DOPAS: Full-scale demonstration of the feasibility and performance of plugs and seals – German contribution:

CH/HM coupled behaviour of shaft sealing materialsO. Czaikowski, K. Jantschik, H. C. Moog, K. Wieczorek & C.-L. Zhang

7th US/German Workshop on Salt Repository Research, Design, and Operation

Washington, DCSeptember 7-9, 2016

ABSTRACT

THE EC-cofinanced DOPAS project includes design, implementation and assessment of five experiments on plugs and seals. Three large-scale experiments are performed in crystalline rock by SKB (Sweden), POSIVA (Finland) and SURAO/CTU (Czech republic), one performed by ANDRA (France) addresses a seal in clay rock. The fifth (German) contribution includes lab experiments and performance assessment studies related to salt. In this frame, the investigation programme of GRS addresses sealing materials planned to be used in the shaft seals. The issues addressed comprise the chemical-hydraulic (CH) and the hydro-mechanical (HM) behaviour of cement based sealing materials in rock salt. A third topic, the hydro-mechanical behaviour of claystone-bentonite-mixture as seal material, is not discussed here.

Samples of rock salt and salt concrete were obtained from an existing drift seal element finished in 1992 and underwent testing with respect to long-term deformation and damage behaviour. In the meantime, the CH behaviour of sorel concrete (MgO-based concrete) samples prepared in the laboratory was investigated by corrosion and diffusion experiments. In particular, advection experiments with NaCl and MgCl2-saturated brines were performed. MgCl2-saturated brine is corrosive for salt concrete, while NaCl brine is corrosive for sorel concrete. When sorel concrete is subjected to NaCl brine, corrosion leads to an expected increase in permeability after 7 – 60 days, however, afterwards no additionally permeability increase is observed. The interpretation is that the solution passes the sample fast enough that the brine in the already corroded concrete is not replaced, which protects the material farther away from the pathways. A major part of the experiments is dedicated to the investigation of the combined system of plug, contact zone (the main pathway) and surrounding (damaged) rock salt. Hollow salt cylinders equipped with salt concrete cores were used for this. Samples are subjected to radial load and axial flow (gas or brine). At dry conditions, reconsolidation is slow, while a fast reduction of permeability is observed, if brine is present and the seal element is intact.

In one of the experiments, the sample is subjected to radial load and axial flow with NaCl and afterwards MgCl2-saturated solution. In the first phase (with NaCl solution), permeability decreases with time due to creep of the salt and closing of the contact zone. In the second phase, the solution is replaced by corrosive MgCl2-saturated brine, which leads first to a permeability decrease because of brucite precipitation. The associated pH decrease to 8-9, however, causes decomposition of portlandite and CSH phases and leads to permeability increase in the longer term.

Future work includes dismantling of the samples and microscopic inspection for pathways.