RD&D-Programme 2004 - SKB

These phenomena have been studied both in basic laboratory tests and in model tests on
different scales. The former were done either with constant flow rate or constant water pressure
in drilled holes with a diameter of 2–4 millimetres through 5–10 centimetre long bentonite
pellets. The results showed that only very low water flow rates or very low water pressures
could be stopped by the bentonite. Since these tests can be regarded as conservative extremes,
model tests were performed, first on a scale of 1:10 and then on full scale. The result of the
model tests on a scale of 1:10, where the gap at the top of the simulated distance block was
four millimetres, was that a pressure increase rate of about 200 kPa/h was the upper limit below
which piping did not occur, but also that the water fill-up rate was an important parameter.
When the test was scaled up to full scale, however, the upper limit was only 5 kPa/h, since the
gap there was four centimetres. At the same time, the leakage rate during fill-up was very great.
In other words, the requirements cannot be met without an engineering solution, and a collar
was applied to the “rock” in the most recent test. The collar was attached to the “rock” and its
annular disc was made so big that it closed off the gap between the “rock” and the spacer block
and extended another five centimetres into the spacer block. This arrangement could cope with
50 kPa/h up to a pressure of 2 MPa without piping occurring and without any leakage occurring
before fill-up was finished.
The conclusion is thus that a gap on the order of several centimetres between the roof and the
distance block cannot handle the base scenario without an engineering solution, but also that it
should be fully possible to either devise solutions that meet the requirements or to make spacer
blocks with smaller gaps against the rock (for example, wedge-shaped three-piece blocks).
Independent tests have been conducted within the Colloid Project, as well as field tests at Äspö.
The results clearly show that spontaneous dispersal of montmorillonite from a bentonite buffer
does not occur if the calcium concentration in the groundwater exceeds 1 mM. The results
complement and confirm previous investigations both qualitatively and quantitatively /17-19/.
The problem of spontaneous release of montmorillonite is thereby probably reduced to the
concentration of divalent ions in the groundwater.
Programme
The tests with piping and erosion for KBS-3H will continue both on a small scale and on full
scale to gain a better understanding of these processes. If it proves difficult to guarantee that
piping will not occur, a series of erosion tests must be done for the purpose of being able to
estimate the quantity of eroding bentonite in different situations.
Quantitative modelling of a possible removal of montmorillonite colloids from the buffer is
planned.
17.2.20 Radiation-induced montmorillonite transformation
Montmorillonite in the buffer can be broken down by gamma radiation. The result is a decrease
in the montmorillonite concentration. Experiments have shown that the accumulated radiation
dose to which the bentonite will be exposed in a deep repository does not cause any measurable
changes in the montmorillonite concentration.
17.2.21 Radiolysis of pore water
Gamma radiation that penetrates through the canister can decompose pore water by radiolysis,
forming OH radicals, hydrogen, oxygen and several other components. The oxygen is consumed
rapidly by oxidation processes which affect the redox potential, while the hydrogen is transported
away. The canister’s wall thickness is, however, sufficient so that the effect of gamma
radiolysis on the outside is negligible, see section 17.1.4.
222 RD&D-Programme 2004