Chemical evolution of the SF / HLW near field

5.3.4.1 Evolution of the porewater chemistry of the SF / HLW near field Overview

The mineralogy of Opalinus Clay and the composition of its porewater have been discussed in Section 4.2.6. In the far field, removed from the influence of elevated temperatures and repository materials such as bentonite and steel, the porewater chemistry is expected to change only very slowly (a small decrease in salinity may occur in the long term (Nagra 2002a)). In the near field, by contrast, chemical changes will occur as a result of temperature-dependent inter-actions. The air entrapped in bentonite at the time of repository closure will lead to a short period of oxidising conditions, and some mineral impurities present in bentonite will dissolve in the groundwater that gradually saturates the near field. The effects of these changes on repository performance are discussed in the following sections.

NAGRA NTB 02-05 134

Evolution of redox conditions in the near field

Redox conditions in the near field will initially be oxidising, as a result of the presence of air trapped in pores in the bentonite. MX-80 bentonite contains a small amount of pyrite (0.3 wt %) and siderite (0.7 wt %) (Müller-Vonmoos & Kahr 1983)⁸¹, oxidation of which would be expec-ted to speed up the consumption of oxygen. Other factors affecting the redox state include microbial activity, which can catalyse attainment of reducing conditions, corrosion of the steel canisters and the presence of pyrite (~ 1 wt %) and other reduced minerals in the Opalinus Clay.

Microbial studies suggest that oxygen consumption by microbes is far more rapid than by inor-ganic reactions. For example, Pedersen (2002) suggests that it will occur in less than a year in sealed tunnels in crystalline rock after repository closure. Nonetheless, evidence for the lack of significant microbial activity in highly compacted bentonite (West et al. 2002, Pedersen 2002) indicates some uncertainty regarding whether this will occur as rapidly in the case of slowly resaturating SF / HLW emplacement tunnels in Opalinus Clay.

The low moisture content of bentonite will initially hinder consumption of O₂ by the steel canister; if saturation is slow, this situation may last for a considerable time. If moisture inflow is relatively fast, much of the O₂ is likely to be rapidly consumed by the canister and redox-active minerals in the bentonite. In the expected case of slow water inflow rates, the relatively high reactivity of pyrite towards O₂ (Wersin et al. 1994a), the high moisture content of Opalinus Clay (compared to the bentonite), and the high rate of O₂ diffusion in unsaturated bentonite will favour consumption of O₂ by pyrite present in the EDZ of the Opalinus Clay and by steel in the emplacement tunnels (rails, mesh). Depending on the rate of saturation of the bentonite, the consumption of O2 may take a few years (if bentonite saturation is rapid) to a few decades (for a longer unsaturated period). Mass balance calculations indicate that corrosion of the steel canisters to a depth of tens of µm or oxidation of pyrite contained in a ~ 2 cm thick region of Opalinus Clay at the excavation boundary would be sufficient to consume all the O₂ in the near field (Wersin et al. 2003).

Once reducing conditions are established, a number of redox reactions may influence the redox potential, but the most important couples are expected to be Fe(metal)/Fe₃O₄ at the canister surface, Fe(II)/green rust⁸² (Cui & Spahiu 2002), Fe(II)/ Fe₃O₄ and H₂/H₂O. The H₂/H₂O couple is generally considered to be rather unreactive, but the high partial pressure of H₂ (> 10 MPa), resulting from anaerobic corrosion of steel, may considerably increase its reactivity (Spahiu et al. 2000). The surface of the canister will become covered with Fe3O4, but galvanic coupling between steel corrosion and reduction of Fe(III) oxide phases that might form on Fe₃O₄ will prevent passivation and the surface of the magnetite is expected to remain reactive.

After canister breaching, additional influences on the redox chemistry are radiolysis at the spent fuel / water interface and redox reactions involving uranium. As noted in Section 5.3.4.5, high partial pressures of H2 are seen to suppress fuel oxidation very effectively in the presence of radiation (King et al. 1999, Spahiu et al. 2000 and Röllin et al. 2001), implying that reduced U(IV) species may be dominant in solution. The presence of some U(VI) in solution is, how-ever, considered possible, because of radiolysis at the fuel surface. The reduction of such oxidised species is expected to occur on magnetite covering the steel surfaces during diffusion out of the canister and on bentonite clay surfaces. This is confirmed by the studies of Morrison et al. (2001), which show that U(VI) is reduced to U(IV) during transport through a permeable Fe(0) barrier and Cui & Spahiu (2002), who found similar results for green rust formed on

81 The actual bentonite used would be analysed to determine which of these minerals are present, and a similar reducing mineral could be added if deemed necessary.

82 Green rust is an Fe(II)/Fe(III) hydroxy-carbonate compound of variable composition.

135 NAGRA NTB 02-05

corroding steel. The possibility of an oxidised zone (redox front) migrating from the canister thus appears very remote, even if relatively high dissolution rates are assumed for spent fuel (Johnson & Smith 2000).

There are significant difficulties in defining a unique redox potential in the bentonite porewater.

Close to the canister surface, Fe(0) forces a very low redox potential leading to the decomposi-tion of water and the formadecomposi-tion of corrosion products (anaerobic iron corrosion). However, it is unlikely that the solutes in the porewater are in equilibrium with respect to the redox potential at the canister surface. Close to the host rock, redox conditions in the bentonite porewater will be influenced by diffusion of redox-active species from the surrounding Opalinus Clay. Based on thermodynamic modelling, Wersin et al. (2003) derived a bentonite porewater Eh range of -127 mV to -282 mV. This range includes uncertainties in pH as well as those related to the mineral phases controlling the Fe(III)/Fe(II) equilibria. Table 5.3-1 shows the derived Eh values for the magnetite/Fe(II) equilibrium where the dissolved Fe(II) concentration is assumed to be the same as in the surrounding Opalinus Clay. The results indicate that the uncertainty in pH affects redox potentials to a larger extent than do uncertainties related to Fe-bearing minerals.

Tab. 5.3-1: Calculated redox potentials within bentonite under the assumption of magnetite/

Fe(II) equilibrium for Fe (II) concentrations equal to those estimated for Opalinus Clay (Wersin et al. 2003)

Possible deviations from expected redox chemistry in the near field

The possibility that oxidising species produced by radiolysis diffuse out of the canister and into the surrounding bentonite cannot be completely excluded, although this could only occur in the case of passivation of magnetite such that it becomes unreactive (Johnson & Smith 2000).

5.3.4.2 Porewater composition in the bentonite Evolution of the bentonite porewater in the near field

Water taken up from the surrounding Opalinus Clay will induce dissolution of mineral impuri-ties in the bentonite, such as gypsum, NaCl, carbonates and quartz, and in the EDZ, e.g. gypsum from pyrite oxidation. Interaction with the clay fraction and other silicate impurities will cause only slight changes in porewater composition. On the other hand, ion exchange and protona-tion/deprotonation reactions occurring at the montmorillonite surfaces will strongly affect porewater composition. Furthermore, dissolution of calcite and dissociation of carbonic acid are important reactions which will effectively buffer the solution against pH variations. Because of the similar geochemical properties of bentonite and Opalinus Clay, the compositions of their porewaters are not expected to be very different. Tab. 5.3-2 shows a modelled bentonite porewater composition (Curti & Wersin 2002) and, for comparison, the porewater composition of Opalinus Clay as presented in Section 4.2.6. The geochemical modelling assumed an open

Parameter Nominal case Lower pH limit Higher pH limit

pH 7.25 6.9 7.8

Eh [mV] -193 -127 -282

NAGRA NTB 02-05 136

system concerning CO₂ partial pressure, i.e., it was assumed to be in equilibrium with Opalinus Clay. Tab. 5.3-2 also gives the expected extreme ranges of composition of bentonite porewater.

The main uncertainty is related to the pCO₂ conditions in the surrounding host rock which have not been precisely determined (cf. Section 4.2.6). The slightly increased sulphate content in bentonite compared to Opalinus Clay is due to dissolution of gypsum impurities.

Tab. 5.3-2: Compositions of Opalinus Clay reference water (Pearson 2002) and bentonite porewater (Curti & Wersin 2002)

The bentonite reference water was derived by a thermodynamic model which includes ion exchange and surface complexation reactions and corresponds to an early stage after saturation. The expected maximum variations of bentonite porewater composition are also given. Total concentrations of dissolved components are in mol l^-1. Redox potentials are given in Tab. 5.3-1.

The temporal evolution of the porewater has been assessed with two simple models, a water exchange cycle model and a diffusion-reaction model (Curti & Wersin 2002). The results of both calculations indicate that the change in composition is small. This is because of the similar chemistry and the chemical stability of the argillaceous environment. Redox reactions, such as those involving Fe(II) and Fe(III), will not have a significant effect on the chemistry of the major ions.

Microbial activity in highly compacted bentonite is expected to decrease gradually with time, because pore sizes are smaller on average than typical cell diameters of microbes and because pores are poorly connected (Stroes-Gascoyne 2002). This is confirmed by experiments performed with a number of species, including sulphate-reducing bacteria (SRB), that might

Bentonite

Since the applied model does not distinguish between the neutral external water and the diffuse double layer, the porewaters are slightly positively charged which is compensated by the negatively charged clay surface.

*

* *

*

137 NAGRA NTB 02-05

contribute to corrosion of metal canisters. For example, Pedersen et al. (2000a) have shown in experiments lasting 15 months at temperatures of up to 70 °C that only spore-forming bacteria survive in highly compacted bentonite and that their numbers are reduced markedly over time.

In other studies, the activity of SRB was observed to cease at saturated densities higher than 1.5 Mg m^-3 (Pedersen et al. 2000b). Furthermore, Pusch (1999) has shown that SRB are immobile in bentonite with a saturated density exceeding 1.9 Mg m^-3, thus they will not be able to migrate towards the canister from the surrounding rock. As a result, it is expected that bacteria will have a negligible impact on canister corrosion and on radionuclide transport in highly compacted bentonite.

Possible deviations from expected porewater composition in the near field

The similar mineralogical and porewater compositions of bentonite and Opalinus Clay suggest that the porewater composition in the near field will remain close to the Reference Case in Tab. 5.3-2. Both geochemical systems display a high buffering capacity towards acid-base and redox reactions. The main uncertainty is related to pH and pCO₂ conditions of the Opalinus Clay formation, which at present can be only roughly estimated. However, strong changes of these parameters with time are not expected. No realistic mechanism exists for intrusion of a groundwater of a significantly different composition for a repository closed and sealed as planned.

5.3.4.3 Mineralogical changes in bentonite

Maintaining the swelling properties and plasticity of at least the outer half of the bentonite barrier is considered important in relation to its functions of providing a low permeability diffu-sion barrier around the canister and providing a degree of swelling to limit the deformation of Opalinus Clay surrounding the excavations. There are several types of processes that might degrade swelling and reduce the plasticity of bentonite backfill over time. These include:

• dissolution and precipitation of silica and soluble trace minerals (e.g. CaCO₃, CaSO₄⋅2H₂O, and FeCO₃);

• ion exchange of Ca in porewater with Na that is initially present in montmorillonite;

• alteration of montmorillonite to other clay minerals such as illite;

• reaction between Fe(II), from magnetite dissolution, and silica or montmorillonite;

• alteration of swelling properties by heating in the unsaturated state.

These processes and their consequences are summarised here.

Minerals such as CaCO₃, CaSO₄⋅2H₂O, and FeCO₃ are present in rather small quantities in bentonite (~ 1 wt %). Some dissolution of these minerals can be expected, along with ion ex-change of some Ca with Na present on the exex-change complex of montmorillonite, although Na remains the dominant cation. In the unlikely case that early saturation of the buffer occurs, then the temperature gradient in the bentonite may also lead to precipitation of calcium sulphates at the canister surface. This has been observed in four-year long heater tests at maximum temperatures of 180 °C (Pusch et al. 1992), above the temperatures expected at the canister surface in the proposed repository. The resulting cementation of the clay in their study extended only a few centimetres from the canister surface. It is also possible that silica dissolving from quartz and smectite near the hot canister surface may migrate to cooler regions of the backfill and precipitate as chalcedony or quartz. The possible alteration of smectite to illite, a clay with

NAGRA NTB 02-05 138

very limited swelling capacity, also needs to be considered. Because of low potassium contents in bentonite and in Opalinus Clay porewaters (~ 5 × 10^-3 mol l^-1),the supply of which is required for the reaction to proceed, the extent of illitisation will be negligible. Even if saturation occurs immediately after waste emplacement, the degree of illitisation of bentonite at the canister surface calculated with the method of Pusch & Madsen (1995) is only 5 % after ~ 10⁵ years. As a result, illitisation is expected to be very limited. The most important effects of illitisation are reduced swelling capacity and the release and subsequent precipitation of silica, which can lead to an increase in strength (Pusch et al. 1998), presumably due to cementation between crystals.

Nonetheless, the effects are considered to be unimportant for bentonite in the repository, because the amount of silica released and precipitated will be small. Observations from natural analogues such as the Kinekulle bentonite confirm this (Pusch et al. 1998). This bentonite, although experiencing even higher temperature than that expected in the proposed repository, leading to 20 to 40 % conversion to illite, still retains a plasticity and swelling capacity comparable to high density bentonite.

Magnetite will be formed on the steel canister surface as a product of the anoxic corrosion reac-tion. Under reducing conditions, magnetite may dissolve as Fe(II), which could favour forma-tion of nontronite, a smectite with reduced swelling capacity, and other Fe-silicate phases (Grauer 1986). However, Müller-Vonmoos et al. (1991), in experiments performed at 80 °C over 6 months, found no evidence for Fe uptake by montmorillonite contacted with magnetite.

Studies by Couture (1985) and Oscarson & Dixon (1989) show clearly that uncompacted bento-nite loses some of its swelling capacity due to silica cementation after even a few days of heating in a partially saturated state at temperatures above 110 °C. Compacted bentonite, in contrast, does not lose its swelling capacity at temperatures of 90 to 125 °C (Oscarson & Dixon 1990, Pusch 2000). Studies of the reference bentonite backfill, made up of ~ 80 % dense granules and ~ 20 % powder, also show that the swelling pressure is not reduced by exposure to steam at 125 °C, although it decreases by 50 % at 150 °C (Pusch et al. 2002).

The maximum temperature reached in the bentonite midway between the canister and the tunnel wall is 115 °C; midway between canisters along the tunnel axis, the maximum value reached is

~ 95 °C (Section 5.3.2). Thus, despite the likelihood of some local bentonite degradation, each canister is effectively surrounded by tens of cm of bentonite that will experience no significant degradation over time. Bentonite will also be used in the construction of seals at the ends of emplacement rooms and in other locations in the repository. All these seals will experience maximum temperatures of no higher than ~ 70 °C, thus the bentonite would not be thermally altered.

With respect to the significance of reduced swelling capacity of some of the bentonite closest to the canisters, the effect on near field hydraulic conductivity and radionuclide transport is likely to be small, because the outer portion of the bentonite will maintain its high swelling capacity, and because reduced average bentonite swelling capacity is likely to be compensated by increased tunnel convergence.

Possible deviations from expected physical and chemical behaviour of bentonite

The extent of alteration of bentonite is expected to be rather small, and is unlikely to effect its plasticity or swelling pressure, except close to the canister surface. It is difficult to quantify the effects or extent, but embrittlement seems possible near the canister surface only if early satura-tion of bentonite occurs. In addisatura-tion, in the region of bentonite that may reach temperatures above ~ 125 °C (approximately the inner third), some reduction of swelling pressure could occur from cementation due to unsaturated heating effects. At distances greater than 25 cm from

139 NAGRA NTB 02-05

the canister, where temperatures are approximately 110 °C or less, bentonite clay is expected to remain essentially unaltered.

5.3.4.4 Corrosion of SF and HLW canisters

It is known that, in air, corrosion of mild steel is extremely slow provided the relative humidity is less than a critical value of ~ 60 % (Brown & Masters 1982). The 2 % initial moisture content of the bentonite backfill corresponds to a relative humidity of ~ 5 % (Marshall & Holmes 1979).

In contact with bentonite, the critical relative humidity for initiation of aqueous corrosion may be reduced to about 30 to 40 % due to absorption of water by hygroscopic salts, in particular, trace quantities of CaCl₂ or NaCl present in bentonite (Mansfeld & Kenkel 1976). Such a high moisture level at the canister surface is unlikely to be reached for many years (see Section 5.3.3.1), because the high temperature gradient in the bentonite maintains low moisture levels in the hottest part of the bentonite, even when saturation of the outer bentonite is approached (Börgesson & Hernelind 1999). The corrosion of the steel canisters is therefore expected to be limited (< 100 µm) for the first decades, because the surfaces of canisters will remain dry until the humidity increases sufficiently that a thin film of water can condense and initiate both local and general corrosion (Johnson & King 2003). As discussed in Section 5.3.4.1, during this time period, much of the oxygen initially present in the bentonite will be consumed by reaction with pyrite and siderite in the bentonite and in the Opalinus Clay immediately surrounding the tunnel (Wersin et al. 2003) and by other steel materials present in emplacement tunnels (e.g. mesh and rails).

Corrosion of the canisters due to sulphide appears to be extremely improbable in the long term, because of the inability of sulphate-reducing bacteria (SRB) to thrive and be mobile in bentonite backfill (see Section 5.3.4.2). If a steady-state flux of sulphide to the canister surface is maintained as a result of SRB activity in the adjacent Opalinus Clay, this would lead to less than 1 mm corrosion in 10 000 years (Johnson & King 2003, Wersin et al. 1994b).

The effects of γ-radiation on corrosion of the SF canisters (wall thickness ~ 15 cm, compared to 25 cm for the HLW canister) are expected to be insignificant, because the radiation field at the canister surface is only ~ 35 mGy hr^-1 at the time of canister emplacement (Kühl et al. 2003).

This is well below the critical dose rate of 3 Gy hr^-1, which the studies of Marsh & Taylor (1988) suggest is the threshold for enhanced corrosion due to radiolysis.

The possibility that stress-corrosion cracking (SCC) could occur in the weld region has been examined. A summary of studies of SCC by JNC (2000) suggests that its occurrence is highly unlikely. This is supported by the discussion in Johnson & King (2003), which notes that, of the various forms of SCC, only high and low pH SCC in HCO₃^-/CO₃^-2 need be considered in a repository environment. This type of SCC occurs only at slightly acid (pH ~ 6) and moderately alkaline (pH ~ 10 – 11) conditions, significantly below and above the expected bentonite pore-water pH of ~ 7.3. Furthermore, cyclic loading, which would not occur in a repository, is believed to be necessary for crack propagation, thus SCC of the weld region is considered to be only a remote possibility.

After several decades or longer, water reaching the canister surface will initiate corrosion. In the unlikely event that oxygen still remains at this time, it will cause rapid aerobic corrosion at a

After several decades or longer, water reaching the canister surface will initiate corrosion. In the unlikely event that oxygen still remains at this time, it will cause rapid aerobic corrosion at a

Im Dokument TECHNICALREPORT 02-05 (Seite 194-200)