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Workreport 2019-4

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Name:

Effect of Polyurethanes on Groundwater Chemistry and Long-Term Safety - Risk Assessment

Writer:

Keith-Roach, M., Polido, E., Höglund, L-O.

Language:

English

Page count:

60

Summary:

Water leakages from fractures in the Olkiluoto tunnels and shafts have occurred that involved quite high flows of water. Polyurethane foams (PUF) could be beneficial for grouting these flowing fractures when the working conditions or water flow rates make the use of silica or cement grout difficult. However, the introduction of foreign materials to the repository is restricted and the risks for the long term safety of the repository need to be assessed.

This study provides an overview of the chemistry of PUF, an introduction to the PUF of interest to Posiva and the associated hazards. The literature is then reviewed to identify chemical and biological degradation mechanisms for PUF, leaching of PUF, and the potential for PUF leachate/ degradation products to act as complexing agents for radionuclides. Finally, the information gathered is used to carry out a highly conservative assessment of the risks associated with PUF. The approach taken calculated the maximum amount of risk species that could form from the degradation of 100 kg PUF, the amount that is expected to be used to grout a flowing fracture, or the resultant stimulation of sulphate-reducing bacteria (SRB). The length of tunnel required to dilute each potential risk species to a concentration defined as favourable for the repository was then calculated. The risk species considered were complexing agents, acetate, reduced nitrogen species, sulphide and dissolved organic carbon (DOC).

The conservative assessment showed that complexing agents and reduced nitrogen species only required dilution in relatively short lengths of tunnel to reach acceptable concentrations. Sulphide produced from the stimulation of SRB required the most dilution of all risk species, but precipitation of iron sulphides is expected to remove the HS- from solution soon after its formation. Acetate also required a relatively high level of dilution, but as PUF is unlikely to degrade entirely to acetate and acetate is highly biodegradable, these conservative concentrations were judged to be unrealistic. Therefore, the level of dilution required for DOC was considered to be the limiting factor, and with the conservative assessment applied, the PUF could be used at a distance > 60 m from the deposition holes.

The risks associated with the degradation products migrating into the fracture network are also discussed. Although the risks are thought to be low, this may need further consideration depending on the intended location for the use of PUF.

Keywords:

Polyurethane, degradation, complexation, DOC, sulphide, nitrogen

File(s):

WR 2019-04_web (pdf) (14.9 MB)


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