RD&D-Programme 2004 - SKB

19.2.20 Colloid formation – colloids in groundwater
Conclusions in RD&D 2001 and its review
Colloids are tiny particles that do not sediment. This means that they can accompany the
groundwater and even act as carriers of radionuclides. SKB has been conducting studies
and measurements on colloids for more than ten years. The conclusion from these studies,
both nationally and internationally, is that the colloid content of the groundwater in Swedish
granitic bedrock mainly consists of clay, silicon and iron hydroxide particles, and that the mean
concentration is 20–45 ppb, which is low. The concentration is limited by the fact that the
colloids adhere to the fracture surfaces, which reduces their stability and transport capacity. In
Nevada in the USA, where hundreds of underground nuclear bomb tests have been performed,
measurements of plutonium show that colloid fractions in the groundwater could be detected
1.3 kilometres from the detonation site. This is an indication of rapid colloid transport.
Newfound knowledge since RD&D 2001
The Colloid Project was initiated in the autumn of 2000 and will be concluded in 2004.
The purpose of the project is to clarify the stability of colloids, their potential to transport
nuclides, and the potential of the bentonite to generate colloids. The role of bentonite clay as
a colloid source was investigated at varying groundwater salinities (NaCl/CaCl) in laboratory
experiments. Chemical changes, size distributions and the effects of sodium- and calcium-rich
bentonite were studied, among other things.
The background concentration of colloids in different water types and fracture zones was
measured with a high-resolution laser. The results show that the natural colloid concentration
declines with increasing salinity and depth. The colloid concentration on Äspö is less than
300 ppb, and at repository depth the concentration is less than 50 ppb.
A total of six bentonite reactors were installed in the Äspö HRL and in Olkiluoto, Finland. It
was found that colloid production from the bentonite under natural conditions is small and that
it was not affected by the flow rate.
Programme
There are plans to study transport of colloids between two nearby boreholes at the Äspö HRL.
The boreholes penetrate the same fracture zone, known as True feature A, which has a relatively
homogeneous geology. The experiment will be supplemented by laboratory experiments and
experiments with colloid transport of radionuclides in a drill core in the Chemlab apparatus in
the Äspö tunnel.
19.2.21 Colloid formation – radionuclide transport with colloids
Conclusions in RD&D 2001 and its review
It is stated in RD&D 2001 that some model development, mainly of an analytical nature, will
take place to be able to quantify the importance of colloidal transport.
In its review of RD&D 2001, SKI concludes that SKB should acquire a better understanding of
the processes that control transport and mobility of colloids, and if necessary incorporate these
processes into the safety assessment.
Newfound knowledge since RD&D 2001
Generic modelling of nuclide transport with colloids has been carried out during the current
three-year period. It is shown in /19-93/ that colloids can have a significant impact on plutonium
transport under certain combinations of parameters and conditions that are otherwise comparable
to the situation on Äspö. Specifically, it is shown that the filtration rate of colloids is an
266 RD&D-Programme 2004