RETRACTED: Superior techniques for disposal of highly radioactive waste (HLW)

Progress in Nuclear Energy 59 (2012) 75e85

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Superior techniques for disposal of highly radioactive waste (HLW) Roland Pusch a, *, Richard Weston b a b

Drawrite AB, Luleå Technical University, St Södergatan 57A, 22223 Lund, Sweden Division of Production and Mechanical Engineering, University of Lund, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 October 2011 Accepted 22 January 2012

The Swedish Nuclear Fuel and Waste Company (SKB) has recently worked out a concept, KBS-3V, for disposal of highly radioactive waste in the form of spent reactor fuel, and asked for the Government’s approval and licensing. It implies blasting of tunnels at about 400 m depth and boring of large-diameter canister deposition holes extending vertically from the tunnel floor. The rock stresses will be critically high in the construction phase and lead to failure by spalling when the heat pulse from the canisters evolves. The canisters will be surrounded by dense expansive “buffer” clay for minimizing groundwater flow around and along them but the long-term performance of either of them is not adequately proven and the placement is impractical and risky. Four major changes of the concept would make it satisfactory. One involves reorientation of the deposition holes from vertical to 45
inclination in two directions for reducing the risk of rock failure. A second is to prepare ready-made stiff units of “supercontainers” with highly compacted blocks of clay tightly surrounding the canisters for simpler and safer installation of clay blocks and canisters. A third is to surround the supercontainers by clay mud that provides the dense buffer with water from start and supports the surrounding rock when the thermal pulse begins to raise the rock stresses. A fourth is to replace the proposed smectite-rich buffer by clay with higher chemical stability and lower but sufficient expandability. A possible fifth change can be to manufacture homogeneous copper canisters of HIPOW type, which would radically reduce the risk of contamination of groundwater by released radionuclides. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Clay Canisters Disposal Radioactive waste

1. SKB’s concept KBS-3V for disposal of HLW 1.1. General The need for safe disposal of high-level nuclear waste (HLW) has been in focus of the International Atomic Energy Agency for decades and of a number of national authorities. Various concepts have been proposed for deep deposition in salt, argillaceous rock and crystalline rock but no repository has yet been constructed (Svemar, 2005). The interest of nearly all countries that consider HLW disposal in crystalline rock, exemplified by China, Korea, Czech Republic, Japan, Finland, Korea, Ukraine, and India, is focused on the principal design proposed by the Swedish Nuclear Fuel and Waste Management (SKB) represented by its concept KBS-3V. Other concept types of multibarrier disposal are illustrated in Fig. 1 but they have not been worked out to nearly the same degree of detailing and accuracy as SKB’s version KBS-3V and are not official concepts. SKB, which is owned by power-producing private

* Corresponding author. Tel.: þ46 462111406. E-mail address: [email protected] (R. Pusch). 0149-1970/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.pnucene.2012.01.005

companies, the state and communities that utilize nuclear energy, began to develop techniques for safe disposal of high-level nuclear waste (HLW) in the form of spent reactor fuel already back in the sixties. The goal has been to work out a multibarrier concept for isolating HLW from the biosphere for at least 100,000 years. An early political decision implies that no processing shall be made of the spent fuel, which shall instead be placed under water for 40 years intermediate storage in order to reduce the radioactivity and generated heat and then be encapsulated for placement in a repository at 400e500 m depth in rock. Crystalline rock will be used and the repository site has recently been decided to be in the Forsmark area north of Stockholm, where very tight granitic rock has been found. It serves as a barrier by providing mechanical protection and delaying migration of possibly released radionuclides. The waste itself is also taken as barrier because of its low solubility. The major engineered barriers are the copper-shielded iron canisters and the surrounding dense “buffer” clay that surrounds them. It provides a tight ductile embedment that evens out mechanical stresses in the canisters that can be generated by seismic events. The concept worked out has been refined in various ways through the years to yield the present version, which is shown in Fig. 2.

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Backfilled drift Fracture

Deposition hole with canister surrounded by ”buffer” of dense smectite-rich clay

Canister

Fig. 2. Section and view of a deposition hole according to the original SKB’s KBS-3V concept. The dimensions have undergone several changes through the years and the diameter of the hole is presently 1900 mm. The bentonite buffer consists of big monolithic clay blocks of high density. The rock fracture symbolizes the major threat to the integrity of the canisters.

diameter canister deposition holes are bored to about 8 m depth from the floor of them with a spacing of 6e8 m depending on the thermal properties of the rock. SKB does not make use of any categorization scheme like those used by the sister organizations ENRESA (Spain) and NAGRA (Switzerland) and can therefore not refer to specific rock structural features of value for engineers in describing suitable and unsuitable positions of the deposition holes (cf. Brady and Brown, 1985). Using only the unconfined compressive strength, 150e350 MPa, as practical parameter for assessing hole positions, application of statistical methods has shown that only a few percent of them have to be avoided in the site selection process since the theoretical maximum hoop stress, calculated by use of conventional rock mechanics, does not exceed 120 MPa for the primary rock stresses 20 and 40 MPa in the horizontal plane and 10 MPa vertically, assuming best possible direction of the tunnels. When the heat pulse comes from the radioactive decay the hoop stress will rise, however, and breakage occur in a large number of deposition holes, causing fracturing and fissuring. The porosity and hydraulic conductivity will thereby increase very significantly. 1.3. The engineered barriers Fig. 1. Concepts proposed for disposal of HLW. The vertical hole with several canisters has three versions with one, two or several canisters.

The scientific and technical evolution of the basis of the KBS-3V concept is extensive and has resulted from experiments on laboratory, bench-scale and full-scale in rock in international cooperation (Svemar, 2005). Much of the design, including selection of rock excavation methods and performance modelling, is based on non-proven construction techniques and use of invalid numerical codes, largely disregarding long-term thermodynamic impact on the buffer clay. The present paper illustrates these shortcomings and aims at providing a safer basis of developing concepts of the discussed type.

Canisters of iron lined with copper represent the most important engineered barrier and will not leak radionuclides as long as they are intact. The liner is a copper tube with 1.05 m outer diameter1 and 50 mm thickness and a length of about 4.84 m, surrounding an insert of cast iron with channels for 12 fuel bundles of BWR fuel2 (Fig. 3). The manufacturing involves sealing of the insert in an environment of 90% noble gas, which limits the water content per canister to 600 g. The gap between the iron insert and the copper lining is a couple of millimeters wide and will be closed by the water pressure and the swelling pressure exerted by the surrounding clay except near the ends where the solid lids prevent radial compression. Since the repository will be located at 400e500 m depth the water pressure will be 4e5 MPa. The swelling pressure of the expandable clay can be higher than that.

1.2. The rock SKB’s concept implies that 20e25 m2 tunnels are blasted at about 400 m depth a few tens of meters apart and that 1.9 m

1 2

The diameter has been changed by a small amount in recent years. The number and type of fuel bundles can vary.

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Fig. 3. The canister. Schematic picture of the present design (Svemar, 2005).

The canisters are surrounded by blocks of highly compacted smectite-rich clay. The mineral grains resemble mica particles, consisting of stacks of thin lamellae held together by cations like potassium, magnesium and calcium. The difference is that the smectite crystal lattices have charge deficiencies that allow uptake of cations and water molecules in the interlamellar space, which makes them expand (Pusch and Yong, 2006). The very strong hydrophilic potential means that a large fraction of the porewater is bonded to the minerals and has a high viscosity. The fineness of the particles makes the channels formed by the voids very narrow and tortuous. The hydraulic conductivity is therefore very low and diffusion the major ion-transport mechanism. The tunnel backfill presently proposed by SKB consists of piled smaller clay blocks and clay grains (“pellets”) blown to fill the space between the blocks and the rock. The clay material is proposed to be the same as in the buffer blocks. 1.4. Installation of engineered barriers The order of installation is that most of the buffer blocks are placed first and then the canisters in the hollow space left after placing the lower, monolithic blocks and the stack of annular blocks. The 30e50 mm gap that remains between the blocks and the rock is planned to be filled with air-dry granular smectite-rich clay. It is essential to prevent water from the wet tunnel floor and fractures intersecting the holes to enter the space to be filled with granules since this would make the filling heterogeneous. A plastic sheet will therefore be lowered in the space and tightly connected to the bottom plate of concrete on which the buffer blocks are stacked. It will be removed in the course of the filling of granules

but if it gets stuck the rate and distribution of the water saturation of the granular fill and dense buffer will be affected. 1.4.1. Buffer clay Placement of the first set of buffer blocks in a deposition hole is tightly scheduled to fit the procedure of installing the tunnel backfill. Filling of the 6e8 m length of a deposition tunnel from the latest installed canister is scheduled to take one day during which the placement of buffer blocks with surrounding granular fill and installation of the canister in the deposition hole next to the advancing backfill front have to be completed. The transport and handling of the blocks are a logistical problem implying risks of delay that have become obvious at testing the technique on full scale. Fig. 4 shows how a block can be lowered into a deposition hole under non-radiation conditions. In a repository, placement of the uppermost blocks must be made remotely because of the gamma radiation from the canisters, and there is an obvious risk of dropping them or of getting them stuck before they are on site. Vacuum technique is possible but involves even greater risk. One of the risks associated with handling of the buffer blocks is caused by the inherent weaknesses in the blocks resulting from the compaction technique, i.e. uniaxial compression of clay granules. Detailed inspection of blocks prepared under 100e150 MPa pressure has shown presence of numerous fine fractures that can be explained by use of numerical calculation technique. It has confirmed the existence of overstressing of the upper edge of the large blocks as shown in Fig. 5. The weaknesses can cause loss of fragments in the placement phase. Isostatic compaction using HIP technique does not give such fracturing as demonstrated by the quality control of the clay products (big electric insulators) or the

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base of the hole is not perfectly horizontal. This causes the stack of big clay blocks to tilt, implying, in turn, a risk that the canister touches the blocks when placed and that fragments of clay can fall and hinder placement. The small space between canister and blocks, only 10 mm according to the design (Svemar, 2005), makes this risk obvious and the moistening and expansion of the blocks in the prevailing humid environment will cause further reduction of this space. 1.5. Function of engineered barriers

Fig. 4. Buffer block with 1.85 m diameter and 0.4 m height (2000 kg) ready for installation in a deposition hole in SKB’s test site at Äspö (Svemar, 2005).

numerous blocks manufactured for the international large-scale test of buffer and backfill in the Stripa mine (Pusch et al., 1985; Pusch, 1994). 1.4.2. Canister Placement of the 24,600 kg canisters in the vertical deposition holes has to be made remotely by use of equipment that can transport and put the canisters in the hollow space of annular clay blocks and cover it by placing additional blocks on top (Fig. 6). For bringing the canisters down ramps in the rock are required as shown in the figure. They will generate very high local stresses and risk for spalling and related hindrance in the placement phase. A major problem is that there is no room for delay or mishap. Thus, a stop for one or a few days would lead to significant inflow of water in the tunnels and softening of the placed backfill because of the successively raised water pressure in the surrounding rock. One realizes that the proposed way of installing clay, canisters and backfill is not rational and involves risks for delay causing serious problems especially if retrieval of blocks and gamma-radiating canisters is required. Among the problems that will appear already when the first canister is to be installed in the stack of buffer blocks, is that the

1.5.1. Canister The most important barrier to radioactive contamination of the biosphere is the canister. As long as it is tight radionuclides cannot escape but if it leaks they will migrate into the surrounding clay buffer and rock and soon continue to the ground surface via fracture zones in the rock. Leakage also implies that water enters the canisters, producing highly pressurized vapour that migrates into and through the buffer, creating channels that serve as paths for diffusive transport of radionuclides. The manufacturing involves sealing of the iron insert in an environment of 90% noble gas, which limits the water content per canister to 600 g. The gap between the insert and the copper lining is a couple of millimeters and will be closed by the water pressure and the swelling pressure exerted by the surrounding clay except near the ends where the lids prevent radial compression. Since the repository will be located at 400e500 m depth the water pressure will be 4e5 MPa and the clay pressure can be even higher than that. Two mechanisms can threaten the integrity of the copper/iron canister: corrosion, and mechanical strain. SKB has deemed corrosion to be a non-problem for the first 100,000 years while mechanical strain is estimated to be critical if it takes the form of shearing of a rock fracture that intersects the deposition hole, as indicated in Fig. 2, by more than about 50 mm (Svemar, 2005). The structural constitution of the rock is hardly such that instantaneous shearing by more than a few millimeters can be generated by even severe earth shocks and risks related to tectonics can be ruled out in practice (Svemar, 2005; Pusch et al., in press). However, tension of the copper liner will take place in the way indicated in Fig. 7, which illustrates that very high swelling pressure of the buffer causes significant upward displacement of the tunnel backfill, the larger

Fig. 5. Large strain of the upper edge of blocks identified by numerical stress/strain calculation of the compressed block (After L. Börgesson, Clay Technology AB).

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Canister being placed in deposition hole

EDZ

Gap between buffer blocks and rock filled with smectite pellets according to SKB’s concepth KBS-3V

Blocks of highly compacted smectite clay

Fig. 6. Placement of blocks of highly compacted clay and canister in deposition holes. EDZ represents excavation damage caused by the tunnel blasting.

part of which hydrates and mobilizes counter-pressure much slower than the buffer. The tight grip of the buffer on the lower part of the canister keeps it fixed while the upper end is pulled up, generating high tension stresses in the uppermost part. An extension by 0.25 m may take place causing linear strain by about 5%, which can be sustained by the liner but cause cavern corrosion and fissuring (Sandström and Wu, 2007). Failure associated with cavitation occurs even down to 75
C, demonstrating the sensitivity of copper/iron canisters of SKB’s type. Fissures and holes, illustrated in Fig. 8 represent weaknesses that can develop and yield breakage. 1.5.2. Buffer A number of experimental studies have demonstrated the excellent tightness and expandability of dense smectite-rich buffer under isothermal conditions and temperatures well below 90e100
C, while exposure to temperature gradients and temperature of this and higher orders initiates early degradation.

Upward movement of expanding buffer and compression of the tunnel backfill

Buffer clay Canister

Fig. 7. Axial tension of canister by upward expansion of the buffer clay at its upper part in conjunction with compression of the tunnel backfill. The lower part of the canister is firmly held by the clay.

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There is natural evidence of various geological ages showing that heating to about 100
C for some hundred years does not totally degrade the buffer but reduces the amount smectite and causes significant to substantial loss of expandability and tightness (Pusch, 2008). The examples show that stiffening had taken place by silicification or precipitation of other cementing agents, like Fecompounds (Pusch et al., 2010). A recently reported study comprising determination of the composition and properties of three hydrothermally treated expandable clays, two smectite-rich (MX-80, GMZ) and one mixed-layer illite/smectite (FIM3) clay, has revealed the mechanisms involved in the alteration under repository-like conditions (Xiaodong et al., in press). Samples were prepared by compaction of air-dry powder under the same pressure (1.30 MPa) in cells, followed by heating the cells at one end to 85e95
C and circulating 3.5% CaCl2 solution through a filter at the opposite end, which was held at a constant temperature of about 50
C for 3 weeks. After termination of the hydrothermal tests the samples were water saturated at room temperature and then sectioned for determining the hydraulic conductivity, swelling pressure and compressibility of the least and most heated parts and for investigation of micro-structural and mineralogical changes. The results showed that while the mixed-layer clay was not noticeably affected by the hydrothermal treatment, the smectites had lost a significant part of the swelling pressure and experienced an increase in hydraulic conductivity and stiffness. The most obvious chemical and mineralogical changes were an increase in silica content in the coldest part and precipitation of silicious minerals, gypsum and kaolinite in the most heated parts (Fig. 9, Table 1). The silicification is believed to have caused stiffening (Xiaodong et al., in press). Table 2 summarizes the results from the determinations of the hydraulic conductivity and swelling pressure for the hot and cold end specimens at their respective densities after saturation and percolation with 3.5% CaCl2 solution for 14 days, including also data for the virgin clays of corresponding densities. It demonstrates an important impact on the density distribution in axial direction: the “hot” samples are denser than the “cold” ones since the latter were early saturated with water and consolidated the hot ones which could not expand when they finally got saturated because of the reduction in expandability caused by the hydrothermal treatment. The low density of GMZ was caused by the very low compressibility of the granular powder in the sample preparation phase. As to the swelling pressure one finds that it was very low, 0.11e0.13 MPa, for the “hot” GMZ and MX-80, which had similar original smectite contents. For the “cold” MX-80 it was on the same order of magnitude as for “virgin” MX-80 (1.2 MPa) with a density of 1844 kg/m3. The higher swelling pressure of the “hot” FIM than of the “cold” FIM is exclusively explained by the difference in density. One concludes that while the “hot” part of both GMZ and MX-80 had lost a significant fraction of the original expandability, the “cold” parts were nearly unaffected. The difference in evaluated hydraulic conductivity between the “hot” and “cold” parts was nearly none for all the clays but the significant difference in density shows that the “hot” parts had become more permeable, except for FIM, for which the reduced conductivity of the “hot” part was due to the higher density of this part. In summary, FIM was largely unaffected by the hydrothermal treatment, which may be due its homogeneous microstructure with few collapsible gel components (Pusch, 2008). Uniaxial loading tests were made of the hot-end samples and of samples of virgin clays prepared with approximately the same

3 Friedland clay, a hugh deposit of mixed-layer illite/smectite/chlorite clay in north-eastern Germany.

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Fig. 8. Examples of intergranular creep cavities and cracks identified by scanning electron microscopy (SEM). Width of micrographs about 75 mm (Sandström and Wu, 2007).

density as the hot-end samples. For relevant comparison the virgin samples were treated by percolation with 3.5% CaCl2 for one week and then permeated with distilled water for two weeks. Table 3 summarizes the stress/strain data from the rheological study from which one concludes that the hydrothermally treated clays were all stiffer than the virgin clay samples and exhibited brittle behaviour at failure. The virgin clays were ductile and cohered until failure was reached. Summing up the major chemical and physical changes that one can expect for buffer heated up to about 100
C in a repository, the change in content of smectite and expandable part of the mixedlayer minerals would be very limited, while there will be an increase in hydraulic conductivity and stiffness, especially of the most heated part. Since these changes have been found in shortterm experiments it is required that THMC experiments of long

duration be performed for working out a valid geochemical model to be used in predicting the evolution of chemical and physical properties during and after the long time of geological disposal, i.e. 100,000 years (cf. Pusch, 2008). This means that the repository host rock will be exposed to one or several glaciations implying temperature reduction to around zero centigrades and percolation with low-electrolyte meltwater when the glaciers retreat. Dense smectite clay will not freeze as demonstrated by laboratory tests (Yong et al., 2010).

1.5.3. Backfill The backfill in the tunnels will not be exposed to higher temperature than about 50
C and will not freeze at glaciation (Yong et al., 2010).

Fig. 9. Examples of precipitated salt in the hot parts of the clays (Xiaodong et al., in press). a: The SEM electron image of the MX-80 Hot End sample indicates formation of minute flat gypsum crystals precipitated on the surface of montmorillonite crystals; b: The EMPA secondary electron image of the MX-80 Hot End sample shows aggregated (Gyp) crystals precipitated in a void between of montmorillonite particles; c: GMZ Hot End, very fine silica particles precipitated on smectite (montmorillonite); d: GMZ Hot End, hexagonal kaolinite crystal aggregations precipitated on smectite (montmorillonite).

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Table 1 Chemical composition of the clay samples by EPMA Analyses (weight %). C ¼ cold end, H ¼ hot end, V ¼ virgin. Sample

SiO2

Al2O3

Fe2O3 (FeO)

CaO

MgO

K2O

Na2O

MnO

TiO2

Total

GMZ, C GMZ, H GMZ, V MX-80, C MX-80, H MX-80, V FIM, C FIM, H FIM, V

71.16 68.44 68.40 63.58 60.63 61.69 57.18 58.70 57.59

15.63 19.58 21.13 21.75 20.09 22.36 27.12 26.98 27.89

2.18 1.90 1.41 3.29 2.77 3.50 6.56 4.71 4.70

2.35 1.97 0.54 1.31 0.89 0.31 0.60 0.75 0.16

4.10 4.80 4.97 2.57 3.53 2.29 2.30 1.88 2.25

0.18 0.10 0.02 0.05 0.06 0.20 3.87 2.43 2.86

0.91 1.03 1.01 0.66 0.77 0.68 0.46 0.53 0.40

0.01 0.05 0.01 0.02 0.03 0.02 0.01 0.01 0.01

0.06 0.00 0.02 0.14 0.10 0.10 0.14 0.31 0.28

97.18 98.37 97.40 94.15 90.76 91.14 98.55 97.33 96.20

1.6. Conclusive remarks concerning the KBS-3V repository concept The major obstacles respecting the function are:  fracturing of the deposition holes implies quick transport of possibly released radionuclides from the canisters to and along the deposition holes to the permeable EDZ and further out in fracture zones to the biosphere,  the copper/iron canisters can break under tension,  stiffening of montmorillonite-rich buffer reduces its selfsealing potential. Only the outer part will perform as planned with respect to permeability and expandability. The part exposed to higher temperature will be significantly stiffer and more permeable than virgin clay, which must be taken into account in the safety analysis,  placement of the buffer blocks and surrounding granular fill as well of the canisters will cause severe problems or be impossible if there are unplanned stops in the backfill placement. 2. A superior concept, termed KBS-3i, for disposal of HLW 2.1. General Maintaining the major engineering barriers of KBS-3V a number of features can be improved, firstly the geometry for reaching better rock stability but also the canister design and the selection and placement of buffer. 2.2. Canister The copper/iron canister’s main weaknesses are the potential of breakage by tension, and that local (pitting) through-corrosion will expose the entire waste content to water and hence make unlimited release of radionuclides possible. The risk of leakage of composed canisters was realized in the evolution of SKB’s canister design and led to a concept implying manufacturing of a solid copper body homogeneously encapsulating spent fuel. Even very deep pitting corrosion would not cause exposure of more than a very small fraction of the fuel to water, namely where corrosion happens to hit

Table 2 Hydraulic conductivity (K)a, swelling pressure (ps) and density (r ¼ density at water saturation, rd) for GMZ, MX-80 and FIM after the hydrothermal experiment and subsequent saturation with 3.5% CaCl2 solution. Property

r/rd

(kg/m3) K, (m/s) ps (MPa) a

GMZ

MX-80

FIM

Hot end

Cold end

Hot end

Cold end

Hot end

Cold end

1871/ 1232 2.6E-11 0.11

1788/ 1233 2.8E-11 0.53

1951/ 1375 1.2E-11 0.13

1844/ 1310 2.0E-11 1.14

1958/ 1412 1.8E-11 0.43

1875/ 1392 4.0E-11 0.28

Hydraulic gradient 30e50.

a fuel element. A technique for producing canisters of this type was suggested in 1976 by the Swedish company ASEA Atom AB, implying application of hot isostatic compression, termed HIPOW (Lönnerberg et al., 1983). The idea was to place the fuel bundles in their zirconium claddings in a copper tube in the compression chamber of a QuintusR-type press with the fuel kept in constant position in the tube while pouring copper powder into it, followed by heating the object to 600
C under an isostatic pressure of 100 MPa. Half-scale tests by ASEA in the early eighties demonstrated the feasibility of the HIPOW process for encapsulation of spent nuclear fuel as illustrated by Fig. 10. The porosity of the copper was as low or even lower than that of cast copper. The copper column was only 1.6 m long and 1 m in diameter because of the limited size of the press but production of canisters with the same length as ordinary KBS-3 canisters seems possible (Burström, 2011). For getting good metallic bonding between the copper powder grains, which is needed for achieving a non-brittle, high-strength product, surface oxide coatings of the copper grains must be removed before compaction. This can be made by reduction of the powder in hydrogen or some other gas at 250
C. Alternatively, the whole process of preparing a canister by compacting copper powder can be made in inert gas environment although this has not yet been demonstrated. The tensile strength of HIPOW canisters is at least 100 MPa. In addition to the totally eliminated risk of exposure of the confined spent fuel to water for hundreds of thousands of years or more, the 100% copper canisters have the advantage of being stronger than the copper/iron canisters but still relatively ductile. Fractures generated by extreme deformation would not expose more than an insignificant fraction of the waste to the surroundings. A further advantage, that has to do with the need for nearly perfect straightness for bringing the canisters down in deposition holes with good fitting, is that it is easily achieved by light lathing of HIPOW canisters, while copper/iron canisters are not straight and cannot be straightened. Admittedly, there are practical difficulties in manufacturing HIPOW canisters because of the severe radioactivity conditions but reasonably extensive R&D is expected to overcome them (Burström, 2011).

Table 3 Densities and heights of the 30 mm diameter samples used for determining the stress/strain properties. Clay

Height, mm

Dry density, kg/m3

Pressure at failure, kPa

Average compression at failure, %

GMZ Hot End GMZ Virgin MX-80 Hot End MX-80 Virgin FIM Hot End FIM Virgin

6.0 7.3 11.3 9.7 9.3 8.3

1232 1330 1375 1360 1412 1400

142 142 575 >650 142 284

1.5 6.7 3.2 6.4 2.0 3.5

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Fig. 11. Schematic picture of the two cases, vertical hole KBS-3V to the left and inclined hole KBS-3i to the right.

implies significantly safer conditions with respect to the risk of spalling since the Mises stress is more than 20% lower than for KBS-3V. Fig. 10. Disc sawn from a 1.6 m long column of a HIPOW canister made by compressing copper grains in a copper tube. The spent fuel, simulated by steel pellets in the experiment, is the black spots in the copper mass. The original boundary between the copper tube and the copper matrix is not detectable (Photo: SKB).

2.3. Rock stability The critically high hoop stresses in the walls of the deposition holes is the greatest problem for the rock and a geometry that reduces the hoop stresses is required for making the concept trustworthy. We will deal here with “case C” in Fig. 1 illustrating various constellations of canisters in deposition holes and tunnels. It implies that the deposition holes are inclined in two directions, sideways and in the axial direction of the tunnels. The concept, which is termed KBS-3i here, has a number of advantages compared to the vertically oriented holes represented by the KBS3V concept:  the average rock stress is considerably lower than for KBS-3V,  maintaining the distance between the upper ends of the holes as 6e8 m reduces the overlap of the local temperature fields and thereby the hoop stress of the holes,  the inclined deposition holes make the ramps at the deposition holes of KBS-3V unnecessary and provide simpler placement of buffer and canisters. The more moderate stress levels in the rock surrounding inclined deposition holes than vertical ones of the same size have been identified by FEM technique. The two concepts are illustrated in Fig. 11. Von Mises stresses at three levels in each hole (1 m from upper end, at mid-height, and at the bottom, respectively) are shown in Figs. 12 and 13. Fig. 12 shows Von Mises stresses for the KBS-3V hole with 1.9 m diameter in rock with the primary rock stresses 40, 25 and 10 MPa. The calculation was made by use of ANSYS Finite element code taking the modulus of elasticity to be E4 MPa and Poisson’s ratio as 0.3. Fig. 13 shows the corresponding Von Mises stress values for the KBS-3i hole. Comparison of the stress levels for the two cases shows that while the Von Mises stress in all parts of the KBS-3V hole exceeds 60 MPa locally, the maximum Von Mises stress of the KBS-3i hole is significantly lower and nowhere reaches up to 60 MPa. The latter concept

2.4. Buffer material The most important property of the buffer is to be less hydraulically conductive than the rock surrounding the deposition holes. The boring-induced EDZ means that the conductivity of the rock within about 10 mm from the holes is about E-10 m/s, which hence calls for a lower conductivity value for the buffer (Pusch, 2008). This criterion is fulfilled by clays with a high degree of expandable minerals and a density at water saturation higher than 1900 kg/m3 (Pusch and Yong, 2006). This density is also high enough to make it impossible for microbes to multiply and migrate in the buffer, which is claimed to be a threat to its waste-isolating function (Pusch, 1994). A minimum density of this order is required for preventing the canisters to sink in the buffer while an upper limit of 2050 kg/m3 has been set for not causing damage to the canister and rock by exposing them to a too high swelling pressure (<7 MPa). Smectite-rich clay in virgin form fulfils these requirements like the North-American MX-80 from Wyoming/ South Dakota. It is being processed and delivered by American Colloid Co and has been the buffer material favoured by SKB since the start of the development of the concept for disposal of HLW. Comparable clay materials are presently considered. The problem is that neither the hydraulic conductivity nor the expandability, which is needed for maintaining tight contact between the rock and the buffer and between the buffer and the canisters, are maintained after the several hundred years long hydrothermal period This is demonstrated by the investigation of the GMZ and FIM clays mentioned earlier in the report, and by studies of other smectite-rich candidate buffer clays (Kolaríková et al., 2010; Herbert et al., 2008; Xiaodong et al., in press). It is obvious from these studies that smectites with beidellite as major clay mineral species are unsuitable and that those dominated by saponite, which is rich in magnesium, are chemically more stable. Less expandable clays like the smectite-poor mixed-layer FIM clay also appear to be more stable (Pusch, 2008). A recently identified possible degradation process is that smectite clay can migrate out from the buffer into open fractures in the surrounding rock and be eroded and carried out into the rock by flowing groundwater. This effect can only be practically important after a major glaciation cycle when electrolyte-poor meltwater can possibly percolate the repository rock and give the buffer

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Fig. 12. Von Mises stress at the top and mid of the vertical hole. Upper left: 1 m below the top of the hole. Upper right: mid-height. Lower: bottom of hole. The range is from 10 MPa (blue) to 60 MPa (Red). The grey areas indicate higher stress levels than 60 MPa. Highest stresses are near the tunnel floor (72.3 MPa), second highest at mid height of the holes (68.3 MPa), and lowest at their bottoms (62.1 MPa). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. Von Mises stress for the KBS-3i hole tilted as shown in Fig. 9. Upper left: 1 m below the top of the hole. Upper right: mid-height. Lower: bottom of hole. The range is from 10 MPa (blue) to 60 MPa (Red). The grey areas indicate higher stress levels than 60 MPa. Highest stress is near the tunnel floor (53.2 MPa), second highest at mid height of the holes (50.1 MPa), and lowest at their bottoms (39.3 MPa). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 4 Major properties of buffer candidates in virgin form, saturated with 3.5% CaCl2 porewater (Pusch, 2008). Dominant clay mineral

Density, kg/m3

Hydraulic conductivitya, m/s

Swelling pressure, MPa

Potential to undergo loss in density by migration into fractures

Montmorillonite Montmorillonite Montmorillonite Saponite Saponite Mixed-layer (FIM) Mixed-layer (FIM) Mixed-layer (FIM)

2000 1900 1800 2000 1800 2100 2000 1900

2E-13 5E-12 5E-11 5E-13 E-12 E-12 2E-11 1.5E-10

4.7 3.0 1.0 8.8 2.5 2.0 1.0 0.3

Very substantial Very high High Medium high Low Low Low Low

a

Determined by oedometer tests with hydraulic gradients lower than 50.

Mechanical packer removed in the backfilling phase, allowing slight axial displacement

Tunnel floor

Supercontainer of perforated tube of navy bronze with clay blocks and canister

Temporary fill Blocks of highly compacted blocks of smectite-rich clay in the supercontainer Mud is pressed up along the supercontainer when it is placed in the hole

Mud filled before submerging the supercontainers Canister

Fig. 14. Schematic picture of placing a supercontainer in smectitic mud, which is pressed up along the container when it submerges.

maximum expansion and dispersion potential. The spontaneous and comprehensive dispersion of clay particles that is a prerequisite of the phenomenon to take place is strongest for montmorillonite-rich smectite buffer since its expansion is stronger and the internal friction angle lower than for smectitepoor clays like the FIM clay. The results from the various tests and performance analyses are compiled in Table 4 for ranking of the major expandable clay material candidates. For reaching the required minimum swelling pressure of 1.0 MPa the density of montmorillonitic buffer at water saturation needs to be at least 1900 kg/m3, while for saponite it should not be higher than 1800 kg/m3. For the mixed-layer (FIM) clay the density should be at least 2050 kg/m3, all the densities being achievable by uniaxial or isostatic compaction of sufficiently dry granular clay material. The note that montmorillonite buffer has the highest potential to undergo dispersion makes it more sensitive to erosion and loss. Using Table 4 as a basis of ranking the clays and considering also the chemical stability, which determines the risk of stiffening by precipitation of dissolved mineral particles, the most suitable buffer clay material is deemed to be saponite as indicated by the THMC experiments referred to and concluded from the experience of using saponite drilling muds in deep boreholes (Pusch et al., in press). The second best is the mixed-layer FIM clay, while montmorillonite-rich

clay, represented by MX-80 type clays, has the lowest ranking. The differences are small, however, and the content of accessory constituents, like sulphur-bearing minerals and organic matter, is an important factor in selecting a suitable buffer material. They are all available in nearly unlimited quantities on most continents. 2.5. Installation of engineered barriers While installation of buffer blocks and canisters in the vertical deposition holes according to the KBS-3V concept is a complicated and tedious procedure that can go completely wrong if it is not perfectly synchronal with the tunnel backfilling operation or hindered by unforeseen events, like rock fall from the highly stressed rock around the deposition holes, the placement of containers with buffer blocks and canisters is believed to be simple and quick. The container technique, which was proposed already in the nineties in conjunction with working out a concept for the Spanish organization ENRESA4, involves manufacturing of perforated containers under or above ground in the repository area and transporting them at a rate that is adaptated to the rate of

4 The concept was outlined by the former Man. Dir of SKB Lars B. Nilsson, and the senior author.

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backfilling of the deposition tunnels. The preparation of the containers would involve work in “hot lab” environment comprising placement of very well fitting clay blocks in the container, leaving sufficient space in the center for the canister. By temporary storing of the prepared container units in atmosphere with appropriate, high RH, the clay blocks will start to hydrate and tightly confine the canister. The container, which is a strong cage of copper or Navy Bronze tightly filled with clay blocks, is a stable monolith that requires a transport casc for shielding of gamma radiation as in the placement of the canisters according to the KBS3V concept. By equipping the container with copper fens extending from its periphery it can be slipped down from the casc into the inclined holes under complete control and with a minimum of steering. The gap between rock and supercontainer does not have to exceed a few millimeters. The buffer in KBS-3V deposition holes will not get much water for hydration and maturation until after weeks or months and only via the surrounding pellet fill. This can give very heterogeneous wetting of the buffer blocks and significant movement of both blocks and canister, which can be exposed to a strongly varying swelling pressure. The advantage of inclined deposition holes with ready-made containers is fully reached if the space between the containers and rock is initially filled with a clay mud that provides uniform access to water for early wetting of the buffer blocks (Fig. 14). 3. Discussion and conclusions A concept for disposal of high level radioactive waste must be based on identified and solved problems respecting constructability and safe performance. The proposed way of isolating such waste by SKB implies blasting of tunnels at about 400 m depth and boring of large-diameter canister deposition holes extending vertically from the tunnel floor. The rock stresses will be critically high for some holes in the construction phase and lead to failure by spalling in all holes when the heat pulse from the canisters evolves. SKB’s concept implies that the canisters are surrounded by dense montmorillonite-dominated buffer clay blocks that are difficult to place with required accuracy. In a long-term perspective the clay will undergo chemically induced changes leading to very significant loss of the waste-isolating potential. Most deposition holes will not be wetted until after several months or even centuries, which can cause permanent loss of expandability and sealing potential of the buffer. The copper liner of the canisters can yield under the tension caused by the strongly expansive clay when it becomes wetted.

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Four major changes of this concept would make it more constructable and safer from functional points of view. One involves reorientation of the deposition holes from vertical to 45
inclination in two directions for reducing the risk of rock failure. A second is to prepare ready-made stiff units of “supercontainers” with highly compacted blocks of clay tightly surrounding the canisters for simpler and safer installation of clay blocks and canisters. A third is to surround the supercontainers by a clay mud that early provides the dense buffer with water and supports the surrounding rock when the thermal pulse begins to raise the rock stresses. A fourth is to replace the proposed montmorillonite-rich buffer by clay with higher chemical stability and lower but sufficient expandability. A fifth option can be to manufacture homogeneous canisters of HIPOW type, which would radically reduce the risk of contamination of groundwater and release of radionuclides to the biosphere.

References Brady, B.H.G., Brown, E.T., 1985. Rock Mechanics for Underground Mining. George, Allen & Unwin, London and Sydney, ISBN 0-04-622004-6. Burström, M., 2011. Personal communication. Herbert, H.-J., Kasbohm, J., Sprenger, H., Fernández, A.M., Reichelt, C., 2008. Swelling pressures of MX-80 bentonite in solutions of different ionic strength. Physics and Chemistry of the Earth 33, S327eS342.  Kolaríková, I., Svandová, J., Prikryl, R., Vinsová, H., Jedináková-Krí zová, V., Zeman, J., 2010. Mineralogical changes in bentonite barrier within Mock-Up-CZ experiment. Applied Clay Science 47, 10e15. Lönnerberg, B., Larker, H., Ageskog, I., 1983. Encapsulation and Handling of Spent Nuclear Fuel for Final Disposal. SKB/KBS Technical Report 83-20. SKB: Stockholm. Pusch, R., Börgesson L., Ramqvist, G., 1985. Final Report of the Buffer Mass Test e Volume II: Test Results. Stripa Project Technical Report TR-85-12. SKB: Stockholm. Pusch, R., 1994. Waste Disposal in Rock. Developments in Geotechnical Engineering, vol. 76. Elsevier Publ. Co., ISBN 0-444-89449-7. Pusch, R., Yong, 2006. Microstructure of Smectite Clays and Engineering Performance. Taylor & Francis. ISBN 10: 0-415-36863-4. Pusch, R., 2008. Geological Storage of Radioactive Waste. Springer-Verlag, Berlin, Heidelberg, ISBN 978-3-540-77332-0. Pusch, R., Yong, R.N., Nakano, M., 2010. Stiffening of smectite buffer clay by hydrothermal effects. Engineering Geology 116 (1e2), 21e31. Pusch, R., Yong, R.N., Nakano, M., in press. Geological Storage of Radioactive Waste. Wessex Institute of Technology. Sandström, R., Wu, R., 2007. Origin of the Extra Low Creep Ductility of Copper Without Phosphorous. SKB TR-07e02, 2007. Svemar, Ch, 2005. Cluster Repository Project (CROP). Final Report of European Commission Contract FIR1-CT-2000, Brussels, Belgium. Xiaodong, L., Prikryl, R., Pusch, R., in press. THMC-testing of three expandable clays of potential use in HLW repositories. Applied Clay Science. Yong, R.N., Pusch, R., Nakano, M., 2010. Containment of High-level Radioactive and Hazardous Solid Wastes with Clay Barriers. Spon Press. ISBN: 10: 0-415-45820X (hbk).