Reactive- diffusion simulations performed in the current scoping seem to provide useful information about the time scale in which key chemical evolutions would take place within the considered NF. However, there are uncertainties and limitations to the approach need to be care about.
Firstly, it has been recognized that plausible porosity occlusion has an important role to play in determining the time scale within which considered chemical evolutions would take place. Although the model accounts for the porosity change in the calculation of diffusion, the time scale and the extent of the reaction zone simulated are expected to be far away from the reality. More realistic mechanisms describing porosity clogging in terms of kinetics, models that simultaneously account for altered surface area of involved minerals, and more complicated/realistic 2D or 3D models are probably needed to replicate porosity reduction and occlusion. At the time being, simulations considering porosity sealing are only complementary to the simplified simulations where only mass action/balance and diffusion are considered.
Secondly, boundary conditions applied were not realistic because of difficulties to be implemented in the computer simulations and unpractical long computational time needed. Use of a realistic boundary condition needs to implement both cement model and Boom Clay model in the same model domain and that is still difficult to realize satisfactorily in the current version of the code. Also, due to a very fine gridding needed and very detailed chemical simulation that normally takes more than 95% of the computational time, a computer run for a typical problem described in this work took very long computational time, e.g., a tens of hours to a few days. In addition, because of normally encountered sharp chemical gradient across the interface of interest, e.g., the NF- clay boundary, a computation hardly converges without needed simplifications made in the conceptual model. More realistic boundary conditions, for example, considering the PCO2 alteration in the host Boom Clay would be pursued in the future simulations.
Thirdly, as mentioned in the text that the consistency of the thermodynamic data base was not systematically checked. Although a complete consistency of any thermodynamic data base is probably unreachable, work is currently going on to ensure that at least the part of data base related to the cement/concrete phases should be checked thoroughly. Stability constants of Si and Al aqueous species at high pH should be reviewed, modified, and improved if desirable in the future work.
In addition, uncertainties remain in the value of diffusion coefficient in concrete, the variation of diffusion coefficient with system porosity, and retardation mechanisms of detrimental anions to metallic barriers within the NF.
Lastly, temperature effect is currently not systematically taken into account and should be one of the priorities for the future work.
7 Summary
Calculated pore water compositions and related parameters estimated are summarized in Table 11.
The pH value
The pH value at the overpack and the SS envelope should be about 12 in the thermal phase considering an average temperature of 80 °C. After the temperature drops, the SS envelope and the overpack may experience a pH of 13.5 for at lest a few hundreds to 1000 years and very probably 10,000 years. If effective sealing occurs at the concrete- clay boundary due to carbonation, the pH of 13.5 might last for a very long time. In case the amount of dissolved alkalis completely diffused away from the NF, and if the overpack and the SS envelope still exist until then, the NF pH should be around 12.5 buffered by the dissolution of portlandite.
The concentration of chloride
Chloride concentration in Boom Clay pore water is too low to affect corrosion of the overpack and the SS envelope significantly. In the most conservative case, i.e., considering a very early perforation of the SS envelope plus a fast ingress of the ‘worst-case’ Boom Clay pore water, a Cl concentration of 12 mM might reach the overpack in 400 years. With a retardation of Cl generally observed in a cementitious system, the same Cl concentration only reaches the overpack after 8000 years. Considering a more realistic ‘mixed case’, the maximum Cl concentration at the overpack should not be more than 4 mM.
The concentration of sulphate
The concentration of sulphate in the ‘worst-case’ water (~10 mM) may reach the overpack and the SS envelope in about 500 and 300 years respectively considering no retardation. This is conservative
Table 11: Simulated pore water compositions of the NF concrete and related parameters
time scale (yr) unsaturated t = 0 saturated t = 5~10 young concrete water t = 1000~10,000 evolved concrete water t > 80,000 CSH water [element], mM Ca Na K Al Si* Mg Fe C (TIC) SO42- S2O32- HS-/S2- Cl- pH Eh, mV 0.7 141 367 0.06 0.05/0.3 ~10-7 10-5 0.3 2 - - - ~12§ 100~200 0.7 141 367 0.06 0.05/0.3 ~10-7 10-5 0.3 2 -/1.9# -/0.15# 0.2/3.6# ~12§ 100~200 0.7 141 367 0.06 0.05/0.3 ~10-7 10-5 0.3 2 1.9~6.4 0.15~0.5 0.2~12 13.5 ~ -800 15.3 15.1 0.2 0.005 6 10-3/3 10-3 4 10-6 10-6 8 10-3 7 10-3 - - 0.2 12.5 ~ -800 0.8/1.3* 15.1 0.2 9.4/2.9* 0.8/6.3 10-6 10-7 0.02 0.05 - - 0.2 ~12 ~ -800 *controlled by afwillite and CSH_1.8
#saturated with the reference or the ‘worst-case’ water §temperature at 80 °C of thermal phase
as sulphate will likely interact with cement constituents. Taking ettringite as the sulphate controlling mineral in the model system, the sulphate concentration is about 2 mM when the pH is regulated by KOH/NaOH and 7 µM if the pH is buffered by the portlandite dissolution.
The concentration of sulfide species
The concentration of sulfide in the ‘worst-case’ water is 0.5 mM. This concentration might reach the overpack and the SS envelope in about 500 and 300 years respectively if no retardation of the species is considered. Mechanism of chemical interactions between sulfide species and cementitious materials is not known to us so is not modeled at this stage. In the mixed case where the ‘worst-case’ water was only available for the first 20 years followed by the undisturbed Boom Clay pore water, about 0.2 mM of sulfide can reach the SS envelope and the overpack in about 40 and 70 years respectively.
The concentration of thiosulphate
The highest thiosulphate concentration of 6.4 mM in the ‘worst-case’ water that may arrive at the overpack and the SS envelope in about 500 years considering no retardation. In the mixed case, the maximum concentration of thiosulphate of ~3 mM might arrive at the SS envelope and the overpack in about 40 to 70 years respectively. No retardation mechanisms of thiosulphate are taken into account.
The concentration of carbonate
The concentration of inorganic carbon is controlled by the dissolution of calcite within the domain so is dependent on pH. At 25 °C and at the beginning of 1000 to 10,000 years in which the pH is controlled by KOH/NaOH, the concentration of inorganic carbon at the overpack and the SS envelope is about 0.4 mM. The concentration decreases to 8 µM afterwards when the pH is regulated by the dissolution of portlandite.
The redox in the NF
The redox potential within the NF should be very reducing at more negative value than -800 mV as long as metallic barriers are not totally corroded away. Afterwards, the redox potential in the NF will be the same as that of far filed Boom Clay at about -300 mV.
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