Bentonite clay has been proposed as buffer material in several concepts for nuclear high level waste repositories. The “Long Term Test of Buffer Material” (LOT) series aims at validating models and hypotheses concerning physical properties in a bentonite buffer material and of related processes regarding mineralogy, microbiology, cation transport, copper corrosion and gas transport under conditions similar to those in a the Swedish KBS3 repository.

The test series comprises 7 test parcels, which will be run for 1, 5 and 20 years. The testing principle is to place parcels containing heater, central copper tube, pre-compacted clay buffer, instruments, and parameter controlling equipment in vertical bore-holes in granitic rock. The parcels are equipped with heaters in order to simulate the decay power from spent nuclear fuel at standard KBS3 conditions (90°C) and adverse condition (130°C). Adverse conditions in this context refer also to high temperature gradients over the buffer, and additional accessory minerals leading to i.a. high pH and high potassium concentration in clay pore water. Temperature, total pressure, water pressure and water content are measured during the heating period. At test termination standard chemical and mineralogical analyses, and physical testing are made. The 2 pilot tests (1-year tests) have been completed and preliminary results concerning the bentonite analyses and tests are presented. The 5 test parcels in the main test series have been started during the fall, 1999.