/sup 238/Pu-substituted zirconolite (CaPuTi/sub 2/O/sub 7/) was stored near ambient temperature for 231 days, equivalent to an alpha decay dose of 3.1x10/sup 25/ ..cap alpha../m/sup 3/ or 3x10/sup 5/ years of storage time for SYNROC ceramic nuclear waste. Periodic indentation testing showed that hardness was decreased by alpha decay-induced conversion to the metamict state, while fracture toughness and resistance to cracking were increased, apparently as a consequence of the formation of a heterogeneous microstructure. These results imply improved stability of this nuclear waste phase as a result of self-irradiation damage. 21 references, 4 figures

Clinard Jr., F. W.

Hurley, G. F.

Kise, C. D.

Rankin, J.

Tucker, D. S.

UNT Digital Library

/.LA-UR--84-3383bl)E85 002410 c’ mlg-.zqhw- - 9La AWno@ *t!onml L9bmt0fy h qmrawd by lhg Unh9rslly 01 0auforn19 fof * lM S@* DopwtmcrII 04 Enofw UIWW contmm W-740E.ENG3S,TITLE:AUTHOR(8):WBMITTEO TO:IRRADIATION-INDUCEDREDUCTIONOF MICROCRACKINGIN ZIRCONOLITEF. W. Clinard, Jr., D. S. Tucker, G. F. Hurley,C. D. Kiee, and J. RankinAnnual Meeting of the Meteriale Research SocietyNovember 26-30, 1984 Boston, MA(Proceedings to be publiehed)IMSCLAIMERllhmwup raftudumnswount dworkspHofd bymra8wcy#hr Unltal SbtaCbrmo9tm *thU*Wk-Mlmmnym-,muydMrmplopahmskatsymmnly, 9WwlorlmPlbd,wunrm MyhmlllddutyormpomkWhytbriksccurmy, omplmmofudul- ofanylnhfmdcm,wwmlus, walwLorp ~ W rcpmmti lIM1 Ih urn wouldnot Infrlnp phtdy oweodrkbb. Rdbr=rn bdII tow EwOIilowmmuold produotowor9Nsof mrdos by Ird rmmiwWlaosrLL auhotww, w Mkwka h IWMImau9rlly oomtltuteor Imply Iti adawnmL rmni-madltb w rmww’lmby Ilm llultd stats Clwarnnmator arty qcnoy Itmrd ‘rho Vkwn9do@ubM ofdtwDox~kdndfJn0t ~rllyusb orrofktthocodlhoU* Ststa Govwnmomor ●ny u9noy thraf.Bycwoptmv of trtloWtbm,ltmpubhhor?OooenlsssIhti rtw U,@ OovO?nM.nl mtalnIs nononaluslvo,ro@rY.froQ ltoonw to putwh or wroduootho Pubwwr tofm of W oomrlbtilon, w 10 sllw ONISTI to do w% for US @ofnmmrl DWPOWThg Los Alamoo PWIOMI MbOftlOW rqwm NW lfw wbllohof Idcntifb MB wtlols m wwh psrfwmod under thommpkw d Iho U B 09BWIm~1 ot EnergyMmLosAlamosNationalLaboratorlmsAlamos,NewMexico 8754~IRRADIATION-lNM.JCEDREDUCTION OF M1CI?OCRACKIN6F. M. Cllnard, Jr., D. S. Tucker, 6. F. Hurley, C. D.Materials Sci~nce and Technology Division, Los AlamosLos Alamos, Ntl87544ABSTRACT -.IN ZIRCONOLITE*Kfse and J. RankIn+National Laboratory,238Pu-substitutedzirconolite (CaPuTi,O ) was stored near amblenJ52!ternerature or 231 days, equivalent to an a pha decay dose of 3.lxIO9 ~u/m or 3x1O years of storage time for SYNQOC ceramic nuclear waste.Periodic indentation testing showed that hardness was decreased by alphadecay-induced conversion to the metamlct state, while fracture toughness andresistance to cracking were increased, apparently as a consequence of theformation of a heterogeneous microstructure. These results Imply Improvedstability of this nuclear waste phase as a result of self-irradiationdamage.INTRODUCTIONThe zirconollte phase of the SYNROC [1] cerarnlcnuclear waste form will bea host for various actlnlde isotopes. As a result thts material, which has thenominal composition CaZrTl O , will suffer self-irradiationclamagefromalpha decay events during ~t&rage. Transmission electron microscopic studiesof Pu-doped [2] and Pu-substituted [3] forms of zlrconollte have shown thatdamage proceeds prlmarlly by accumulation of alpha recoil tracks, whichgradually convert the structure from crystalline to metamlct, As a consequencethis ceramic undergoes extensl~e swelling, up to 5.4 vol % at room temperature[3].Dlfferentlal swelllng among th~ component phases of a multlphase wastoform such as SYNROC could lead to microcracklng, as has been demonstrated In asimulated two-phase material [2]. Mlcrocracklng Is of concern because theresultfng Increase In surface area could lead to Increased leach rates.Therefore the ablllty of a waste form to resist cracking Is an Important factorto consider In selectlon of an optimal waste form.Fracture strength of ceramics Is given by the relationship(1;where E = elastlc modulus, Y ● fracture energy, c ■ crltlcal flaw size to causefracture, and A Is a geometrical factor. It may be seen that strength Isproportional tory, the fracture twghness, at constant flaw size. In thework reported here, fracture toughness and hardness of Pu-substitutedzlrc nullte werk monitored for 231 days, equivalent to a storagR time of~3Y1O y for SYNROC-C containing 10 wt % commercial waste Calcirre. Resultsare discussed In terms of the evolving damage state, nnd l~l~pllcatlonsfor wastestablllty are consl(lerecl..... . . . . . ..- ---Samples of Pu-doped zirconolite (CaPuTi O ) were fabricated by atechnique similar to that described elsewher6 13]. Briefly, powders of PuO ,CaO, and Tlo were ball-milled together. calcined, cold pressed, and si~$~;~dfor 24 h at !675 K in air. Isotopic purity of the plutonium was -80 %(half-life= 88 y), with the remainder being made up of isotopes of longerhalf-llfe. The resulting product contained -10 % porosity, along with minorphases [3].The test cylindfis were metallographicallymounted and a series of 25indentationsmade each week with a Vlckers diamond indenter. Test temperaturewas -350 K, the increase above ambient being attributable to the significantself-heat of the plutonium [3]. Initially both 0.050 and 0.100 kg loads wereemployed, but it was found that the heavier load resulted in crack patternsextensive enough to interact with pores and minor phase inclusions, so thatsubsequent tests were restricted to 0.050 kg loads, Typically, 15 to 20 of the25 indentationswere usable when the lower load was employed. Because ofconcern that cracks might extend with time before measurements could be maoe,initial indentationswere re-examined after storage under ambient gloveboxconditions for 84 days; however, no changes in crack dimensions were observed.Hardness values were determined each week from average dimensions of theindentations,while fracture toughness was calculated from average crack sizesusing the relationship,-3/2~ H al/2 (E/H)2’5 [(),057(c/a) ],‘Ic (2)where H ■ hardness, a = half-diagonal of the indentation, c = distonce from thecenter of the indentation to the t~p of the crack,, and E = Young’s modulus.This equation, used byldeter et al. in a study of nuclear waste glasses [4],was \’riginallyderived for use when the cla ratio is large. However, aslightly-modifiedrelationship is also applicable even with c/a <2 [5]. At anyrate the exact formulation chosen is not of primary importance, since Interestis not so much in the absolute value of fracture toughness as in its changewith accumulation of radiationc~amage,Since Young’s modulus for zirconolite In either the crystalline ormetamict form is not known, it was necessary to estimate these parameters. Avalue for the urlirrcdiatedmaterial of 209 GPa was chosen as appropriate for aceramic with a meltlng point of -1800 K [3], and the reduction of modulus uponmetu~ictization was taken to be 10 %. This estimetew~s based on the fact thatthe modulus ofmetamlct quartz Is -30% less than that of its crystallinecounterpart [6], Since swelling of quartz as a result of metamictizatlon isroughly three times that for zirconollte [7] and since both slllcates andtit.anatesdlsordar as & result of disorientation of cation-ox qen structural{units [8,9], a s!mple proportional correlation seems reasonab e.Optical photomicrographs of Indentations and crack pattf?rnsmade 28 daysand 189 days after fabrication of the test material are shown In Fig. 1. TwoIrradlntlon-indursdchanges are immediately apparent: the higher-doseIndentations are Iar er, indicating a softening of the m?terldl, and the extent?ef cracking Is great y reduced, Actually, as in the exampl~ shown in F! ,!l(b), lnde~tatlons In high-dose mater’lalcaused no discernible cracking ntwo-thirds of the tests.Mlcrohardnws and crack length as a function of stor~ge time,corrospondlng alphn decay dose, and equivalent SYNROC age with 10 wt % wasteloading are pre$ented In Fig. 2. Cracu length here refers to the averagelen th of one crack emanating from one corner of the!Indentation, and does not?Inc udethedlagonal dimension of the Indentation Itself; this parameter isIncluded to Illustraix!the extent towhlch mlcrocracklng was suppressed by\ ---- —. .. ..-—-.4-j;’.-.. . ..- ..-..4. . 4_.. . ..4I,.;.-j. ..-1...!.-.. .I i,I(a)mumi1I—4I. . ..4-.-144.-iI--7I—.:-,120{l!!30(F{..-.,-!ALPHA DECAY DOSE (%IO* a/m%I 2 3t~8 ~-sljBsTITuTED\ZIRCONOLITE00 000\6● °●CRACK.-.000●●-o ● @-1 1 ● 160 200 2?!STORAGE TIME (days). . -. .-.,..-.IO*OE=$YNf?OC AGE (yQUr8)Fl$j.2. Vlckers hardness number and length of Induced cracks as a functionof storage time, alpha decay dose, and equivalent SYNROCSTORAGE TIME (dayo)shown in Fig. 3. Initial value of cla was 2.9, dropping to 1.6 at 70 days.Results at higher doses were not calculated because of the low values of c/abeyond that point.DISCUSSIONThe extent of softening of zirconolite as a function of self-damage issimilar to that obse~ved by Ewing [10] in a comparison of crystalline andmetamict euxenite. Apparently the lattice disorder, dilation, and decrease inelastic modulus that accompany metamictization combine to bring about acharacteristic reduction in hardness.The observed increa~e in fracture toughness appears to be related to theformation of a composite damage microstructure. It has been established inearlier work [3,11] that accumulation of damage t~~$ks resulting from thestopping of the -100 KeV alpha recoil particles ( U in the presentexperiment)gradually converts zirconolite from a crystalline to a metamictstructure. Stages of damage and approximate alpha decay doses are:Stage I -- Isolated tracks in a crystalline matr~~ (O.? to 5x1024a/m3)Stage II -- Significant track overlap (5 to 10xIO alm )Stage III -- IsO~gted ~emanent crystallite in ametamict matrix (1 to 5Xlo a/m )Stage IV -- Completely metamict (>5x1025 a/m3).The damage level achieved to date in this study, 3.1x1025 u/m3, correspondsto the middls of Stage III,The amount of damage per decay event is obviously a function of the sizeof the recoil tracks. Swelling studies [3] have established that half of2~hetot~l growth of this material (2.7 vol %) is act,ievedat a dose of 7.5x1Oalm . If it is assumed that half of the volume has been converted at thisdose and that the tiaackshave a spherical configuration, then each damage eventresults in metamictization of ~ microvolume of diameter of 5.4 nm, This resultwas used as the basis for generation of computer images of the damagefilicrostructureas a function of alpha decay dose, with the results shown inFig, 4. These images represent opaque sections, where white areas correspondto undamaged material, gray spots to damage tracks (some smaller than full sizebecause of sectioning), and black areas to multiply-damaged zones.A number of mechanisms have been propcsed to explain toughening ofceramics with heterogeneous microstructure. These include crack bowing[12-14]; crack deflection [15,16]; and internal stresses associated withcoherency of second-phase particles, mismatches in thermal expansioncoefficient, or differences in elastic properties of these phases [17-19].Recently, observations of increases in fracture toughness of neutron-irradiatedceramics have been attributed to interaction of cracks with strain fieldsaround dislocation loops and to impedance of crack propagation by interactionwith voids [20~,The tough~ning r~ported here may be attributable to more than onemechanism, It seems likely that some crack bowing and deflection would occurin the heterogeneous microstructure: however, the extet]tof conversion at 28days (Fig. 4a) was ~pparently insufficient to-cause significant toughening.Stage 11 damage is characterized by track overlap (Fip. 4b), and is accompaniedb) significant strengthening, Swelling at 60 days is -3 vol % [3], indicativeof a high level of internal strain, so that a major contribution to tougheningfrom this source is likely, In support of this hypothesis, it can be pointedout that evidence of considerable internal strain was observed in earlier TEM. . —..... ..... ..\— ——._. _-— .. ——---- .-, - -— -IAl) .4.J-..!-.IJI-,-J-1-,-!8-!—. .-.. .-- .,. .Ii.-- ,I.-:,.. .-1..:-/“1!..,(a)[c)I.—. ----.I!-(b)(d),. ,,, ,,,..,, .,, ,,,,, .!, ., ...Fig. 4. ~,m ~*8 ~ay~), ~b~ ;;;;!4;:,;3Computer-generated images of2~amag~ evolutlon asalpha decay dose: (a)2~*7x~9(60 days); (C) 1.3x1U a/m (100 days); (dj ;.lxIO altnphase.As accumulation of damage tracks proceeds, introduction of these dilatedregions would initially increase the level of internal strain, some of whichwould be accommodated by expansion of the matrix. As a result the increment ofstrain per decay event should decrease. Further damage would eventually resultin a metamict matrix; at that point the presence of remanent crystallite ofrelatively high density would lead to tensile rather than compressivestresses. Nadeau and Dickson [19] showed that strengthening can result fromeither condition. It is apparent from these considerations that accumulation ofdamage is accompanied by evolution of a complex and ever-changing internalstress condition, as ~ell as a change in the quantity, morphology, and natureof the dispersed phase.Qualitative observations as to change of fracture toughness withself-damage can be made beyond the point where toughness can be calculated(Fig. 3). The continued rapid decrease in crack length (Fig. 2) initially morethan compensates for the decrease in hardness, so t~$t toughening wouldcontinue to occur. Eventually, hcwsver (at~l.6x10 cdm ), crack lengthbecomes constant while softening continues, with the result that fracturetoughness would begin to show a slight decrease. Thus beyond an equivalentSYNROC age of -5x1O y the benefits of self-damage will begin to be reducedby a small amount. However, since the relative toxicity of the waste will havedecreased to 0.1 % ~f that of the starting material (i.e., to about that ofuranium ore) at -10 y, the biological hazard will be greatly reduced by thattime. At any rate, extent of microcracking may be a more important parameterthan fracture toughness for this applicat~~n, a~d cracking remained near zeroto the highest dose attained here (3.1x1O alm , or 3x1O y for SYNROCwaste).At very high damage levels conversion to the metamict state is complete[3]. The contributiofito toughening from the presence of a heterogeneo~smicrostructure should disappear, but internal strain may still be great enoughhave a toughening effect. Eventually, however, the redamaging process, alreadyprevalent at the 231-day level attained to date in this study (Fig. 4d), couldlead to relaxation of these strains and loss of r~+iation-induced toughening,litthe much greater times corresponding to nuclear waste storage, it ispossible that thermal annealing.coula also bring about such relaxation;however, the similarity of damage in P~-substituted and natural zirconolitewith respective ages of 0.4 and 1.3x1O y [3,11] argues that no major changeswill occur at ambient temperature.CONCLUSIONSAlpha decay damage in 238Pu-substituted zirconolite causes a significantdecrease in hardness and increase ir fracture toughness. The former resultsfrom conversion to the metamict state, and the latter from the heterogeneousmicrostructure ‘nd internal strains that accompany metamictization, Thus themicrocracking that is likely to accompany differential swelling of multlphaseceramic waste forms such as SYNROC should be reduced by self-irradiationdamage, with the result that the probability of increased leaching wll’1belessened. .ACKNOWLEDGEMENTSThe authors wish to thank D, E, Peterson and C, C, Land of this Laboratoryfor their sunoort of this Dro:i~ct.●✎ REFERENCESn[1] A. E. Ringwood, S. E. Kesson, N. G. Ware, W. O. Hibberson, and A. Major,Geochem.J. 13, 141 (1979).[2] F. W. Clinard, Jr., D. L. Rchr, and R. B. Roof, Nucl. Instruments andMethods Bl, 581 (1984)[3] pi Clinard, Jr., D. E. Peterson, D. L. Rohr, and L. W. Hobbs, J. Nucl.in press (1984).[4] ~i9~i~~eber, Hj. Matzke, and J. L. Routbort, J. Mater. Sci. 19, 2533[5] ~. Lan~ford, J: Mater. Se:. Lett. 1, 493 (1982).[6] R. B. Sosman, “The Properties of Silica” (The Chemical Catalog Co., NewYork, 1927).[7] M. Wittels and F. A. $herrill, Phys. Rev. 93, 1117 (1954).[8] L. W. Hobbs anf M. R. Pascucci, J. de Physique 41, C6-237 (1980).[9] R. B. Greegor, F. W. Lytle, R. C. Ewing, and R. F. Haaker, Nucl.Instruments and Methods Bl, 587 (1984).[10] R. C. Ewing, Amer. Mineralogist 58, 942 (1973).[11] T. J. Headley, G. W. Arnold, and C. J. M. Northrup, in: “ScientificBasis for Nuclear Waste Management”, ed. W. Lutze (Elsevier, New York,1982) p. 379.[12] F. F. Lange, J. Amer. Ceram. Sot. 54, 614 (1971).[13] A. G. Evans, Phil. Msg. 26, 1327 (1972).[14] Cl.J. Green, J. Amer. Ceram. Sot. 66, C-4 (1983).[15] K. T. Faber and A. G. Evans, Acts. Fletall 31, 565 (1983).[16] K. T. Faber afidA. G. Evans, Acts Metall. 31, 577 (1983).~17] G. K. Bansal and A. H. Heuer, Phil. Msg. 29, 709 (1974).[18] D. J. Green and P. S. Nicholson, in: Fracture Mechanics of Ceramics”,Vol. 4, eds. R. C. Bradt, D. P. Hasselman, and F. F. Lange (Plenum, NewYork, 1978) p. 945.[19] J. S. Nadeau and J. 1. Dickson, J. Amer. Ceram, Sot. 63, 517 (1980).[20] F. W. Clinard, Jr., G. F. Hurley, R. A. Youngman, and L, W. Hobbs, FirstInternational Conference on Fusion Reactor Materials, Tokyo, Japan,December 3-6, 1984 (Proceedings to be published by Journal of NuclearMaterials),[21] J. L. Routbort, P. Offerman, and Hj. Matzke, in: Scientific Basis forNuclear Waste Management”, ed. D. G. Brookins (Elsevier, New York, 1983)p. 415.

Irradiation-induced reduction of microcracking in zirconolite

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Irradiation-induced reduction of microcracking in zirconolite

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