Fissure experiments

Table I. Am Adsorption by Gray Hornblende Schist Fissure Experiments (static)...

The actinide retardation data for the hydrothermally-altered repository component experiments (4 and 5) in Table VI are preliminary and their interpretation is hindered by the lack of attainment of a steady-state condition for actinide migration in the systems These data indicate that only a small fraction of the actinides are retained by the rock core Comparing these results to those of the unaltered fissure experiments, where Pu and Np were almost completely retarded by the rock core, one could conclude that altering the rock tends to lower its ability to retard actinide migration Another way to discuss these same data, and one that leads to a completely opposite conclusion, is in terms of the amount of activity retained by the rock core, or, the rate of actinide loss from the groundwater in terms of A (dpm/mL) of 2 Pu and 237Np ... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

Fissure Adsorption Experiments. To perform fissure sorption experiments, one end of each fissure was connected via small bore tubing to a microliter pipetter as is shown in Figure 2. Rock-equilibrated water, described below, was injected into each fissure and the fissure walls were allowed to equilibrate with the solution for a period of two days before the adsorption experiments were performed. 

Adsorption experiments were performed by removing rock-equilibrated water from the fissures and injecting stock solution which was made by dissolving tracer amounts of americium-241 in rock equilibrated water. The stock solution was allowed to equilibrate within the fissures for different periods of time and was then removed from each fissure. The stock solution was assayed before injection and after removal from the fissures so that the change in americium concentration was determined for a different time in each fissure. Ten adsorption experiments were performed in this manner and the results are presented in table I. Figure 3 is a graphical representation of the initial part of the adsorption curve. 

Figure 2. Apparatus used in static fissure adsorption experiments...

Fissure Desorption Experiments. Desorption experiments were performed with two fissures by injecting rock equilibrated water into the fissures immediately after removing the americium-bearing solution from the fissures in adsorption experiments. After injection, the rock-equilibrated water was allowed to react within the fissures for a period of time before removal. The rock-equilibrated water was assayed after removal from the two fissures and the amount of nuclide desorbed as a function of contact time was determined. Between five and seven desorption experiments were performed for each of six time periods between ten seconds and sixty minutes for each of the two fissures. The results of the desorption experiments are presented in table 2 as the percent of 3 (at equilibrium) that desorbed from the fissures in a period of time. 

Fissure Elution Experiments. The migration characteristics of americium by water transport in fissures fabricated from gray hornblende schist were determined. Fissures not used in the previous sorption experiments were used for these elution experiments. A diagram of the experimental apparatus is shown in Figure 4. Solution reservoirs were attached above the fissures and the small bore tubes affixed to the bottom of the fissures were connected to solution metering pumps. 

Before the elution experiments were performed, the "schist"-equilibrated water was pumped through the fissures for two days to allow the fissure walls to interact with the rock-equilibrated water. 

Figure 4. Diagram of apparatus used for fissure elution experiments...

The second set of fissure-elution experiments was performed in the same manner as described for the first set of fissure-elution experiments. The difference in the second set of experiments was that after 0.67 fissure volumes of stock solution were drawn into the fissures at their respective flow rates, the stock solution in the reservoirs was replaced with rock-equilibrated water. The solution metering pumps were not turned off during the exchange. Subsequently, a total of twenty fissure volumes of solution was drawn through each fissure before the metering pumps were turned off. After the metering pumps were stopped, the rock-equilibrated water was removed from the reservoirs and the solution in the fissures was rapidly drawn off. 

The fissures were dried under vacuum at 25°C and were dismantled. The distributions of americium on the fissure surfaces in the second set of experiments were determined as they were in the first set, that is first qualitatively by autoradiography and then quantitatively by gamma scanning the fissure faces through a 0.3 cm slit. 

Autoradiographs of the fissure surfaces from the second set of experiments are presented in Figures 6, 8, and 10. Histograms representing the quantitatively determined distribution of americium on the fissure surfaces are also presented in Figures 6, 8, and 10. 

Model Predictions. The rate for desorption of americium from the fissure surfaces into solution was assumed to equal the rate for the adsorption of americium from solution by the fissure surfaces. The sorption rate and the equilibrium fractionation of americium that were determined in the static experiments were used to determine input parameters to the ARDISC model. The ARDISC model predictions for the distributions of americium on the fissure surfaces in both sets of experiments are presented in Figures 5 through 10 along with the autoradiographs and the experimental histograms representing the various distributions of americium on the fissure surfaces. 

Figure 13. Plot of In fraction of americium in solution vs. time for the data obtained in the static fissure absorption experiments...

During the first experiment it was found that to maintain flow through the basalt fissure it was necessary hold the fissure open by placing two strips of 0.14-mm thick gold ribbon... 

The fissured basalt cores and bentonite were altered by placing them in an autoclave under simulated groundwater at 320°C for 30 and 60 days to simulate 1000 y and 2000 y aging, respectively The waste-form wafers for both experiments were aged in the same manner by treating them for 17 days in saturated steam at 340°C The details of these procedures and the rationale for their use have been published previously (7). The effects of saturated steam on borosilicate glass were discussed in a recent publication (8) ... 

Changes in Analog Groundwater Composition The data in Figure 2 for the first three analog experiments (which used unaltered waste form, bentonite, and basalt cores) show that the groundwater exiting the basalt fissure reaches a steady-state composition for [Na+] and [Ca +] soon after the start of the experiment ... 

An auxiliary experiment, performed at 90°C with ground-water and a column of crushed basalt chips and with a basalt surface area to groundwater volume ratio equivalent to that of the analog basalt fissure (2 x 10 cm" ), has elucidated this phenomenon Data from this experiment, which are presented in Figure 3, show that the sodium-ion concentration of the ground-water does not decrease gradually from its initial value of... 

Table VI shows the concentrations of plutonium, neptunium, and uranium measured at the inlet and outlet of the unaltered and hydrothermally-altered basalt core fissures in the first five analog experiments (see Table I) Under conditions simulating a repository that was unaltered by groundwater interaction (Table I, Exp 1-3), both Np and Pu, in the concentrations developed in these analog experiments from the leaching of the waste form, were substantially retarded within the 14.6-cm basalt fissure In fact, as can be seen from Figure 4, almost all of Np activity was sorbed on the first one-third of the rock fissure The data in Figure 4 have an estimated error, based on counting statistics above, of approximately 2 counts per 1000 seconds Uranium retardation was determined to be not as complete ...

Also, laboratory experiments will be run with rock cores containing naturally-aged fissures. This will be an important step in testing the appropriateness of laboratory aging of rock surfaces to simulate natural aging processes. 

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