Alternative Buffer Materials

In the Alternative Buffer Material test, ABM, eleven different buffer candidate materials with different amount of swelling clay minerals, smectite counter ions and various accessory minerals are tested.

The test series is performed in the rock at repository conditions except for the scale and the adverse conditions (the target temperature is set to 130°C). Parallel to the field tests, laboratory analyses of the reference materials are going on.

The main objectives with the project are:

• Compare different buffer materials concerning mineral stability and physical properties, both in laboratory tests of the reference materials but also after exposure in field tests.
• Study the interaction between metallic iron and bentonite. This is possible since the central heaters are placed in tubes made of straight carbon steel.
• ABM is an SKB project with several international partners collaborating in the part of laboratory experiments and analyses.

Canister Retrieval Test

The Canister Retrieval Test is aiming at demonstrating the readiness for recovering of emplaced canisters also after the time when the bentonite is fully saturated.

In the Canister Retrieval Test two full-scale deposition holes have been drilled, at the -420 m level, for the purpose of testing technology for retrieval of canisters after the buffer has become saturated.

These holes have been used for studies of the drilling process and the rock mechanical consequences of drilling the holes.

Canister and bentonite blocks were emplaced in one of the holes in 2000 and the hole was sealed with a plug, heater turned on and artificial water supply to saturate the buffer started.

In January 2006 the retrieval phase was initiated and the canister was successfully retrieved in May 2006. The saturation phase had, at that time, been running for more than five years with continuous measurements of the wetting process, temperature, stresses and strains.

Long Term Test of Buffer Materials

The project Long Term Test of Buffer Material (Lot) aims to validate models and hypotheses concerning mineralogy and physical properties in a bentonite buffer.

Seven test parcels containing heater, central tube, clay buffer, instruments and parameter controlling equipment have been placed in boreholes with a diameter of 300 mm and a depth of around 4 m. The test concerns realistic repository conditions except for the scale and the controlled adverse conditions in four parcels.

Temperature, total pressure, water pressure and water content, are measured during the heating period. At termination of the tests, the parcels are extracted by overlapping core-drilling outside the original borehole. The water distribution in the clay is determined and subsequent well-defined mineralogical analyses and physical testing of the buffer material are made.

The test parcels are also used to study other processes in bentonite such as cation diffusion, microbiology, copper corrosion and gas transport under conditions similar to those expected in a deep repository.

Prototype Repository

The Prototype Repository is located in the TBM-tunnel at the -450 m level and includes six full scale deposition holes. The aims of the Prototype Repository are to demonstrate the integrated function of the repository components and to provide a full-scale reference for comparison with models and assumptions.

The Prototype Repository should, to the extent possible, simulate the real repository system regarding geometry, materials and rock environment.

Instrumentation is used to monitor processes and properties in the canister, buffer material, backfill and the near-field rock. The evolution will be followed for a long time.

The inner tunnel (Section I) was installed and the plug cast in 2001 and the heaters in the canisters were turned on one by one. The outer tunnel (Section II) was backfilled in June 2003 and the tunnel plug with two lead-throughs was casted in September the same year.

Temperature Buffer Test

The Temperature Buffer Test (TBT) is a heated full-scale field experiment presently carried out jointly by SKB/ANDRA and DBE, formerly also by ENRESA, at the SKB Äspö Hard Rock Laboratory in Southeast of Sweden. An existing 8 m deep, 1.8 m diameter KBS3-type deposition hole located at ‑420 m level has been selected for the test.

The objectives are to improve the general understanding of Thermo-Hydro-Mechanical, THM, behaviour of buffer materials submitted to severe thermal conditions with temperatures well over 100°C during water uptake of partly saturated bentonite-based buffer materials, and to check, in due time, their properties after water saturation.

The test includes two carbon steel heaters, each 3 m high and 0.6 m in diameter, surrounded by MX-80 buffer material. There is a 0.2 m thick sand shield between the upper heater and the surrounding bentonite, while the lower heater is surrounded by bentonite only. On top of the stack of bentonite blocks is a confining plug anchored to the rock.

In the slot between buffer and rock wall is a sand filter equipped with pipes to control the water pressure at the boundary, which is seldom done with an EBS in situ experiment. Both heater mid-height planes are densely instrumented in order to follow, with direct or indirect methods, buffer THM evolution. Temperature, relative humidity, total pressure, pore pressure, cable forces and lid displacement have been monitored since the test start in March 2003. Total water inflow is also monitored.

Rock Mechanics Projects

Clay Technology has for many years assisted SKB in assessing the behaviour of the rock mass surrounding the engineered barriers of the HLW repository. The services include in particular numerical modeling of effects in the near-field and in the far-field of different loads that are potentially important to the safety assessment:

• Loads occurring during construction/excavation of the repository
• Thermal loads occurring as a result of the heat generated by the spent nuclear fuel
• Loads occurring as a result of future ice covers
• Seismic loads