Hydromechanical evolution of SF / HLW near field

5.3.3.1 Evolution of the near field rock and bentonite backfill system Effects of excavation

The region of rock immediately surrounding the emplacement tunnels (the excavation-disturbed zone or EDZ) will become partially desaturated as a result of evaporation due to ventilation during the construction and operation phase. Stress re-distribution due to excavation will lead to the formation of micro- and macro-scale fractures in the EDZ and desaturation will result in stiffening of the clay. For unsupported SF / HLW emplacement tunnels, the rock is sufficiently strong that the deformation immediately upon excavation and during the operational phase (1 to 2 years for a given tunnel) is limited (approximately 1 to 2 % convergence), although fracturing may extend outwards approximately 1.6 tunnel radii, equivalent to 2 m, from the roof and floor of the tunnels (Nagra 2002a).

160 140 120 100 80 60

40 0.25 0.5 0.75 1.0 1.25

Temperature[°C]

Radial distance from canister surface [m]

Bentonite

backfill Opalinus

Clay 10 years

100 years 1000 years

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Fig. 5.3-5: Temperature distribution for a disposal tunnel with canisters containing 4 PWR SF assemblies (3 UO₂ plus 1 MOX) at 268 years after emplacement, for a bentonite thermal conductivity of 0.4 W m^-1 K^-1

The canister is represented by the rectangle at the centre of the tunnel and the solid line and dashed line represent the tunnel boundary (R = 1.25 m) and the assumed outer boundary of the EDZ (R = 1.75 m), respectively. The time of 268 years represents the time of maximum temperature of the bentonite at the mid-point between canisters and in the surrounding rock.

The vertical cross-sections are at mid-tunnel and midway between canisters (Johnson et al.

2002).

Some oxidation of pyrite may occur on fracture surfaces, leading to the formation of small amounts of gypsum and iron hydroxide, which has been studied in detail. In the case of SF / HLW emplacement tunnels, which will be open for only one to two years, estimates of the extent of oxidation have been derived from various field studies. Results indicate that gypsum formation in the EDZ is limited to open fracture surfaces (Mäder & Mazurek 1998). Calcula-tions based on field studies of tunnels open from a few years (Mont Terri) to over 100 years (Hauenstein railway tunnel) permit bounds to be placed on the extent of oxidation. For SF / HLW emplacement tunnels, only about 1 % of the pyrite originally present will be altered, thus long-term impacts will be insignificant (Mäder 2002, Nagra 2002a).

Near field saturation

After emplacement of the bentonite, resaturation of the partially desaturated EDZ will gradually occur. The fracturing of the rock upon excavation and its low strength when resaturated, combined with the ~ 1 to 7 % swelling capacity of Opalinus Clay (Nagra 2001 and 2002a, Meier et al. 2000) is expected to result in effective homogenisation and self-sealing of the EDZ

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and gradual convergence of the tunnels. This deformation process is illustrated on a small scale in Fig. 5.3-6, which shows the behaviour of a small diameter (8 mm) borehole in Opalinus Clay under a confining stress. When the hole is dry, the material is sufficiently strong to be self-supporting with a circumferential stress of 30 MPa. When water is present, weakening of the material leads to creep and borehole convergence.

Fig. 5.3-6: Small-scale demonstration of strength reduction of Opalinus Clay due to water weakening of stressed material at an excavation boundary

Sample diameter = 3 cm (Nagra 2002a). In both cases, a confining stress is applied to the sample.

The reduction in strength of the Opalinus Clay when saturated is greater at elevated tempera-tures. The impact of the tunnel convergence and self-sealing process on the hydraulic properties of the EDZ have not yet been examined at full scale⁸⁰, but, based on the process understanding and hydraulic modelling results, the long-term effective hydraulic conductivity of the 2 m thick EDZ is expected to be increased by approximately one order of magnitude relative to the undisturbed rock (Nagra 2002a).

High temperatures near the canisters would initially cause water vapour to migrate away from the canister surfaces (Börgesson & Hernelind 1999). As the bentonite approaches approximately 50 % saturation, the highly compacted granules will begin to swell and a homogeneous micro-structure will quickly develop once saturation is achieved, as has been observed in studies by Pusch et al. (2002) and Dereeper & Volckaert (1999). At full saturation, a swelling pressure of

~ 2 to 4 MPa will develop and the hydraulic conductivity will be 10^-12 m s^-1 (Pusch et al. 2002).

80 The SELFRAC experiment, which is examining these processes, was still underway at Mont Terri when this chapter was developed.

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The tunnel convergence process will compact the bentonite to a higher density, likely in concert with the resaturation process. The final saturated density of the bentonite will not exceed

~ 2.15 Mg m^-3, because at this density the swelling pressure of ~ 15 MPa will approximately balance the external stress field (~ 16 MPa vertical and 15 MPa minimum horizontal).

Assuming that this stress balance defines the limiting state of convergence, the tunnel radius will be reduced from 1.25 to 1.15 m. Concurrent with the slow compaction of the bentonite, its porosity would decrease from ~ 0.45 to ~ 0.36, and its permeability from 10^-12 m s^-1 to less than 10^-13 m s^-1 (Dixon 2000).

Because of the very low hydraulic conductivity of the Opalinus Clay (≤ 10^-13 m s^-1), water up-take by the bentonite will be slow. The range of possible timescales for saturation of the bentonite has been estimated using various models, including a transient analytical model that does not include temperature effects and TOUGH-2, a hydraulic model partially considering thermal effects (Nagra 2002a). The models suggest resaturation times ranging from ~ 100 years to many hundreds of years, reflecting, in particular, the uncertainties in some of the values of the hydraulic parameters of the Opalinus Clay and bentonite. The impact of uncertainty in water inflow rates on canister corrosion and bentonite behaviour is discussed in Section 5.3.4.

The hydraulic conductivity of bentonite increases with increasing salinity, although the effects are not significant for the salinity of Opalinus Clay porewater at saturated densities of bentonite greater than 1.9 Mg m^-3 (Dixon 2000). The porewater salinity may initially be slightly increased (~ 2 %) as a result of evaporation during the operating phase (Gribi & Gautschi 2001), but this does not affect the conclusion.

It is possible that some compaction of the bentonite backfill by tunnel convergence will occur while its moisture content remains low, because of the slow water inflow rate. Under such con-ditions, the partially saturated backfill may be compacted to a higher dry density (to a maximum of ~ 1.7 Mg m^-3, cf. 1.5 Mg m^-3 at emplacement). This represents a saturated density similar to the values noted above, thus the same stress balance is expected to result in the long term.

The temperature rise in the Opalinus Clay surrounding the disposal tunnels is expected to pro-duce a thermal expansion of porewater, which would increase fluid pressures and compressive stresses. Calculations based on the approach described by Horseman (1994) suggest that excess pore pressures may reach about 5 MPa after several hundred years (Nagra 2002a). The pressures are likely to dissipate gradually with cooling and are not sufficient to reactivate existing dis-continuities or fractures in the Opalinus Clay (Nagra 2002a).

Gas production and transport through bentonite

Various gases will be produced in the SF / HLW near field as a result of metal corrosion, radio-lysis of water and radioactive decay. The gas produced in largest quantity is H₂, arising pre-dominantly from anaerobic steel corrosion, with lesser amounts from α-radiolysis of water after canister breaching. Other gases produced in much smaller quantities after breaching of the canister include Rn, He, Xe, Kr and, possibly ¹⁴C (as CH₄ or CO₂), all present in, or arising from, radioactive decay of the waste. Once the near field becomes partially saturated with groundwater (after some hundreds of years), anaerobic corrosion of steel canisters at a rate of 1 µm a^-1 (Johnson & King 2003) will produce H₂ at a rate of ~ 4 mol a^-1 per canister. The comprehensive measurements of Smart et al. (2001) suggest that lower rates are more likely, but at a pH of ~ 7.5 a rate of 1 µm a^-1 cannot be excluded, because the passive film of magnetite may be more porous than at higher pH values. Hydrogen will initially dissolve in the porewater, but calculations show (Nagra 2003a) that for a corrosion rate of > 0.1 µm a^-1, the production rate exceeds the diffusive transport rate of dissolved gas, because of the low diffusion rates of

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dissolved H₂ in both bentonite and the surrounding Opalinus Clay. Thus the concentration of hydrogen in bentonite porewater at a hydrostatic pressure of 7.5 MPa will reach the solubility after several hundred years, after which a gas phase will form (see Section 5.5.2).

Laboratory studies show that gas breakthrough within bentonite occurs at a pressure approxi-mately equal to the sum of the bentonite swelling pressure and the hydrostatic pressure (Pusch et al. 1985, Horseman et al. 1999 and Tanai et al. 1997). The swelling pressure of bentonite will increase from several MPa at a time shortly after saturation to ~ 15 MPa in the very long term after compaction caused by deformation and creep of the surrounding Opalinus Clay under the lithostatic load. There is some uncertainty about whether the gas breakthrough mechanism in bentonite involves capillary flow (displacement of water in capillaries) or microfracturing and pathway dilation (Rodwell et al. 1999), but recent studies discussed in Swift et al. (2001) pro-vide some epro-vidence for the latter mechanism. Irrespective of uncertainties about the details of the mechanism, there is agreement that there is a threshold pressure required for gas break-through, that gas entry and gas flow result in very little desaturation of the clay, and that gas pathways will reseal if the gas pressure drops and water is available (Swift et al. 2001). The discrete gas pathways (or capillaries) formed by gas breakthrough are believed to have a dia-meter of < 1 µm. Ortiz et al. (1997) also present evidence that the extent of water expulsion is very small (< 1 %). The transport of gas through host rock is discussed in Section 5.5.2.

Possible deviations from expected hydromechanical behaviour

The uncertainties related to the hydromechanical behaviour of the near field are largely related to the time over which the various processes take place. Although the duration of tunnel conver-gence and the associated compaction of bentonite to a higher density is uncertain, the process is expected to occur concurrently with resaturation and be largely complete within several thousand years, i.e., prior to breaching of the waste canisters.

A possible, although unlikely phenomenon, involves potential gas-driven accelerated release of porewater containing radionuclides from SF canisters, in the event that breaching of the canister wall occurs only on the underside. In this case, flow of water into the canister could occur, followed by H2 gas production from corrosion of internal canister surfaces, which could expel water from the canister.

5.3.3.2 Hydromechanical evolution of the SF and HLW canisters

As the bentonite saturates, it will develop a swelling pressure of ~ 2 to 4 MPa, increasing to a maximum of ~ 15 MPa as a result of convergence of the tunnels. This pressure, combined with the hydrostatic pressure of 7.5 MPa, will impose a load of ~ 22 MPa on the canisters. The load may be somewhat anisotropic initially, because of differences between the swelling pressure of highly compacted bentonite blocks (~ 10 MPa) and granular bentonite backfill (2 to 4 MPa), but will gradually become isotropic because of the plasticity of saturated bentonite. Canister sinking as a result of consolidation and creep is expected to be very limited (~ 1 cm), based on the information presented by Pusch & Adey (1999).

The HLW canister is designed to withstand an isotropic load of 30 MPa with safety margins higher than required by the corresponding ASME pressure vessel code, even after reduction of the wall thickness by 5 cm due to corrosion (Steag & Motor Columbus 1985).

The cast steel SF canister design is described in Section 4.5.3.1 and has a minimum wall thick-ness of ~ 15 cm. Calculations for an external pressure of 40 MPa indicate that when the wall

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thickness is eventually reduced to ~ 5 cm by corrosion, stresses may reach the failure criterion (Johnson & King 2003). No analysis has been performed for cases where loads on the canister might be anisotropic, such as during resaturation or as a result of tunnel convergence. None-theless, the results summarised below for Cu/cast iron canisters suggest this will not lead to canister damage.

Stresses in excess of the sum of lithostatic and hydrostatic load could arise eventually as a result of the volume of canister corrosion products, because magnetite has a lower density than steel.

The effects of volume increase on stresses have been examined in the H-12 study (JNC 2000) for the case of a steel canister surrounded by bentonite. The results show that the additional forces generated are relatively small for bentonite thicknesses of > 50 cm; this process is there-fore considered unimportant for the Nagra steel canister designs and tunnel diameters.

The Cu/cast iron canister of SKB, considered as a design variant, has a Cu shell with a thickness of 5 cm and a cast iron insert. Calculations discussed in SKB (1999) indicate that it has a collapse pressure of 80 MPa in the case of isotropic loading. For various anisotropic loading scenarios, the stresses in the insert were found to lie far below the yield strength in all cases.

Gases released from spent fuel while the canisters remain unbreached include fission gases, He and Rn, all of which may accumulate in the void space in the canister if the Zircaloy cladding is breached by, e.g. creep rupture. Internal pressures in the canister would be expected to reach only ~ 1 MPa in 100 000 years.

It is thus clear that both the SF and HLW canisters have a large initial margin of safety in relation to structural strength. Nonetheless, structural failure is expected to occur after

> 10 000 years, because the wall thickness will be reduced by corrosion. The various corrosion processes considered and their projected rates are discussed in Section 5.3.4.4.

Possible deviations from expected hydromechanical behaviour of canisters

Large safety margins exist in relation to structural integrity of SF and HLW canisters; there therefore appears to be little likelihood of structural failure until extensive corrosion has occurred. The remote possibility of stress-corrosion cracking in the weld region causing a reduced lifetime is discussed in Section 5.3.4.4.