Technical Report NTB 15-03

The Colloid Formation and Migration (CFM) project involves one of a series of experiments per­formed within the GTS Investigation Phase VI, which focuses on repository-relevant boundary conditions. CFM is dedicated to the study of colloid formation/bentonite erosion, the ground­water – porewater mixing zone, colloid migration (filtration), and colloid-associated radio­nuclide transport.
 

The CFM field experiments are conducted within a permeable shear zone structure that has been the subject of previous tracer migration experiments which focussed on bentonite colloid and colloid-associated radionuclide transport. Colloid tracer tests were performed using borehole dipoles with spacing ~ 2 m and high extraction flow rates resulting in travel times of about 1 hour for conservative species. The aim of CFM has been to extend these results to timescales of a few weeks to years and to examine the colloid formation process under close to repository-like conditions at relatively low hydraulic gradients and flow velocities. In order to create the required flow conditions it was necessary to engineer one or more low gradient but high recovery controlled dipoles. This has been achieved with a novel tunnel packer system. The main objectives of this report are to present this system along with all other instrumentation installed at the CFM site in preparation for the Long-term In-situ Test (LIT), and to describe the initiation of the LIT in May 2014.

The tunnel packer system currently in operation at the CFM site was conceptualised, imple­mented and optimised over the course of a decade. Itcomprises (1) surface packers to collect, control and monitor the flow from the main channels in the shear zone into the tunnel, (2) a resin layer on the tunnel surface to stop groundwater inflow into the tunnel from the matrix and less perme­able parts of the shear zone, (3) the 5 m long megapacker to provide mechanical support to the resin and further isolation between the shear zone and the tunnel, and (4) surface equipment to control flow from the surface packers and monitor groundwater chemistry. Five new bore­holes at the experiment site were drilled to meet specific CFM requirements. They augment the nine operational boreholes from previous experiments. All are equipped with multi-packer systems.

The operating and monitoring installations include packer control boards with pressure trans­ducers, a flow controller for measuring and controlling the shear zone inflow rates, a differential pressure sensor for high accuracy measurements of the hydraulic gradient, cabinets with hydro­chemistry sensors and fluorometers for tracer test monitoring, an automatic sampler for sampling during tracer tests, a data acquisition system, sensors to monitor the climatic conditions in the tunnel, and radioprotection measures. Radionuclide/colloid tracer test samples are collected by a fraction collector in a glovebox, i.e. under controlled atmosphere, to minimise oxidation.

Reducing the inflow from the shear zone to the tunnel resulted immediately in a significantly lowered hydraulic gradient in the shear zone. A total of 27 tracer tests have been performed within CFM under a range of controlled hydraulic boundary conditions, one purpose being repeated testing of the tunnel packer system. In the end, the tunnel packer system has proven to be a reliable and flexible system for managing hydraulic gradients in the shear zone. Since 2009 conditions have been varied from almost totally shut in (3 ml/min inflow) to free inflow when boreholes were open. The system has performed well throughout with no evidence of any signi­fi­cant leakages and thus demonstrated its adequacy and robustness with respect to the require­ments of the LIT.

For the LIT, a source consisting of prefabricated highly-compacted bentonite rings was mounted on a specially designed multiple packer system and emplaced in a shear zone borehole interval within a flow field controlled by extraction from a surface packer. The source was traced with radionuclides, synthetic clays and conservative tracers. Bentonite colloids generated by swelling and erosion of the traced bentonite migrate from the source into the shear zone. The chemistry and colloid content of groundwater around the source is monitored by sampling from three nearfield monitoring boreholes. The extraction flow from the Pinkel surface packer is also monitored.

When the LIT is terminated in early 2018, the three monitoring boreholes will be used for resin injection to stabilise the bentonite source and shear zone. It is planned to then overcore the interval to retrieve the source and surrounding rock for further analysis.