112 Reactive transport calculations of sulphide availability and fluxes in the near-field of a SF/HLW repository M. Pekala1, P. Wersin1, V. Cloet2, P. Smith3 1University of Bern, Institute of Geological Sciences, Bern, 2Nationale Genossenschaft für die Lagerung radioaktiver Abfälle (Nagra), Wettingen, 3Safety Assessment Management (SAM Switzerland) GmbH, Klingnau, Switzerland Presenting author e-mail:
[email protected] Background and Objectives: In Switzerland spent nuclear fuel (SF), vitrified high-level waste (HLW) and intermediate level waste (ILW) are planned to be co-disposed in a repository constructed in the Opalinus Clay (OPA) at a depth of about 600-900 m. SF and HLW will be encased in canisters, emplaced into horizontal tunnels (drifts) excavated in the OPA, and backfilled with bentonite. Nagra is exploring various options with regard to possible canister materials with copper-coated steel being a strong candidate (Nagra 2014). Sulphide has been recognised as a potential corroding agent to copper under anaerobic conditions, and its effect has been evaluated by various nuclear waste disposal organisations who consider copper as canister material (SKI 1996, SKB 2010, SSM 2011, Posiva 2014). Sulphide cycling under repository conditions is affected by a complex system of bio-chemical reactions and transport processes. Reactive transport (RT) calculations offer a convenient framework within which such processes and their coupling can be realistically represented. The objective of this work is to employ RT calculations to evaluate the fate of sulphide in the near-field of the SF/HLW repository in the OPA and the bentonite. Potential copper canister corrosion due to sulphide fluxes is also assessed. Key Processes: The model assumes that sulphide can be generated in the repository's near-field due to metabolic activity of Sulphate Reducing Bacteria (SRB), according to the overall reaction (where CH2O(aq) is a simplistic formula representing dissolved organic matter - DOM): 2CH2O(aq) + SO42-(aq) ⇒ HS-(aq) + H+(aq) + 2HCO3-(aq) In the reference calculations SRB activity is only considered within a narrow “excavation disturbed zone” (EDZ) of the host rock. Alternative calculation scenarios also explore the effect of potential bacterial activity within the bentonite or crushed OPA backfill. Most calculations consider DOM as the electron donor to sulphate, but an additional case explores the possibility of hydrogen (generated by anaerobic steel corrosion) as the dominant electron donor. Gypsum and celestite present in the backfill and OPA are sources of sulphate. DOM concentration in the porewater of both the OPA and backfill is assumed to be maintained at an approximately constant concentration by dissolution of solid organic matter (SOM). Generated sulphide can be precipitated as mackinawite (FeS) or elemental sulphur, depending on chemical conditions in the porewater. Iron for mackinawite precipitation is provided by dissolution of siderite and goethite. Sulphate reduction has an additional effect of depressing pH and elevating alkalinity, affecting sulphide solubility. The sole mass transport process considered is diffusion in fully water saturated pore space (single porosity) under isothermal conditions according to concentration gradients. At the canister surface sulphide is assumed to undergo a fast chemical reaction with copper, and the model assumes that canister corrosion rate is proportional to the sulphide flux. Implementation: Fully-coupled reactive transport calculations are performed using the PFLOTRAN code (http://www.pflotran.org/) for a modelling period of 100,000 years. Calculations consider a 1D radial geometry (perpendicular to the main drift axis), which includes the canister, backfill, EDZ, and OPA. The chemical model includes 14 master and 41 secondary species, considers cation exchange and surface protonation reactions, as well as dissolution/precipitation reactions for 10 minerals. Chemical reactions are performed at 25 ºC using the Thermochimie (https://www.thermochimie-tdb.com/) thermodynamic database v.9b with the extended Debye-Hückel aqueous activity model. Selected Results: Considerable quantities of celestite (OPA) and gypsum (bentonite backfill) ensure that sulphate availability is not rate limiting for sulphide production during 100ꞌ000 years. Similarly, sufficient quantities of SOM result in a steady availability of DOM. Consequently, the model predicts continuous sulphide production at a pessimistically high rate. However, the results also indicate that sulphide concentrations in the repository near-field would be strongly limited by the precipitation of mackinawite. This process constitutes an effective sink for sulphide due to relative abundance of iron in the OPA (siderite). The solubility of mackinawite controls therefore sulphide release rates and significantly reduces the calculated corrosion depth of the canister. Moreover, precipitation of elemental 214
sulphur in the bentonite backfill is predicted as a result of reductive dissolution of goethite, defining a second sulphide sink. Reference calculations (bentonite backfill with SRB activity restricted to the EDZ) predict total canister corrosion depth on the order of several micrometres at 100ꞌ000 years. Alternative pessimistic calculations considering SRB activity in the backfill and increased backfill diffusivity forecast an increased canister corrosion depth of around 0.1 mm at 100,000 years due to the combined effect of bacterial activity directly at the canister surface and increased sulphide fluxes. A calculation case considering crushed OPA as backfill material and assuming SRB activity in the backfill predicts total corrosion depth of the canister of several tens of micrometres at 100ꞌ000 years. The RT calculation results corroborate simplified calculations based on more conservative assumptions regarding bio-geochemical reactions and transport (e.g. instant sulphate reduction to sulphide, fixed sulphide solubility and steady state sulphide diffusion towards the canister), which include wide ranging parameter sensitivity analyses. Conclusions: In the calculations performed, relevant conceptual and model parameter uncertainties are represented in a way which tends to overestimate canister corrosion. For example, the calculations assume a large and constant SRB population size, although many factors could lead to its significant reduction (e.g. availability of key nutrients). Another example is the assumption of fast dissolution kinetics of SOM and near-constant availability of DOM (while the quantity of leachable SOM and its dissolution rate could be much lower leading to a slowdown/termination of sulphide production). Furthermore, the assumption of mackinawite being the only sulphide-controlling mineral is pessimistic (possible equilibrium with pyrite would greatly reduce sulphide concentrations). In addition, a number of alternative calculations are performed to bracket the potential effect of adverse assumptions regarding the backfill (e.g. presence of SRB and increased diffusivity). Despite the model's conservatism all calculations that have been performed suggest that canister corrosion over 100,000 years would be small (well below 1 mm). This conclusion relies to a large extent on the hypotheses made about siderite (and goethite) dissolution and mackinawite precipitation. These hypotheses are corroborated by an abundant body of experimental and field observations, and a realistic geochemical model for these reactions can be constructed with reasonable confidence. Acknowledgments: Financial support by Nagra is acknowledged. References: Nagra, 2012. Canister Design Concepts for Disposal of Spent Fuel and High Level Waste. Nagra Technical Report 12-06. Nagra 2014. Feasibility evaluation study of candidate canister solutions for the disposal of spent nuclear fuel and high level waste - A status review. Arbeitsbericht NAB 14-90. SKI, 1996. CAMEO: A Model of Mass-Transport Limited General Corrosion of Copper Canisters. Swedish Nuclear Power Inspectorate, SKI Report 96:46. SKB, 2010. Corrosion calculations report for the safety assessment SR-Site. SKB Technical Report TR10-66. SSM, 2011. Is copper immune to corrosion when in contact with water and aqueous solutions? Swedish Radiation Safety Authority, SSM Report 2011:09. POSIVA, 2014. Sulphide fluxes and concentrations in the spent nuclear fuel repository at Olkiluoto. POSIVA Report 2014-01.
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