Riege, U.

Scheffler, K.

English

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April 1977Abteilung Behandlung radioaktiver AbfälleProjekt Wiederaufarbeitung und AbfallbehandlungKFK 2422Investigations on the Long-term RadiationStability 01 Borosilicate Glasses against Alpha mEmiUersK. Scheffler, U. RiegeAls Manuskript vervielfältigtFür diesen Bericht behalten wir uns alle Rechte vorGESELLSCHAFT FÜR KERNFORSCHUNG M. B. H.KARLSRUHEKernforschungszentrum KarlsruheKFK 2422PWA 108/76Abteilung Behandlung radioaktiver AbfälleProjekt Wiederaufarbeitung und AbfallbehandlungInvestigations on the Long-Term Radiation Stabilityof Borosilicate Glasses against Alpha-EmittersK. SchefflerU. RiegeGesellschaft für Kernforschung mbB, KarlsruheAbstractBy accellerated experiments with Cm-242 doped HLW glas ses thelong-range behaviour of borosilicate glas ses during finaldisposal has been simulated. To cover those questions con-cerning long-term safety aspects the properties of theseglasses have been investigated on a time-hpsescale corres-ponding to 10.000 years storage of real HLW borosilicateglasses. The effects of radiation on structure, helium build-up, mechanical strength, energy storage and leachability ofthe glas ses have been studied.Zus~mmenfassungUntersuchungen zur Langzeitstrahlenbeständigkeit von Boro-sili~atg~äsern gegen~r Alpha-StrahlernDurch Zeitrafferexperimente mit Cm-242-haltigen hochaktivenGläsern wurde das Langzeitverhalten von Borosilikatgläsernwährend der Endlagerung simuliert. Um die mit der Langzeit-sicherheit in Zusammenhang stehenden Fragen zu beantworten,wurden die Glaseigenschaften im Zeitraffermaßstab entsprechend10.000 jähriger Lagerung echter hochaktiver Borosilikatgläseruntersucht. Die Strahlungseinflüsse auf Struktur, Heliumaufbau,mechanische Festigkeit, Energiespeicherung und Auslaugungdieser Gläser wurden bestimmt.Des recherches sur la stabilite des verres borosilicateaux emetteurs alpha a longue echeanceDans des experiences accelerees avec des verres de hautesactivites, fondus avec Cm-242, le comportement des verresborosilicate a longue echeance pendant le stockage finalest simule. La qualite des verres est examinee dans uneperiode acceleree correspondant a 10.000 annees de stockagedes verres de hautes activites typiques en reponse a laquestion posee sur la securite pendant de longues periodesde temps. Les influences d1irradiations sur la structure,la formation d'helium, la resistance mechanique, les energiesemmagasinees et la lixiviation de ces verres sant etudiees.Table of ContentsIntroductionPage12.3 •4.5.6.General Aspects of Radiation Effects in Boro-silicate Glasses during Final DisposalSimulation of Long-Term Irradiation,Preparation of Test SpecimensPost-Irradiation Investigations4.1 Effects of Radiation on Structure,Microscopy4.2 Effects of Radiation on Composition,Autoradiography4.3 Helium Buildup and Helium ReleaseRates4.4 Effects of Radiation on MechanicalProperties, Crack-Test Evaluation4.5 BUildup of Stored Energy4.6 Effects of Radiation on Leachabilit~Leach Rates of ActinidesSummary and ConclusionsReferences235555141919222311. IntroductionAt present, glass-systems are considered as the most convenientsolidification products for the final disposal of high-levelwastes arising from the reprocessing of spent nuclear fuel.The process technology for the immobilization of high-levelwaste in glass is most advanced and just demonstrated in somecountries.The basic glass compositions have been optimized for differenttypes of high-level wastes and different types of technologicalprocesses aiming at the incorporation of a high percentage ofwaste oxides into glass. At the same time aseries of require-ments are expected from the final glass products with respectto final disposal safety considerations. In this context leacha-bility, mechanical and physical properties are of particularinterest, and special attention is paid to the long-term inte-grity and stability of the HLW glasses.This report presenb some appropriate procedures and resultson HLW glass behaviour over 10.000 years, that has been simu-lated by time-lapse experiments.22. General Aspects of Radiation Effects in BorosilicateGlasses durin~ Final DisposalThere is the question, what might happen to HLW glasses withinthousands of years due to the bombardement by a-and ß-par-ticles or x-rays or recoil nuclei. A quantitative evaluationperformed on the basis of theoretical calculations {1} demon-strates that the number of atomic displacements followinga-disintegration of the long-lived actinides takes more than95% of the total number of atomic displacements caused byall particle interactions in the HLW glass matrix. Thereforeit seems to be a good approach that only the actinides aretaken into account when any radiation damage in the glasseswould occur.As the lifetimes of the interesting actinides espe americiumand plutonium exceed those of human beings by some orders ofmagnitude it is obviously necessary to start with acceleratedexperiments on realistic models of HLW glasses doped withshort-lived actinides.From the point of view of their solubility in glass and theiravailability the curium-242 and curium-244 isotopes are speciallyfavoured to achieve similar a-decay yields on curium spikedglass specimens within months as on real HLW glasses withincenturies { 2}33. Simulation of Long-Term Irradiation, Preparation of~est SpecimensAs indicated in chapter 2 the most realistic assessment to thelong-range stability of HLW glas ses will be met by acceleratedexperiments carried out on curium doped glass sampIes with theproper composition of HLW glasses and similar preparation tech-niques.A detailed description of the preparation procedure and thecomposition of the specimens which are subject of the post-irradiation studies reported below is given in lit. (2). Table 1shows a compilation of some relevant chemical, physical andirradiation properties of the three different types of specimens.The curium spiked specimens, prepared in January 1975, wereanalyzed on their properties at certain irradiation intervals.After one yeamstorage at room temperature the cumulativenumber of a-disintegrations and the total absorbed a-dosewere corresponding to 2.300 years and 4.000 years disposal re-spectively when compared to real HLW glasses, and after two yearsstorage ofthe specimens the quivalent disposal periods of HLWglasses would be 8.500 years or 13.000 years respectively.Thus, the results of the post-irradiation investigations onthese a-emitter doped sampIes are representative for thoseeffects that might happen during final disposal of HLW glassesdue to the a-decay of actinides within about 10.000 years. Thisdisposal period has to be considered as the very important onein view of the decrease of the a-toxity of HLW glasses withrepect to the a-toxity of naturally occuring 3% uranium ore (2):the relative a-toxity is decreasing by about three orders ofmagnitude to a level only slightly above that of uranium ore.kind of composition density a-activity (Ci/g glass) cumulative number ofspecimen (wt. %) (g/cm3 ) initial after after a-disintegrations1 year 2 years after 1 year after 2 yearsbasic glass 95,1'1 98 "(1) 2,58 2,63 0,67 0,25 1 ,64 · 1018 2,12 · 10 18"981\4,9 (Am,On)02HLW glass78,5H98" 10 18 10 18li98~ 2,74 2,63 0,67 0,25 1,64 · 2,12 ·16,6 HL~- (2)oX1des4,9 (Am,On)02HLW-Gd glass69,0 "9 8~ 10 18 1018119 8'/ 3,06 2,63 0,67 0,25 1 ,64 · 2,12 ·16,6 HLW-oxides9,5 Gd203(3)4,9 (Am,Cm)° 2~( 1 )(2)(3)b . 1 . t . " 9 8" ( .aS1C g ass compos1 lon, wt.%): 50,5 S102 , 4,2 Ti02j 1,4 A1 203/13,6 B203/2,8 CaO, 27,5 Na20HLW-oxides composition: 87,9 wt.% f.p.-oxides from LWR-fuel(burn-up 33.000 MWd/t),8,4 wt.% Fe20 3 , 2,4 wt.% Cr20 3 , 1,3 wt.% NiOGd 20 3 is added in the form of Gd-nitrate solution as homogeneous neutron poison duringdissolution of spent nuclear fuel.Table 1: Data Sheet on Curium Doped HLW Glass Specimens54. Post - Irradiation Inve~tigationsThe surface of polished discs of the glass cylinders was ob~served through the light microscope (fig. 1,2,3). Supplemen-tary photographs of artificially produced crack areas weretaken by the scanning electron microscope (fig. 4).All together the microscopic observation of the specimens asa function of a-disintegration shows no significant radiationeffects on the glass structure even in the sphere of metallicinclusions which may be expected as nucleation centers.Alpha-Autoradiographs taken from polished sampIes after dif-ferent decay periods of the actinides dissolved in the glassspecimens are demonstrating the unaltered homogeneous distri-bution of these a-emitters (fig. 5,6,7).By electron capture the a-particles from actinide decay areconverted into uncharged helium atoms. The solubility anddiffusion coefficients of helium in borosilicate glasses areunknown. Therefore sometimes an influence of helium generationon the mechanical strength of HLW glasses is discussed {3}.In order to get an idea on the helium permeation rate theleakage of helium from the three curium doped glass specimenswas measured by gas chromatography. The glass cylinders (~ 10 mm,average weight 5g) were confined in welded stainless steel cap-sules. After 530 days storage at room temperature these sampleswere inserted into the gas chromatography measuring assembly.The whole system was evacuated and afterwards the capsulestapped and rinsed with argon for helium measurement.continued p. 136at start ofexperiment;lfter two years3torageF ig. 1: . " "M1croscopy of Curium Doped Basic Glass 987as start ofexperimentafter two yearsstorageFig. 2: Microscopy of Curium doped HLW Glass "98"Sat start ofexperimentafter two yearsstorageFig.3: Microscopy of Curium doped HLW-Gd Glass /9S"9basic glassHLW glass'9a'HLW-Gd glass" 9a,lFig. 4: Scanning Electron Microscopy on Cracks at CuriumDoped Specimens after two years storage10at start ofexperimentafter two yearsstorageFig. 5: Alpha Autoradiographs of Curium Doped Basic GlasJ98~11at start ofexperimentafter two yearsstorageFig. 6: Alpha Autoradiographs of Curium Doped HLW GlassU98"12at start ofexperimentafter two yearsstorageFig. 7: Alpha Autoradiographs of Curium Doped HLW-Gd Glass "98"13For all samples no eseaped helium eould be deteeted exeeedingthe sensitivity of the measuring deviee, which eorrespondsto a total of 20 rnrn3 He per sample.With the following ealeulation based on Fiek's law an appro-ximate value for the diffusion eoeffieient of He in the HLWborosilieate glass system has been estimated:= -V 2 Dt mean path lengthof He atoms {ern}D He diffusion eoeffieient{em2/s}t time {s}D =a radius of eylindrieal= r . ra o samplea measured He release perg glassaverage He produetion pera o g glass within the time t2 • t .r = 0,5 ema <: 4 rnrn3/g glass= 46,5 rnrn3/g glass (see fig. 8)a ot = 530 days = 4,58 . 10 7 sD <: 2 • 1o~ 1 em2i§14From the estimated value for D increasing internal stressin real monolithic HLW glass blocks seems reasonable ifthe unknown ratio of helium solubility and helium productionis low. To answer the question for internal stress and radiationinduced changes of the mechanical properties of HLW glassescrack tests have been performed on the irradiated samples -see chapter 4.4 •4.4 ~~~~2E§_Q~_B~g!~E!Q~_Q~_M~2h~~!2~1_EEQE~!E!~§L_g!~2~­~~§E_~Y~lg~E!Q~As mentioned above the mechanical properties of the final glassproducts may be changed by radiation effects during final dis-posal. There are some common testing procedures for mechanicalstrength used in glass industry. For these tests it is in anycase presupposed that the samples have to undergo very carefulpreparation, after-treatment and dimensioning prior to testing.But at the expense of these pretreatment procedures only littleinformation is achieved in view of the essential problem.On the other hand the reasonable objectives are establishedby the histroy and the future of the products themselves:Comparison of different kinds of productsIntercomparison of products from different processesIntercomparison of products before and ~er irradiation.An additional marginal restriction has to be taken into accountwhich refers to the more difficult handling of products andequipments in glove boxes or hot cells.Following these argumentations a simple procedure - the"Crack-Test" - has been elaborated, tested and applied:15400 500 600 700 800 900 [days]530100 200 300o7040306020105046,5 --I7-7"7/-r7""7/'""7"""7~'-'--'--~"--"-'.....o......>..",,--,,-"'-'-"'-'-~[mm He~[g glas~80Fig. 8: HeliumGene~tion in Curium Doped Glass Samples16A cylindrical or cube-shaped specimen cut from a biggersample is inserted into the testing device (see fig. 9)The dimensions of the specimen shall be such that thevalue of the ratio ~~igh~ shall be about 1. And the~ame ervalue of the ratio area of Eress die shall be about 10.area of specimenThe specimen and the press die respectively are exposed toapressure, that shall be ten times larger than the pressurenecessary for rupture of the specimen. On the crushed specimena sieve analysis is performed and evaluated utilizing theRRS-grain size distribution.For the intercomparison of different types of specimens theaverage grain diameter d has to be standardized to the volume(1 cm3 ) of each specimen, since the crushing of the specimenpeaces after rupture is dependent on the total particle volume.In table 2 the results of the crack-tests evaluated on thebasis of the RRS plots following the equation 19 (lg 100/R)=n.lg d + C are listed (R = weight of cumulative sieving re-sidue in % of total weight,n = expression for the uniformityof grain distribution, d = grain diameter or mesh of thescreen) •Apparently the mechanical properties of the Curium doped speci-mens have changed slightly after irradiation for 2 years (seealso table 1) compared to the non-irradiated samples. But theirmechanical strength is still in the range of other glass-likeproducts.- 17 -Pressureep 29Testing Device for Mechanical StabilityFig. 9Specimen- 18 -average graindiameter d{p}degree of uniformityof grain distributionn-----------.---11----------+-------- .----------1basic glass "98"curium doped basic glass"98" after two yearsstorageHLW glass "98"curium doped HLW glass"98" after two yearsstorageHLW-Gd glass "98"curium doped HLW-glass"98" after two yearsstoragecownercial DURAN glasscommercial fused silicacommercial sinteredalumina ALSINTbasic glas "98",exposedto a thermo-shock of400 0 C43246635553445050027026058003050,750,780,670,900,740,780,780,730,780,81-------------------1. .__--'- _Table 2: Crack - Test Results19On the samples listed in table 1 energy storage followinga-disintegration was measured as a function of the total in-tegrated dose up to a dose equivalent to 2 • 10 18 a-disinte-grations/g glass {1}. A saturation value for stored energy isalready obtained after about 1 • 10 18 a-disintegrations/gglass { 1 }. Final values are given in table 3.Specimen stored energy(cal/g glass)basic glass "98" 64 + 3--HLW glass "98" 83 + 3-HUv-Gd glass "98" 94,5 + 2~.Table 3: Stored Energy in Borosilicate Glassesafter 2,12 • 10 18 a-Disintegrationsper g Glass4.6 ~EE~S~~_2E_g~9!~~!2~_2~_~~~S~~e!!!~YL_~~~S~_g~~§~of ActinidesThe leaching rates of Curium and Americium were determinedin a run of long-term leach tests on the three glass compo-sitions (see table 1).A somewhat modified leach test when compared to the IAEA-re-commended procedure {4} was applied:Glass specimens were imbedded in Araldite, grinded and polished.The so prepared surface of the specimens was exposed to the dif-ferent leachants (table 4) at room temperature (23 ± 30 C) •20In certain intervals the suspended specimens were withdrawnand introduced into new leachant containers with fresh leachants.The spent leachants were acidified with nitric acid within theleachants containers. Homogenized sampIes of these leach so-lutions were taken for a-counting an a-spectrometry for Ameri-cium and Curium determination.Fig. 10 shows the leaching rate versus time plot from leach testswith demineralized water. Within the total leaching periodcomprising 8 months the cumulative number of a-disintegrationswas increasing up to a value of 1,3 • 10 18 a's/g glass. Never-theless the leaching rates remained obviously constant indicatingthat no significant changes of the glass matrix with respect toany leacheability - radiation dose dependence could be detected.basic glass HLH glass HLW-Gd glass1198 11 1198 11 1198 113,8 · 10-8 5,5 · 10-7 6,0 · 10-71 ,4 · 10-6 1 ,4 · 10-6 1 ,4 · 10-6tion 1 ,4 · 10-6 4,0 · 10-6 22,0 · 10-60,7 · 10-6 0,6 · 10-6 1 ,8 · 10-61 ,8 · 10-6 5,7 · 10-6 24,0 · 10-6.leachantsaturatedCarnallite-solution+1 M MgCI 2-solution1 M NaCI-solu0,01 M NaCI-solutiondemineralizedwater+) composition of saturated carnallite solution: 62,83 wt.% H20,2,04 wt.% MgS0 4 , 34,3 wt.% Mg C1 2 , 0,62 wt.% K Cl, 0,21 wt.%NaCI'l'able 4: Leach Rates (g/cm2d) of Americium from Curium DopedGlass Specimens in different Leachants.21The long-term leaching rates of americium are listed intable 4.10-4,------------------------------------,9Am-241 CDCm-242 ®Am-241 (j)Cm-242 <2>Am-241 Ci'>B7..."- __ ---0-----...,, Cm-242 CD4 5 6Time [months]3",A./' ""\"\2lO-B,-t---"---.-----r--~------>r_--.__--,_--__,_--.....,..--___r--­o"lJNEu......ClCD basic glass "98"® HLW glass "98"Q) HLW-Gd glass "98"Fig. 10: Long-Range Leaching Rates for Am and Cm forCurium Doped Glass Specimens (Leachant: Water)225. Summary and ConclusionsBy time-Iapse experiments and subsequent post-irradiationstudies on Cm-242 doped HLW glas ses it has been demonstratedthat borosilicate glasses are stable against long-term radi-ation from a-decay of the long-lived actinides.The mechanical strength of the glass products is alteredslightly but within the limits of mechanical strength for commonglass systems. Structure and homogeneity of the borosilicateglasses as weIl as their leachability are not affected byradiation.The number of atomic displacements due to a-disintegration ismacroscopically indicated by the stored energy in HLW glass.The formation and annealing of atomic displacements gets upto an equilibrium at about 10 18 a-disintegrations per g glasrepresented by the saturation values for stored energies. Atthevalues measured, even if a spontaneous release of storedenergies would take place, the temperature increase in theglasses would be below 300oC.bcknowleqgeme~sWe thank for the help in the field and technical assistancein the laboratory:H.Enderlein, R. Pejsa, B. SchweigelAbteilung Reaktorbetrieb und Technik/Heiße ZellenH. KreiselmeierAbteilung Strahlenschutz und SicherheitLabor für Elektronenmikroskopie,Europäisches Institut für Transurane, (EURATOM), Karlsruhe236. References{1} K. Scheffler, U. Riege, W. HildEnergiespeicherung in BorosilikatgläsernKFK - 2333 (August 1976){2} K. Scheffler, U. Riege, W. Hild, A.T. JakubickZur Problematik der sicheren Beseitigung a-haltigerAbfälle aus Wiederaufbereitung und Brennelementfertigungam Beispiel des Langzeitverhaltens hochaktiver GläserKFK - 2170 (Juli 1975){3} A. M. PlattQuarterly Progress Report - Research and DevelopmentActivities - Waste Fixation Program July throughSeptember 1973BNWL - 1788 (Qctober 1973){4} E.D. HespeLeach Testing of Immobilized Radioactive WasteSolidsAtomic Energy Review 9, 195 - 207 (1971)

Investigations on the long-term radiation stability of borosilicate glasses against alpha-emitters

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Investigations on the long-term radiation stability of borosilicate glasses against alpha-emitters

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