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Bentonite as a containment barrier for the disposal of highly radioactive wastes
Applied Clay Science, 4 (1989) 157-177
157
Elsevier SciencePublishersB.V., Amsterdam-- Printed in The Netherlands
B e n t o n i t e as a Containment B a r r i e r for the Disposal of Highly R a d i o a c t i v e Wastes FELIX BUCHER
and M A X
M~LLER-VONMOOS
Institut fi~r Grundbau und Bodenmechanih, ETH-HSnggerberg, CH-8093 Zi~rich (Switzerland) Tonmineralogisches Labor des Institutes fiir Grundbau und Bodenraechanih, ETH-Zentrum, CH-8092 Zi~rich (Switzerland)
(ReceivedNovember28, 1988;acceptedFebruary 10, 1989)
ABSTRACT Bucher, F. and Mfiller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. Appl. Clay Sci.,4: 157-177. The project for the storage of highly radioactive waste in Switzerland is characterized by a system of successive safety barriers.One of these barriersis the highly compacted bentonite backfill.The functions of thisbentonite backfillare described and some investigationsof the important properties as swelling and water uptake are presented. In order for the radioactivematerial to leak out into the biosphere, water from the surrounding rock has to reach the steelcanisters,and these have to be broken through by corrosion. Conservative estimates call for a time of at least 1000 years. After this period the activityof the waste comes mostly from actinides.Breakthrough time for the actinides is estimated at 104-10e years. The long-term stabilityof montmorillonite under elevated temperature and pressure has been examined by the investigation of K-bentonites of Kinnekulle (Sweden).
INTRODUCTION The project "Gew~ihr 1985" (Nagra, 1985 ) was developed by the Nagra (National Cooperative for the Storage of Radioactive Waste) in view of problems arising with the disposal of highly radioactive wastes. This project is here briefly presented, to show where and in which capacities bentonite is to be used as a containment barrier for the disposal of highly radioactive waste. A schematic, three-dimensional representation of a model repository is given in Fig. 1. The main features of the installation are the surface receiving facilities, two vertical shafts, and a system of parallel galleries at a depth of approximately 1,200 m. These galleries, with a diameter of 3.7 m, are bored through a widely undisturbed granite formation, and serve for the storage of the canisters containing the highly radioactive waste material. 0169-1317/89/$03.50
© 1989 Elsevier Science Publishers B.V.
158
Fig. 1. Schematic representation of a final disposal for highly radioactive waste according to project "Gew~ihr" (Nagra, 1985).
The canisters are as a rule installed in regular intervals along the gallery axis, and the remaining space between them is sealed with bentonite. Fig. 2 gives a simplified picture of the installation procedure for a container. Preformed bentonite pieces are fabricated at the surface (in three types, with different geometries ), and are transported to the site. These component pieces are well protected from humidity during all of the handling. The pieces are installed in the gallery with a mechanical arm. First, a base is prepared, upon which the canisters are placed; when these are installed, the remaining bentonite parts are fit into place, and the remaining open space, especially in the crown of the gallery, is filled with a bentonite powder. The storage gallery is
159
Fig. 2. Installation of a container with highly radioactive waste upon pieces of preformed compacted bentonite (Nagra, 1985).
thus gradually filled, and then sealed with a concrete plug where it joins the main gallery. Finally, the main tunnel itself is filled and sealed. The project "Gew~ihr 1985" is characterized by a system of successive safety barriers. This system consists of the glass matrix in which the radionuclides are imbedded, the steel canister that encloses the glass matrix, the bentonite fill material, the host rock, and the sedimentary overburden. The following details pertain solely to the bentonite fill material. THE FUNCTION OF BENTONITE AS A CONTAINMENT BARRIER
The bentonite has many functions, both mechanical and hydraulic, to fulfill as a barrier (Miiller-Vonmoos, 1986). First is the structural function of holding the containers in place and preventing collapse of the excavation. A plastic deformability of the bentonite is desired, however, in order to redistribute the stresses which can occur as a result of creep in the rock, and to prevent excessive stresses to be transferred to the canisters themselves. Secondly, a very high level of watertightness is required, to limit the access of water to the containers, and retard it as much as possible. The containers corrode very slowly
160 and are expected to be fully tight for at least 1,000 years. During this time span, the toxicity of the highly radioactive wastes decreases to 1 percent of its original value. The remaining radioactivity comes mainly from actinides with a long half-life. The very low hydraulic conductivity of the compacted bentonite ensures that the actinides can reach the surrounding rock only by diffusion through the containment layer. This diffusion takes place as a cation exchange into the montmorillonite, a process which is calculated to raise the time needed for the actinides to traverse the bentonite layer to 104 to 106 years. The toxicity of the containers after a span of 104 years corresponds to that of natural uranium ore deposits. The bentonite must retain its swelling potential and its cation exchange capacity over a span of several hundred thousand years, if it is to fulfill its role as a containment barrier. It must be kept in mind that the bentonite is exposed to high pressures due to the overburden, and to high temperatures due to the radiation of the waste which temporarily raises the temperature well over the 50°C naturally occurring in the rock at the planned depth of 1,200 m. Observations on natural formations of swelling clay minerals show that these clays can lose a great part of their innercrystalline swelling potential and of their ion exchange capacity with increasing overburden (i.e. pressure) and temperature. A critical question at hand is therefore the tong-term stability of the bentonite under conditions as are expected in the planned waste repositories.
Fig. 3. Compaction mold for uniaxial compaction.
161
Fig. 4. Decomposed compaction mold with the highly compacted bentonite sample for investigations in the laboratory. HIGHLY COMPACTED BENTONITE
From the geological aspect, most bentonite clays are the products of the weathering of volcanic ashes. They consist mainly of montmorillonite, and are therefore highly swelling clays. Bentonite is used in a highly compacted form for the sealing and filling of radioactive waste deposits. In the laboratory, the compaction is carried out uniaxially, in the specially designed mold shown in Figs. 3 and 4 (Bucher et al., 1982). Pressures of 100-300 kN (1 kN--0.1 t) are necessary for the production of cylindrical laboratory samples with an area of 25 cm 2 and heights of 20-25 mm. Larger preformed pieces, as are planned for the repository design described earlier, can be compacted with isostatic pressures of 500-1,500 bar, and subsequently worked into the desired shapes. Dry densities of up to 2 Mg m -~ can thus be achieved (1 Mg m -3 = lg cm -3). THE SWELLING POTENTIAL OF BENTONITE
An important property of the highly compacted bentonite is its swelling potential. This swelling potential should be as high as possible, to guarantee the sealing of any cracks occurring in the fill material or in the storage gallery, and thus to ensure a good watertightness. Under conditions of restrained volume
162
change and high swelling potentials, however, swelling stresses which could exceed the allowable stress values for the canisters and even for the surrounding rock could be generated. Thus, upper and lower bounds must be set for the swelling potential of the bentonite used. Extensive tests were conducted in the laboratory, in order to gather basic
Fig. 5. Experimental setup used to measure the swelling stress of highly compacted bentonites.
163
Fig. 6. Swellingstress apparatus decomposed. data on the swelling behaviour of highly compacted bentonite (Bucher and Spiegel, 1984). The experimental setup used to measure the swelling stress is shown in Figs. 5 and 6. Fig. 7 shows a cross-section of the apparatus. In the test, water is introduced to the compacted bentonite sample, and the resulting swelling stress is recorded. The accompanying small deformations of the apparatus are also measured. Fig. 8 shows the results of tests conducted on a highly compacted sodium bentonite sample from Wyoming, USA (designation MX-80), over a 270-day period. The initial dry density of the bentonite was 1.92 Mg m -~. Only the temperature was varied during the time of the test; from the initial 20°C, it was raised to 90 ° C after 90 days, and held at that level for the remaining time. The pressure in the water supply was held at 0.7 N mm -2 (1 N m m - 2 = 10 kg cm -2). The swelling stress built up to about 35 N mm -2, and changed little over the duration of the test. Fig. 9 shows a test in which several parameters were varied. The sample in question is a calcium bentonite from Bavaria, West Germany, designated by the name Montigel. The sample was first compacted to a dry density of 1.93 Mg m-a; cores amounting to 13.5% of the volume were then bored out. These open spaces closed up during the test as a result of the swelling of the bentonite, and the dry density thus was reduced to # ~ d = 1.70 Mg m -3. Tests showed that
164
Fig. 7. Cross-section of swelling stress apparatus: 1. bentonite sample, D = 56.4 mm, H = 25 mm; 2. heating device; 3. water supply; 4. pressure transducer; 5. deformation dial gauge.
this reduced density controlled to a large extent the ultimate value of the swelling stress. The swelling stress measured in the laboratory setup is shown as a function of Pred in Fig. 10 for MX-80, and Fig. 11 for Montigel. The values shown were all measured at room temperature and for a water supply pressure of 0.6 N m m -2. Demineralized water was used for these tests. The rapid increase of the swelling stress with increasing dry density is remarkable; this increase is more marked in calcium bentonite than in sodium bentonite. Besides direct, experimental measurement, the possibility also exists to analytically estimate the swelling stress from the water vapor adsorption isotherm (Kahr et al., 1986; Kraehenbuehl et al., 1987). The following relation is adopted: q=
R'T
--In Mw • vw
(P/Po)
165
MX-80
q z,O'
N/rnm2]
Swelling stress
30 20 102'0
I [.~m]
4o
iZo
1~o
2~o
2~o [Doy~]
1~o
1~o
2~o
2~o ;~
i~o
rio
26o ;~ [Days]
rio
2~o :
Deformati on
0'41 0.2
I
io PI [Nlmm2]
z~'Ot 2.0 0.6
16o
do
[ooy~]
Hydrostatic pressure
20
•[oc]
~6o
16o
~o
i~o
Temperature
1501
':t
20
6'o
I
I 16o
i~o
[Days]
Fig. 8. Results of a swelling stress test conducted on highly compacted Na-bentonite MX-80.
where R is the gas constant, T the temperature, Mw the molecular weight of water, Vwthe specific volume of adsorbed water in the clay, and P/Pothe relative water vapor pressure. This relation reduces to: q = - 134 In
(p/po)
where q is expressed in N m m -2. The swelling pressures thus calculated for MX-80 and Montigel are represented in Figs. 10 and 11. If a pressure p is applied to the water supply, an increase of the measured swelling stress takes place, with respect to the swelling stress existing for a water supply pressure of zero. The increase in swelling stress q is equal to the increase in water pressure only for samples with a dry density under 1.40 Mg m -3. The increase is less t h a n proportional for higher densities, and
166
Montigel 20-
10-
~
t [mmj 4
p &IN/ram 2]
32
t [Doys]
~8
3'2
'~o9~]
~'8
3'2
~[Doy~]
Deformation
8
12
16
2'o
~
Hydrostatic pressure
10,0806.040 ~ 200
6
~
2'0 I Eoc] 1501
I007
2'~
Temperature
l
Fig. 9. Results of a swelling stress test conducted on a highly compacted Ca-bentonite Montigel.
amounts to only 60-70% of the applied water pressure for a density of 1.90 Mg m -3 (Fig. 12). This deviation from the standard effective stress relation can not yet be satisfactorily explained. A major role, however, can surely be attributed to the extremely small pore space, and therefore the lack of free pore water in the highly compacted bentonite. Additional tests, on a larger scale, were conducted besides the ones performed in the laboratory, in which the examined samples were only of the order of 200 g mass. A 1: 20 scale model of a 5-m section of the repository gallery was built, and is shown in Fig. 13. Fig. 14 shows a cross-section of the test cylinder, which contains about 12 kg bentonite. The absorption of water, the degree of saturation, and the development of swelling stress were examined more closely in a first test, over a period of 118 days, on a highly compacted sample of MX-80 (without the model canister).
167
q ~[N/ram 2] 60" A meosured 50.
•
colcutoted
,x
40 ¸
30.
M×-80 20 ¸
A 10 ¸
0
1.2
1:4
1:6
1:8
2'.0
2:2
Pr,d
Fig. 10. Measured and calculated swelling stress q as a function of the reduced dry density P,,d for highly compacted Na-bentonite MX-80. Temperature 20 °C, hydrostatic pressure 0.6 N mm-2. q,
, [N,,m2]
60" A meclsured 50-
•
• colcu[oted
40-
30-
Montigel 20-
10-
o
,.2
,:4
,:6
,:8
2'.0
2:2 Pr,d
Fig. 11. Measured and calculated swelling stress q as a function of the reduced dry density Prodfor highly compacted Ca-bentonite Montigel. Temperature 20 ° C, hydrostatic pressure 0.6 N mm-2. T h e water uptake of the b e n t o n i t e across the six filter strips is s h o w n as a f u n c t i o n of time in Fig. 15. A b s o r p t i o n of water is determined by a n o n s t e a d y diffusion process; for the horizontal cylinder, it is described by the following differential equation:
~C
1 ~/
5C~
6t-r~r~rD~r)
168
=qo÷api
q
•
!
° "--.--.i~......, ql."
08~ i
%\
06! i
!
0.4~
[Mg/m~] 02i 3
, 1.4
1'5
1'6
, 17
, ~.8
, 19
2'0
--Pred
Fig. 12. C o e f f i c i e n t a a s a f u n c t i o n o f t h e r e d u c e d d r y d e n s i t y Pred'
F i g . 13. S e t u p for t e s t s o n a 1 : 2 0 - s c a l e m o d e l .
The concentration C, i.e. the water content, for example, is a function of time, the distance r from the center of the cylinder, and the diffusion coefficient D. The solution to this equation is known for a variety of boundary conditions (Crank, 1975 ), and can be used to calculate the water absorption by the bentonite as a function of time. Using the diffusion coefficient (D = 3.5-10-to m 2 s - t ) obtained from earlier work (Kahr et al., 1985), one obtains the water
169
Fig. 14. Longitudinal section of the test cylinder: 1. highly compacted bentonite, D= 185 mm, L = 250 ram; 2. model canister; 3. filter strips; 4. pressure transducer.
Wateruptake wI [g]
1600
1407g f~r 5r : 1.0
1200-
800-
400-
Oq
o
2'0
,~o
~o
8'o
~Go
i~o
t [boys]
Fig. 15. Water uptake of the highly compacted bentonite as a function of time measured in the 1 : 20-scalemodel test and calculated using the theory of diffusion. absorption curve in Fig. 15. This curve is only an approximation since, in the test, water was brought to the sample only through the filter strips, and not over the entire surface of the cylinder. The consequence, which is manifest especially in the early stages of the test, is a retardation of the uptake of water by the sample. The calculation of the quantity of water required for saturation of the sample is based on a specific density of the bentonite of 2.755 Mg m -'~ (Miiller-Vonmoos and Kahr, 1983 ). The development of swelling stress is shown in Fig. 16. The swelling stress, which was measured along the axis of the cylinder shown in Fig. 14, started building up only 30 days after the begin of the test, to reach 15.8 N mm -2 at the end of the test. This value, which was still increasing slightly at the end of
lTt
Swelling stress /
%4
,s
i
~e
/ •
4
10
20
30
4'0
50
6'0
7'0
8'0
9'0
100
11'0
1'20
t [Doy~]
Fig. 16. Development of swelling stress as a function of time in the 1 : 20-scale model test.
[N~mm 2]
q
60t measured 50
•
calculated
40 ) 30MX-80
2O i
i
01---I~
,
14
r -
1:6
118
20
-
212
Pred
Fig. 17. Maximal swelling stress of Na-bentonite determined in the 1 : 20-scate model test marked with a star. For comparison swelling stress values measured by using the swelling stress apparatus are given.
the test, corresponds within the order of magnitude (Fig. 17 ) to what could be expected according to the relation between swelling stress and dry density shown in Fig. 10. T H E R E IS NO POREWATER IN HIGHLY COMPACTED B E N T O N I T E
MX-80 Wyoming bentonite samples tested with an initial dry density of 1.6 Mg m -3 reached a water content of 25% at saturation and generated a swelling stress of about 5 N mm-2. The water taken up contributes to the innercrys-
171 talline swelling; the result is a decrease of the space between the montmorillonite particles, i.e. the pore volume. The pore volume for samples with an initial dry density of 1.6 Mg m -3 is very small, and no pore space remains for initial dry densities above 1.8 Mg m -3. Nearly all the water is in the interlayer space of the montmorillonite. It is bound to the cations, as hydration water, and to the polarized layer surfaces, and it fills the space between the hydrated ions. All of the water is in a more or less ordered state (Kraehenbuehl et al., 1987). There is no free pore water in highly compacted bentonite. The water in the interlayer space of montmorillonite has properties that are quite different from those of free pore water; this explains the extremely high swelling pressures that are generated. The water molecules in the interlayer space are less mobile than their free counterparts, and their dielectric constant is lower. The water and the exchangeable cations in the interlayer space can be compared to a concentrated salt solution. The sodium content of the interlayer water, at a water content of 25%, corresponds approximately to a 3-n salt solution, or six times the concentration in natural seawater. This more or less ordered water is fundamentally different from that which engineers usually take into account; in the latter case, pore water in a saturated soil is considered as a freely flowing fluid. References to the porosity in highly compacted bentonite are therefore misleading. Highly compacted bentonite is an unfamiliar material to the engineer. WATER IN THE SURROUNDINGROCK TAKES ABOUT200 YEARS TO REACH THE CANISTERS In order for the radioactive material to leak out into the biosphere, water from the surrounding rock has to reach the steel canisters, and these have to be broken through by corrosion. Two important questions were therefore how the water is taken up by the compacted bentonite and how it moves within it. The predominant answer sought was how long it takes for the water to travel from the surface of the clay barrier to the canister. Various investigations at the IGB resulted in an estimated time of about 200 years (Kahr et al., 1986). Since the hydraulic conductivity of water is much lower than its diffusivity at a permeability of 10-13 m s-1, travel of water occurs by diffusion. The corrosion of the steel container can thus be assumed to be very slow. Conservative estimates by the Nagra (Nagra, 1985) call for a time of at least 1000 years before a breakthrough of the 25 cm thick steel container that holds the vitrified waste material can occur. BREAKTHROUGHTIME FOR THE ACTINIDESIS 104-106YEARS In this 1000-year period, the activity of the radioactive waste decreases to 1% of the original value. The activity comes mostly from thorium, uranium,
172
plutonium, americum, neptunium, and other actinides. The actinides must diffuse through the 135-cm thick compacted bentonite before reaching the surrounding rock. The diffusivity of the cationic actinides is strongly reduced in the process by the adsorption potential of the montmorillonite. The diffusivity increases hundredfold when the water content is raised from seven percent (about one water layer), to a total of 25% corresponding to four layers of water molecules in the interlayer space of the montmorillonite (Kahr et al., 1985). The diffusivity of anions is much higher than that of cations, since the former are most probably not adsorbed to the montmorillonite surface. Compared to the diffusivity of C1- and I - , the diffusivity of potassium is 3 times lower, that of Sr 2+ 10 times, ofCs + 50 times, ofUO~ + 2,000 times, and that of Th 4÷ 12,000 times lower. The actinides in question therefore diffuse very slowly. The time required for breakthrough of the actinides to the surface of the bentonite barrier is estimated at 104-106 years, and their activity will have greatly decreased during that time. THE LONG-TERM STABILITY OF MONTMORILLONITE UNDER ELEVATED TEMPERATURE AND PRESSURE
The innercrystalline swelling potential and the adsorption potential of the montmorillonite must be maintained over several hundred thousand years, if the compacted bentonite is to fulfill its barrier function. In this regard, one must keep in mind that the bentonite will be exposed to elevated temperatures and pressures at the 1,200 m depth of the repository. The temperature of granite at 1,200 m is about 50°C in the natural state. This temperature is temporarily increased by the heat emissions of the radioactive waste. The innermost layer of bentonite is heated to about 150 ° C after one year. This temperature decreases progressively to 77 °C after 100 years, and to 65 °C after 1,000 years (Fig. 18) (Nagra, 1985). As sediment petrographic examinations have revealed, montmorillonite is changed to illite under increasing overburden, i.e. raised temperatures and pressures. The innercrystalline swelling capacity is lost in the process. The transformation is clear in the examined Mississippi Delta deposits (Fig. 19): mixed layers of montmorillonite and illite, with constantly decreasing proportions of swelling layers, appear with increasing depth. Increase of the layer charge must take place for the change from montmorillonite to illite. This occurs primarily through the isomorphic substitution of aluminum for silicon (Fig. 20). An increase of the charge can also take place as a result of the reduction of trivalent iron to divalent iron in the octahedral sheet. Both bentonites examined (MX-80 from Wyoming and Montigel from Bavaria) contained only very small quantities of pyrite and organic substances, i.e. materials which could, in the oxidation process, reduce the iron in the octahedral sheet (Mtiller-Vonmoos and Kahr, 1983). The increase in layer charge is usually the result of the replacement of silicon by aluminium. The
173 Overburden 12ooi"1 ;te
let
acted bentonite
Fig. 18. Cross-section of the gallery with vitrified waste, steel container and highly compacted bentonite. The temperature of the rock at 1200 m is about 50°C in the natural state. Heating of the innermost bentonite by heat emission of the radioactive waste is also given.
increase in charge has no major direct influence on the innercrystalline swelling capacity; the latter is lost only when potassium enters the interlayer space. The potassium in the pore fluid is selectively taken up into the highly charged interlayer spaces through an ion exchange process. Potassium is a large, monovalent cation; it is surrounded only by a very thin hydration envelope, and it moves relatively freely in the interlayer space. Its high polarity makes it comparable to divalent ions, such as calcium. The potassium ion fits conveniently in the hexagonal holes of the tetrahedral sheet after it is stripped of its hydration envelope. As a result, the distance between layers decreases, the electrostatic attraction of the positively charged potassium ion on neighboring layers increases, and the innercrystalline swelling capacity is strongly reduced or disappears. The question remains, as to the time span necessary for this transformation of montmorillonite into illite in the repository.
174
MISSISSIPP! - DEtTA Chevron- borehoLe Depth 1800}- :,I< MontrnoriL[onite
24°° i30°0 i3600
~: Y
1-
42o0 ~4800 I 5400
~
t
I0
I 20
I
I
30 40
1
i
50
60
I__l 70 80
I
90 100
Ctay minerals [%]
Fig. 19. Alteration of the clay mineralogical composition in the Mississippi Delta deposits according to Weaver and Beck (1971). The montmorillonite content decreases and the illite content increases with increasing depth, i.e. raised temperature and pressure.
charge 4
Si
+16
t_etrahedra[ she \
4 0
I~ \IXI/ \/ \ "
\ IY'I. \/\
/
-10
20H
octahedra[
+ 12
. / i \, I \_ / 1\/1\ / i/",1." \ ,/ b / \ / -etrahedrat s
h
e
e
t
t
I
~
2 OH 4o 4 Si 6 0
TOT- UNIT-CELL 0 0 OH • si • A1
t~;
Surface area: Formuta : Weight :
-10
+16 -12 -44
+44
5 , 1 5 x 8 , 9 A2 [A12(OH)2(Si205)212 720 g
Fig. 20. Unit cell of a TOT-layer silicate. Clay minerals have a negative charge in contradistinction to this representation with balanced charges. The negative charge is raised mainly through the substitution of aluminium for silicon.
175 INVESTIGATIONOF ANALOGOUSSYSTEMS Transformations under elevated temperatures and pressures can be achieved in the laboratory. Significantly higher temperatures than are encountered in nature must be applied, however, in order to reach the states of transformation found in natural deposits that have changed over the course of many millions of years. Furthermore, temperatures below 100 °C applied over millions of years do not lead to the same changes of the montmorillonite structure as when the sample is subjected to temperatures of more than 200 ° C over only several weeks or months. Thermodynamic calculations are not sufficiently exact either, to determine the long-term stability in the repository. The only sensible way to obtain accurate results is to examine ancient bentonite deposits with wellknown pressure-temperature histories. Two such sites have been examined by the IGB up to date, one in Sweden, and the other in the USA. THE KINNEKULLE {SWEDEN) BENTONITES The Kinnekulle bentonites were deposited in the Ordovicium, 450 million years ago. They were then covered by Silurian and Devonian deposits. These later deposits were subsequently carried off, and their thickness is therefore not known. From the high degree of compaction of the bentonites, which were deposited in horizontal layers, it can be inferred that the former overlying layers, and especially the Devonian, must have been very massive. A diabase intrusion then occurred, in the Permian, 280 million years ago. The base of this intrusion lies about 90 m over the examined bentonite layers. Various Swedish investigations show that the bentonite layers were probably heated for several hundred years, by the diabase intrusion, to temperatures in the range of 100 ° C. The increase in pressure and temperature due to the thick, overlying Silurian and Devonian formations is not known. Investigations by Brusewitz (1986) and by Velde and Brusewitz (1982) suggest that the potassium for the illitization of the montmorillonite diffused into the bentonite from the surrounding rock. The Kinnekulle bentonites are therefore a good system on which to base future predictions, because: (1) the temperature in the bentonite was temporarily raised by the diabase intrusion; (2) the potassium was taken up from the surrounding rock by diffusion. An increase in temperature, caused by the radiation from the canisters, also takes place in the repository. This warming, however, will be less marked than that caused by the diabase intrusion. Furthermore, the potassium must also predominantly diffuse into the bentonite from the porewater in the surrounding rock. Samples were taken from different depths in a 2-m thick bentonite layer and in a 10-cm thick band located about 1 m over the layer (Miiller-Vonmoos and Kahr, 1985). The clay minerals isolated from these samples were almost ex-
176 TABLEI Cation exchange capacity of the montmorillonite-illitemixed layers, the fixed interlayer potassium, the interlayer cations, the charge per half unit cell, and the interlayer potassium given as a percentage of the interlayer cations in the 10 cm thick bentonite band A:~ and in the 2 m thick bentonite layer B with increasing depth Sample: A~ (10cm) Cation exchange capacity (meq./100 g) Fixed interlayer potassium (meq./100 g) Interlayer cations (meq./100 g) Charge per half unit cell Interlayer potassium as percentage of interlayer cations
B (0-5cm)
B (9-30cm)
B (45-60cm)
B (ll0cm)
50.4
59.4
72.1
72.0
75.5
103.6
80.7
73.3
72.0
57.8
112.1 0.415
116.3 0.430
115.2 0.426
106.7 0.395
123.2 0.451 84
72
63
62
54
clusively mixed layers of montmorillonite-illite. Table I shows the exchange capacity of the montmorillonite-illite mixed layers, the fixed interlayer potassium, the interlayer cations, the charge per half unit cell, and the interlayer potassium given as a percentage of the interlayer cations. The interlayer cations were calculated by subtracting 20% off the sum of the cation exchange capacity and the interlayer potassium, to account for exchange at both outer surfaces and at the edges. The exchange capacity increases with the depth in the thicker bentonite layer B, and the fixed potassium decreases accordingly. X-ray and thermal analyses show that illitic layers were formed especially in the top 10 cm. The amount of potassium as a proportion of the interlayer cations is 54% in the center of the thick layer, and 72% in the upper 5 cm. This shows that the portion of illitic layers increased from the middle to the edge of the thick bentonite bed, and the swelling potential therefore decreased. The charge only slightly increased, and can still be referred to as smectitic. A typical montmorillonite has about 0.30 charges per half unit cell, while the charge on illite ranges from 0.60 to 0.90. The low exchange capacity and the high potassium content of the interlayer cations in the 10-cm thick A2 band are attributed to the relatively small distance across which the potassium needs to diffuse. The incorporation of potassium through diffusion into the layer was the key step in the illitization of the compacted Kinnekulle bentonites. As the example shows, this was a very slow process. THE TRANSFORMATION INTO THE POTASSIUM FORM IN THE REPOSITORY
Water flow in the granite in which the repository is planned is very small. It is estimated at 4 litres per year per metre of gallery. Potassium content of the
177 water is 1.20 meq per litre. Considering a dry density at installation of 1.7 Mg m -3 and an exchange capacity of 76 meq./lO0 g Wyoming bentonite, and assuming an immediate and complete uptake of the potassium carried by the groundwater, the fixed potassium would amount to a third of the interlayer cations after one million years. Such a bentonite would still have a considerable swelling capacity.
REFERENCES Brusewitz, A.M., 1986. Chemical and physicalpropertiesof paleozoicpotassium bentonites from Kinnekulle, Sweden. Clays Clay Miner., 34: 442-454. Bucher, F. and Spiegel,U., 1984. Quelldrnck von hochverdichtetenBentoniten. Nagra Tech. Ber., 84-18. Bucher, F.,Jeger,P.,Kahr, G. and Lehner, J. 1982. Herstellungund Homogenitiit hochverdichteter Bentonitproben. Nagra Tech. Ber.,82-05. Crank, J., 1975, The Mathematics of Diffusion.Clarendon Press, Oxford, 2nd ed. Kahr, G., Hasenpatt, R. and Mtiller-Vonmoos,M., 1985.Ionendiffusionin hochverdichtetem Bentonit.Nagra Tech. Ber.,85-23. Kahr, G., Kraehenbuehl, F.,Mtiller-Vonmoos, M. and Stoeckli,H.F., 1986. Wasseraufnahme und Wasserbewegung in hochverdichtetem Bentonit. Nagra Tech. Ber.,86-14. Kraehenbuehl, F., Stoeckli,H.F., Brunner, F., Kahr, G. and Mtiller-Vonmoos, M., 1987. Study of the water-bentonite system by vapour adsorption, immersion calorimetry and X-ray techniques,1.Micropore volumes and internalsurfaceareas,followingDubinin'stheory.Clay Miner., 22: 1-9. Miiller-Vonmoos, M., 1986. Bentonit und radioaktive Abfalle.Forschung und Technik, Neue ZiircherZeitung, 1-10-1986. Miiller-Vonmoos, M. and Kahr, G., 1983. Mineralogische Untersuchungen yon Wyoming Bentonit M X - 8 0 und Montigel. Nagra Tech. Bet.,83, 12. Mtiller-Vonmoos, M. and Kahr, G., 1985. Langzeitstabilitiityon Bentonit unter Endlagerbedingungen. Nagra Tech. Bet.,85-25. Nagra, 1985. Projekt "Gewtihr 1985". Endlager fiirhochaktive Abf~ille:das System der Sicherheitsbarrieren.NGB, 85-04. Velde, B. and Brusewitz, A.M., 1982. Metasomatic and non-metasomatic low-grade metamorphism of Ordovician meta-bentonites in Sweden. Geochim. Cosmochim. Acta, 46: 447-452. Weaver, C.E. and Beck, K.C., 1971. Clay-water diagenesisduring burial;how mud becomes gneiss. Geol. Soc. Am. Spec. Pap., 134, 96 pp.
157
Elsevier SciencePublishersB.V., Amsterdam-- Printed in The Netherlands
B e n t o n i t e as a Containment B a r r i e r for the Disposal of Highly R a d i o a c t i v e Wastes FELIX BUCHER
and M A X
M~LLER-VONMOOS
Institut fi~r Grundbau und Bodenmechanih, ETH-HSnggerberg, CH-8093 Zi~rich (Switzerland) Tonmineralogisches Labor des Institutes fiir Grundbau und Bodenraechanih, ETH-Zentrum, CH-8092 Zi~rich (Switzerland)
(ReceivedNovember28, 1988;acceptedFebruary 10, 1989)
ABSTRACT Bucher, F. and Mfiller-Vonmoos, M., 1989. Bentonite as a containment barrier for the disposal of highly radioactive wastes. Appl. Clay Sci.,4: 157-177. The project for the storage of highly radioactive waste in Switzerland is characterized by a system of successive safety barriers.One of these barriersis the highly compacted bentonite backfill.The functions of thisbentonite backfillare described and some investigationsof the important properties as swelling and water uptake are presented. In order for the radioactivematerial to leak out into the biosphere, water from the surrounding rock has to reach the steelcanisters,and these have to be broken through by corrosion. Conservative estimates call for a time of at least 1000 years. After this period the activityof the waste comes mostly from actinides.Breakthrough time for the actinides is estimated at 104-10e years. The long-term stabilityof montmorillonite under elevated temperature and pressure has been examined by the investigation of K-bentonites of Kinnekulle (Sweden).
INTRODUCTION The project "Gew~ihr 1985" (Nagra, 1985 ) was developed by the Nagra (National Cooperative for the Storage of Radioactive Waste) in view of problems arising with the disposal of highly radioactive wastes. This project is here briefly presented, to show where and in which capacities bentonite is to be used as a containment barrier for the disposal of highly radioactive waste. A schematic, three-dimensional representation of a model repository is given in Fig. 1. The main features of the installation are the surface receiving facilities, two vertical shafts, and a system of parallel galleries at a depth of approximately 1,200 m. These galleries, with a diameter of 3.7 m, are bored through a widely undisturbed granite formation, and serve for the storage of the canisters containing the highly radioactive waste material. 0169-1317/89/$03.50
© 1989 Elsevier Science Publishers B.V.
158
Fig. 1. Schematic representation of a final disposal for highly radioactive waste according to project "Gew~ihr" (Nagra, 1985).
The canisters are as a rule installed in regular intervals along the gallery axis, and the remaining space between them is sealed with bentonite. Fig. 2 gives a simplified picture of the installation procedure for a container. Preformed bentonite pieces are fabricated at the surface (in three types, with different geometries ), and are transported to the site. These component pieces are well protected from humidity during all of the handling. The pieces are installed in the gallery with a mechanical arm. First, a base is prepared, upon which the canisters are placed; when these are installed, the remaining bentonite parts are fit into place, and the remaining open space, especially in the crown of the gallery, is filled with a bentonite powder. The storage gallery is
159
Fig. 2. Installation of a container with highly radioactive waste upon pieces of preformed compacted bentonite (Nagra, 1985).
thus gradually filled, and then sealed with a concrete plug where it joins the main gallery. Finally, the main tunnel itself is filled and sealed. The project "Gew~ihr 1985" is characterized by a system of successive safety barriers. This system consists of the glass matrix in which the radionuclides are imbedded, the steel canister that encloses the glass matrix, the bentonite fill material, the host rock, and the sedimentary overburden. The following details pertain solely to the bentonite fill material. THE FUNCTION OF BENTONITE AS A CONTAINMENT BARRIER
The bentonite has many functions, both mechanical and hydraulic, to fulfill as a barrier (Miiller-Vonmoos, 1986). First is the structural function of holding the containers in place and preventing collapse of the excavation. A plastic deformability of the bentonite is desired, however, in order to redistribute the stresses which can occur as a result of creep in the rock, and to prevent excessive stresses to be transferred to the canisters themselves. Secondly, a very high level of watertightness is required, to limit the access of water to the containers, and retard it as much as possible. The containers corrode very slowly
160 and are expected to be fully tight for at least 1,000 years. During this time span, the toxicity of the highly radioactive wastes decreases to 1 percent of its original value. The remaining radioactivity comes mainly from actinides with a long half-life. The very low hydraulic conductivity of the compacted bentonite ensures that the actinides can reach the surrounding rock only by diffusion through the containment layer. This diffusion takes place as a cation exchange into the montmorillonite, a process which is calculated to raise the time needed for the actinides to traverse the bentonite layer to 104 to 106 years. The toxicity of the containers after a span of 104 years corresponds to that of natural uranium ore deposits. The bentonite must retain its swelling potential and its cation exchange capacity over a span of several hundred thousand years, if it is to fulfill its role as a containment barrier. It must be kept in mind that the bentonite is exposed to high pressures due to the overburden, and to high temperatures due to the radiation of the waste which temporarily raises the temperature well over the 50°C naturally occurring in the rock at the planned depth of 1,200 m. Observations on natural formations of swelling clay minerals show that these clays can lose a great part of their innercrystalline swelling potential and of their ion exchange capacity with increasing overburden (i.e. pressure) and temperature. A critical question at hand is therefore the tong-term stability of the bentonite under conditions as are expected in the planned waste repositories.
Fig. 3. Compaction mold for uniaxial compaction.
161
Fig. 4. Decomposed compaction mold with the highly compacted bentonite sample for investigations in the laboratory. HIGHLY COMPACTED BENTONITE
From the geological aspect, most bentonite clays are the products of the weathering of volcanic ashes. They consist mainly of montmorillonite, and are therefore highly swelling clays. Bentonite is used in a highly compacted form for the sealing and filling of radioactive waste deposits. In the laboratory, the compaction is carried out uniaxially, in the specially designed mold shown in Figs. 3 and 4 (Bucher et al., 1982). Pressures of 100-300 kN (1 kN--0.1 t) are necessary for the production of cylindrical laboratory samples with an area of 25 cm 2 and heights of 20-25 mm. Larger preformed pieces, as are planned for the repository design described earlier, can be compacted with isostatic pressures of 500-1,500 bar, and subsequently worked into the desired shapes. Dry densities of up to 2 Mg m -~ can thus be achieved (1 Mg m -3 = lg cm -3). THE SWELLING POTENTIAL OF BENTONITE
An important property of the highly compacted bentonite is its swelling potential. This swelling potential should be as high as possible, to guarantee the sealing of any cracks occurring in the fill material or in the storage gallery, and thus to ensure a good watertightness. Under conditions of restrained volume
162
change and high swelling potentials, however, swelling stresses which could exceed the allowable stress values for the canisters and even for the surrounding rock could be generated. Thus, upper and lower bounds must be set for the swelling potential of the bentonite used. Extensive tests were conducted in the laboratory, in order to gather basic
Fig. 5. Experimental setup used to measure the swelling stress of highly compacted bentonites.
163
Fig. 6. Swellingstress apparatus decomposed. data on the swelling behaviour of highly compacted bentonite (Bucher and Spiegel, 1984). The experimental setup used to measure the swelling stress is shown in Figs. 5 and 6. Fig. 7 shows a cross-section of the apparatus. In the test, water is introduced to the compacted bentonite sample, and the resulting swelling stress is recorded. The accompanying small deformations of the apparatus are also measured. Fig. 8 shows the results of tests conducted on a highly compacted sodium bentonite sample from Wyoming, USA (designation MX-80), over a 270-day period. The initial dry density of the bentonite was 1.92 Mg m -~. Only the temperature was varied during the time of the test; from the initial 20°C, it was raised to 90 ° C after 90 days, and held at that level for the remaining time. The pressure in the water supply was held at 0.7 N mm -2 (1 N m m - 2 = 10 kg cm -2). The swelling stress built up to about 35 N mm -2, and changed little over the duration of the test. Fig. 9 shows a test in which several parameters were varied. The sample in question is a calcium bentonite from Bavaria, West Germany, designated by the name Montigel. The sample was first compacted to a dry density of 1.93 Mg m-a; cores amounting to 13.5% of the volume were then bored out. These open spaces closed up during the test as a result of the swelling of the bentonite, and the dry density thus was reduced to # ~ d = 1.70 Mg m -3. Tests showed that
164
Fig. 7. Cross-section of swelling stress apparatus: 1. bentonite sample, D = 56.4 mm, H = 25 mm; 2. heating device; 3. water supply; 4. pressure transducer; 5. deformation dial gauge.
this reduced density controlled to a large extent the ultimate value of the swelling stress. The swelling stress measured in the laboratory setup is shown as a function of Pred in Fig. 10 for MX-80, and Fig. 11 for Montigel. The values shown were all measured at room temperature and for a water supply pressure of 0.6 N m m -2. Demineralized water was used for these tests. The rapid increase of the swelling stress with increasing dry density is remarkable; this increase is more marked in calcium bentonite than in sodium bentonite. Besides direct, experimental measurement, the possibility also exists to analytically estimate the swelling stress from the water vapor adsorption isotherm (Kahr et al., 1986; Kraehenbuehl et al., 1987). The following relation is adopted: q=
R'T
--In Mw • vw
(P/Po)
165
MX-80
q z,O'
N/rnm2]
Swelling stress
30 20 102'0
I [.~m]
4o
iZo
1~o
2~o
2~o [Doy~]
1~o
1~o
2~o
2~o ;~
i~o
rio
26o ;~ [Days]
rio
2~o :
Deformati on
0'41 0.2
I
io PI [Nlmm2]
z~'Ot 2.0 0.6
16o
do
[ooy~]
Hydrostatic pressure
20
•[oc]
~6o
16o
~o
i~o
Temperature
1501
':t
20
6'o
I
I 16o
i~o
[Days]
Fig. 8. Results of a swelling stress test conducted on highly compacted Na-bentonite MX-80.
where R is the gas constant, T the temperature, Mw the molecular weight of water, Vwthe specific volume of adsorbed water in the clay, and P/Pothe relative water vapor pressure. This relation reduces to: q = - 134 In
(p/po)
where q is expressed in N m m -2. The swelling pressures thus calculated for MX-80 and Montigel are represented in Figs. 10 and 11. If a pressure p is applied to the water supply, an increase of the measured swelling stress takes place, with respect to the swelling stress existing for a water supply pressure of zero. The increase in swelling stress q is equal to the increase in water pressure only for samples with a dry density under 1.40 Mg m -3. The increase is less t h a n proportional for higher densities, and
166
Montigel 20-
10-
~
t [mmj 4
p &IN/ram 2]
32
t [Doys]
~8
3'2
'~o9~]
~'8
3'2
~[Doy~]
Deformation
8
12
16
2'o
~
Hydrostatic pressure
10,0806.040 ~ 200
6
~
2'0 I Eoc] 1501
I007
2'~
Temperature
l
Fig. 9. Results of a swelling stress test conducted on a highly compacted Ca-bentonite Montigel.
amounts to only 60-70% of the applied water pressure for a density of 1.90 Mg m -3 (Fig. 12). This deviation from the standard effective stress relation can not yet be satisfactorily explained. A major role, however, can surely be attributed to the extremely small pore space, and therefore the lack of free pore water in the highly compacted bentonite. Additional tests, on a larger scale, were conducted besides the ones performed in the laboratory, in which the examined samples were only of the order of 200 g mass. A 1: 20 scale model of a 5-m section of the repository gallery was built, and is shown in Fig. 13. Fig. 14 shows a cross-section of the test cylinder, which contains about 12 kg bentonite. The absorption of water, the degree of saturation, and the development of swelling stress were examined more closely in a first test, over a period of 118 days, on a highly compacted sample of MX-80 (without the model canister).
167
q ~[N/ram 2] 60" A meosured 50.
•
colcutoted
,x
40 ¸
30.
M×-80 20 ¸
A 10 ¸
0
1.2
1:4
1:6
1:8
2'.0
2:2
Pr,d
Fig. 10. Measured and calculated swelling stress q as a function of the reduced dry density P,,d for highly compacted Na-bentonite MX-80. Temperature 20 °C, hydrostatic pressure 0.6 N mm-2. q,
, [N,,m2]
60" A meclsured 50-
•
• colcu[oted
40-
30-
Montigel 20-
10-
o
,.2
,:4
,:6
,:8
2'.0
2:2 Pr,d
Fig. 11. Measured and calculated swelling stress q as a function of the reduced dry density Prodfor highly compacted Ca-bentonite Montigel. Temperature 20 ° C, hydrostatic pressure 0.6 N mm-2. T h e water uptake of the b e n t o n i t e across the six filter strips is s h o w n as a f u n c t i o n of time in Fig. 15. A b s o r p t i o n of water is determined by a n o n s t e a d y diffusion process; for the horizontal cylinder, it is described by the following differential equation:
~C
1 ~/
5C~
6t-r~r~rD~r)
168
=qo÷api
q
•
!
° "--.--.i~......, ql."
08~ i
%\
06! i
!
0.4~
[Mg/m~] 02i 3
, 1.4
1'5
1'6
, 17
, ~.8
, 19
2'0
--Pred
Fig. 12. C o e f f i c i e n t a a s a f u n c t i o n o f t h e r e d u c e d d r y d e n s i t y Pred'
F i g . 13. S e t u p for t e s t s o n a 1 : 2 0 - s c a l e m o d e l .
The concentration C, i.e. the water content, for example, is a function of time, the distance r from the center of the cylinder, and the diffusion coefficient D. The solution to this equation is known for a variety of boundary conditions (Crank, 1975 ), and can be used to calculate the water absorption by the bentonite as a function of time. Using the diffusion coefficient (D = 3.5-10-to m 2 s - t ) obtained from earlier work (Kahr et al., 1985), one obtains the water
169
Fig. 14. Longitudinal section of the test cylinder: 1. highly compacted bentonite, D= 185 mm, L = 250 ram; 2. model canister; 3. filter strips; 4. pressure transducer.
Wateruptake wI [g]
1600
1407g f~r 5r : 1.0
1200-
800-
400-
Oq
o
2'0
,~o
~o
8'o
~Go
i~o
t [boys]
Fig. 15. Water uptake of the highly compacted bentonite as a function of time measured in the 1 : 20-scalemodel test and calculated using the theory of diffusion. absorption curve in Fig. 15. This curve is only an approximation since, in the test, water was brought to the sample only through the filter strips, and not over the entire surface of the cylinder. The consequence, which is manifest especially in the early stages of the test, is a retardation of the uptake of water by the sample. The calculation of the quantity of water required for saturation of the sample is based on a specific density of the bentonite of 2.755 Mg m -'~ (Miiller-Vonmoos and Kahr, 1983 ). The development of swelling stress is shown in Fig. 16. The swelling stress, which was measured along the axis of the cylinder shown in Fig. 14, started building up only 30 days after the begin of the test, to reach 15.8 N mm -2 at the end of the test. This value, which was still increasing slightly at the end of
lTt
Swelling stress /
%4
,s
i
~e
/ •
4
10
20
30
4'0
50
6'0
7'0
8'0
9'0
100
11'0
1'20
t [Doy~]
Fig. 16. Development of swelling stress as a function of time in the 1 : 20-scale model test.
[N~mm 2]
q
60t measured 50
•
calculated
40 ) 30MX-80
2O i
i
01---I~
,
14
r -
1:6
118
20
-
212
Pred
Fig. 17. Maximal swelling stress of Na-bentonite determined in the 1 : 20-scate model test marked with a star. For comparison swelling stress values measured by using the swelling stress apparatus are given.
the test, corresponds within the order of magnitude (Fig. 17 ) to what could be expected according to the relation between swelling stress and dry density shown in Fig. 10. T H E R E IS NO POREWATER IN HIGHLY COMPACTED B E N T O N I T E
MX-80 Wyoming bentonite samples tested with an initial dry density of 1.6 Mg m -3 reached a water content of 25% at saturation and generated a swelling stress of about 5 N mm-2. The water taken up contributes to the innercrys-
171 talline swelling; the result is a decrease of the space between the montmorillonite particles, i.e. the pore volume. The pore volume for samples with an initial dry density of 1.6 Mg m -3 is very small, and no pore space remains for initial dry densities above 1.8 Mg m -3. Nearly all the water is in the interlayer space of the montmorillonite. It is bound to the cations, as hydration water, and to the polarized layer surfaces, and it fills the space between the hydrated ions. All of the water is in a more or less ordered state (Kraehenbuehl et al., 1987). There is no free pore water in highly compacted bentonite. The water in the interlayer space of montmorillonite has properties that are quite different from those of free pore water; this explains the extremely high swelling pressures that are generated. The water molecules in the interlayer space are less mobile than their free counterparts, and their dielectric constant is lower. The water and the exchangeable cations in the interlayer space can be compared to a concentrated salt solution. The sodium content of the interlayer water, at a water content of 25%, corresponds approximately to a 3-n salt solution, or six times the concentration in natural seawater. This more or less ordered water is fundamentally different from that which engineers usually take into account; in the latter case, pore water in a saturated soil is considered as a freely flowing fluid. References to the porosity in highly compacted bentonite are therefore misleading. Highly compacted bentonite is an unfamiliar material to the engineer. WATER IN THE SURROUNDINGROCK TAKES ABOUT200 YEARS TO REACH THE CANISTERS In order for the radioactive material to leak out into the biosphere, water from the surrounding rock has to reach the steel canisters, and these have to be broken through by corrosion. Two important questions were therefore how the water is taken up by the compacted bentonite and how it moves within it. The predominant answer sought was how long it takes for the water to travel from the surface of the clay barrier to the canister. Various investigations at the IGB resulted in an estimated time of about 200 years (Kahr et al., 1986). Since the hydraulic conductivity of water is much lower than its diffusivity at a permeability of 10-13 m s-1, travel of water occurs by diffusion. The corrosion of the steel container can thus be assumed to be very slow. Conservative estimates by the Nagra (Nagra, 1985) call for a time of at least 1000 years before a breakthrough of the 25 cm thick steel container that holds the vitrified waste material can occur. BREAKTHROUGHTIME FOR THE ACTINIDESIS 104-106YEARS In this 1000-year period, the activity of the radioactive waste decreases to 1% of the original value. The activity comes mostly from thorium, uranium,
172
plutonium, americum, neptunium, and other actinides. The actinides must diffuse through the 135-cm thick compacted bentonite before reaching the surrounding rock. The diffusivity of the cationic actinides is strongly reduced in the process by the adsorption potential of the montmorillonite. The diffusivity increases hundredfold when the water content is raised from seven percent (about one water layer), to a total of 25% corresponding to four layers of water molecules in the interlayer space of the montmorillonite (Kahr et al., 1985). The diffusivity of anions is much higher than that of cations, since the former are most probably not adsorbed to the montmorillonite surface. Compared to the diffusivity of C1- and I - , the diffusivity of potassium is 3 times lower, that of Sr 2+ 10 times, ofCs + 50 times, ofUO~ + 2,000 times, and that of Th 4÷ 12,000 times lower. The actinides in question therefore diffuse very slowly. The time required for breakthrough of the actinides to the surface of the bentonite barrier is estimated at 104-106 years, and their activity will have greatly decreased during that time. THE LONG-TERM STABILITY OF MONTMORILLONITE UNDER ELEVATED TEMPERATURE AND PRESSURE
The innercrystalline swelling potential and the adsorption potential of the montmorillonite must be maintained over several hundred thousand years, if the compacted bentonite is to fulfill its barrier function. In this regard, one must keep in mind that the bentonite will be exposed to elevated temperatures and pressures at the 1,200 m depth of the repository. The temperature of granite at 1,200 m is about 50°C in the natural state. This temperature is temporarily increased by the heat emissions of the radioactive waste. The innermost layer of bentonite is heated to about 150 ° C after one year. This temperature decreases progressively to 77 °C after 100 years, and to 65 °C after 1,000 years (Fig. 18) (Nagra, 1985). As sediment petrographic examinations have revealed, montmorillonite is changed to illite under increasing overburden, i.e. raised temperatures and pressures. The innercrystalline swelling capacity is lost in the process. The transformation is clear in the examined Mississippi Delta deposits (Fig. 19): mixed layers of montmorillonite and illite, with constantly decreasing proportions of swelling layers, appear with increasing depth. Increase of the layer charge must take place for the change from montmorillonite to illite. This occurs primarily through the isomorphic substitution of aluminum for silicon (Fig. 20). An increase of the charge can also take place as a result of the reduction of trivalent iron to divalent iron in the octahedral sheet. Both bentonites examined (MX-80 from Wyoming and Montigel from Bavaria) contained only very small quantities of pyrite and organic substances, i.e. materials which could, in the oxidation process, reduce the iron in the octahedral sheet (Mtiller-Vonmoos and Kahr, 1983). The increase in layer charge is usually the result of the replacement of silicon by aluminium. The
173 Overburden 12ooi"1 ;te
let
acted bentonite
Fig. 18. Cross-section of the gallery with vitrified waste, steel container and highly compacted bentonite. The temperature of the rock at 1200 m is about 50°C in the natural state. Heating of the innermost bentonite by heat emission of the radioactive waste is also given.
increase in charge has no major direct influence on the innercrystalline swelling capacity; the latter is lost only when potassium enters the interlayer space. The potassium in the pore fluid is selectively taken up into the highly charged interlayer spaces through an ion exchange process. Potassium is a large, monovalent cation; it is surrounded only by a very thin hydration envelope, and it moves relatively freely in the interlayer space. Its high polarity makes it comparable to divalent ions, such as calcium. The potassium ion fits conveniently in the hexagonal holes of the tetrahedral sheet after it is stripped of its hydration envelope. As a result, the distance between layers decreases, the electrostatic attraction of the positively charged potassium ion on neighboring layers increases, and the innercrystalline swelling capacity is strongly reduced or disappears. The question remains, as to the time span necessary for this transformation of montmorillonite into illite in the repository.
174
MISSISSIPP! - DEtTA Chevron- borehoLe Depth 1800}- :,I< MontrnoriL[onite
24°° i30°0 i3600
~: Y
1-
42o0 ~4800 I 5400
~
t
I0
I 20
I
I
30 40
1
i
50
60
I__l 70 80
I
90 100
Ctay minerals [%]
Fig. 19. Alteration of the clay mineralogical composition in the Mississippi Delta deposits according to Weaver and Beck (1971). The montmorillonite content decreases and the illite content increases with increasing depth, i.e. raised temperature and pressure.
charge 4
Si
+16
t_etrahedra[ she \
4 0
I~ \IXI/ \/ \ "
\ IY'I. \/\
/
-10
20H
octahedra[
+ 12
. / i \, I \_ / 1\/1\ / i/",1." \ ,/ b / \ / -etrahedrat s
h
e
e
t
t
I
~
2 OH 4o 4 Si 6 0
TOT- UNIT-CELL 0 0 OH • si • A1
t~;
Surface area: Formuta : Weight :
-10
+16 -12 -44
+44
5 , 1 5 x 8 , 9 A2 [A12(OH)2(Si205)212 720 g
Fig. 20. Unit cell of a TOT-layer silicate. Clay minerals have a negative charge in contradistinction to this representation with balanced charges. The negative charge is raised mainly through the substitution of aluminium for silicon.
175 INVESTIGATIONOF ANALOGOUSSYSTEMS Transformations under elevated temperatures and pressures can be achieved in the laboratory. Significantly higher temperatures than are encountered in nature must be applied, however, in order to reach the states of transformation found in natural deposits that have changed over the course of many millions of years. Furthermore, temperatures below 100 °C applied over millions of years do not lead to the same changes of the montmorillonite structure as when the sample is subjected to temperatures of more than 200 ° C over only several weeks or months. Thermodynamic calculations are not sufficiently exact either, to determine the long-term stability in the repository. The only sensible way to obtain accurate results is to examine ancient bentonite deposits with wellknown pressure-temperature histories. Two such sites have been examined by the IGB up to date, one in Sweden, and the other in the USA. THE KINNEKULLE {SWEDEN) BENTONITES The Kinnekulle bentonites were deposited in the Ordovicium, 450 million years ago. They were then covered by Silurian and Devonian deposits. These later deposits were subsequently carried off, and their thickness is therefore not known. From the high degree of compaction of the bentonites, which were deposited in horizontal layers, it can be inferred that the former overlying layers, and especially the Devonian, must have been very massive. A diabase intrusion then occurred, in the Permian, 280 million years ago. The base of this intrusion lies about 90 m over the examined bentonite layers. Various Swedish investigations show that the bentonite layers were probably heated for several hundred years, by the diabase intrusion, to temperatures in the range of 100 ° C. The increase in pressure and temperature due to the thick, overlying Silurian and Devonian formations is not known. Investigations by Brusewitz (1986) and by Velde and Brusewitz (1982) suggest that the potassium for the illitization of the montmorillonite diffused into the bentonite from the surrounding rock. The Kinnekulle bentonites are therefore a good system on which to base future predictions, because: (1) the temperature in the bentonite was temporarily raised by the diabase intrusion; (2) the potassium was taken up from the surrounding rock by diffusion. An increase in temperature, caused by the radiation from the canisters, also takes place in the repository. This warming, however, will be less marked than that caused by the diabase intrusion. Furthermore, the potassium must also predominantly diffuse into the bentonite from the porewater in the surrounding rock. Samples were taken from different depths in a 2-m thick bentonite layer and in a 10-cm thick band located about 1 m over the layer (Miiller-Vonmoos and Kahr, 1985). The clay minerals isolated from these samples were almost ex-
176 TABLEI Cation exchange capacity of the montmorillonite-illitemixed layers, the fixed interlayer potassium, the interlayer cations, the charge per half unit cell, and the interlayer potassium given as a percentage of the interlayer cations in the 10 cm thick bentonite band A:~ and in the 2 m thick bentonite layer B with increasing depth Sample: A~ (10cm) Cation exchange capacity (meq./100 g) Fixed interlayer potassium (meq./100 g) Interlayer cations (meq./100 g) Charge per half unit cell Interlayer potassium as percentage of interlayer cations
B (0-5cm)
B (9-30cm)
B (45-60cm)
B (ll0cm)
50.4
59.4
72.1
72.0
75.5
103.6
80.7
73.3
72.0
57.8
112.1 0.415
116.3 0.430
115.2 0.426
106.7 0.395
123.2 0.451 84
72
63
62
54
clusively mixed layers of montmorillonite-illite. Table I shows the exchange capacity of the montmorillonite-illite mixed layers, the fixed interlayer potassium, the interlayer cations, the charge per half unit cell, and the interlayer potassium given as a percentage of the interlayer cations. The interlayer cations were calculated by subtracting 20% off the sum of the cation exchange capacity and the interlayer potassium, to account for exchange at both outer surfaces and at the edges. The exchange capacity increases with the depth in the thicker bentonite layer B, and the fixed potassium decreases accordingly. X-ray and thermal analyses show that illitic layers were formed especially in the top 10 cm. The amount of potassium as a proportion of the interlayer cations is 54% in the center of the thick layer, and 72% in the upper 5 cm. This shows that the portion of illitic layers increased from the middle to the edge of the thick bentonite bed, and the swelling potential therefore decreased. The charge only slightly increased, and can still be referred to as smectitic. A typical montmorillonite has about 0.30 charges per half unit cell, while the charge on illite ranges from 0.60 to 0.90. The low exchange capacity and the high potassium content of the interlayer cations in the 10-cm thick A2 band are attributed to the relatively small distance across which the potassium needs to diffuse. The incorporation of potassium through diffusion into the layer was the key step in the illitization of the compacted Kinnekulle bentonites. As the example shows, this was a very slow process. THE TRANSFORMATION INTO THE POTASSIUM FORM IN THE REPOSITORY
Water flow in the granite in which the repository is planned is very small. It is estimated at 4 litres per year per metre of gallery. Potassium content of the
177 water is 1.20 meq per litre. Considering a dry density at installation of 1.7 Mg m -3 and an exchange capacity of 76 meq./lO0 g Wyoming bentonite, and assuming an immediate and complete uptake of the potassium carried by the groundwater, the fixed potassium would amount to a third of the interlayer cations after one million years. Such a bentonite would still have a considerable swelling capacity.
REFERENCES Brusewitz, A.M., 1986. Chemical and physicalpropertiesof paleozoicpotassium bentonites from Kinnekulle, Sweden. Clays Clay Miner., 34: 442-454. Bucher, F. and Spiegel,U., 1984. Quelldrnck von hochverdichtetenBentoniten. Nagra Tech. Ber., 84-18. Bucher, F.,Jeger,P.,Kahr, G. and Lehner, J. 1982. Herstellungund Homogenitiit hochverdichteter Bentonitproben. Nagra Tech. Ber.,82-05. Crank, J., 1975, The Mathematics of Diffusion.Clarendon Press, Oxford, 2nd ed. Kahr, G., Hasenpatt, R. and Mtiller-Vonmoos,M., 1985.Ionendiffusionin hochverdichtetem Bentonit.Nagra Tech. Ber.,85-23. Kahr, G., Kraehenbuehl, F.,Mtiller-Vonmoos, M. and Stoeckli,H.F., 1986. Wasseraufnahme und Wasserbewegung in hochverdichtetem Bentonit. Nagra Tech. Ber.,86-14. Kraehenbuehl, F., Stoeckli,H.F., Brunner, F., Kahr, G. and Mtiller-Vonmoos, M., 1987. Study of the water-bentonite system by vapour adsorption, immersion calorimetry and X-ray techniques,1.Micropore volumes and internalsurfaceareas,followingDubinin'stheory.Clay Miner., 22: 1-9. Miiller-Vonmoos, M., 1986. Bentonit und radioaktive Abfalle.Forschung und Technik, Neue ZiircherZeitung, 1-10-1986. Miiller-Vonmoos, M. and Kahr, G., 1983. Mineralogische Untersuchungen yon Wyoming Bentonit M X - 8 0 und Montigel. Nagra Tech. Bet.,83, 12. Mtiller-Vonmoos, M. and Kahr, G., 1985. Langzeitstabilitiityon Bentonit unter Endlagerbedingungen. Nagra Tech. Bet.,85-25. Nagra, 1985. Projekt "Gewtihr 1985". Endlager fiirhochaktive Abf~ille:das System der Sicherheitsbarrieren.NGB, 85-04. Velde, B. and Brusewitz, A.M., 1982. Metasomatic and non-metasomatic low-grade metamorphism of Ordovician meta-bentonites in Sweden. Geochim. Cosmochim. Acta, 46: 447-452. Weaver, C.E. and Beck, K.C., 1971. Clay-water diagenesisduring burial;how mud becomes gneiss. Geol. Soc. Am. Spec. Pap., 134, 96 pp.