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Working reports


Posiva's working reports are used to publish research results related to the practical aspects of the final disposal of spent nuclear fuel. Approximately one hundred working reports are published every year, usually in Finnish or English. The reports are written by Posiva employees or specialists from third party organisations. The working reports are an important tool for distributing the results of research work. All working reports are published on the Posiva website.



Workreport 2011-68

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Name:

Laboratory Tests to Determine the Effect of Olkiluoto Bounding Brine Water on Buffer Performance

Writer:

Martikainen, J. & Schatz, T.

Language:

English

Page count:

24

Summary:

This report presents a set of results from laboratory studies on the effect of bounding brine water exposure on buffer performance.

In order to evaluate the effect of bounding brine water exposure on compacted bentonite buffer performance a series of experiments were conducted using swelling pressure and hydraulic conductivity measurements as follows:

-    Direct exposure measurements were performed on MX-80 bentonite samples encompassing a range of dry density values from 1334 to 1585 kg/m3. These samples were saturated directly with a 70 g/L solution composed of calcium and sodium chloride at a Ca2+/Na+ mass ratio of 3:2.

-    Direct exposure measurements were performed on IBECO RWC samples encompassing a range of dry density values from 1314 to 1564 kg/m3. These samples were saturated directly with a 68.45 g/L solution composed of calcium and sodium chloride at a Ca2+/Na+ mass ratio of 3.2:2.

-    A set of MX-80 and IBECO RWC samples encompassing a range of dry density values from 1018 to 1607 kg/m3 were initially saturated with tap water followed by (indirect) exposure to a 68.45 g/L solution composed of calcium and sodium chloride at a Ca2+/Na+ mass ratio of 3.2:2. Sample swelling pressures were continuously monitored and hydraulic conductivity measurements were performed at each constant swelling pressure condition.
In some cases, exchangeable cation analyses were performed.

The specific test results are summarized as follows:

-    All of the measured swelling pressure values upon exposure to bounding brine water were lower than the corresponding values after saturation with tap water.

-    The relative swelling pressure decrease for the IBECO RWC samples was approximately half of the corresponding decrease for the MX-80 samples.

-    On exposure to bounding brine water, hydraulic conductivity values were increased at the lowest densities, for both the MX-80 and IBECO samples, while at higher densities hydraulic conductivity values were essentially equivalent, for both the MX-80 and IBECO samples, and unchanged.

-    Swelling pressure ranges of 3300 to 10 000 kPa, 2200 to 7700 kPa, 5600 to 18 000 kPa, and 4300 to 16 000 kPa are estimated, based on exponential fits to the data, for MX-80 saturated with tap water, MX-80 exposed to bounding brine water (both directly and indirectly), IBECO RWC saturated with tap water, and IBECO RWC exposed to bounding brine water (both directly and indirectly), respectively, over the saturated buffer density design interval (1950 – 2050 kg/m3).

-    A hydraulic conductivity range for both MX-80 and IBECO RWC materials at saturated buffer density design interval, over tap to bounding brine water compositions, is estimated to be 6.5 × 10-14 m/s ± 50 %.

-    The exchangeable calcium content in the MX-80 and IBECO RWC materials was uniformly increased on exposure to bounding brine water. Furthermore, the exchangeable sodium content in the MX-80 samples was uniformly decreased on exposure to bounding brine water whereas sodium content in the IBECO RWC material remained largely unchanged.

Keywords:

Groundwater, bounding brine water, tap water, bentonite buffer, swelling pressure, hydraulic conductivity, performance.

File(s):

Laboratory Tests to Determine the Effect of Olkiluoto Bounding Brine Water on Buffer Performance (pdf) (2.9 MB)


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