Fried, S.

Friedman, A. M.

Weeber, R.

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THE DISTRIBUTION OF PLUTONIUM IN A ROCK CONTAINMENT ENVIRONMENTbyS. Fried, A. M. Friedman, and R. WeeberUoiC-AUA-USAEC«For Presentation At: f\\]High Level Radioactive Waste Management Symposium,Los Angeles, California, March 31, 1974MASTERARGONNE NATIONAL LABORATORY, ARGONNE, ILLINOISWSIBBWION OF THIS DOCUMENT IS UNL.M.TED HThe Distribution of Plutonium in a Rock Containment EnvironmentS= FRIiSD, A . M. FRIEDMAN* AND R. WEEBER**Chemistry DivisionArgonne National Laboratory, Argonne, Illinois 60439INTRODUCTIONThe increasing amounts of radioactive waste material accumulatingfrom reactor operations makes the disposal and safekeeping of thismaterial of vital importance. This is especially true for thosenuclides of long half life.Regardless of the technical details of a particular disposalmethod it is obvious that in a "depository" the ultimate secondarycontainer must be some rock stratum.It is necessary not only to consider the rock strata asbarriers but also as possible conduits for ground waters and asproviding for the dispersal of radionuclides and their ultimateincorporation into the biosphere. With this in mind studiesWork performed under the auspices of the U. S. Atomic EnergyCommission.and College of Environmental and Applied Sciences, Govex r'sState University, Park Forest, Illinois 60466.**Thesis Parts Student, Governor's State University, Park Forest,Illinois.- N O T I C E -Thls report was prepared as an account of worltsponsored by the United Slates Government. Neilhoithe United States nor the United Sutei Atomic EnergyCommission, nor any of their employees, nor any oftheir contractors, subcontractors, or their employed,makes any warranty, express ot implied, or assumes anylegal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus,product or process disclosed, or represents that ill usewould not infringe privately owned rights.DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDI >^jhave been undertaken to determine the behavior of long-livedradionuclides, particularly Pu and Am (T, 24,000y and 450 y)in rock strata.The present paper presents some of the results obtained onthe migration and absorption of Pu in certain rocks.EXPERIMENTALThe experiments were of two types. In the first set,studies were made of migration of solutions of Pu through coresof Niagara limestone and basalts. A high pressure chromato-graphic absorption apparatus was constructed and is shown diagram-matically in Figure 1. The function of the piston is to exert pres-sure on the solution while isolating it from the pressurizing gas.At the start of an experiment a small amount of Pu(NO-)4 •;tracer in neutral aqueous solution was placed on the surface of !]jthe disk of stone and allowed to dry at room temperature. Sue- \cessive increments of H-0 were then forced through the limestone Jijdisk and the depth of penetration of the Pu tracer was measured ;jby the X-ray absorption technique. ijIsThis technique depends upon the fact that the three uranium \\238 h'L X-rays, which accompany 25% of all Pu decays are of slightly jidiffering energies and therefore have different absorption flI.Icoefficients in the limestone. Figure 2 shows the spectrum of jjthese L X-rays observed through several samples of wet lime- \\fistone. These were thin measured slices of the limestone which \\238 '̂were interposed as absorbers between a source of Pu radiation UMj :•;and the detector. As is easily seen the ratio of the inten-sities of the L./L, X-rays decreases with the thickness of thestone, similarly the L,/L2 ratio increases with thickness. Itis a simple matter to calibrate the change in these ratios asa function of thickness (penetration), since the X ray intensityratios change logrithmically with thickness. In the time in-terval after each increment of H_0 had passed through the sample,the L X-ray spectrum was measured through the surface of thestone and the average depth of penetration of Pu tracer wasdetermined by comparing the measured L,/L-, and h^/h2 ratios tothe calibration curve.The results of these experiments yielded a migration coef-ficient, m = 30 ± 10 micrometers/meter (y/m) of water flow forthe limestone and 61 ± 8 y/m for the basalts. Where m is theaverage distance traveled by the Pu atoms for every meter traveledby the H-0 molecules.The second set of experiments consisted of measurements ofthe surface absorption coefficient of Pu on the,stones. Inthese experiments disks of the stone were immersed in solutionsof .00004 M Pu(NO3)4. Small aliquots (.05%) of the solutionswere removed, dried on Ta planchets and then placed in an inter-nal alpha proportional counter. When the counting rate of sam-ples taken at 12 hour intervals had become constant this wasregarded as evidence of the attainment of equilibrium.We will define an absorption constant as:k = (activity of Pu/ml of solution)/(activity of Pu/cm of stone)ifIt was found that the value of k for pure solutions of Pu(NO3).at .00004 M was .10 ± .02 for limestone and .07 ± .02 for ba-salts. In order to observe the effect on this absorption con-stant of other ions the value of k was measured for solutionscontaining NaCl, Cad-, LaCl3 and ZrCl. at various molarities.Figures 3, 4, and 5 illustrate the variation in k as a functionof the concentration of these salts for the limestones andbasalts used.Computer Model and Experimental Results with FissureThe computer model, MARGIE, is a simplified variation of acalculation we have used for predicting multioruer buildup ofisotopes in reactors. It traces the distribution of Pu activityover many small increments of the face of a fissure or leakduring the addition and washing cycles. The only input neededfor the program is the surface absorption coefficient, dimensionof the fissure, and volume of solution in which the Pu is addedor with which it is eluted.Figure 6 is an example of the output of the program for atypical calculation. 10 ml of solution containing the activitywas presumed to be introduced into* the top of the fissure andallowed to flow through, this was then presumed to be followedby 20 ml of water wash. The curves represent the distributionof the activity on the face of the rock after the wash period;it is calculated for three systems with three different absorp-tion coefficients, k = .04, .066 and .133. In Figure 6, d isthe distance along the rock face in the direction of flowexpressed in millimeters, and f is the fraction of the originalactivity absorbed per 0.25 nun strip across the fissure. As isexpected the higher the value of k the further and more rapidlythe activity moves. It should be remembered that k is defined2as (Activity of Pu/ml of solution)/(Activity of Pu/cm of rock).To test this model of the relationship of the surface absorp-tion coefficient to the migration rate through the fissure weconstructed a model fissure. This was done by sealing asmooth slab cut from a basalt core to the face of a teflon blockthat had a .0127 cm depression milled down its face. This then238formed a fissure that was .0127 x 1 x 4 cm in size. Pu tracerwas dissolved in 10 ml of water and allowed to flow through thisfissure at a rate of 3 ml per day. This was then followed by20 ml of water wash. After this the slab of rock was removed,dried, and the surface was scanned by a scanning alpha counterwith a spatial resolution of 0.25 mm. Figure 7 is a plot of theactivity observed in these .25 mm strips. The left hand peak at.20 inches (5.8 mm) from the top of the fissure corresponds tothe calculated position of a peak of activity with k = .04 whichis close to the value for our solutions. The second peak at 1.7inches (43 mm) from the top corresponds to a peak of activity withk = .35. Since the solutions were composed of pure water andtracer this was an unexpected result.Our present feeling is that this more rapidly moving peak,if found to be reproducible, is composed of a chemical form ofplutonium which does not absorb strongly, perhaps a (Pu)n polymer;and that the flow through the fissure is so fast (1.71 m/day IJvof water flow) that it does not have time to reach equilibrium.We are now working on a method to identify the chemical formof the plutonium in this peak.The migration of the plutonium through a fissure of thissize, a .0127 cm crack, is much more rapid than through the smallpores of the stone. Instead of 60 microns/meter of water flowas found in part 2 even the slowly moving peak (k = .04) moves217 microns/meter of water flow and the rapidly moving peakmoves over 2000 microns/meter of water flow. These rates dependon the (volume/surface area) ratio of the fissure and will belarger for larger fissures. This also implies an average porediameter of 30-60 microns for the sample of rock used in themeasurements of part 2, if the migration is through intercon-necting pores.SUMMARYThe results of these studies indicate that our basic assump-tion of the intercorrelation of the migration coefficients andsurface absorption coefficients to the migration through fissuresis verified. However, they also point out that a great deal ofeffort must be spent studying the effects of other solute species,the chemical nature of the plutonium itself and the kinetics ofthe absorption process before any understanding of the macro-scopic characteristics of the transport of plutonium can bereached.3/26/74/ljcFIGURE CAPTIONSFigure 1. Schematic diagram of the high pressure chromato-graphic column. The apparatus was constructedof stainless steel.238Figure 2. L X-ray spectra of the Pu source using 9, 990 pm,and 1727 urn thick limestone absorbers between thesource and detector. In all cases the spectra werenormalized to the same number of counts in the L_X-ray peak.Figure 3. Surface absorption coefficients of .00004 M Pu(N0,).solutions on limestone as a function of the concen-tration of other salts, i.e. ZrCl-, LaCl,, CaCl,, orNaCl.Figure 4. Behavior of the absorption coefficient of Pu as afunction of the concentration of other salts fordense basalt cores.Figure 5. Behavior of the absorption coefficient of Pu forporous basalt cores.Figure 6. Calculated distribution of Pu on surface of fissure.Figure 7. Measured distribution of Pu on surface of fissure.APPARATUS FOR FORCING WATERTHROUGH SAMPLES OF SOLID ROCK"0" RINGSPERFORATEDSUPPORT PLATEFOR ROCKTO HIGH PRESSUREN 2 TANKPISTONL~^H WATERROCK SAMPLEASSEMBLYSUPPORT PLATEFIGURE 1COUNTS/CHANNEL§oIzoc:70f\JCDO8- . 10 ifi">J to c8OOP8oI I I I I I I I I I I I i I T I l I I I••••"•.."••M"!• £ JL *I I I I I I I I I I I i I I I I J.-J I I I.,; •„.- 'II'.NaCit i0 0.4 0.8 !.2 1.6 2.0 2.4 2.8kFIGURE 31.010"F n f I I I i I i i rBcsolt CoresA,B,CD Z r ( N 0 3 ) 4La(NO 3 > 3-7 I I I 1 I I II0 0.4 0.8 1.2. 1.6 2.0 2.4FIGURE 41-0 crJO"1 H-Z IO"2toI0 - 510- 610 - 7z 1 1 1 1 1 1~/ S^* C0CI2¥ri-tL[ 1 1 1 1 1 11 I 1 i I 1 ZBasalt CoresD, E, F—La(N0 3 ) 3—ll 1 Mil——1 1 1 1 1 10 0.4 0.8 1.2 f.6 2.0 2.4FIGURE 5rn1000 TOP_ I100ZUiQ.(0z3Ou10j L_j I I I IBOTTOM1 I ,0 0.2 0.4 0.6 0.8 1.0 1.2 1.4INCHES FROM TOP1.6 1.8 2.0fo50t-n0.100.09-0.080.070.060.050.040.030,020.010k = 0.0410 ml addition20 ml wash4 8 12 16 20d24 28 32 36

Distribution of plutonium in a rock containment environment

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Distribution of plutonium in a rock containment environment

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