Characterization data for particulates formed under a variety of laboratory leaching conditions that simulate glass reaction in a repository environment are presented. Data on the particle size distributions and filterable fractions for neptunium, plutonium, americium, and curium were obtained by filtrations through a series of filters with pore sizes ranging from 1 {mu}m to 3.8 nm. The neptunium was found to be largely nonfilterable. Americium and plutonium were associated with filterable particles. The particles with which the americium, plutonium, and curium were associated were characterized using transmission electron microscopy (TEM) examination techniques. 8 refs., 1 fig., 2 tabs

Bates, J.K.

Cunnane, J.C.

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--ItANL/CP—72166DE91 015547IDENTIFICATION OF COLLOIDS INNUCLEAR WASTE GLASS REACTIONSI t s feSSS«-|=«.fii»1ii « a S R •§ 5 cJ. C. Cunnane and J. K. BatesChemical Technology DivisionArgonne National Laboratory9700 South Cass AvenueArgonne. IL 60439-4837Tftt subf»>ned manuscript has teen authoredby a contractor of thft U. 5< Govtfnmtntundo contract No. W-31-109-ENG-3S-AccordinsW. the U. S. Government retains anonexclusive, royalty-free license to publishor reproduce the puMWied form of thiscontribution, or atfow others to do to. totU. S- Government purpoMt.For Presentation atthe 93rd Annual MeetingAmerican Ceramics SocietyCincinnati, OhioApril 28-May 2. 1991mDISTRiSUTiOS OF THIS DOCUMENT iS UNLIMITEDhIDENTIFICATION OF COLLOIDS IN NUCLEAR WASTE GLASSREACTIONSJ. C. Cunnane and J. K. Bates. Argonne National Laboratory. 9700 South CassAvenue. Argonne. IL 60439ABSTRACTCharacterization data for participates formed under a variety of laboratory leachingconditions that simulate glass reaction in a repository environment are presented.Data on the particle size distributions and filterable fractions for neptunium,plutonium, americium. and curium were obtained by filtration through a series offilters with pore sizes ranging from 1 pm to 3.8 nm. The neptunium was foundto be largely nonfilterable. Americium and piutonium were associated withfilterable particles. The particles with which the americium. piutonium, and curiumwere associated were characterized using transmission electron microscopy (TEM)examination techniques.1. INTRODUCTIONIt is well known that colloidal particles are mobile in subsurface environments andcan play an important role in the transport of contaminants."' However, theimportance of colloids in assessing the radionuclide release performance of theengineered and natural barriers for the geologic disposal of high-level radioactivewaste is not well established. While the role of colloid formation and transport inthe release of radionuclides has been discussed in the literature.I2'3J there is littleexperimental information on the formation and characteristics of colloids formed inrepository-relevant glass testing. Current performance assessment calculationsassume that the mobile concentrations of most sparingly soluble radioelements inany groundwater that contacts the waste will be "solubility limited."'*'5' Animportant implicit assumption in this solubility-limited approach is that colloids arenot formed or play an insignificant role in radionuclide release.This paper is intended to examine the formation and characteristics of colloidalparticles under a variety of waste glass testing conditions that are relevant to thecandidate Yucca Mountain repository. It provides an initial basis for evaluating theneed to consider the role of colloids in transporting radionuclides out of the wastepackages and the repository engineered barrier system.2. EXPERIMENTALThe experimental results that are discussed in Section 3 were obtained by filtrationof leachates. from a variety of waste glass tests, through a sequence of membranefilters with pore sizes ranging from 5 /im to 2.8 nm. The filterable andnonfilterable fractions for Np-237. Pu-239. Pu-238. Am-241. and Cm-244 in theleachate were determined using alpha spectroscopy. In addition, the particles thatwere isolated by wicking small leachate droplets through a holey carbon grid wereexamined using the TEM. An outline of the types of waste glass that weretested, the test conditions, and the ultrafiltration and counting methods that wereutilized follows.2.1 The Waste Glass Characteristics and Testing ConditionsA variety of waste glasses, designated ATM-8. 131/11. 165/42. 200R. and 202Awere utilized. These designations are based mainly on a range of Savannah RiverLaboratory (SRL) waste glass composition recipes^' and are described below:* the 165/42 glass denotes a composition based on the 165 type frit andsludge from tank 42. It represents the glass durability target that was usedduring frit development at SRL;* the 131/11 glass denotes a composition based on the 131 type frit andsludge from tank 11. It represents a lower bound on the acceptabledurability range:* the 200R glass denotes a composition based on the 200 type frit and sludgefrom tanks 8 and 12 with a nonradioactive analog additive for the PHA feed;* the 202A glass denotes a composition based on the Defense WasteProcessing Facility (DWPF) startup frit with additions to make it similar tothe target glass that is identified in the Waste Compliance Plan for theinitial DWPF production campaigns:* the ATM-8 glass is a reference glass composition documented in reference 7.It is not a DWPF-based glass.Samples of these glasses in the form of monolithic disks or crushed (sieve size100-200 mesh) powders were leached in J-13 well water under the matrix of testconditions shown in Table 1. Each leach test was performed in conventional 304Lstainless steel Parr vessels which were sealed using a copper compression gasketand kept, for the test duration, in a thermostatically controlled oven. At the endof each test, approximately 2 mL of the leachate was removed, weighed, and setaside for filtration purposes.Table 1. Matrix of Test ConditionsTest Glass lestNumber Type/Form SA/V (m'1} Duration (d)DP16 131/11-powderDP61 165/42-powderDP62 165/42-powderDP122 200R-powderDP123 200R-powderDP158 200R-powderPY11 131A-powderPY12 131A-powderBY11 202A-powderG523 ATM-8-diskG537 ATM-8-disk200020002000200020002000200020002000303028028028014141414014014014601460*AII tests were conducted at 90°C.**Duplicate tests.*Test similar to G523 with tuff disk included.2.2 Ultrafiltration and Counting MethodsThe leachaie sample from each test was filtered sequentially through a series ofNuclepore © (polycarbonate) and Amicon Diaflo ® (YM-cellulose) filters. Ingeneral, the-filter pore size sequence utilized was 5. 1 . 0.1, and 0.015 pmNudeopore® and Amicon YM 30 (pore size of approximately 3.8 nm). Forfiltration the filters were placed and sealed in a Gelmann filter housing. Theleachate solution was forced through the filters by using a syringe to supply theleachate and the necessary filtration overpressure. After filtration, the amount offiltrate collected was weighed and an aliquot was deposited and dried on astainless steel planchette for alpha counting. Also, the filters were removed fromthe filter housing and mounted on steel planchettes in a configuration suitable foralpha counting.The filters and filtrate aliquots were counted using a calibrated Ortec model 576alpha spectroscopy system. The observed count rates for the alpha peakscharacteristic of Np-237. Pu-239. Pu-238, Am-241. and Cm-244. were used tocalculate the filterable and nonfilterable concentrations for each isotope.Samples, suitable for T E M examination, were prepared from selected leachatesolutions by carefully wicking a small droplet of the leachate through a holeycarbon T E M grid.f3. RESULTSAs mentioned in Section 2. alpha spectroscopy counting data were used tocalculate the percentages of the leachate concentrations that were filterable (i.e..removed by the filter) and nonfilterable (i.e.. transmitted by the filter) at eachfiltration step. The results obtained are shown in Table 2. The nonfilterablefractions are based on the alpha spectroscopy data for the filtrate aliquots thatwere collected after each filtration step. Data for the percent filtered wereobtained, by difference, from the concentration in the solution that was filtered andthe filtrate concentration. The Pu results for the DP-16 test are discussed here toillustrate how the Table 2 data should be interpreted. For this test. 4% of thePu leachate concentration was removed (i.e.. filterable) by the 5 /xm filter. 7% bythe 1 fim filter. 80% by the 0.1 /im filter, and 3 and 4% respectively by the0.015 and 0.0038 /im filters. Only 2% of the original Pu concentration remainedin the solution (i.e.. was nonfilterable) after the 0.0038 /im filtration stage. Thiscorresponds to a Pu concentration of 2.5E-12 moles/liter.The alpha spectroscopy data obtained by counting filters was used to check forclosure of the mass balance after each filtration step. The filter and filtrate datawere generally consistent with mass balance closure. However, in some cases,particularly when the filtrate data indicated that significant fractions (>20%) werefilterable by the Amicon YM 30 filters, mass balance closure was not achieved.We believe that this may be due to trapping of the particles within the membranewhich results in counting errors due to attenuation or stopping of some of thealpha particles in the filter.Figure 1 shows a brightfield electron micrographs of particles that were collectedon a T E M grid for the DP-158 test (see Table 2). These micrographs wereobtained using a 200 keV analytical electron microscope. The larger particleshown in Figure lb is believed to be a piece of the reacted layer that has spalledfrom the glass surface. The particles shown in Figure la are believed to betypical of those that account for the filtration results presented here. They rangein size from about 2000 A to less than 100 A. The elemental composition ofthese particles is mostly sodium, potassium, aluminum, and silicon. Because theyare amorphous, the phases involved could not be identified.The residual concentrations in the filtrate solution after passage through the lastfilter in the filtration series provides an indication of the true solutionconcentrations. These data are tabulated in Table 2.4. DISCUSSION AND CONCLUSIONSThe results presented here were obtained through an initial scoping study for whichthe objective was to provide an overview of how colloidal particles, formed under avariety of repository-relevant test conditions, influence the total concentrations ofactinide elements in leach solutions. Many potentially important issues, such asthe effects of temperature changes and leachate storage on the colloidal particles,remain tc be investigated.Table 2. Results for Filtration SeriesTest NumberDP16DP61/62*"DP122/123***DP158FilterPore Size(/jm)5.01.00.10.0150.00381.00.10.050.0150.00385.01.00.10.00385.01.00.1NpNonfilterable FractionPu968996(2.5E-12)21028380(5.7E-11)"1011510391(4.6E-11)2610011962(%)*Am10013865Cm89991410(3.5E-15)4100807777(1.9E-13)*'71007859(9.4E-13)19Filterable Fraction,Np Pu47803401930700012650057Am0073(%)*Cm1854602030700221940ConfdTableTest2 (Confd)NumberFilterPora SizeNpNonfilterablePuFraction (%)*Am CmFilterable Fraction,Np Pu Am(%)'CmG523G537PY11/12BYil5.01.00.10.00385.01.00.10.00385.01.00.10.00385.01.00.10.0038869085(4.9E-7)85103112112(1.1E-6)92872115(1.0E-10)111107548(2.6E-10)101008837(1.1E-9)121018062(7.6E-9)201009427(7.3E-12)7897775(1.4E-10)1414 130 661 603527380125125021180667201112242 51'The filterable and nonfilterable fractions are shown as percentages of the concentration in the leachate." T h e numbers in parentheses are the corresponding nonfilterable concentrations expressed as moles/liter.'"Average data from duplicate tests.# 0.1pm>: •&••(a)Figure 1. TEM Micrographs of Particulate Material Isolated on a Holey CarbonTEM GridThe data for Np (see the results for the G523 and G537 tests in Table 2)indicate that less than 15% of the solution concentration is filterable even at the3.8 nm filter pore size. This result is consistent with Np results reportedpreviously.(8J In contrast to Pu. Am. and Cm. the Np data are consistent withthe expectation that the concentration is dominated by the material in true solutionand that colloidal formation and transport may not play an important role in Nprelease from the waste packages and EBS.Most (up to 100%) of the Pu. Am. and Cm concentration in glass leachatesolutions is associated with particulate material in the colloidal size range. Thesize distribution (as indicated in Table 2 by the fractions that are filterable by thedifferent pore size filters) changes significantly from test to test. The sizedistributions are probably influenced by a broad range of factors, including glasstype, test duration, and test conditions, which are not well understood at thispoint. However, the fractions of these elements (i.e., Pu, Am, Cm) associatedwith the different size fractions do not appear to be element specific. Where dataare available for more than one of these elements they appear to follow the sametrend.Although the work reported here is in an early scoping phase, the results indicatethat the performance assessment assumption that groundwater concentrations ofactinide elements will be solubility limited should be reexamined. This conclusionapplies particularly to modeling the release of Pu, Am, and Cm isotopes fromwaste packages containing nuclear waste glass.5. ACKNOWLEDGMENTSWork supported by the U.S. Department of Energy. Office of EnvironmentalRestoration and Waste Management, under Contract W-31-109-ENG-38.6. REFERENCES1. Hazardous Materials Control. Vol. 3, Number 4. July/August 1990.2. A. Avogado and F. Lanza. "Relationship Between Glass LeachingMechanism and Geochemical Transport of Radionuclides." ScientificBasis for Nuclear Waste Management V. W. Lutze. ed. (1982).3. J. D. F. Ramsay. "The Role of Colloids in the Release of Radionuclidesfrom Nuclear Waste." Radiochimica Ada 44/45, 165-170 (1988).4. M. J. Apted and D. W. Engel. "Mass-Transfer Analysis of WastePackages Containing Defense Waste Processing Facility Glass as aWaste Form," Proc. of the Internal. Topical Mtg. on High-LevelRadioactive Waste Mgmt.. Vol. 1. Las Vegas. Nevada. April 8-12, 1990.5. W. J. O'Connell. "Status of Integrated Performance Assessment ofWaste Packages and Engineered Barrier System." ibid.6. C. M. Jantzen. "Glass Compositions and Frit Formulations Developedfor DWPF." memo. C. M. Jantzen to M. J. Plodinec. Savannah RiverLaboratory report DPST-88-952 (1988).7. J. W. Wald. "Fabrication and Characterization of MCC ApprovedTesting Material - ATM-8 Glass." PNL-5577-8 (1985).8. J. C. Cunnane and J. K. Bates. "The Role of Laboratory AnalogExperiments in Assessing the Performance of Waste Package Materials.'Scientific Basis for Nuclear Waste Management XIV, T . A. Abrajanoand L H. Johnson, eds. (1991).

Identification of colloids in nuclear waste glass reactions

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Identification of colloids in nuclear waste glass reactions

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