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Geochemical Journal, Vol. 15, pp. 229 to 243, 1981 229

Alpha-recoil damage in natural and

W. SINCLAIR and A. E. RINGWOOD

Research School of Earth Sciences, Australian National University , Canberra, A.C.T. 2600, Australia

(Received October 13,1980: Accepted July 20, 1981)

Zirconolite (CaZrTi207) and perovskite (CaTi03) are key in SYNROC, a ceramic material developed for the immobilization of high level nuclear reactor wastes. When these are incorporated in SYNROC, the long-lived radioactive actinide elements are preferentially partitioned into zirconolite and perovskite which are therefore subjected to the effects of alpha-recoil, resulting from the decay of these elements. These effects have been studied via X-ray and electron diffraction investigations of natural samples of zirconolite and perovskite of varying ages and varying uranium and contents . The samples studied have received cumulative alpha doses ranging from 1.0 X 1018 to 1.1 X 1020a/g. The upper limit corresponds to the alpha irradiation which would be received by the zirconolite in SYNROC containing 10 percent of high level waste over a period of 5 X 108years. These studies show that zirconolites remain crystalline up to and beyond alpha doses of 2 X 1019a/g. This dose would have accumulated in such a SYNROC zirconolite after a million years of storage. Elec tron microscopy revealed that the grains were composed of small crystalline domains which possessed the defect fluorite-type structure. After a dose exceeding that which would be received by SYNROC in 100 million years, zirconolites appeared metamict when studied by X-ray diffraction . However, the elec tron micrographs and diffraction patterns clearly demonstrate that the continues to retain a large degree of short range order and in no way resembles a glass. The density changes produced in these zir conolites by irradiation are small and range from 0 to 3 % at saturation. Perovskite samples which have SYNROC ages up to 20,000 years decrease in density by 1.8 ± 0.1 %. Their X-ray powder patterns are essentially unaffected. Comparative studies show that the perovskite lat tice is even more resistant to the effects of alpha-recoil than the zirconolite lattice. The results demonstrate that zirconolite and perovskite are extremely resistant to the effects of nuclear radiation and will provide stable crystal structures for the containment of the radioactive waste elements during the time required for the radioactivity to decay to safe levels (typically 101_106years).

INTRODUCTION emitted by these elements, notably neptunium, plutonium, americium, and curium isotopes, are Zirconolite (CaZrTi2O7) and perovskite associated with a recoil of the nucleus. The (CaTi03) are used in the SYNROC process for displacements due to the recoil may cause con the immobilization of elements occurring in siderable damage to the lattice and decrease the high-level nuclear wastes (RINGWOOD et al., stability of the synthetic phases. The alpha 1979). The radioactive wastes are incorporated emitting elements are strongly partitioned into into the crystal structures by forming dilute zirconolite and perovskite and these crystals will solid solutions with the SYNROC phases. The therefore be subjected to most of the alpha waste elements are tightly bound within the recoil damage. crystal lattices and are extremely resistant to We have assembled a collection of naturally leaching by hydrothermal solutions. occurring samples of zirconolite and perovskite These experiments, however, do not take containing the alpha emitting elements, uranium into consideration the important effects of and thorium. The cumulative radiation doses nuclear radiation arising mainly from alpha received by these minerals have been calculated decay of the actinide elements. Alpha particles and cover a considerable range from 1.0 X 1018 230 W. SINCLAIR and A. E. RINGWOOD to 1.1 X 1020 alphas per gram (OVERSBY and Furthermore, the authors supported the con RINGWOOD, 1981). In this paper we describe clusion that the ability for a material to become the effects of increasing doses of alpha radiation metamict was strongly dependent on the crystal on the crystal structures of these minerals using structure. The fact that the mineral thorianite X-ray diffraction and electron microscopy Th02 (whose fluorite-type structure is the techniques. parent structure of zirconolite) is not found to REEVE and WOOLFREY (1980) have ap be metamict in the natural state was said to be proached the same problem by irradiating explained by this hypothesis. SYNROC mineral assemblages with fast neu COMESet al. (1967) have reported the ef trons to simulate alpha particle and actinide fects of fast neutron irradiation on single crystal recoil damage. Their results form a comple quartz. These authors observed a gradual break mentary study and will be discussed later. down of the structure into a heterogeneous Many other phases have also been examined mixture of crystalline and metamict areas. After for the effects of radiation damage. PYATENKOvery strong irradiation (> 1020n/cm2) the crystal (1970) has suggested that as minerals (including becomes entirely metamict and exhibits a dif zirconolite) become metamict there is a break fuse diffraction ring on X-ray photographs. down of the leading to a They concluded that the Si04 tetrahedra remain segregation of new phases on a very small scale. nearly undamaged during irradiation. In con These new phases may represent component trast, the ionic crystal LiF remains nearly un oxides or a more complex configuration of ions. distorted when subjected to radiation damage. Extensive studies previously carried out on In view of the variety of conclusions existing zircon (ZrSi04) have led to several differing in the literature, it seemed possible that an inde conclusions. HOLLAND and GOTTFRIED (1955) pendent detailed study on new mineral types and PELLAS (1965) invoked a multistage process such as zirconolite and perovskite may provide occurring with increasing radiation dose. BUR necessary information to resolve the effects of SILL and MCLAREN (1966) have supported this large doses of nuclear radiation. Moreover, multistage process and provided evidence from since these minerals are key components of electron microscopy for the existence of small SYNROC, the information so obtained should crystallites of zircon, even in the metamict state. have an important bearing on the long-term PELLAS (1965) concluded that zircon ultimately behaviour of SYNROC after incorporation of decomposes to a mixture of its component high-level nuclear reactor wastes. oxides, SiO2 + Zr02. WASILEWSKIet al. (1973) Zirconolite, CaZrTi2O,, is closely related to have supported these conclusions from infrared the defect fluorite-type (CaF2-X) structure absorption spectral studies. (PYATENKO and PUDOVKINA, 1964; ROSSELL, Alternatively, VANCE and BOLAND (1975) 1980a, b). It can be derived from the simple and VANCE (1975) have suggested a progressive lattice by distortion of the parent cubic cell and disordering of the lattice with increasing radia ordering of the cations. The structure so derived tion dose. These authors did not find a second is monoclinic and has eight times the volume of new phase for zircons as suggested by HOLLAND the original cube. and GOTTFRIED (1955) nor the breakdown of Perovskite (CaTi03) is orthorhombic con the lattice into its component oxides. The con sisting of a 3-dimensional framework of corner clusion of progressive lattice disorder was also joined Ti06 octahedra with Ca atoms occupying made by VANCE and BOLAND (1978) in studies the spaces between them. In nature, perovskite of Zr02. shows a considerable range of ionic substitu More recently, CARTZ and FOURNELLE tions. The rare earths and alkalis commonly (1979) did not observe a breakdown of ZrSi04 replace while small sized cations such as into its component oxides in the metamict state. and tantalum replace . Alpha-recoil damage 231

The ability of these minerals to accom containing 10% high level waste which would modate a large range of elements in their crystal have received a similar radiation dose. structures is of primary importance to their inclusion in the SYNROC process. EXPERIMENTAL AND RESULTS

1.1 Radiation dose as a function of SYNROC A suite of natural samples was examined age using X-ray diffraction and electron microscopy. The SYNROC process proposes to incorpo The X-ray diffraction techniques included single rate 10% of high level radioactive wastes as crystal precession, rotation and Weissenberg dilute solid solutions in the constituent minerals. photography, and Debye-Scherrer and Guinier Zirconolite and perovskite strongly partition the powder photography. Refinement of cell para actinide elements present in the waste, and will meters was done by the method of least-squares therefore absorb most of the alpha-recoil dam on a Hewlett-Packard 9825 calculator. Heating age. To evaluate the effect of alpha-recoil on experiments were carried out in air, in unsealed these phases, OVERSBY and RINGWOOD (1981) platinum capsules. Chemical analyses, Table 1, have calculated the cumulative alpha dose per were obtained by energy-dispersive electron gram of SYNROC mineral as a function of time. microproble. Figures 1 and 2 from OVERSBY and RINGWOOD Electron microscope studies were carried (1981) show the variation of radiation dose with out under the guidance of JOHN FITZGERALD age for SYNROC containing 10% of high level using a 200keV JEOL instrument. The samples wastes. The samples described in the following were first roughly thinned by manual polishing -sections have been placed on the SYNROC to about 40µm and were then ion-thinned with curves. 5 keV Ar ions. Other samples were crushed and The radiation dose received by the natrual placed onto a carbon grid. Density measure zirconolite and perovskite samples can be cal ments were determined by the method of culated from their age and U and Th concentra Archimedes, using toluene as the immersion tions. The dose, in alphas per gram, can then be liquid. directly related to the age of a SYNROC sample A summary of relevant information for the

a Dose vs SYNROC Age for Zirconolite & Perovskite 14

12

Total Jacupiranga Dose 10 zirconolite a/g x 1018 8

6 high-U zirconolite Kaiserstuhl

Baikal Perovskite 4 Loparite

2 Jacupiranga Perovskite

103 104 105 106 SYNROC Age (years)

Fig. 1. Total alpha dose vs. age for SYNROC samples between 103 and 106yrs. (After OvERSBYand RINGWOOD, 1981. 232 W. SINCLAIR and A. E. RINGWOOD

14 a Dose vs SYNROC Age

12 T Zirconolites Sri Lanka ota I high T h -_~ Dose 10 a/g x 1019 Sri Lanka 8 high U

6 Russian A

4

2

106 107 108 109 SYNROC Age (years)

Fig. 2. Total alpha dose vs. age for SYNROC samples between 106 to 109yrs. (After OVERSBY and RINGWOOD, 1981).

minerals has been compiled in Table 2. The defect fluorite-type diffraction patterns unlike volume changes of the zirconolite and perovskite the samples studied by PUDOVKINAet al. (1974) lattices, caused by the effects of alpha-recoil, are which were metamict. Secondly, the grains con presented in Table 3. Some of the key samples, tinued to display single crystal diffraction pat however, require more detailed discussion. terns when reconstitued at temperatures up to 1,100T. These new observations are of con (a) zirconolite siderable importance, showing that after intense Zirkelites from Jacupiranga, Brazil were radiation damage and suffering extensive meta first reported by HuSSAK and PRIOR (1895). mictization, the lattice retains a large degree of The minerals received little attention until order. PUDOVKINAet al. (1974),using HUSSAK'Ssamples, At 1,200°C the original cubic defect fluo carried out chemical analyses and X-ray studies rite-type cell completely transformed. The pat of heated sepcimens in order to establish their terns obtained seemed at first much simplified relationship to zirconolite. These authors in comparison to synthetic zirconolite. found that in the natural state the crystals were PUDOVKINAand PYATENKO(1966) were able to metamict, although one specimen showed a describe the simplification of X-ray patterns faint, diffuse reflection from Laue photography. from Aldan zirconolites in terms of a distortion When heated to 800'C the samples produced along the 3-fold axis of a cube. The resultant cubic defect fluorite-type patterns with a = X-ray pattern displays a strong hexagonal ap 5.08A. At 1,200°C the X-ray diffraction pat pearance while at the same time appears pseudo tern of the heated crystals was identified as the cubic. The "hexagonalisation" of the X-ray monoclinic zirconolite. patterns were also encountered by PUDOVKINA The latter results have been confirmed by et al. (1974) for zirconolites from Brazil. our analyses. However, several significant addi Preliminary electron microscopy of a natural tional observations were also made. Firstly, the unheated Brazilian zirconolite revealed that the natural samples in this investigation gave cubic specimen was composed of a number of regions Alpha-recoil damage 233

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Table 2. Sample descriptions

1Age Dose (a/g) SYNROC Age Sample (my) x.1018 (Y) Comments ZIRCONOLITE: , Kaiserstuhl 16 1.2 1X103 Monoclinic structure; low 20 angle X-ray diffraction complex reflections are sharp; high angle data show broaden ing and weakening of reflections. Brazil, Jacupiranga carbonatite 133 7.6 4x105 See text. complex NNorway , Stavern and Larvik 295 17. 1x106 Similar X-ray observations to Brazil samples with presence of new orthorhombic intermeoiate phase at 880°C: a = 10.08, b = 14.28, c = 7.40k space group Aba2 or Cmca. 3Russian A (Aldan?) 640 18. 1x106 Similar X-ray observations to Brazil samples. Sri Lanka 550 80-110 4-7 x 108 See text. PEROVSKITE: Lovozero alkaline massif, Kola 300 1.3 2 x 103 X-ray powder diffraction patterns show no sign of peninsula (Loparite)4 lattice damage. Brazil, Jacupiranga 133 1.6 5x103 Sharp X-ray powder diffraction patterns. Lake Baikal, Tazheran 500 2.6 1.7 x 104 See text. 1. Ages from OVERSBYand RINGWOOD(1981); Age for Lovozero loparite from KoMLEVet al. (1961). 2. Samples from Norway are known as polymignite (DANA and DANA, 1944; VLASOV,1966; LIMA DE FARIA, 1964). 3. Origin of sample unknown; X-ray and chemical properties suggest they originate from the Abarastakh Massif, Aldan (BORODINet al., 1960; PUDOVKINAand PYATENKO,1966). 4. Loparite is a variety of perovskite rich in rare earth elements.

with different properties. The appearance (b) Sri Lanka zirconolite of some of these regions. Figs. 3-5, were simi Zirconolites from Sri Lanka were first de lar to those described by BURSILL and MCLAREN scribed by BLAKE and SMITh (1913). X-ray (1966) for zircon. Still other areas of the photographs indicated that these minerals were Brazilian specimen gave only diffuse powder completely metamict. The powder patterns gave rings and were of low contrast. These areas are only diffuse rings. Electron microscopy of sam interpreted to be the final state of radiation ple 83800, however, displayed regions similar damage. to those of the Brazilian zirconolite. These are The d-spacing of ^• 2.92A of the inner ring shown in Figs. 6 and 7. in Fig.4, corresponds to the distance between Heating to 700-800°C for two hours caused the metal-only (111) planes of the fluorite-type the cubic defect fluorite-type powder pattern to structure. In the zirconolite structure the planes appear. After further heating to 1,100-1,200°C which alternately contain Ti and Ca + Zr are the X-ray patterns corresponded to the mono derived from these close-packed (111) fluorite clinic zirconolite. Two samples, numbers 83 800 type planes. Thus, even in the metamict state and B20392, gave the 'hexagonalized' zirco the cations within these planes still retain a nolite pattern while sample SL3-12 produced a short range periodic arrangement. monoclinic pattern similar to a pure synthetic To check to see if the crystal had segregated zirconolite. into several new phases such as its component oxides as suggested by PYATENKO (1970), large (c) Baikal perovskite areas of the specimen were moved under the Although this sample has received a higher electron beam. The cubic subcell did not change alpha dose than the loparite, the X-ray powder and remained orientated throughout the pro patterns are extremely sharp with no indication cedure. No other phases were observed to be of lattice damage. Comparison of these X-ray present. patterns with those of Kaiserstuhl zirconolite Alpha-recoil damage 235

Table 3. Unit cell dimensions of natural zirconolites and

Sample Temperature (°C) a(A) b(A) c(A) a(°) V(A3) %Vol. change Germany XIRCONOLITES R.T. 12.544 (1) 7.288 100.26 (2) 1046.7 (6) Kaiserstuhl (1) 11.636 (4) ± 0.1 1200 12.571 (2) 7.300 (1) 11.523 (5) 100.62 (2) 1039.3 (8) }0.7 Brazil Jacupiranga R.T. 5.06 (4)a 1036 (25). ± 2.4 1200 12.592 (1) 7.270 (1) 11.451 (1) 100.56 (1) 1030.5 (3) }0.5 Norway R.T. 5.10 (3)a 1061 (18) Stavern 2.8 ± 1.7 1300 12.616 (2) 7.284 (1) 11.424 (2) 100.60 (1) 1031.9 (4) Norway Larvik 1200 12.581 (1) 7.264 (1) 11.416 (1) 100.59 (1) 1025.3 (3) Russian A R.T. 5.03 (1)b 1018 (6) 1200 12.548 (1) 7.245 (1) 11.404 (1) 100.57 (1) 1019.2 (3) 1 0 ±0.6 Sri Lanka 1200 12.566 (1) 7.255 (1) 11.432 (1) 100.56 (1) 1024.6 (3) (B20392) Sri Lanka 1140 12.568 (1) 7.256 (1) 11.435 (1) 100.56 (1) 1025.1 (3) (83800) Sri Lanka 1200 12.451 (2) 7.243 (1) 11.386 (2) 100.54 (2) 1009.5 (4) 2.5 ± 2c (SL3-12) PEROVSKITES Lovozero 58.59 (2) Loparite R.T. 3.8839 (5)d 1.56 ± 0.07 1200 3.8636 (5)d 57.67 (2) Brazil Jacupiranga R.T. 5.4791 (6) 7.6873(15) 5.4066 (8) 227.72 (8) 1.03 ± 0.06 1200 5.4517 (5) 7.6617(10) 5.3956 (5) 225.37(6) } Baikal R.T. 5.4926 (5) 7.6972 (7) 5.4049 228.51 (6) (5) 1.82 ± 0.05 1200 5.4486 (5) 7.6475 (7) 5.3844 (5) 224.35(6) } Errors are given in parentheses;Cell dimensionsobtained using a GuinierHdgg focussing camera, Cu Kal radiation (A = 1.54060A) Si standard (a = 5.43088 A), unless otherwise stated; Natural Sri Lanka and Larvik crystals gave diffuse patterns. (a) Measuredfrom precession photograph, MoKa radiation (A = 0. 7107A); cubic cell is 118 the volume of the monoclinic cell. (b) Cell edge derived from 57.3 mm Debye-Scherrer pattern, FeK& radiation (A = 1.9373 A); Si standard. (c) Vol. change obtained from density measurements using the method of Archimedes. (d) Pseudo-cubic cell edge, 114.6mm Debye-Scherrer pattern, CuKZWradiation (A= 1.5418A). indicates that the perovskite mineral is even kite. These results suggest that both for zir more resistant to radiation damage than zirco conolite and perovskite most of the lattice ex nolite. The volume expansion obtained for this pansion occurs after a relatively small radiation sample, 1.82 ± 0.05%, compares favourably with dose. After lattice expansion of less than 3 the results of REEVE and WOOLFREY (1980). percent there is little further expansion even for These workers reported an increase in volume of very large alpha doses. approximately 2.6% for synthetic perovskites irradiated with fast neutrons for a period of time DISSCUSSION equivalent to a SYNROC age of 10,000 years. Although this dose was accumulated in 22 days, X-ray diffraction and electron microscopy the above results support the relevance of ac studies suggest that zirconolites retain a high celerated irradiation testing. degree of lattice order even after they have The volume changes reported here are quite received a radiation dose equivalent to 106 small in relation to the alpha dose received and SYNROC years. The very small volume changes demonstrates the structural stability of perovs observed for the samples which have received 236 W. SINCLAIR and A. E. RINGWOOD

I

~ a

11

~ Fg+ " P ....

Fig. 3. Dark field image and diffraction pattern of a well crystallized area of natural Brazil zirconolite. The dif fraction pattern corresponds to the cubic defect fluorite-type lattice plus extra weak diffuse reflections. Beam direction along (011) fluorite.

a 8

v g~

m k" g

F

a F

i

Fig. 4. Dark field image illustrating the domain structure of natural Brazil zirconolite. The contrast is speckled on a scale of about 50k The defect fluorite-type diffraction pattern exhibits a strong powder ring whose d-spacing corresponds to the (111) fluorite-type reflection. Beam direction along (011) fluorite.

Alpha-recoil damage 237

Fig. 5. Dark field image of a sector of the (111) powder ring of Fig. 4. Only areas of low contrast in Fig. 4 con tribute to the powder ring.

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Fig. 6. Bright field image of sample 83800. The crystallites (^•140A in diameter) are radomly orientated giving rise to the sharp defect fluorite-type powder pattern. The radiation dose accumulated by the sample is 6 times higher than the expected dose received by zirconolites inSYNROC containing 10 percent high level waste and stored for 106 years. The specimen, however, still contains areas of crystalline material.

238 W. SINCLAIR and A. E. RINGWOOD

large doses of radiation indicates that only a is another desirable characteristic of the com small degree of internal rearrangement has oc pound. curred. Moreover, the zirconolite samples re The fluorite-type structure exhibits these constituted, as single crystals, to their original characteristics. It possesses a high ionicity, undamaged state after being heated for less (NAGUIB and KELLY,1975), and the Ca atoms than one hour at temperatures as low as 950°C. are arranged in cubic closest-packing. Thorianite Even the highly damaged Sri Lanka zirconolites adopts this structure and, in nature, is not found reconstituted to a single phase of polycrsystal in the metamict state. Several other simple line material. This is in contrast to the behav oxides with the fluortie-type structure have also iour of heavily damaged zircons. These miner been found to remain crystalline when bom als, when heated, reconstitute to a complex mix barded with high doses ( 10"ions/cm2) of ture of the oxides (CARTZ and FOURNELLE, energetic heavy ions (NAGUIBand KELLY,1975). 1979). The close relationship between the zirco The effects of nuclear radiation on the nolite structure and the fluorite-type structure properties of inorganic compounds are highly has already been mentioned in a previous sec variable. As early as 1924, however, GoLDSCH tion. The fluorite-type subcell, upon which zir MIDT (1924) suggested that those crystals which conolite is based, is only slightly deformed from contain a high degree of ionic binding were more an undistorted face-centred cubic lattice. A resistant to the effects of radiation. This same rhombohedron results with an angle of about observation has since been made by many 92° (PUDOVKINA and PYATENKO, 1966). The authors (e.g. WITTELs and SHERRILL, 1959; ability of natural zirconolite to withstand the BILLINGTONand CRAWFORD,1961; BUDYLIN and damaging effects of large doses of alpha radia VOROB'EV,1964 and NAGUIB and KELLY, 1975). tion is probably due to this association. Close packing of the atoms within the structure The perovskite structure, like that of zirco

Fig. 7. Electron micrograph of the featureless areas of sample 83800. The corresponding diffraction pattern dis plays diffuse rings which approximate the cubic defect fluorite-type pattern. The inner ring (I) is equivalent to the combination of the (111) and (200) fluorite-type reflections. Similarly, the intermediate ring (M) corresponds to the (220), (311) and (222) defect fluorite-type reflections. Rings (I) and (M) have been overexposed to show the outer ring (0). Alpha-recoil damage 239

nolite, is based upon a cubic lattice. In this the bonding and type of distortion from the structure the and calcium atoms are ideal cubic perovskite-type compound. More arranged approximately in cubic close-packing, over, the extremely high dose rate may have i.e. a face-centred cubic array (BLoss, 1971). been a contributing factor. In contrast all ti The distortion to the observed orthorhombic tanate perovskites which have received much structure involves a very small degree of shearing larger doses, but over longer periods of time, ( 48') of the cubic pseudo-cell (KAY and have been shown to remain highly resistant to BAILEY,1957). WITTELsand SHERRIL(1959) the effects of large doses of nuclear radiation. irradiated a number of perovskite-type crystals. Furthermore, the volumes of all the perovskite These included BaTi03 , KNb03 and PbTi03 . type compounds mentioned increase by only They demonstrated that the above compounds small amounts. transformed to their high-temperature cubic The term metamict is usually applied to modification when irradiated with fast neutron minerals which have lost their crystallinity due dosages in excess of 1019n/cm2. The a and c to the action of nuclear radiation arising from axial lengths of tetragonal BaTi03 increased by the radioactive elements contained within them. approximately 1.9% and 0.9% respectively after PABST(1952) and BERMAN(1955) have given a receiving a neutron dose of 1.2 X 10"n/cm'. detailed discussion of the term and some of the At this stage, all axes were equivalent (cubic) characteristics associated with such minerals. and after 1.8 X 1020n/cm2 the value of the cubic There are many degrees of the metamict state axis increased by a further 0.3 %. This radiation and the physical properties usually change con dose is close to three times the dosage received tinuously in response to the amount of radia by the perovskites used in the experiments of tion dose. With the use of electron microscopy, REEVE and WOOLFREY(1980). The BaTi03 minerals that were once thought to be metamict crystals remained single throughout the entire are now found to display strong diffraction pat irradiation. terns typical of well crystallized materials. Recently, loparites containing greater than Recently, several authors (CARTZ and 1 % U02 have been found in the Lovozero intru FOURNELLE, 1979; EWING, 1975, 1976) have sion, Kola Penisula. Although these minerals suggested that minerals in the metamict state have accumulated a dosage of 8.9 X 1014/g, have a glass-type random atomic arrangement. they still continue to produce sharp diffraction However, electron diffraction patterns of the patterns (KOGARKO,pers. com.). The radiation featureless apparently metamict areas of zir dose exceeds that received by the zirconolites conolite (Fig. 7) led to a different interpreta from Jacupiranga. tion. These samples have received extremely In contrast to the perovskites dis high doses of alpha radiation and it has been cussed above, the rhombohedral perovskite-type found that the atoms remain sufficiently or compound CmA1O3 became metamict after it dered to display broadened powder rings. The had received a dose of 1.6 X 1014/g. This dose rings are identifiable with a cubic fluorite-type was derived in 8 days from the decay of 244Cm, pattern. The Mossbauer study of ANSALDO and is equivalent to a SYNROC age of 1,000 (1975) on a sample of X-ray amorphous thoro years. The effect of nuclear radiation caused a steenstrupineINa2Ce(Mn,Fe,Ta)(La,Th ...) [(Si, small increase in volume (3.2% at saturation) P)0413H2 } also supportsan ordered atomic and a phase transformation to the cubic modifi arrangement in the metamict state. The author cation before the compound became amorphous has shown that Fe 31 occurs in two different to X-rays (MosLEY,1971). distinct sites and that the Mossbauer spectrum The behaviour of CmA1O3 to nuclear radia has not been affected by the radiation damage. tion may be a consequence of its differing chemical composition and reflects the nature of 240 W. SINCLAIR and A. E. RINGWOOD

investigated by X-rays has nevertheless been CONCLUSIONS found to retain a high degree of short range Results described in previous sections have order when studied by electron diffraction. This elucidated several of the effects of increasing state in no way resembles a true glass as sug doses of nuclear radiation on the crystal struc gested by EWING(1975, 1976) and CARTZ and tures of naturally occurring zirconolite and FOURNELL(1979). The electron diffraction perovskite minerals. When subjected to an alpha pattern of the metamict areas of highly irradi dose of 8 X 1018a/g equivalent to a SYNROC age ated zirconolite gave distinct powder rings and of 4 X 105yr, zirconolite transformed to the could be identified with the cubic defect fluo cubic defect fluorite-type structure. Despite the rite-type pattern. very large radiation doses received, these crystals One of the primary concerns expressed continued to display single crystal X-ray dif about proposals to incorporate nuclear waste fraction patterns. Electron microscopy of these into a crystalline ceramic waste form, has been zirconolites revealed that the mineral was main the possibility that radiation damage could ly composed of crystalline domains coherently destroy the crystal structures and seriously orientated. impair their ability to immobilize the waste Sri Lanka zirconolite which had recieved a elements. It has been suggested that because radiation dose exceeding that which would be radiation damage ultimately leads to metamicti experienced by SYNROC in 4 X 108 yr were ap zation, the ability of these metamict minerals to parently metamict when studied by X-ray dif retain the waste elements may be no greater fraction. However, electron diffraction investi than that of glass. Our results on zirconolite gations demonstrated that they contained areas and perovskite effectively dispel those concerns, of crystalline domains. These, however, were in at least, for these particular minerals. They random orientations and gave a spotty cubic show that the atomic structures of the irradiated defect fluorite-type powder pattern. The micro minerals are still essentially those of the crystal graphs of other areas of this sample studied by line state and are in no way analogous to those electron microscopy were of low contrast but of silicate or borosilicate glasses. Further sup the diffraction patterns exhibited diffuse pow port for this conclusion is provided by the der rings also identifiable with the defect fluo results of OVERSBYand RINGWOOD(1981) on the rite-type pattern. These apparently metamict Pb/U isotopic systematics and leachability of areas have suffered an extremely large radiation the natural zirconolites and perovskites used in dosage and yet the atoms retain a large degree their study. They demonstrated that the min of short range order. erals display a remarkable ability to immobilize These results have a direct bearing upon U and Pb under geological conditions. More alternative hypotheses dealing with the effect over, the Sri Lanka zirconolites which have suf of radiation damage in crystals. In particular, fered the most extensive radiation doses were they are best explained by progressive disorder found to be less leachable than borosilicate ing of the structure with increasing radiation glasses in water and brines at 200'C by a factor dose, as suggested by VANCE (1975) and VANCE exceeding one thousand. and BOLAND (19,75, 1978). There is no evidence For SYNROC containing 10 percent of high that the minerals become segregated into simple level waste, it is usually estimated that a storage oxide phases even after receiving extremely large period of 105 -106yr would be required for the doses of radiation as proposed by PYATENKO wastes to decay to a safe level. The natural (1970), WASILEWSKIet al. (1973) and PELLAS samples described here provide us with a suitable (1965). model with which to assess the crystalline integ The atomic arrangement of zirconolite in a rity of SYNROC phases during this storage state which would be classed as metamict when interval. After 1,000 years the crystal struc Alpha-recoil damage 241

tures of zirconolite and perovskite in SYNROC a rapid time scale (1 month) and the damage containing 10% high level waste are essentially caused by alpha-particles over periods of geo unaffected by the nuclear radiation received logical time (up to 1 O$yrs) were found to be in (chiefly from alpha particle recoils). After good agreement. Although there is a factor of receiving a radiation dose equivalent to a SYN 109 difference between the two time scales, ROC age of one million years zirconolite is these results support the relevance of accelerated still crystalline but undergoes a change in struc irradiation testing. The rate of accumulation of ture to the defect fluorite-type lattice. The radiation damage in SYNROC containing high volume increase is less than 3%. After a radia level waste is intermediate between the above tion dose equivalent to 100 million years SYN dose rates. In view of these results we conclude ROC age zirconolite appears metamict. Elec that the responses to nuclear radiation observed tron diffraction, however, shows that the atoms on the natural minerals over much longer time still maintain short range order, as previously scales are directly applicable to the long term described. The volume increase after this period behaviour of these phases in SYNROC. of time has already reached saturation level and remains smaller than 3 %. Acknowledgements-Wewish to record our appreciation At a radiation dose equivalent to that which of the persons and institutions who made it possible to SYNROC would accumulate in 10,000 years of obtain the samplesof mineralsused in this investigation. storage, perovskite expands in volume by less In this respect, we are particularly indebted to JORG than 2% but the crystallinity remains unaf KELLER, GENE KAULA, CARL FRANCIS, BRIAN fected. After receiving a radiation dose equiv MASON,ARNE RAHEIM,T. R. CUTTILAN,H. D. N. PATHIRANA,and to the British Museum (Natural alent to a SYNROC age of half a million years, History), the Smithsonian Institution, Harvard Uni perovskite is still crystalline. Comparison of versity, Mineralogisk-GeologiskMuseum (Oslo), Uni X-ray diffraction photographs of perovskites versity of Freiburg, SerranaS/A de Mineracao,Sri Lanka and zirconolites which have received similar GeologicalSurvey, and the USSR Academyof Science. radiation doses indicates that perovskite is even We are also indebted to Dr. R. A. EGGLETONof the Department of Geology, and Professor B. G. HYDE, more resistant to nuclear radiation than zirco Dr. G. B. ROBERTSONand Mr. G. M. MCLAUGHLINof nolite. Thus, the basic properties of the crystal the Research School of Chemistry,Australian National line state of these phases are not affected over Universityf6r the use of the X-ray equipment. One of the time period required for the waste to decay us (W.S.) would like to thank Dr. EGGLETONfor criti to a safe level. cally commenting on sectionsof the manuscript. Final The volume change of SYNROC containing ly, we wish to thank Mr. P. WILLISfor technicalassist 25% perovskite, 35% zirconolite and 40% Ba ance. The researchesdescribed in this paper were sup ported by grants provided by the National Energy hollandite*, buried for 105-106yrs would be Research Developmentand DemonstrationProgram of less than 2%. Because of the very slow strain the Commonwealth Department of National Develop rate in the presence of confining pressure when ment and by the AustralianAtomic Energy Commission. buried at depth in a geological repository it is This support is gratefullyacknowledged. expected that this expansion will cause SYN ROC to deform by plastic flow rather than by brittle . Indeed, this behaviour is dis REFERENCES played by large (-60 gram) single crystals of ANSALDO,E. J. (1975) Mossbauer absorption in a Sri Lanka zirconolite which are found as resist metamict mineral. Nature 254, 501. ate minerals in the gem gravels. BERMAN,J. (1955) Identification of metamict min Results of fast neutron damage produced on erals by X-ray diffraction. Am. Mineral. 40, 805

* Ba-hollandite expanded by less than 2% after being irradiated for a period of time equivalent to 106yr SYN ROC age REEVE and WOOLFREY,1980). 242 W. SINCLAIR and A. E. RINGWOOD

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