Geochemical Journal, Vol.15, Pp. 229 to 243, 1981 229
Geochemical Journal, Vol.15, pp. 229 to 243, 1981 229
A lp h a-re c oil d a m a g e in n a tu r al zir c o n olite a n d p er o v sk ite
W . SlNCLAIR and A. E. RlNGWOOD Research SchoolofEarth Sciences,Austraiian National University,Canberra,A.C.T.2600,Australia
(R eceived October_13,1980.' Accepted July 20,1981)
Zirconolite (CaZrTi207) and perovskite (CaTi03) are key minerals in SY NR OC, a ceramic m aterial developed for the im m obilization of high level nuclear reactor w astes. Wh en these are incorporated in SY NR OC,the long-1ived radioactive actinide elem ents are preferentially partitioned into zirconolite and perovskite which ~re therefore subjected to the effects of alpha-recoil,resulting from the decay ofthese elem ents. These effects have been studied via X-ray and electron diffraction investigations of natural sam ples of zirconolite and perovskite of varying ages and varying uranium and thbrium contents. The sam ples studied have received cum ulative alpha doses ranging from 1.0 X 1018 to I.1 X I020alg. The upper limit corresponds to the alphairradiation which would bereceived by the zirconolite in SY NRO C containing 10 percent ofhigh levelw aste overaperiod of5 X I08years. These studies show that zirconolitesrem ai n crystalline up to and beyond alphadosesof2 X 1019a/g. This dose would have accum ulated in such a SY NROC zirconolite after a million years ofstorage. Elec- tron microscopy revealed th at the grains were com posed of sm all crystalline dom ains w hich possessed the d efect fluorite-type structure. After a dose exceeding that w hich would be received by SY N ROC in 1OO m illion years,zirconolites appeared m etam ict w hen studied by X-ray diffraction. H owever,the elec- tron microgr.aphsand diffraction patternsclearly dem o nstrate that the m ineral continuesto retain alarge
degree of short range order and in no way resem bles a glass. The density changes produced in these zir- conolites byirradiation aresm allandrangefrom Oto 3% atsaturation. P erovskite sam ples which have SY NR OC ages up to 20,000 years decrease in density by I.8:!:0.1%. Their X-ray pow der patterns are essentially unaffected. C om parative studies show thatthe perovskitelat- ticeiseven m oreresistanttothe_effectsofalpha-recoilthanthezirconolitelattice. Theresultsdem onstrate that zirconolite and perovskite are extrem ely resistant to the effectsofnuclearradiation and w illprovide stable crystal stru ctures for the containm entoftheradioactivew asteelem ents duringthetim ere quired for the radioactivity to decay tosafelevels(typically 105-106years).
INTRODUCTION em itted by these elem ents, notably neptunium , plutonium , am ericium , and curium isotopes, are Zirconolite (CaZrTi20 7) and perovskite associated with a recoil of the nucleus. The (CaTi03) are used in the SY N R O C process for displacem ents due to the recoil m ay cause do n- the im m obilization of elem ents occurring in siderable day rage to the lattice and decrease the high-1evel nuclear w astes (RlNGW OOD et al., sta bility of the synthetic phases. The al pha 1979). The radioactive w astes are incorp orated emitting elem ents are strongl y p artitioned into into the crystal structures by form ing dilute zirconolite and perovskite and these cry s tals will solid solutions with the SY N R O C phases. The therefore be subjected to m ost of the alpha- w aste elem ents are tightly bound within the recoil dam age. cry stal lattices and are extrem ely resistant to W e have assem bled a collection of naturally leaching by hydrotherm alsolutio ns. occurri ng sam ples of zirconolite and perovskite T hese experim ents, ho wever, do not take containing the alpha em itting elem ents, uranium into consideration the im port ant effects of and thorium . The cum ulative radiation doses nuclear radiation arising m ai nly from alpha- received by these m inerals have been calculated decay ofthe actinide elem ents. Alpha particles and cover a considerable range from I.O X I018
230 W.SlNCLAIR and A.E. RlNGWOOD to I.I X 1020 alphas per gram (O VERSBY and Furtherm ore, the authors support ed the con-
RlNGWOOD, 1981). In this paper w e describe clusion that the ability for a m a terialto becom e the effects of increasing doses ofalpha radiation m etam ict was strongly dependent on the cry stal o n the crystal stru ctures of these m inerals using structure. The fact that the m ineralthorianite
X-ray diffraction and electron m icroscopy Th02 (w hose fluorite-type structure is the techniques. paren t structure of zirconolite) is not found to
R EEVE and W OOLFREY (1980) have ap- be m etam ict in the natural state w as said to be proached the sam e problem by irradiating explained by this hypothesis.
SY N R O C mineral assem blages wi th fast neu- COMES et al. (1967) have report ed the ef- trons to sim ulate alpha particle and actinide fects of fast neutron irradiation on single cry stal recoil dam age. Their res ults form a com ple- quartz. These authors observ ed a gradual break- m entary study and will be discussed later. dow n of the structure into a heterogeneous
M a ny o ther phases have al so been examined m ixt ure ofcrystalline and m etam ict areas. A fter for the effects of radiation dam age. PYATENKO very strong irr adiation (> I020n/cm 2) the cry stal
(1970) has suggested that as m ine rals(including becom es entirely m etam ict and exhibits a dif- zirconolite) becom e m etam ict there is a break- fuse diffraction ring on X-ray photographs. dow n of the cry stal structure leading to a T hey concluded that the Si04 tetrahedra r em ain segregation of new phases on a very sm all scale. nearly undam aged during ir adiation. In con-
These new phases m ay represent com ponent trast, the ionic crystal LiF rem ains nearly un- oxide s or a m or e com plex configuration ofions. d ist ort ed w hen subjected to radiation dam age.
Extensive studies previously carried out on In view ofthe va riety ofconclusions existi ng zircon (ZrSi0 4) have led to several differi ng in the literature, itseem ed possible that an inde- conclusions. H OLLAND and G OTTFRIED (1955) pendent detailed study on new m ineral types and PE LLA S (1965) invoked a m ultistage p rocess such as zirconolite and perovskite m ay provi de occurring with increasing radiation dose. BuR- necessary inform ation to resolve the effects of
SILL and M CL AREN (196 6) have support ed this large doses of nuclear radiation. M oreover, m ultistage process and provided evidence from since these m inerals are key com ponents of electron m icroscopy for the existence of sm all SY N R O C, the inform ation so obtained should crystallites of zircon,even in the m etam ict state. have an im port ant bearing on the long-term
PELLAS (1965) concluded that zircon ultim ately behaviour of SY N R O C after incorp oration of decom poses to a m ixture of its com ponent high-level nuclear reactor w astes. oxid e s, Si02 + zr02' W ASILEWSKI et al. (1973) Zi rcon olite, CaZrTi207, is closely related to h ave support ed these conclusions from in frared the d efect fluorite-type (CaF2_*) stru cture absorp ti on spectral studies. (PYATENKO and PUDOVKlNA, 1964; R OSSELL,
A lternatively, V ANcE and B OLAND (1975) 1980a, b). It can be derived from the sim ple and V ANCE (1975) have suggested a prog ressive lattice by distortion of the parent cubic cell and disordering of the lattice wi th increasing radia- orde ring of the cations. The stru cture so derived tion dose. These authors did not find a second is m ono clinic and has eight tim es the volum e of new phase for zircons as suggested by H OLLAND the originalcube. and G OTTFRIED (1955) nor the breakdow n of Perovskite (CaTi0 3) is orthorhom bic con- the lattice into its com ponent oxides. Th e con- sistin g of a 3-dim ensional fram ew ork of corn er- clusion of progressive lattice disorder w as also joined Ti06 octahedra with Ca atom s occupying m ade by V ANCE and BOLAND (1978) in studies the spaces betw een them . In nature,perovs kite of Zr02- sh ows a con siderable range o f ioni c su bstitu-
M ore recently, CARTZ and FOURNELLE tions. The rare earths an d alkalis com m only
(1979) did not observ e a breakdow n of ZrSi04 replace calcium w hile sm allsized cationssuch as into its com ponent oxidesin the m etam ict state. niobium and tantalum replace titanium .
Alpha-recoildam age 231
T he ability of these m inerals to accom - containing IO% high level w aste w hich w ould m odate a large range of elem entsin their crystal have received a sim ilar radiation dose. stru ctures is of prim ary im po rtance to their inclusion in the SY N R O C process. EXPERIM ENTAL AND RESULTS 1.1 R adiation dose as a function of S YN R O C A suite of natural sam ples was ex am ined age using X-ray diffraction and electron m icroscopy. T he SY N R O C process proposes to incorp o- The X -ray diffraction techniquesincluded single rate 10% of high level radioactive w astes as crystal precession, rotation and W eissenberg dilute solid solutionsin the constituent m inerals. photography, and D ebye-Scherrer and G uinier Zirconolite and perovskite strongly partition the pow der photography. R efinem ent of cell para- actinide elem ents present in the w aste, and will m eters w as done by the m ethod ofleast-squares therefore absorb m ost of the alpha-recoil dam - on a H ewlett-Packard 9825 calculator. H eating age. To evaluate the effect of alpha-recoil on experim ents were carried out in air, in unsealed these phases, O VERSBY and RlNGW OOD (1981) platinum capsules. Chem ical an aiyses, Table I, have calculated the cum ulative alpha dose per w ere obtained by energy-dispersive electron- gram of SY N R O C m ineral as a function oftim e. m icroproble.
Figures I and 2 from O VERSBY and RlNGW OOD Electron m icroscope studies w ere carried (1981) show the variation o fradiation dose with out under the guidance of JOHN FITZGERALD age for SY N R O C conta ining 10% of high level using a 200 keV JE O L instrum ent. T he sam ples w astes. The sam ples described in the following ivere first roughly thinned by m an ual polish in g sections have been placed on the SY N R O C to about 40pm and w ere then ion-thinned with cu rv es. 5keV Ar ions. Other sam ples w ere cru shed and T he radiation dose received by the natrual placed onto a carbon grid. D ensity m easure- zirconolite and perovskit e sam ples can be cal- m ents were determ ined by the m ethod of culated from their age and U and T h concentra- Archim edes, using toluene as the im m ersion tions. The dose,in alphas per grar n,can then be liquid. directly related to the age ofa SY N R O C sam ple A sum m ary of relevant inform ation for the
a D ose ., SY N RO C A ge f'* Zirconolite & Perovskite 14 12 TotaI Dose lo zJiarccuopniroalintega alg xl018 8 high-Uzirconolite 6 Kaiserstuhl BaikalPerovskite 4 Loparite 2 Jacupiranga Perovsk'te 103 104 105 106
SYN RO C Age (years) Fig.1. Totalalpha dose vs.ageforSYNR O C sa mplesbetween 103 and 10~)lrs. (AfterO VERSBY and RINGWOOD, 1981.
232 W. SlNCLAIR and A. E.RlNGWOOD
14 a D o se .= S Y N R O C A g e 12 zirconolites Sri Lanka Total high Th¥_~ ~ D ose ~~ 10 u/g SriLanka ~ ~ ~~ xl019 8 high U
6 Russian A
4
2 ~(H - Norway
106 10 7 10 8 109 SY N Ro c Age (years)
Fig. 2. Totalalpha dose vs. age for SYNR O C sam ples between 106 to 10~)lrs. (After O VERSBY and RING WOOD, 1981).
m inerals has been com piled in Table 2. The defect fluorite-type diffraction pattern s unlike volum e changes ofthe zirconolite and perovskite the sam ples studied by PUDOVKlNA etal. (1974) lattices, caused by the effects of alpha-recoil,are w hich w ere m etam ict. Secondly,the grainscon- presented in T able 3. Som e ofthe key sam ples, tinued to display single cry stal diffraction pat- how ever,require m ore detailed discussion. terns w hen reconstitued at tem peratures up to 1,100'C. T hese new observ ations are of con- (a) Brazil zirconolite siderable im portance, showing that afterintense Zirkelites from Jacupiranga, Brazil w ere radiation dam age and suffering extensive m eta- first reported by H USSAK and PRIOR (1895). m ictization, the lattice retains a large degree of The m inerals received little attention until order. PUDOVKINA etal.(1974),using H USSAK'S Sam ples, At I,200'C the original cubic defect fluo- carried out chem ical analyses and X-ray studies rite-type cell com pletely transfo rm ed. The pat- of heated sepcim ens in order to establish their terns obtained seem ed at first m uch sim plified relationship to zirconolite. T hese authors in com parison to synthetic zirconolite. found that in the natural state the cry stals w ere PUDOVKlNA and PYATENKO (1966) w ere able to m etamict, although one specim en showed a describe the sim plification of X-ray pattern s faint, diffuse reflection from Laue photography. from Aldan zirconolites in term s of a distortion W hen heated to 800'C the sam ples 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. A t 1,200'C the X-ray diffraction pat- pearance w hile atthe sam e tim e appears pseudo- tern ofthe heated cry stals w as identified as the cubic. The "hexagonalisation" of the X-ray m onoclinic zirconolite. pattern s were also encountered by PUDOVKlNA T he latter results have been confirm ed by etal. (1974) for zirconolitesfrom Brazil . our analyses. H ow ever, several significant addi- Prelim inary electron microscopy ofa natural tional observ ations w ere also m ade. Firstly,the unheated Brazilian zirconolite revealed that the natural sam ples in this in vestigation gave cubic specim en w as corn posed of a num ber ofregions
Alpha-recoildam age 233
~1 tf) e~Q c:(7:¥; ,(:H5 ¥~O: I (':5H C~:; ~ I ~ I CS e(~ oC;~ Lf) ~ O~
$ce:~;O r~:t･~cl c~r ,cH; crf{) I ,OH cO~ ~H:) l (~1 I co; ,~Ho. (c~~a ~:1~ v) c~f) o~ ce ~ h O i~S)Q ~ ~.e HOi-:~ ~ ~O NO4)'~- ~c; ,CH; oHd I ,C~:; c~; l vo~) cCOf; c~~ Qrq; c~f; (c~; C~)~ > ~ ~r (~ o Q~:f, ~O1~:O, 'H Cel'~ ~ I(H,~ ce:q; c-; ~vi) e~(o; c';l ¥~o: r~r; c:~; ¥co; 'c¥Hi I l ~lC~f)l rn !1 ~ o~~ eo C':IH!-l~C'!~ :t~L~) ~ ~ ~~I ;e¥, ~ ~c) ｭ~c:eo.~ ~) ~O c~ o ~r oo ~ o (~ v) ~ c~ ~ ~. ~!l~ c~ I r: cS * ･i ~: c:; c~ r: I l c~
234 W.SlNCLAIR and A. E.RINGW OOD
Table 2. Sample descriptions 1Age Dose(alg) SYNROC Age Sam ple (my) x,1018 (y) Comments ZIRCONOLITE: Germ any,Kaiserstuhl carbonatite 16 1.2 1x 103 Monoclinicstru cture;Iow 20 an gle X-ray diffraction com plex reflectionsaresharp;high an gledatashow broaden- ingandweak eningofreflections. Braz il,Jacupiran gacarbonatite 133 7.6 4x10= Seetext. com plex 2Norway,Stavern an d Larv ik 295 17. 1xl06 Sim ilarX-rayobservationsto Brazilsam pleswith presenceofnew orthorhombicinterme｢iatephaseat 8800C: a=10.08,b= 14.28,c=7.40A;space groupAba2 orCmca. 3Russian A(Aldan ?) 640 18. 1x l06 Sim ilar X-ray observationstoBrazilsam ples. SriLank a 55O 80-110 4-7x l08 Seetext. PEROVSKITE: Lovozeroalk alinemassif,Kola 300 1.3 2xl03 X-raypowderdiffractionpatternsshow nosignof peninsula(Loparite)4 latticedam age. Brazil,Jacupiranga 133 1.6 5 xl03 Sharp X-ray powderdiffractionpatterns. Lake Baikal,Tazheran 5OO 2.6 1.7x104 Seetext. 1. Agesfrom O VERSBYand RINGWOOD (1981),AgeforLovozero loparitefrom KOMLEV etal.(1961).
2, Sam ples from N orway are known as polymignite(D ANA and DANA, 1944, VLASov, 1966, LIM A DE FARIA, 1964). 3. origin of sam ple unknown; X-ray and chemical properties suggest they originate from the A barastakh M assif
Aldan(BORODIN etal..196 0,PUDO VKINA and P YATENKO.1966). 4. Lopariteisa variety ofperovskiterich inrareearth elem ents. w ith different properties. T he appearance (b) Sri Lanka zirconolite of som e of these regions. Figs. 3- 5, w ere sim i- Zirconolites from Sri Lanka w ere first de- lar to those described by B URSILL and M CLAREN scribed by BLAKE an d SMlm (1913). X- ray (1966) f or zircon. Still other areas of the photographs indicated that these m inerals w ere
Brazilian specim en gave only diffuse pow der com pletely m etam ict. The pow der patterns gave ri ngs and w ere oflow contrast. T hese areas are only diffuse rings. Electron m icroscopy ofsam - interpreted to be the final state of radiation ple 83800, how ever, displayed regions sim ilar dam age. to those of the Brazilian zirconolite. T hese are The d-spacing of - 2.92A ofthe inner ring show n in Figs. 6 and 7. in Fig.4, cor esponds to the distance betw een H eating to 700-800'C for tw o hours caused the m etal-only (111) planes ofthe fluorite-type the cubic defect fluorite-type pow der pattern to stru cture. In the zirconolite structure the planes appear. A fter further heating to I,IOO-I,200'C w hich alternately contain Ti and Ca + Zr are the X-ray patterns corresponded to the m ono- derived from these close-packed (111) fluorite- clinic zirconolite. T w o sam ples, num bers 83800 type planes. Thus, even in the m etam ict state and B 20392, gave the 'hexagonalized' zirco- the cations within these planes still retain a nolite pattern w hile sam ple SL3-12 produced a short range periodic arrangem ent. m onoclinic pattern sim ilar to a pure synthetic To check to see ifthe crystalhad segregated zir conolite. into several new phases such as its com ponent oxides as suggested by PYATENKO (1970),Iarge (c) Baikal perovskite areas of the specim en were m oved under the A lthough this sam ple has received a higher electron beam . The cubic subcell did not change alpha dose than the loparite,the X-ray pow der
and rem ained orientated throughout the pro- patterns are extrem ely sharp with no indication cedure. N o other phases were observ ed to be of lattice dam age. Com parison of these X-ray present. patterns with those of Kaiserstuh l zirconolite
Alpha-recoil dam age 235
Table3. Unitcelldim ensionsof naturalzirconolitesand perovskites Sample Temperature('C) a(A) b(A) c(A) p(') v(A~) %Vol.change Germany XIRCONOLITES Kaiserstuhl R.T. 12.544 (1) 7.288 (1) 11.636 (4) 100.26 (2) 1046.7 (6) 1200 12.571 (2) 7.300 (1) 11.523 (5) 100.62 (2) 1039.3 (8) 0.7 i0.1 Brazil Jacupiranga R.T. 5.06 (4)a 1036 (25) 0.5 i 2.4 1200 12.592 (1) 7.270 (1) 11.451 (1) 100.56 (1) 1030.5 (3) Norway Stavern R.T. 5.10 (3)a 1061 (18) 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) RussianA R.T. 5.03 (1)b 1018 (6) o ~:0.6 1200 12.548 (1) 7.245 (1) 11.404 (1) 100.57 (1) 1019.2 (3) Sri L anka 1200 12.566 (1) 7.255 (1) 11.432 (1) 100.56 (1) 1024.6 (3) (B20392) SriLanka 1140 12.568 (1) 7.256 (1) 11.435 (1) 100.56 (1) 1025.1 (3) (83800) SriLanka 1200 12.451 (2) 7.243 (1) 11.386 (2) 100.54 (2) 1009.5 (4) 2.5 d:2c (SL3-12) PEROVSKITES Lovozero Loparite R.T. 3.8839 (5)d - 58.59 (2) }1.56i 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 (5) 228.51 (6) }1.82:!:0'05 1200 5.4486 (5) 7.6475 (7) 5.3844 (5) 224.35 (6)
Errors are given in parentheses; Celldim ensionsobtained using a Guinier-Hdgg focussing cam era, Cu K al radiation (X = 1.54060A) Sistandard (a = 5.43088A), unless otherwise stated,N atural SriLanko and Larvik cry stalsgave diffusepatterns. (a) M easu red from precession photog7laph, M oK ~ radiation (X = O.7107A); cubic cellis 1/8 the volum e of the m onocliniccell. (b) Celledgederivedfrom 57.3m m D ebye-Scherrerpattern,FeK~ radiation (X = 1.9373A),Sistandard. (c) Vol.change obtained from density m easu rem entsusing the method ofArchim edes.
(d) Pseud o-cubiccelledge,114.6m m D ebye-Scherrerpattern, CuK ~ radiation (X = 1.5418A).
indicates that the perovskite m ineral is even kite. T hese results suggest that both for zir- m ore resistant to radiation dam age than zirco- conolite and perovskite m ost of the lattice ex- nolite. T he volum e expansion obtained forthis pansion occurs after a relatively sm all radiation sam ple, I.82 :t 0.05%, com p ar es favourably with dose. A fter lattice exp an sion of less than 3 the results of R EEVE and W OOLFREY (1980). percent there is little further expan sion even for T hese w orkers reported an increase in volum e of very large alpha doses. approxim ately 2.6% for synthetic perovskites irradiated with fast neutrons for a peri od oftim e DISSCUSSION equivalent to a SY N R O C age of 10,000 years. Although this dose was accum ulated in 22 days, X-ray diffraction and electron m icroscopy studies suggest that zirconolites retain a high the above results support the relevance of ac- celerated irradiation testing. degree of lattice order even after they have
The volum e changes reported here are quite received a radiation dose equivalent to 106 sm all in relation to the alpha dose received and SY N R O C years. The very sm all volum e changes dem onstrates the structural stability of perovs- observ ed for the sam ples w hich have received
236 W. SlNCLAIR and A.E.RlNGWOOD
Fig.3. D ark field im age and dlffraction pattern of a well crystallized area of naturalBrazilzirconolite. The dif- fraction pattern corresponds to the cu bic defect fluorite-type lattice plus extra weak diffuse reflections. Beam direction along(OI1)fl uorite.
Fig. 4. D or k field im ageillustrating the dom ain sttu cture of natural Brazilzirconolite. Th econtrastisspeckled on a scale of about 50A Th e defectfluorite-type diffraction pattern exhibitsa strong powderring whose d-spacing corrlespondsto the(111)fluorite-typereflection. Beam directionalong(O11)fl uorite.
Alpha-recoil dam age 237
Fig.5. Dark field image of a sector ofthe(111)powderring of Fig.4. Only areas oflow contrastin Fig. 4 con- tributeto thepowderring.
*s~~~=* Fig. 6. Brightfield im age of sam ple 83800. The crystallites (~140A in diameter) are radomly orientated giving rise to the sharp defect fluorite-type powder pattern. Th e radiation dose accum ulated by the sampleis 6 tim es higherthan the expected dosereceived by zirconolitesinSYNR O C containing 10percenthigh levelwaste andstored for106 years. Thespecim en,however,stillcontainsareasofcry stalline m aterial.
238 W. SlNCLAIR and A.E. RlNGWOOD large doses of radiation indicates that only a is another desirable characteristic of the com - sm all degree of internal rearrangem ent has oc- pound. curred. M oreover, the zirconolite sarnples re- T he fluorite-type structure exhibits these constituted, as single cry stals, to their original characteristics. It possesses a high ionicity, undam aged state after being heated for less (N AGUIB and K ELLY, 1975), and the Ca atom s than one hour at tem peratures as low as 950'C. are arranged in cubic closest-packing. T horianite Even the highly dam aged Sri Lanka zirconolites adopts this stru cture and,in nature,is not found reconstituted to a single phase of polycrsystal- in the m etam ict state. Several other sim ple line m aterial. This is in contrast to the behav- oxides with the fluortie-type stru cture have also iour of heavily dam aged zircons. These m iner- been found to rem ain crystalline w hen bom - als, w hen heated,reconstitute to a com plex m ix - barded with high doses (>_ 1017ions/cm 2) of ture of the oxides (C ARTZ and FOURNELLE, energetic heavy ions(N AGUIB and K ELLY, 1975). 1979). T he close relationship betw een the zirco- The effects of nuclear radiation on the nolite stru cture and the fluorite-type stru cture properties of inorganic com pounds are highly has already been m entioned in a previous sec- vari able. A s early as 1924, how ever, G OLDSCH- tion. T he fluorite-type subcell, up on w hich zir- MIDT (1924) suggested that those cry stals which conolite is based, is only slightly deform ed from contain a high degree ofionic binding were m ore an undistort ed face-centred cubic lattice. A resistant to the effects of radiation. This sam e rhom bohedron results with an an gl e of about observ ation has since been m ade by m any 92' (PUDOVKINA and PYATENKO, 1966). The authors (e.g. W ITTELS and SHERRILL, 1959; ability of natural zirconolite to withstand the BILLINGT ON and C RAWFORD, 1961; B UDYLlN and dam aging effects of large doses of alpha radia- V OROB'EV,1964 and N AGUlB and K ELLY, 1975). tion is probably due to this association. Close packing of the atom s within the stru cture The perovskite stru cture, Iike that of zirco-
Fig. 7. Electron micrograph of the featureless areas of sam pl e 83800. The corresponding diffraction pattern dis- plays diffuse rings which approximate the cubic defectfluorite-type pattern. Th einnerring(1)is equivalentto the com bination of the (111) and (200)fluorite-type reflections. Similarly,theinterm ediate ring (M ) corresponds to the(220),(311)and (222)defectfluorite-type reflections. Rings(1)and (M ) have been overexposed to show the outerring(O).
Alpha-recoildam age 239 nolite, is based upon a cubic lattice. In this the bonding and type of distortion from the stru cture the oxygen and calcium atom s are ideal cubic perovsk ite-type com pound. M ore- arranged approxim ately in cubic close-packing, over, the extrem ely high dose rate m ay have i.e. a face-centred cubic array (BLOss, 1971). been a contributing factor. In contrast all ti-
The distortion to the observ ed orthorhom bic tanate perovskites w hich have received m uch stru cture involves a very sm all degree ofshearing larger d oses, but over longer periods of tim e,
(- 48') of the cubic pseudo-cell (K AY and ha ve be e n show n to rem ai n highly resistant to
B AILEY, 1957). W ITTELS and SHERRIL (1959) the effects of large doses of nucle ar radiation. irradiated a num ber of perovskite-type crystals. Furtherm ore, the volum es of all the perovskite-
These included BaTi03, K N b03 an d Pb Ti0 3. type com pounds m entioned increase by only
They dem onstrated tha t the above com pounds sm all am ounts. transform ed to their high-tem perature cubic The term m etam ict is usually applied to m odification w hen irradiated wit h fast neutron m inerals w hich have lost their cry stall inity due dosages in excess of 1019n/cm 2. The a and c to the action of nuclear radiation ar ising from ax ial lengths of tetragonal BaTi0 3 increased by the radioactive elem ents contained within them . approxim ately I.9 % and 0.9% respectively after PABST (1952) and BERMAN (1955) have given a re ceiving a neutro n dose of I.2 X 1020n/cm 2. detailed discussion ofthe term and som e of the
A t this stage, al ax es were equivalent (c ubic) characteristics associated w ith such m inerals. and after I.8 X 1020n/cm 2 the value ofthe cubic There are m any degrees of the m etam ict state ax is increased by a fu rt her 0.3 %. This radiation and the physical properties usually change con- dose is close to three tim es the dosage received tinuously in resp onse to the am o unt of radia- by the perovskites used in the experim ents of tion dose . W ith the use ofelectron m icroscopy,
R EEVE and W OOLFREY (1980). The BaTi0 3 m inerals that were once thoughtto be m etam ict cry stals rem ained sin gl e t hrough out the entire are now f ound to display strong diffha ction pat- irradiation. tern s ty pic al of well cry stallized m aterials.
R ecently, Ioparites contain ing greater than R ecently, several authors (CARTZ and l% U 02 have be en foundin the Lovozero intru - F OURNELLE, 1979; E wr NG, 1975, 1976) have sion, K ola Penisula. Although these m inerals suggested t hat m inerals in the m etam ict state have accum ulated a dosage of 8.9 X I018a/g, ha ve a glass-type random atom ic arrangem ent. they still continue to prod uce sharp diffractio n H owever, elec tron diffraction pattern s of the pattern s (K OGARKO, pers. com .). T he radiation featureless apparently m etam ict areas of zir- do s e exceeds that re ceived by the zirconolites conolite (Fig. 7) Ied to a different interpreta- from Jacupiranga. tion. These sam ples have received extrem ely
In contrast to the titanate perovskites dis- high doses of alp ha radiation and it has been cussed above, the rhom bohedral perovskite-type fo und that the atom s rern ain sufficiently or- com pound C m AI03 becam e m etam ict after it dered to display broadened pow der rings. T he had received a dose of I.6 X 1018a/g. This dose rings are identifiable with a cubic fluorite-type was derived in 8 days from the decay of244C m , pattern . The M 6ssbauer study of A NSAL DO and is equivalent to a SY N R O C age of I,OOO ( 1975) on a sam ple of X-ray am orphous thoro- years. The effect of nuclear radiation caused a steenstrupine{N a2Ce(M n,Fe, T a)(L a,T h ...) [(Si, sm all increase in volum e (3.2 % at saturation) P)04]3 H2} also supports an ordered atom ic and a phase transform ation to the cubic m odifi- arran gem ent in the m etam ict state. The author cation before the c om pound becam e am orp hous has show n that Fe3+ occurs in tw o different to X-rays(M OSLEY, 1971). distinct sites and that the M ossbauer spectru m
The b ehav io ur of Cm AI03 to nuclear radia- has not been affected by the radiation dam age. tion m ay be a consequence of its differi ng chemical com position and reflects the nature of
240 W.SlNCLAIR and A.E. RlNGWOOD
investigated by X-rays has nevertheless been CONCLUSIONS found to retain a high degree of short range
Results described in previous sections have order w hen studied by electron diffraction. T his
elucidated several of the effects of increa s ing state in no w ay resem bles a true glass as sug-
dos es of nuc lear radiation on t he c ry stal st ru c- gested by E WlNG (1975, 1976) and C ARTZ and tures of naturally occurring zirconolite and F OUR NELL (1979). The electron di ffracti on perovskite m inerals. W hen subjected to an alpha pattern of the m etam ict areas of highly irradi- dose of 8 X I018alg equivalent to a SY N R O C age ated zirconolite gave distinct pow der rings and of 4 X 105 yr, zirconolite transform ed to the could be identified with the dubic defect fluo- cubic defect fluorite-type stru cture. D espite the rite-type pattern. very large radiation dosesreceived,these cry stals O ne of the prim ary concerns expressed continued to display single cry stal X-ray dif- about proposals to inco rp orate nuclear w aste fraction pattern s. Electron m ic roscopy ofthese into a c ry stalline ceramic w aste form , has been zirconolit es revealed that the m ineral w as m ain- the poss ibility that radiation dam age could ly com posed of crystalline dom ains coherently destroy the crystal structures and seriously orientated. im pair their ability to im m obilize the was te
Sri Lanka zirconolite w hich had recieved a elem ents. It has been suggested that because radiation dose exceeding that w hich w ould be radiation dam age ultim ately leads to m etam icti- experienced by SY N R O C in 4 X 108yr w ere ap- zation, the ability ofthese m etam ict m inerals to parently m etam ict w hen studied by X-ray dif- retain the waste elem ents m ay be no greater fraction. H ow ever, electron diffraction investi- than that of glass. O ur results on zirconolite gations dem onstrated that they contained areas and perovskite effectively dispelthose concerns, of crystalline dom ains. These,h ow ever, werein at least, for these parti cular m inerals. T hey rando m orientations and gave a spotty cubic show that the atom ic stru ctures of th eirr adiat ed defect fluorite-type pow der pattern. T he m icro- m inerals are still essentially those ofthe crystal- graphs of other areas of this sam ple studied by line state and are in no w ay analogous to those e lectron m icroscopy were of low contrast but of silicate or borosilicate glasses. F urt h er sup- the diffraction patterns exhibited diffuse pow- port for this conclusion is provided by the der rings also identifiable with the defect fl uo- r esults of O VERSBY and RlNGW OOD (1981) on the rite-type pattern . T hese apparently m etam ict Pb/U isotopic system atics and leachability of areas have suffered an extrem ely large radiation the natural zirconolites an d pe rovskite s used in dosage and yet the atom s retain a l arge degree their study. T hey dem on strat ed that the m in- ofshort range order. erals display a rem arkable ability to im m o bilize
These r esults have a direct bearing upon U and Pb under geological conditions. M ore - alternative hypotheses dealing w ith the effect over, the Sri Lanka zirconolites w hich have suf- of radiation dam age in crystals. In particular, fered the m ost extensive radiation doses w ere they are best explained by progressive disorder- found to be less leachable th an borosilicate ing of the stru cture with increasing radiation glassesin w ater and brines at 200'C by a factor dose, as suggested by V ANCE (1975) and V ANCE exceeding one thousand. and B OLAND (19.75, 1978). There is no evidence For S Y N R O C containing IO percent of high that the m inerals becom e segregated into sim ple level w aste,it is usually estim ated that a storage oxide phases even afterreceiving extrem ely large period of 10=-106yr w ould be requir ed for the doses of radiation as proposed by PYA'rENK O w astes to decay to a safe level. The natur al
(1970), W ASILEWSKI et al. (1973) and PELLAS sam ples described here provide us with a suitable
(1965). m odel with which to assessthe cry stalline integ-
The atom ic arrangem ent of zirconolite in a rity of SY N R O C phases during this storage state w hich w ould be classed as m etam ict when interv al. A fter I,OOO years the crystal s tru c-
Alpha-recoildam age 241
tures of zirconolite and perovskite in SY N R O C a rapid tim e scale (1 m onth) and the dam age containing IO % high level w aste are essentially caused by alpha-particles over periods of geo- unaffected by the nuclear radiation received logical tim e (up to I08yrs) w ere found to be in
(chiefly from alpha particle recoils). A fter good agreem ent. A lthough there is a factor of receiving a radiation dose equivalent to a SY N- l09 difference betw een the tw o tim e scales,
R O C ag e of one m illion years zirconolite is these resultssupport the relevance of accelerated still cry stalline but undergoes a change in stru c- irradiation testing. T he rate ofaccum ulation of ture to the defect fluorite-type lat tice. T he radiation dam age in SY N R O C containing high volum e increase is less than 3 %. A fter a radia- level w aste is interm ediate betw een the above tion dose equivalent to IOO m illion year s SY N - dose rates. In view ofthese results w e conclude
R O C age zirconolite appears m etam ict. Elec- that the responses to nuclear radiation observ ed tron dif fraction, howeve r, show s that the atom s on the natural m inerals over m uch longer tim e still m aintain sh ort range order, as previously scales are directly applicable to the long term described. The volum e increase after this peri od behaviour ofthese phasesin SY N R O C. oftim e has already reached satura tion le vel an d rem ains sm allertha n 3 %. Ackn owledgements-We wish to record ourappreciation A t a radiation dose equivalent to that w hich ofthe persons and institutions who m ade itpossibleto SY N R O C w ould accum ulate in I0,000 years of obtain the sam ples ofmineralsusedin thisinvestigation. storage, perovskite expands in volum e by less In this respect, we are particularly indebted to J~RG than 2% but the crystalli nity r em ai ns unaf- KELLER, GENE K AULA, CARL FRANCIS, BRIAN fected. A fter receiving a radiation dose e quiv- M ASON, ARNE R XHEIM, T. R. CUTTILAN, H. D.. N. alent to a SY N R O C age of half a m illion years, PATHIRANA, and to the British M useum (Natural History), th e Smithsonian Institution, Harvard Uni- perovskite is still crys tal line. Com parison o f versity, Mineralogisk-Geologisk M useum (Oslo), Uni- X-ray diffraction photographs of perovskites versity ofFreiburg,SerranaS/A de Mineracao,SriLanka and zirconolites w hich have received sim ilar Geological Survey, and the USSR Academ y of Science. radiation dosesindicates that perovskite is even We are also indebted to Dr. R. A. EGGLETON of the m ore resistant to nuclear radiation than zirco- Departm ent of Geology, and Professor B. G. HYDE, Dr. G. B. ROBERTSON and M r. G. M. M CLAUGHLlN of nolite. Thus, the basic properties ofthe cry stal- the Research School of Chemistry, Australian National line state of these phases are not affected over University forthe use ofthe X-ray equipment. One of the tim e period required for the waste to decay us (W.S.) would like to thank Dr. EGGLETON for criti- to a safelevel. cally com m enting on sections ofthe m anuscript. Final-
T he volum e change of SY N R O C containing ly,we wish to thank Mr. P. WILLIS fortechnicalassist-
25 % Perovskite, 35 % zirconolite and 40% B a- ance. The researches described in this paper were sup- hollandite*, buried for 105-106yrs w o uld be ported by grants provided by the National Energy Research Developm ent and Dem onstration Program of less than 2 %. Because of the very slow strain the Com m onwealth Department of National Develop- rate in the presence of confining pressure w hen mentand by the Australian Atomic Energy Com mission. buried at depth in a geological repository it is This supportisgratefully ackn owledged. expected tha t this exp an sion will cause S Y N-
R O C to deform by plastic flow rather than by brittle fracture. Indeed , this behav iour is dis- REFERENCES played by large (- 60 gram ) single cry stals o f ANSALDO, E. J. (1975) M dssbauer absorption in a Sri Lanka zir c onolite w hich are f ound as re s ist- m etamict mineral. N atur e 254,501. ate m ineralsin the gem gravels. BERM AN, J.(1955) Identification of m etarnict min- Results of fast neutron dam age produced on erals by X-ray diffraction. A m. M ineral. 40, 805-
*Ba-hollandite expanded by less than 2% after being irradiated for a period oftim eequivalentto IO6yrSYN- R OC age(REEVE and W OOLFREY,1980).
242 W. SINCLAIR and A.E. RlNGW OOD
827. LEVICH,S.I.(1961) The age ofthe alkalineintru- BILLINGTON, D. S. and CRAW FORD, J. H.,JR.(1961) sions of the Chibina and Lovozero Tundras (K ola R adiat ion dam ag e in solids. Prince ton U niversity Peninsula). Dokl. A kad. Nauk. SSSR 136, 31-33 Press,P rincet on,New Jersey. (Earth Sciences Trans.).
BLAKE, G. S. and SMITH, G. F. H.(1913) On vari- LIMA DE FARIA, J. (1964) Identification of m eta- eties of z irk eli te fro m Geylo n. M ine ral. M ag. 16, mict minerals by X-ray powder photographs. Junta 309-316. de Invest. do Ultram ar,Portugal;Estud.,Ensaiosand BLOSS , F. D. (1971) Cry stallography and cry stal D ocumentos112,130 pp. che m istly, an in tr oduction. H o lt, Rin ehart and M OSLEY, W. C. (1971) Self-radiation dam age in Win ston Inc. curium-244 oxide and aluminate. J.A m. Ceram. Soc.
BORODIN, L. S., BYKOVA, A. B., KAPITONOVA, T. A. 54,475-479. and PYATEN K O, YU. A. (196 0) Ne w data on zir- NAGUIB, H. M. and KELLY, R. (1975) Criteria for co n olite and its niobi um variety. Do kl.A kad.Nauk. bom bardm ent-induced structural changes in non- SSSR 134 ,1188-1191 (in R ussian) . m etallicsolids. Rad.Effects25,1-12. BUDYL lN, B. V.an d VOROB'EV,A.A.(1964) Effect OVERSBY, V. M.and RlNGWOOD,A. E.(1981) Lead of radia tion on ionic stru ctu res (Israel Progam for isotopic studies of zirconolite and perovskite and
Scientific Translations,Jerusalem). thetr im plications forlong range SY NR OC stability. BURSI LL, L. A. an d M CLA REN, A. C.(1966) Trans- Radioactive WasteM anagem entI,289-307. m ission electron m icroscope study of natural radia- PABST, A.(1952) The m etamictstate. A m.M ineral. tio n dam ag einzircon (ZrSi 04) . Phys.Sta t. Solidi 13, 37,137-157.
331 -343. PELLAS, P. (1965) Etude sur lax recristallisation CARTZ,L.and FOURNELLE,R. (1979) Metamictzir- thermique des zircons m 6tamictes. M em. M us. Natl. co n fo rmed by heavy ion bo rnbardment. Rad. Ef - d HistorieN at.,Series C, 12,227- 253.
fects41,211 -217. PUDOVKlNA,Z.V., DUBAKINA, L. S., LEVEDEVA,S.I. CO MES, R., LAM BERT, M. and GUlNIER, A. (1967) and PYATENKO,Y U. A.(1974) Study ofBrazilian M ec hanism of th e trans form ation of c ryst alline zirkelite. Zap. Vses.M ineral. Obstch. 103,368-372. qua rtz into am orph ous silica by n eutron irradia tion. PUDOVKINA, Z. V. and PYATENKO, Y U. A. (1966)
in Interaction ofradiation with solids (ed. BISHAY), O n zirconolite and its crystalographic features. Tr. Ple nu m Pres s,319-3 3 7. M ineral.M uziyaA kad.Nauk. SSSR 17,124-133. DANA, J.D. and D ANA , E. S.(1944) Thesystem of PYATENKO, Y U. A. (1970) Behaviour of metamict m ineralo~V. 7th ed. Entirely rewritten and e nlarged minerals on heating and the general problem of by PALAC HE, C .,BERMA N,H., FRO N DEL,C.,Vol.1, m etam ictization. Geokhimiya 9,1077-1083. El em en ts, sulfides, s ulfosalts, o xides, New Y or k - PYATENKO, Y U. A. and PUDOVKlNA, Z. V. (1964) Londo n. The lattice m etric of CaZrTi207 cry stals. Kristallo- Ewr NG, R. C.(1975) The cry stal chemistry ofcom- grafiya 9,98-100. plex n iob ium and tantalum oxides. I V. The m eta- REEVE,K.D.and W OOLFREY,J. L.(1980) Accelerat- mict state: Discussion. A m.M ineral. 60, 728-733. ed irradiation of SY NR OC using fast neutrons I. EWlNG, R. C. (19 76) M etamict mineral alte ration: First results on barium holandite,perovskiteand un- an im plicationforradioactive waste disposal. Scie nce doped SY NR OC B. A ust. Ceram. Soc. J. 16,10-15. 192 ,13 3 6-1337. RlNGWOOD, A. E., KESSON, S. E., W ARE, N. G., HIB- GOLDSCHM IDT, V. M.(1924) Uber die Um w an dlung BERSON, W . and M AJOR, A. (1979) Im m obiliza- k ryst allisierte r Minerale in den metami kten Z ustr an d tion ofhigh level nuclearreactorwastesin SYN ROC. (Isotr op isieru ng). V id e nsk.Skr.I.( Kri sti ani a) 1924, Nature 278,219-223.
(5), Geochemisc he Verteilungsgesetze der Elem ente. ROSSELL, H. J. (1980a) Zirconolite - a fluorite-re- III( Anhang) 51-58. latedsuper-structure. Nature283,282-283.
H OLLAND, H. D. a nd GO TTFRIED, D. (1955) The ROSSELL, H. J. (1980b) Cation substitution in zir- effect ofnuclearradiation on the struct ur eofzircon. conolite. Presented at CR YSTAL XII the Twelfth A cta Q ystallo gr.8 ,291- 300. M eeting of Cry stallog7laphers in A ustralia, Canberra, HUSSAK, E. an d PRIO R, G. T.(1895) Le wl site and January 30- February 2. zirkeli te,tw o n ew B razilian m in e rals. M in eral . M a g. V ANCE, E.R.(1975) a-recoildam agein zircon. Rad. 11,80-88. Effects 24,1-6. KAY , H. F. and BAILEY, P.C.(1957) Structure and V ANCE, E. R. an d BOLAND, J. N. (1975) Fission pr operties of CaTi03･ A cta Cty stallogr. 10, 2 19- fragm ent dam age in zircon. Red . Effects 26, 135- 22 6. 139. KOM LEV, L. V., MIKHALEVSKAYA, A. D. and DANI- V ANCE, E. R. and BOLAND, J. N.(1978) Fission
Alpha-recoildam age 243
fragm ent irradiation of single crystal m onoclinic A. N.and ALEXANDER,C.C.(1973) A study of Zr02･ Rad.Effects37,237-239. the natural a-reco il dam agein zircon byinfrared spec- VLASOV, K. A.(ed.)(1966) Geochemistry and min- tra. Red.Effects17,191-199. eralogy of rare elem ents and genetic types of their WITTELS, M. C. and SHERRILL, F. A.(1959) Some deposits. (Israel Program for Scientific Translations, irradiation effects in non-metallic cry stals. in Ad- Jerusalem ). Vol.2. vances in X-ray ana lysis (ed. M UELLER, W. M.), W ASILEWSKI,P.J.,SENFTLE,F.E.,VAZ,J.E.,THORPE, Pitm an,London,3,269-288.