Potential Nuclear Waste Forms Research Articles

An investigation of the long-term aqueous corrosion behaviour of glass-zirconolite composite materials (Fe-Al-BG-CaZrTi2O7) as a potential nuclear wasteform

The long-term aqueous corrosion behaviour of Fe-Al borosilicate glass-zirconolite (Fe-Al-BG-CaZrTi2O7) glass-ceramic composite materials results from incongruent and congruent corrosion mechanisms of the individual zirconolite (CaZrTi2O7) and Fe-Al borosilicate glass (Fe-Al-BG) phases after the Fe-Al-BG-CaZrTi2O7 composite materials were exposed to deionised water for 365 days. A comprehensive investigation of the aqueous corrosion resistance and structural stability of Fe-Al-BG-CaZrTi2O7 composite materials has demonstrated that the proposed glass-ceramic composite materials are chemically durable over long-term exposure to water and can be considered as potential alternative hosts to borosilicate glass for the safe and effective disposal of nuclear waste.

Flux Crystal Growth of the Extended Structure Pu(V) Borate Na2(PuO2)(BO3).

As part of our exploration of plutonium-containing materials as potential nuclear waste forms, we report the first extended structure Pu(V) material and the first Pu(V) borate. Crystals of Na2(PuO2)(BO3) were grown out of mixed hydroxide/boric acid flux and found to crystallize in the orthorhombic space group Cmcm with lattice parameters of a = 9.9067(4) Å, b = 6.5909(2) Å, and c = 6.9724(2) Å. Na2(PuO2)(BO3) adopts a layered structure in which layers of PuO2(BO3)2- are separated by sodium cations. Plutonium is found in a pentagonal bipyramidal coordination environment, with axial Pu(V)-O plutonyl bond lengths of 1.876(3) Å and equatorial Pu-O bond lengths ranging from 2.325(5) to 2.467(3) Å. We find that the Pu(V)-O plutonyl bond lengths are approximately 0.1 Å longer than the reported Pu(VI)-O plutonyl bond lengths and shorter by approximately 0.033 Å than the corresponding U(V) uranyl bond lengths. Raman spectroscopy on single crystals was used to determine the PuO2+ plutonyl stretching and the equatorial breathing mode frequencies of the pentagonal bipyramidal coordination environment around plutonium. Density functional theory calculations were used to calculate the Raman spectrum to help identify the Raman bands at 690 and 630 cm-1 as corresponding to the plutonyl(V) ν1 stretch and the equatorial PuO5 breathing mode, respectively. UV-vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.

Synthesis of yttrium titanate stannate (Y2Ti2−xSnxO7, x = 0–2) pyrochlore glass–ceramics as potential nuclear waste form

AbstractY2Ti2−xSnxO7 (x = 0–2) pyrochlore sodium aluminoborosilicate glass–ceramics (GCs) are produced by calcining the pelletized Y–Ti–Sn oxide mixture and glass precursor at 1200 or 1300°C for 4 h. The metal oxide mixture is prepared by a soft chemistry route. X‐ray diffraction, Raman spectroscopy, and electron microscopy are employed to investigate the formation of pyrochlore GCs and local crystal structures. Near phase pure Ti‐rich pyrochlores are produced with minor phase SnO2 observed for Sn‐rich materials. The cell parameters of the pyrochlore structures refined by Le Bail fitting are in good agreement with the published data and increase linearly with the gradual increment of Sn substitution. With progressively increasing Sn proportion on pyrochlore B‐site, Raman characteristic bands of the pyrochlore structure become sharper and well defined. The Raman A1g peak position and its full width at half‐maximum are linearly progressed with increasing x (Sn). The presence of the melting glass facilitates the pyrochlore formation, with ceramic grain sizes ranging from submicron to microns. Transmission electron microscopy and selected area electron diffraction observations indicate the sample possesses a relatively high crystallographic perfection at the atomic level. This new series of pyrochlore GCs and the method disclosed herein may pave the way for further materials development as potential nuclear waste forms.

Surface alteration and chemical durability of hollandite (Cr, Al and Ti) consolidated by spark plasma sintering in acid solution

For the development of pyrochemical reprocessing of spent fuels in which a substantial amount of Cs-containing waste is confronted, appropriate management of the long-lived radioactive Cs is required. In the present study, solid state reaction and Spark plasma sintering (SPS) were applied to prepare three types of dense hollandite samples as advanced waste form matrix including Ba1.23Al2.46Ti5.54O16, Ba1.15Cr2.3Ti5.7O16 and Ba1.15Ti2.3Ti5.7O16 with a relative density higher than 95%. The thermogravimetric analysis (TGA) results show that the three hollandite dense pellets prepared by SPS have good thermal stability and high temperature oxidation resistance, which can be used as potential waste forms for 135Cs. Ba1.15Cr2.3Ti5.7O16 demonstrates the best Ba immobilization among three pellets due to its best chemical durability based on a 240-h leaching test, with a cumulative release amount of 0.62 mmol/m2 for Ba at semi-dynamic leaching (pH = 2). The leaching rates of elements decrease continuously with time, and finally reach a dynamic equilibrium as the long-term leaching rates tend to be constant. It is because the dominant leaching mechanism switches from initial rapid dissolution and surface effects to the long-term diffusion. The Ba1.15Cr2.3Ti5.7O16 sample surface was corroded after 14 days of static leaching in HCl solution with pH = 2, with corrosion pits found on the grain surface of specific crystal orientations, which demonstrates that different crystal orientations have different corrosion resistance. Ba1.15Cr2.3Ti5.7O16 displays good thermal stability and high corrosion resistance, and thus could be used as a potential nuclear waste form for the immobilization of the long-lived and highly mobile Cs.

Monazite-Type SmPO4 as Potential Nuclear Waste Form: Insights into Radiation Effects from Ion-Beam Irradiation and Atomistic Simulations.

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO4 pellets and thin lamellae were irradiated with 1, 3.5, and 7 MeV Au ions, up to fluences of 5.1 × 1014 ions cm−2 to simulate ballistic effects of recoiling nuclei resulting from α-decay of incorporated actinides. Threshold displacement energies for monazite-type SmPO4 subsequently used in SRIM/TRIM simulations were derived from atomistic simulations. Raman spectra obtained from irradiated lamellae revealed vast amorphisation at the highest fluence used, although local annealing effects were observed. The broadened, but still discernible, band of the symmetrical stretching vibration in SmPO4 and the negligible increase in P–O bond lengths suggest that amorphisation of monazite is mainly due to a breaking of Ln–O bonds. PO4 groups show structural disorder in the local environment but seem to behave as tight units. Annealing effects observed during the irradiation experiment and the distinctively lower dose rates incurred in actinide bearing waste forms and potential α-radiation-induced annealing effects indicate that SmPO4-based waste forms have a high potential for withstanding amorphisation.

Chemical durability and surface alteration of lanthanide zirconates (A2Zr2O7: A = La-Yb)

Chemical durability of lanthanide zirconates (A2Zr2O7) (A = La-Yb) under near-field environments is important for evaluating their application as potential nuclear waste forms. In this work, A2Zr2O7 (A = La-Yb) are synthesized by spark plasma sintering with controlled microstructure and their chemical durability are evaluated in a nitric acid solution (pH = 1). Scanning transmission electron microscopy analysis reveals an amorphous passivation film either enriched with Zr or lanthanide. The complex chemistry of the passivation films can be correlated with a transition in corrosion mechanisms from a preferential release of lanthanide in La2Zr2O7 to a preferential release of Zr in Er2Zr2O7 and Yb2Zr2O7. These results suggest a dominant mechanism of incongruent dissolution and surface reorganization for the formation of passivation films. Strong correlations are identified between the leaching rates and cation ionic size, ionic potential, electronegativity differences between A-site cation and Zr, and bonding valence sum of oxygen, suggesting important impacts of structural and bonding characteristics in controlling chemical durability of lanthanide zirconates.

Multicomponent pyrochlore solid solutions with uranium incorporation – A new perspective of materials design for nuclear applications

Multicomponent pyrochlore solid solutions with and without uranium incorporation were fabricated and their thermal-mechanical properties were characterized. Multicomponent pyrochlore solid solutions without uranium exhibit comparable thermal conductivity and higher mechanical strength compared to baseline single component rare-earth titanate pyrochlore (A2Ti2O7). Uranium incorporation reduces hardness as compared with single component compositions. High entropy pyrochlore with uranium displays the highest thermal conductivity within multicomponent pyrochlore solid solutions with significantly better mechanical properties than UO2. The measured thermal conductivity correlates well with A-site cation mixing entropy and a modified size disorder parameter, and thus the size disorder and mixing entropy could be good indicators for predicting thermal conductivity of multicomponent pyrochlore solid solutions. This work opens up the possibility of designing multicomponent oxide solid solutions by controlling their chemical disorder/mixing entropy to achieve acceptable thermal-mechanical properties, desired radiation and corrosion performance for potential nuclear waste form and inert matrix fuel applications.

Corrosion interactions between stainless steel and lead vanado-iodoapatite nuclear waste form part II

This paper studied the release of iodine from lead vanado-iodoapatite (I-APT, Pb9.85(VO4)6I1.7), a potential nuclear waste form for the radioactive waste element of I-129, which can be enhanced when crevice corrosion of stainless steel (SS) occurring nearby. Reference corrosion studies of I-APT were performed in different bulk solutions including DI water, 0.6 M and 6 M NaCl, and 0.1 M HNO3 without metal crevice corrosion interactions. The localized enrichment of Cl−, one of the major consequences of SS crevice corrosion, was found to be the decisive factor that led to the enhanced release of iodine. A surface alteration layer consisting of a mixture of nanocrystalline I-APT and Cl-rich apatite (Cl-APT) formed on I-APT surface. Meanwhile, large Cl-APT crystals formed at the crevice mouth on the I-APT surface. This study reveals a new near-field corrosion mechanism for ceramic waste forms when they are exposed to aggressive local corrosive conditions created by the electrochemical reactions of nearby metals. The insight gained in this study could be beneficial for a more accurate prediction of waste form degradation.

Spark plasma sintering-densified vanadinite apatite-based chlorine waste forms with high thermal stability and chlorine confinement

Effective management of the long-lived radioactive 36Cl is necessary for the development of pyrochemical reprocessing of used fuels and also the molten salt reactors in which a large quantity of Cl-containing waste is encountered. Significant challenge exists in developing advanced waste form matrix with high Cl confinement capability and fabrication technology with minimized loss of Cl. In this work, Cl-bearing vanadinite apatite (Pb5(VO4)3Cl) powder samples are synthesized by solid state reaction at room temperature using high energy ball milling (HEBM), and dense pellets above 99% theoretical density can be consolidated by spark plasma sintering (SPS) at a temperature as low as 450 °C for 3 min. The effects of various sintering temperatures (300–800 °C) on physical density, microstructure, thermal stability and mechanical properties of the SPS densified pellets are investigated. Microstructure analysis shows that the average grain size of nanocrystalline ceramic is less than 190 nm when sintered at 450 °C, and the sintered microstructure is dominated by grain growth at higher sintering temperatures. No phase decomposition and significant chlorine loss can be identified during HEBM and low temperature SPS consolidation processes. Thermal gravimetric analysis (TGA) test indicates that the phase decomposition and chlorine loss occur only at an elevated temperature 830 °C for both HEBM-prepared powders and SPS densified pellets. Vanadinite pellets densified at 450 °C possess the maximum hardness of 4.6 GPa. The SPS-densified Cl-bearing vanadinite apatite displays high confinement of chlorine and a good thermal stability, and thus could be used as a potential nuclear waste form for the immobilization of the long-lived and highly volatile 36Cl.

Rare-Earth Orthophosphates From Atomistic Simulations.

Lanthanide phosphates (LnPO4) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

Assessing the oxidation states and structural stability of the Ce analogue of brannerite

The Ce‐containing analogue of brannerite (ie, UTi2O6) was previously considered to be stoichiometric (ie, CeTi2O6); however, it has recently been determined that the material is O deficient. This oxygen‐deficient material has been suggested to be charged balanced by the presence of a minor concentration of Ce3+ or by the A‐site being cation deficient with the Ce oxidation state being 4+. A variety of Ti‐containing oxides (including brannerite) have been investigated as potential nuclear wasteforms, and it is necessary to understand the electronic structure of a proposed nuclear wasteform material as well as how the structure responds to radiation from incorporated waste elements. The radiation resistance of a material can be simulated by ion implantation. The objective of this study was to confirm the Ce oxidation state in the cation‐ and oxygen‐deficient material (ie, Ce0.94Ti2O6 − δ) and to determine how radiation damage affects this material. X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near‐edge spectroscopy were used to study Ce0.94Ti2O6 − δ before and after being implanted with 2 MeV Au− ions. Analysis of the Ce 3d XPS spectra from the as‐synthesized samples by using a previously developed fitting method has unequivocally shown that Ce adopts both 4+ (major) and 3+ (minor) oxidation states, which was confirmed by examination of magnetic susceptibility data. Analysis of XPS and X‐ray absorption near‐edge spectroscopy spectra from ion‐implanted materials showed that both Ce and Ti were reduced because of radiation damage and that the local coordination environments of the cations are greatly affected by radiation damage.

A Structural Investigation of Hydrous and Anhydrous Rare-Earth Phosphates

Rhabdophane- (REPO4·nH2O; RE = La to Dy), monazite- (REPO4; RE = La to Gd), and xenotime-type (REPO4 and RE'PO4·nH2O; RE = Tb to Lu and Y; RE' = Ho to Lu and Y) rare-earth phosphate materials are being considered for a number of applications including as photonic materials, for biolabeling studies, and as potential nuclear wasteforms. Structural studies of hydrous rare-earth phosphates are rather limited when compared to anhydrous rare-earth phosphates. In this study, rhabdophane- (REPO4·nH2O; RE = La, Nd, Sm, Gd, and Dy) and xenotime-type (REPO4·nH2O; RE = Y and Yb) materials were synthesized by a precipitation-based method and investigated using X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES). Examination of the powder XRD data from rhabdophane-type materials has confirmed that the rhabdophane structure crystallizes in the monoclinic crystal system rather than the hexagonal structure that has most often been reported. Materials adopting the rhabdophane- or xenotime-type structure were studied as a function of temperature to understand how the structure varies with increasing annealing temperature. Local structural information was obtained by collecting P K- and RE L1-edge XANES spectra. Examination of P K-edge XANES spectra from rhabdophane- and xenotime-type materials revealed changes in the local environment around P as a function of temperature. These changes were attributed to the removal of water from these structures as a result of high temperature annealing.

Direct Measurement of Surface Dissolution Rates in Potential Nuclear Waste Forms: The Example of Pyrochlore.

The long-term stability of ceramic materials that are considered as potential nuclear waste forms is governed by heterogeneous surface reactivity. Thus, instead of a mean rate, the identification of one or more dominant contributors to the overall dissolution rate is the key to predict the stability of waste forms quantitatively. Direct surface measurements by vertical scanning interferometry (VSI) and their analysis via material flux maps and resulting dissolution rate spectra provide data about dominant rate contributors and their variability over time. Using pyrochlore (Nd2Zr2O7) pellet dissolution under acidic conditions as an example, we demonstrate the identification and quantification of dissolution rate contributors, based on VSI data and rate spectrum analysis. Heterogeneous surface alteration of pyrochlore varies by a factor of about 5 and additional material loss by chemo-mechanical grain pull-out within the uppermost grain layer. We identified four different rate contributors that are responsible for the observed dissolution rate range of single grains. Our new concept offers the opportunity to increase our mechanistic understanding and to predict quantitatively the alteration of ceramic waste forms.

Ion Beam Irradiation-Induced Amorphization of Nano-Sized KxLnyTa2O7-v Tantalate Pyrochlore

Nano-sized (~10-15 nm) tantalate pyrochlores KxLnyTa2O7-v (Ln = Gd, Y, and Lu) were irradiated with 1 MeV Kr2+ beams at different temperatures and their radiation response behaviors were studied by in-situ TEM observations. All of these nano-sized KxLnyTa2O7-v pyrochlores are sensitive to radiation-induced amorphization with low critical doses (~0.12 dpa) at room temperature and high critical amorphization temperatures above 1160 K. The K+ plays a key role in determining the radiation response of tantalate pyrochlores, in which the K+-rich KLuTa2O7 displays greater amorphization susceptibility than K0.8GdTa2O6.9 and K0.8YTa2O6.9 with lower K+ occupancy at the A-site. The reduced amorphization tolerance of the composition with a greater K+ content is consistent with the prominently larger K+/Ta5+ cationic radius ratio, which may result in more structural deviation from the parent fluorite structure and less capability to accommodate radiation induced defects. An empirical correlation between critical amorphization temperature and ionic size was derived, generally describing the dominant effect of ionic sizes in controlling radiation response of a wide range of pyrochlore compounds as potential nuclear waste forms. The results of the tantalate pyrochlore in this work highlight that nanostructured pyrochlores are not intrinsically radiation tolerant and their responses are highly compositional dependent.

In situ TEM of radiation effects in complex ceramics

In situ transmission electron microscopy (TEM) has been extensively applied to study radiation effects in a wide variety of materials, such as metals, ceramics and semiconductors and is an indispensable tool in obtaining a fundamental understanding of energetic beam-matter interactions, damage events, and materials' behavior under intense radiation environments. In this article, in situ TEM observations of radiation effects in complex ceramics (e.g., oxides, silicates, and phosphates) subjected to energetic ion and electron irradiations have been summarized with a focus on irradiation-induced microstructural evolution, changes in microchemistry, and the formation of nanostructures. New results for in situ TEM observation of radiation effects in pyrochlore, A(2)B(2)O(7), and zircon, ZrSiO(4), subjected to multiple beam irradiations are presented, and the effects of simultaneous irradiations of alpha-decay and beta-decay on the microstructural evolution of potential nuclear waste forms are discussed. Furthermore, in situ TEM results of radiation effects in a sodium borosilicate glass subjected to electron-beam exposure are introduced to highlight the important applications of advanced analytical TEM techniques, including Z-contrast imaging, energy filtered TEM (EFTEM), and electron energy loss spectroscopy (EELS), in studying radiation effects in materials microstructural evolution and microchemical changes. By combining ex situ TEM and advanced analytical TEM techniques with in situ TEM observations under energetic beam irradiations, one can obtain invaluable information on the phase stability and response behaviors of materials under a wide range of irradiation conditions.

Experimental observation of an interface-controlled pseudomorphic replacement reaction in a natural crystalline pyrochlore

Pyrochlore-type (A 2 B 2 O 6 O') ceramics are considered for the immobilization of highly radioactive waste. Understanding the alteration process of such potential nuclear waste form materials in aqueous media is critical for the prediction of their long-term stability in a nuclear repository. Current models on pyrochlore alteration are based on a diffusion-controlled hydration and ion exchange process. However, we present results of a hydrothermal experiment at 200 °C with a natural, polycrystalline pyrochlore and 1 8 O-enriched aqueous solution, which are not compatible with a process based on solid-state diffusion. TOF-SIMS and confocal μ-Raman mapping of the run product revealed the occurrence of 1 8 O-enriched alteration zones with sharp chemical gradients to relict unreacted areas when compared to the extent of the alteration zones. The data are consistent with a pseudomorphic reaction that involves the dissolution of the pyrochlore parent accompanied by the simultaneous reprecipitation of a defect pyrochlore at a moving dissolution-reprecipitation front.

Thermally induced phase decomposition and nanocrystal formation in murataite ceramics

Synthetic murataite, an isometric derivative of the fluorite structure, has been proposed as a host phase for the immobilization of lanthanides and actinides. In this study, murataite polytypes have been synthesized in the system Ca–Ti–U–Mn–Al–Zr–Fe–Ce–O by melting oxide mixtures at 1400–1600 °C followed by crystallization under slightly oxidizing conditions. The phase and structural compositions of the synthetic murataite ceramics were characterized by energy dispersive spectroscopy and transmission electron microscopy (TEM). The thermal stability was studied by in-situ heat treatment under TEM observation. The influence of radiation damage as a result of alpha-decay events of the incorporated actinides on the thermal stability of the synthetic murataite ceramics was simulated by 1 MeV Kr+ irradiation. A thermally induced, Fe-rich phase decomposition and nanocrystal formation was observed in murataite ceramics under vacuum above 573 K. Ce was closely associated with the Fe-rich phase in the Ce-incorporated murataite ceramics. Ion beam irradiation-induced amorphization inhibited the process of phase decomposition. The effects of thermally induced phase decomposition on the radiation resistance of murataite ceramics utilized as potential nuclear waste forms for the immobilization of actinides and, particularly plutonium, were also analyzed.

3He thermal diffusion coefficient measurement in crystalline ceramics by μnra depth profiling

This contribution is devoted to the measurement of the thermal diffusion coefficient of 3He in crystalline nuclear ceramics. The experimental method is based on the observation of the 3He(d,p)α reaction. The detection of the high energy proton (12–14 MeV) is convenient for large implantation depths (5 μm< x<10 μm) and limits the depth resolution around 100–150 nm. After controlled temperature annealing, the thermal diffusion coefficient can be deduced from the broadening of the helium-3 depth profile according to a classical Fick’s law formalism by comparing several computing codes. Several examples concerning simple oxides (UO 2 and ZrO 2) and more complex ceramics considered as potential nuclear waste forms (britholite and zirconolite) are given. Typical thermal diffusion coefficients obtained for britholite, zirconolite, zirconia and uranium dioxide are in the range 10 −14–10 −12 cm 2 s −1 between 300 and 1000 °C.

Application of nuclear reaction geometry for 3He depth profiling in nuclear ceramics

Direct observation of nuclear reactions leading to the emission of charged particles (p or α) allows to determine specifically the spatial distribution of isotopes of light elements from 1H to 23Na and despite low cross section values some heavier isotopes from 24Mg to 68Zn. After a brief overview of the analytical capabilities offered by μNRA, this contribution is focussed on the measurement of the thermal diffusion coefficient of 3He in crystalline ceramics. The experimental method is based on the observation of the 3He(d, p)α reaction. Due to the severe energy loss along the outgoing path, the choice of the detection of the high energy proton or recoil α nucleus depends on the average depth of the 3He distribution. For near surface distributions (<2 μm), the detection of the α-particle gives a good depth resolution (50 nm). For larger depths (5< x<10 μm), the detection of the 12–14 MeV proton limits the depth resolution around 100 nm. After temperature-controlled annealing, the thermal diffusion coefficient can be deduced from the broadening of the helium-3 depth profile according to the classical Fick’s law formalism by using the computing code SIMNRA. Several examples concerning simple oxides and more complex ceramics considered as potential nuclear waste forms or transmutation targets are then presented and discussed.

Heavy Ion Irradiation of Zirconate Pyrochlores

ABSTRACTZirconate pyrochlores, A2Zr2O7, are important potential nuclear waste forms for Puimmobilization. The binary Gd2(Ti2-xZrx)O7 has been shown to have increasing resistance to ionirradiation damage with the increasing Zr content, and Gd2Zr2O7 is radiation resistant to a 1 MeV Kr+ ion irradiation at 25 K to a dose of 5 dpa. In this study, a 1.5 MeV Xe+ irradiation was completed for zirconate pyrochlores A2Zr2O7 (A=La, Nd, Sm, Gd). The radiation resistance decreases with an increase of the ionic radius of A-site cation. La2Zr2O7 is the first zirconate pyrochlore to be amorphized by ion beam irradiation, and the critical amorphization temperature, Tc, is ∼310 K. The susceptibility of La2Zr2O7 to ion beam damage is related to its structure, which shows the largest deviation from the ideal fluorite structure. These results are also consistent with calculations of the cation antisite formation energy in the pyrochlore structure. The ion irradiation-induced pyrochlore-to-fluorite transformation occurred in all of the irradiated zirconate pyrochlore phases. Based on the results for Gd2Ti2-xZrxO7 and A2Zr2O7, the defect fluorite structures are stable when the ionic radii ratio rA/rB≤1.54; beyond this limit, the defect fluorite structure becomes increasingly unstable relative to the amorphous state.