramatic example of the extraordinary
ability of zircon to retain its U-Pb systematics is from studies of
zircons from the Chicxulub impact structure of the Yucatan
Peninsula (58-60~. The shocked zircons were exhumed from the
Chicxulub basement rock during meteorite impact and dispersed
in the fine dust of the impact cloud. Discordancies in the U-Pb
systematics (e.g., Pb loss) are proportional to the extent of
impact-induced shock textures, and isotopic resetting is consistent
with partial lead loss at the time of impact (65 Ma), thus providing
convincing support for the meteorite impact origin of the Chicxu-
lub crater and its being the source of ejected material found at the
Cretaceous-Tertiary (K-T) boundary in North America. Urani-
um-lead dating studies of shocked zircons in the fine-arained
ejecta deposited in areas as wide apart as Colorado, Saskatche-
wan, and Haiti at the Cretaceous-Tertiary boundary have a
predominant age of 545 Ma, in agreement with dates for shocked
zircons from the Chicxulub crater (59-60~. Only a material of
remarkable durability, such as zircon, could preserve the isotopic
signature of an event of such extreme conditions (60~.
Alteration and Disturbed U/Pb Systematics. Any process that
can disturb the U/Pb isotopic systematics of zircon crystals has an
important effect on whether the radiometrically determined dates
are concordant or discordant; thus, there is an extensive literature
that describes this type of alteration (30-32, 61~. Discordant ages
are common, mainly because of Pb-loss and less frequently
because of U-loss. The discordant ages are usually attributed to
episodic loss of Pb or U during thermal events (61) and are
enhanced by (i) physical degradation caused by microfracturing
that is the result of the volume expansion associated with or-decay
damage that results in an increase of surface area (57, 62, 63~; and
(ii) chemical alteration caused by radiation-induced amorph~za-
tion that creates damaged, aperiod domains in which Pb-diffusion
is enhanced and for which bulk leach rates are increased. Thus,
the discordant ages are often the result of the accumulation of
highly damaged regions resulting from or-decay damage; however,
Proc. Natl. Acad. Sci. USA 96 (1999J 3435
improved U/Pb dates may be obtained by removing the radia-
tion-damaged regions either by physical abrasion or etching
techniques (64~. Differential etching experiments (using 48%
HF) have shown by scanning electron microscopy that the
removal of radiation damaged zones improved the concordance
of U/Pb age dates in the remaining, unaltered material (32~.
Additionally, under extreme geologic conditions, e.g., zircons
subjected to deformation in shear zones and altered by hydro-
thermal solutions, disturbed U/Pb systematics are clearly docu-
mented. Detailed experimental studies (65) have shown that
experimentally induced U/Pb isotopic discordance in zircon is a
complex function of zircon durability and the annealing of the
radiation damage. Recent studies have investigated the incorpo-
ration of Pb into zircon (66) and the mechanisms by which Pb may
be lost (67~. The loss of Pb by diffusion is an extremely slow
process (at 1,000°C, the diffusivity is 10-25 m2 s-l), and the low
diffusivities confirm Pb closure temperatures in excess of 900°C
and imply that U/Th/Pb isotopic ages are unlikely to be reset by
thermal events alone except under the most extreme geologic
conditions, i.e., partial melting or granulite-grade metamorphism
(67~. In summary, there are abundant data from geochronologic
studies to show that natural zircons can quantitatively retain Pb
for billions of years in the absence of episodic, thermally induced
Pb loss (404. The minor alteration of zircon over long periods of
time and under rather extreme conditions stands in contrast to
the observations of other potential ceramic nuclear waste forms
such as pyrochlore (68) for which the data of the type summarized
above are simply not presently available.
Diffusion of Tetravalent Cations and Rare Earth Elements.
Even though diffusion rates for Pb are low, the diffusivities of U
and Th at 1,100°C are orders of magnitude lower (69), suggesting
complete containment of actinides for billions of years. The
principal process by which this might change is o`-decay radiation
damage (40~. Elements may preferentially segregate into aperi-
odic, damaged regions that have inherently higher solubilities
than crystalline ceramics (70), and the annealing kinetics of
recrystallization of damaged domains may substantially change
the response of a material to the damage accumulation process
(71~.
The development of a waste form for weapons 239Pu requires
a knowledge of diffusion rates of the actinides, as well as
elements with high neutron-capture cross-sections to control
criticality (e.g., Hf and Gd). Natural zircons can contain up to
several thousand parts per million rare earths, including up to
500 ppm of Gd, and zircon exhibits nearly complete solid
solution with hafnon, HfSiO4. Thus, neutron absorbing nu-
clides may be incorporated into the zircon structure; however,
the neutron absorber must remain within atomic scale prox-
imity of the fissile radionuclide (239Pu or 23su). Recent deter-
minations of diffusion rates for tetravalent cations (Th, U and
Hf) give diffusivities in the order of 10-22-10-2° m2 s-i in the
temperature range of 1,400-1650°C (72~. Based on similarities
in ionic radii, Pu4+ is expected to have a similar diffusivity,
although slightly faster because of its smaller ionic radius,
which means that it is essentially immobile under all but the
most extreme geologic conditions. This is why fine-scale chem-
ical zoning and isotopic signatures are preserved in the inher-
ited cores of zircon crystals that have experienced protracted
thermal events in their past history. In contrast, the rare-earth
elements (REE) diffuse at rates 4-5 orders of magnitude
faster than the tetravalent cations (73~. The diffusion rates vary
among the REEs in a systematic manner as a function of ionic
radius; thus, for Gd, the estimated activation energy and
diffusivity (1,000°C) are 189 kcal/mol and 3.2 x 10-26 m2 s-i,
respectively. Again, the closure temperature of zircon for
REEs is quite high (>1,000°C) for all but the smallest grains.
Other phases that have been suggested as waste form phases,
e.g., titanite and apatite, have closure temperatures many
hundreds of degrees lower than those of zircon.
3436 Colloquium Paper: Ewing
Dissolution Studies. Studies of natural zircons under extreme
laboratory conditions have confirmed the loss of U. Th, and Pb
(30, 31, 65~. The lead loss can be the result of grain boundary or
volume diffusion (in which there is no dissolution of the zircon),
or the bulk dissolution of zircon. However, at lower temperatures
(<80°C) and near neutral pH values, i.e., conditions more
pertinent to nuclear waste disposal, zircon is extremely insoluble.
There are much less leach data in the literature than necessary for
a full evaluation of zircon as a waste form. Crystalline zircon is
stable to such an extent that the equilibrium concentrations of Zr
and Si are in the order of 10-9 moles/liter (0.1 parts per billion)
at 25°C (74~. Dissolution of amorphous zircon followed a first
order reaction based on Si concentrations. Zr concentrations
remained <0.05 ppm, the instrument detection limit, because of
precipitation of ZrO2 and ZrSiO4 (74~. The leach rate of zircon
increases with c-decay damage on the order of one to two orders
of magnitude (75~. Comparing the dissolution rate for a metamict
zircon (74), r = 10-7 moles/m2/day at 80°C, pH 5, with the
long-term rate of a nuclear waste borosilicate glass, r = 10-5
moles/m2/day (which equals 10-3 g/m2/day) in water at 98°C
(76), shows that the dissolution rate of amorphous zircon is still
considerably lower than that of glass in stagnant, silica-saturated
solutions. In an open system (e.g., moving groundwater for which
solubility limits are not reached), the leach rate for zircon (74)
used in this comparison does not increase; however, the leach rate
of borosilicate glass may increase by three orders of magnitude
until reaching the final rate of reaction (21~.
Thus, one of the main advantages of zircon may be its high
durability in an open system in which groundwaters are present
because this allows considerably greater flexibility in disposal
strategy (e.g., deep borehole). For this reason, it is essential to
determine the maximum forward rate of the dissolution reaction.
This is particularly challenging because the dissolution rate for
zircon is low, and precipitation of zirconia removes Zr from the
leaching solution. Recently, a high-temperature Soxhlet extractor
was designed to measure the forward rate of dissolution of zircon
in the range of 120-250°C. The measured rates were 4.1 x 10-4
g/m2/day at 250°C, 1.7 x 10-4 g/m2/day at 200°C, and 7.1 x 10-5
g/m2/day at 120°C. The rate extrapolated to 90°C is 4.6 x 10-5
g/m2/day; therefore, in an open system, in the absence of a
solubility-limiting phase, a 100-,um crystal of zircon would require
150,000 years for complete dissolution (77, 78~. Under static
conditions, the dissolution rate is substantially reduced. The
experimental results are consistent with the high chemical dura-
bility of zircon in a wide range of geologic environments.
Physical Properties. The mechanical properties of zircon
change with increasing a-decay event dose (79) and as a result of
implantation by Pb ions (540 keV) up to fluences of 3.3 x 10~
to 5 x 10~5 ions/cm2 ta dose range that spans the crystalline-to-
amorphous transition (80~. The o`-decay-induced softening leads
to a decrease in hardness (40%) and the bulk elastic modulus
(70%) (81), but there is an increase in fracture toughness
probably caused by crack-tip blunting by the aperiodic domains
(79~. The ion beam irradiation results in softening (70%) and a
decrease in modulus (42%~. The principal effect of the changes
in mechanical properties is the formation of a pronounced
fracture system (62) caused by differential volume expansion in
zones of different c~-decay doses. A model has been developed
that describes the radial and concentric fracture sets that are
characteristic of zircon: principally, a function of the degree of
damage (e.g., the amorphous fraction), the zone thicknesses, and
the confining pressure (82~. This type of analysis is required for
evaluating the development of microfractures as a function of
radiation damage, particularly for disposal in a deep borehole.
Radiation Damage. Radiation damage resulting from the
c~-decay of 239Pu and its daughter products (e.g., 23sU) has an
important effect on the physical and chemical durability of
actinide-bearing zircons. Depending on the waste loading, sig-
nificant doses (>10~8 cY-decay events/g) accumulate, and crys-
talline phases become aperiodic in relatively short periods of time
Proc. Natl. Acad. Sci. USA 96 (1999)
(103 years) (Fig. 3) (83, 84~. Note that natural zircons can
accumulate relatively high ~x-decay doses (horizontal line in Fig.
3), and this allows the comparison of data resulting from accel-
erated irradiation techniques (either actinide doping with short
lived cY-decay nuclides such as 238Pu or 244Cm that have reached
cx-decay doses of 10~9 cr-decay event/g or ion beam irradiation
experiments).
In an o`-decay event, the or-particle dissipates most of its energy
t4.5-5.8 million electronvolts (MeV) for actinides] (1 eV =
1.602 x i0-~9 J) by ionization processes over a range of 16-22 ,um
but undergoes enough elastic collisions along its path to produce
several hundred isolated atomic displacements. The largest num-
ber of displacements occurs near the end of the ~x-particle range.
The more massive but lower-energy a-recoil (86 keV 235U recoil
from decay of 239Pu) dissipates nearly all of its energy ~n elastic
collisions over a very short range, 30-40 nm, causing ~1,000
atomic displacements. The density of energy deposited into the
cascade is high (up to 1 eV/atom) and occurs over an extremely
short time (<10-~2 s). Thus, a single c~-decay event generates
~1,400 atomic displacements, which is significantly greater than
the 0.1 displacements generated per ,B-decay event. Clearly,
al-decay from incorporated actinides will have a profound effect
on the structure of a crystall~ne solid. The cumulative effect of
dose will be time- and temperature-dependent because of an-
nealing and recrystallization of damaged areas.
Studies of radiation effects in zircon have a long history
(86-91~. Zircon undergoes a radiation-induced transformation
from the periodic-to-aperiodic state (metamict state) at doses
over the range of 10~-10~9 or-decay events/g twhich equals
0.2-0.6 displacements per atom (dpa)] with a density decrease
and a corresponding volume expansion of 18%. Previous
studies include analysis of natural zircons that have accumu-
lated cr-decay event damage up to doses of nearly 0.7 dpa over
550 million years (63, 87, 88), 238Pu-doped zircons (half-life =
87.7 years) up to doses of 0.7 dpa in 6.5 years (44, 91, 92), and
heavy ion beam irradiation using 2 MeV He+, 0.8 MeV Ne+,
1.5 MeV Ar+, 0.7-1.5 MeV Kr+, and 1.5 MeV Xe+ up to doses
of 0.2-2.3 dpa in times of <1 hour (91, 93~. All three types of
damage experiments include detailed studies of annealing
kinetics. The natural zircon studies (e.g., of samples that are
550 million years old) and the experimental results for ion
beam irradiation experiments of <1-hour duration have a
range of dose rates > 108. In situ ion beam irradiation combined
with high resolution transmission electron microscopy have
demonstrated that the damage microstructures and the in-
growth of damage with increasing dose can be simulated by
~n24
>~ 1 o20
ct
c'
~ 1019
1 _
-
a' 1 ol8
o
a) 1017
._
1 ol6
15
rT~rcn',,,l ~ .~51 , """'1 """'1 """"1 ' 111111~- 1 """'1 """=
_Limit of Experience /
with Minerals /
/..........
- /."""
-/ .~
,.'
,.
, ,,,,,,d, ,,,,,,,l, ,,,,,,,l, ,,,,,,J, ,,,,,,,l, ,,,,,,,l, ,,,,,,,l, ,,,,,,,l, ,,,,,,,
0° 101 102 103 104 105 106 107 108 109
................
1 0 wt% 239Pu
1 wt% 239PU _
-
Waste Form Storage Time (years)
FIG. 3. Cumulative c'-decay dose as a function of waste form
storage time for a ceramic, e.g., zircon, containing 1 and 10 wt %
loading of 239Pu (as in ref. 85~. The dashed horizontal line indicates
maximum doses reached in natural zircon from decay of uranium and
thorium and daughter products.
Colloquium Paper: Ewing
using heavy ion irradiation (Fig. 4~. Thus, there is a firm basis
for predicting the microstructure of the radiation-damaged
zircon as a function of dose, temperature, and type of radiation
(91, 93, 94~. Systematic studies have been completed for
monazite- and zircon-structure orthophosphates of a wide
variety of A-site end-member compositions (95, 96) and
orthosilicates (97-99~. Zircon, hafnon (HfSiO4), thorite (tet-
ragonal ThSiO4), and huttonite (monoclinic ThSiO4) become
amorphous in a two-stage process with increasing temperature
when irradiated with 800 keV Kr+ or Xe+ ions in the tem-
perature range of 20-1,100 K. The temperature above which
amorphization does not occur (i.e., the temperature at which
the rate of simultaneous annealing is equal to the rate of
damage accumulation) increased in the order huttonite, zir-
con, hafnon, and thorite. When irradiated with heavy ions, all
four orthosilicates may decompose into crystalline oxides:
ZrO2, HfO2, or ThO2 plus amorphous SiO2 (98~. Ion beam
irradiation studies of synthetic and natural zircons and mon-
azites (with impurities) revealed that impurities lower the dose
required for amorphization and correspondingly increase the
temperature above which a material cannot be amorphized.
This suggests that impurities increase susceptibility to amor
Natural zircon damaged by a-decay events
Proc. Natl. Acad. Sci. USA 96 (1999J 3437
phization and inhibit annealing, particularly when coupled
charge balance substitutions are required (96, 99~. Details of
structural rearrangements during damage accumulation and
annealing have been obtained by extended x-ray absorption
fine structure spectroscopy (EXAFS) studies of metamict
zircons (89, 100~. Metamictization is accompanied by major
atomic reorganization: loss of well defined medium range
order, disruption of the immediate environment of Zr, a
decrease of the average Zr-coordination number, and tilting
and distortion of SiO4 polyhedral. A two-stage thermal an-
nealing process was observed that, at lower temperatures
(400-500°C), resulted in the formation of minor Zr-rich
domains (100~. Finally, transmutation effects (239Pu decays to
235U) are important in crystalline materials because they may
lead to phase instability. In the case of zircon, the solubility of
U in zircon is only known approximately (4 + 2 mole percent)
(43~; thus, for higher concentrations of U. one may expect the
formation of USiO4, also with the zircon structure, and UO2.
There are few crystalline ceramics for which such a wide variety
of data on radiation damage are available. Based on these data,
for a waste loading of 10 wt % of 239Pu under ambient conditions,
the zircon will reach the saturation dose of damage (1.2 x 10~9
Synthetic zircon damaged by I.5 MeV K~
FIG. 4. Comparison of radiation damage in natural (A-C) and 1.5 MeV Kr+-irradiated synthetic (D-F) zircon. (A) 5 x 1013 a-decay events/mg
(0.003 dpa). (B) 1.S x 1015 a/mg (0.091 dpa). (C) 6.4 x 1015 c`/mg (0.32 dpa). (D) 5 x 1013 Kr+/cm2 (0.057 dpa). (~) 1.5 x 1014 Kr+/cm2 (0.17
dpa). (F) 3 x 1014 Kr+/cm2 (0.34 dpa). Complete amorphization in both was observed after 0.5-0.55 dpa. Figure courtesy of L. M. Wang (91~.
3438 Colloquium Paper: Ewing
or-decay events/g or 0.8 dpa) in <2,000 years (Fig. 3); thus, the
properties of zircon must be considered in light of its radiation-
damaged, aperiodic state. Based on the extensive database, it is
possible to model the damage accumulation (e.g., percent amor-
phous fraction) as a function of dose and temperature for zircon
and compare the results to those of other phases (e.g., apatite)
(85). Fig. 5 is a plot of the critical amorphization dose for zircon
as compared with a silicate apatite (data for both phases are based
on ion beam irradiation experiments). Above 225°C, zircon with
a 10 wt To loading of 239Pu will remain in the crystalline state
because of thermal annealing. In contrast, silicate apatite anneals
readily and will remain in the crystalline state at ambient tem-
peratures. Such models are very sensitive to the activation ener-
gies for thermal annealing (101), and more recent work has
suggested that temperatures as high as 400°C will be required to
maintain the crystallinity of zircon (98). The efficacy of such an
approach is that modeled results can be confirmed by comparison
to naturally occurring zircons of known thermal history, and the
modeled results are in good agreement with the amorphous
fraction accumulation determined for natural zircons (unless
there has been episodic thermal annealing) (101). The same type
of analysis allows one to calculate minimum storage temperatures
required to maintain the crystallinity of any actinide-bearing
waste form (Fig. 6). At present, sufficient data for this type of
analysis are only available for zircon.
Stored energy values have been carefully determined (102) by
transposed temperature drop calorimetry over the range of the
periodic-to-aperiodic transition on a suite of zircons from Sri
Lanka (550 million years old). The energy released during
annealing varies sigmoidally as a function of or-decay event dose
reaching a saturation value of 322 + 16 J/g at doses >5 x 10~8
a-decay events/g. This is greater than values typical of nuclear
waste glasses, which are generally < 150 J/g and saturate at a dose
of 10~8 a-decay events/g; however, sudden release of this energy
is not anticipated to cause a significant rise in temperature for
either the glass or zircon. The magnitude of the enthalpy of
annealing suggests that the radiation damage is pervasive on the
scale of fractions of nanometers, perhaps leading to the formation
of microdomains of amorphous SiO2-rich and ZrO2-rich regions
in the metamict state. This suggestion is consistent with obser-
vations made by secondary ion mass spectrometry (SIMS) and
high resolution transmission electron microscopy (HRTEM) (70)
and extended x-ray absorption fine structure spectroscopy (EX-
AFS) of annealed zircons (100~.
Summary
There can be little doubt that a phase such as zircon provides a
demonstrable case for its long-term chemical and mechanical 2.
3.
4.
6
en
~ 5
a)
~ 4
A
~2
a)
o
cat 2
o
m1 J J
Q
O , , , , 1 , , , 1 1, , , , 1, , , , 1 , , , , 1 , , , , 1 1 1 1 1 1 , , ,,
a: -50 0 50 100 150 200 250 300 350
Waste Form Temperature (°C)
' 1" " 1' ' '
Silicate
Apatite
Zircon
FIG. 5. Critical amorphization dose vs. storage temperature for
silicate apatite and zircon containing 10 wt % 239Pu (85~.
Proc. Natl. Acad. Sci. USA 96 (1999)
600
_
400
o
-
200
o
Time (years)
F =2 1 TV
8000 1 0000
FIG. 6. Minunum storage temperature of zircon containing 10 and 2
wt % 239Pu, respectively, vs. storage time to ensure complete crystallinity
of zircon. Such modeled calculations are very sensitive to the activation
energy. The dark solid line is for an activation energy of 3.6 eV; the lower,
thinner line if for an activation energy of 2.1 eV (101~.
durability as an actinide waste form. The database for zircon is
unique and extensive, providing ample opportunity to confirm
experimental and modeled results against actual behavior in a
wide range of geologic environments. If the issue is one of
long-term verification of materials performance, no other com-
parable database exists for an actinide-bearing phase, as there is
for zircon.
Are there other mineralogic candidates? Most definitely yes,
and prominent among them are monazite and the polymorphs of
zirconia. I believe that we have only just scratched the surface of
mineralogic applications to issues related to nuclear waste dis-
posal. Are there other mineralogic applications? Much can be
learned about the alteration of UO2 in spent nuclear fuel by
studying the alteration products of uraninite, UO2 +x (102, 103).
I have benefited greatly from collaborations with Werner Lutze, Lumin
Wang, and Bill Weber, and most importantly with students, Gregory
Lumpkin and Al Meldrum, in the development of the ideas presented in
this paper. Lynn Boatner's enthusiasm for this work has kept me in the
game. I thank John Hanchar for very useful discussions of diffusion in
zircon. This work has been sustained by support by Basic Energy Research
Sciences of the Department of Energy (DE-FG02-97ER45656~.
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