Reactive‐transport model for the production, transport, and consumption of sulfide in a spent nuclear fuel deep geological repository in crystalline rock

Abstract

A 1‐D reactive‐transport model has been developed to describe the production and transport of sulfide in a deep geological repository in crystalline rock and the subsequent corrosion of the copper canister. The model accounts for various processes, including: (i) the microbial reduction of sulfate by organotrophic and lithotrophic sulfate‐reducing bacteria, (ii) the supply of sulfate from both the ground water and from the dissolution of gypsum present as an accessory mineral in the bentonite buffer, (iii) diffusive transport of reactants and products, and (iv) sequestration of a fraction of the microbially produced sulfide by precipitation as mackinawite. The results of a base case simulation and of sensitivity analyses indicate that the extent of uniform corrosion is approximately 0.2 mm after 1 million years and that the maximum flux of sulfide to the canister surface is below the threshold for localized corrosion or stress corrosion cracking.

Review of the Modelling of Corrosion Processes and Lifetime Prediction for HLW/SF Containers—Part 1: Process Models

“…2024, 5, FOR PEER REVIEW 43 rigorous than that in the CSM and ISM, which are both based on a simplified treatment of the pore-water chemistry. Despite all of these differences, the various models produce similar results, as revealed by the extensive inter-model comparisons that have been performed [21,178,192]. In all cases, the models predict that the most important source of sulphate initially (for the first 10 5 yr or so) is the gypsum impurity in the bentonite clay, with the sulphate supplied by the ground only becoming significant at longer times.…”

“…Copper Corrosion Model for MIC (CCM-MIC) [172][173][174]. Sulphide production, transport, and consumption models [175][176][177][178] Mechanistically based and supported by analogues in nature. Allows optimization of repository design (for example, buffer density and mineralogical composition).…”

“…More recently, there have been a number of sulphide production, transport, and consumption reactive transport models developed and used to predict the extent of corrosion of copper due to remote SRB activity [79,80,[175][176][177][178][186][187][188][189][190][191]. All of these models share the same Monod kinetic treatment of organotrophic and lithotrophic sulphate reduction, with the dissolution of gypsum and the groundwater acting as sources of sulphate, and with reactions with Fe(II) and/or Fe(III) species acting as potential sinks for the sulphide (Figure 14).…”

“…However, of the sulphide produced, all models predict that the vast majority (90-99%) is sequestered by reaction with Fe(III) minerals and/or by precipitation as mackinawite. As a result, the extent of corrosion is limited and is predicted to be in the range of 0.1-0.3 mm after 10 6 yr [178], with maximum corrosion rates of the order of 1-10 nm/yr. Equally as importantly, the maximum flux of sulphide to the container surface is less than the threshold fluxes for transport control of the rate of uniform corrosion [82] and of the empirically observed threshold for the formation of crack-like defects on copper in sulphide environments [1,164].…”

“…However, there is uncertainty about the location of the source of H2S(g), since the extent of microbial activity will be limited in an unsaturated system, but vapour-filled pores are necessary for the partitioning and transport of H2S(g). There is also uncertainty about the treatment of iron sulphide precipitation and whether the resulting solid (generally assumed to be mackinawite) serves as a permanent sink for a fraction of the total sulphide produced [178]. Mackinawite has a lower solubility [192].…”