Muurinen, Arto

The Long-Term Test of Buffer Material (LOT) is underway in the Äspö Hard Rock Laboratory in Sweden to test the buffer material for nuclear waste disposal. The test parcels contain prefabricated bentonite blocks placed around a copper tube. A heater is placed in the lower part of the copper tube in order to simulate the heating effect of the spent fuel. The parcels are 300 mm in a diameter and placed in 4 m-long vertical boreholes in granitic rock at a depth of 450 m. This report concerns the chemical studies performed on the parcel A2, which was excavated after about five years of experiment period. Two sample blocks were taken from the hot part of the parcel for studies. One of them was without additives while in the other one, cement plugs had been placed.In the block without additives the water content, which was close to the heater 28 – 30 wt% of the dry bentonite, increased to 32 – 33 wt% close to the rock. The total concentrations of dissolving chloride and sulphate in bentonite were determined by dispersing bentonite samples in deionized water. It was obvious that sulphate had redistributed and precipitated close to the heater during the experiment. The initial chloride concentration of bentonite had increased during the experiment approximately by a factor of ten. The increase in the chloride concentration was caused by chloride in the saturating groundwater. Porewaters were squeezed out from the bentonite samples and their Na+, K+, Ca2+, Mg2+, Cl-, SO42- and HCO3- concentrations determined. The chloride, sodium and potassium concentrations were rather independent of the distance from the heater. The calcium, magnesium and sulphate concentrations increased and bicarbonate concentrations decreased proceeding from the outer surface towards close to the heater. The chloride concentration in the squeezed porewater was slightly higher than that determined by the dispersion method, but about half of the concentration in the Äspö groundwater. The Eh and pH were measured directly in bentonite samples. Close to the outer surface, the pH in the bentonite was 7.2 and increased to 8.3 close to the heater. The Eh measurements suggest that the conditions in the bentonite were reducing. With the gold electrode, the Eh varied from -183 to -228 mV and with the Pt electrode from -287 to -366 mV, where the more negative values represent the conditions close to the heater.In the block with the cement plugs the water content was somewhat higher than in the block without cement, which may be caused by the effect of the lower temperature in the block with cement. The concentrations of Na+, K+, Ca2+, Mg2+, Cl-, SO42- and HCO3- in the squeezed porewaters were practically independent of the distance from the cement but clearly higher than in the block without cement. The pH values were generally somewhat higher than in the block without cement, and the increase was clearly seen in the measurement point closest to the cement

Chemical conditions in the A2 parcel of the long-term test of buffer material in Äspö (LOT)

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