Argillite Host Rock and Cement - Engineered Barrier System Experiments: Mineralogical Evolution at Repository Pressures and Temperatures.

The United States initiated the Spent Fuel and Waste Storage and Transport (SFWST) Campaign over ten years to evaluate various generic geological repositories for the disposal of spent nuclear fuel. Most previous international work describes Engineered Barrier Systems (EBS) for repositories focused on low temperature conditions. Our hydrothermal experiments on EBS materials were conducted to characterize high temperature interactions of bentonite clay and argillite wall rock +/- cement with candidate waste container steels (304SS, 316SS, low-C steel).

Over eight years of hydrothermal experiments were performed using Dickson reaction cells at temperatures ranging from 150 to 300°C and pressures of 15 – 16 MPa, respectively, for five to eight weeks. Wyoming bentonite was saturated with a 1,900 ppm K-Ca-Na-Cl solution in combination with stainless and low-C steel coupons to replicate EBS conditions in deep geological disposition of nuclear spent fuel. The solid-reaction products and steel coupons were characterized post experiment via XRD, XRF, SEM, and EMP.

Preliminary mineralogic phase transformations for the experiments are as follows: Smectite clays did not transition to illite,. Clinoptilolite, appears to have formed from the remnant glass which was present in the original bentonite. The Si/Al ratios for the clinoptilolite are dominantly between 4 and 6. The Na/(Na+Ca) values range from 0.55 to 0.75. Calcite and gypsum were also observed as minor reaction products. Aqueous SiO₂ remains saturated with respect to quartz throughout the experiments.

Our experiments focused on bentonite-cement interactions, including 1) Baseline bentonite stability in argillite, 2) reactions with Uncured OPC powder, 3) Cured OPC chip, and 4) Low-pH cement chips. In the Opalinus Clay, Wyoming bentonite + OPC powder experiments (#2), spherical, calcium aluminum silicate hydrate (CASH) phases formed within the fine-grained clay matrix. Based on the composition of this mineral, the C(A)SH phases are likely a hydrated calcium silicate, such as Al-tobermorite (Ca_4.3Si_5.5Al _0.5O₁₆(OH)₂•4(H₂O)). Very Ca-rich hydrous minerals, such as Al-tobermorite, have been observed in experiments involving bentonite and cement with highly alkaline bulk chemistries and pH > ~10 (Savage et al., 2007).

 

The formation of CASH minerals contrasts with the products of previous experiments with Wyoming Bentonite ± Opalinus Clay host rock (#1), in which zeolites (analcime–wairakite solid solution) formed that have similar morphologies and textural contexts.

 

The experiments with cured OPC chips (#3) resulted in Portlandite dissolution during the early elevated pH values, but pH values stabilize at near-neutral values by equilibrium. Montmorillonite was stable, and zeolite formation was observed throughout the groundmass of the reaction products but was not abundant at the bentonite-cement interface. Reactions between the groundwater solution, bentonite, and the low-pH cement (#4) resulted in a lower steady-state silica concentration than reactions between groundwater solution, bentonite, low-pH cement, and Opalinus Clay wall rock. This suggests a controlling input on silica concentrations is affected by Opalinus Clay wall rock that is not advanced by the bentonite or low-pH cement alone.