RD&D-Programme 2004 - SKB

• Reactions between high-pH leaching products from cement and surrounding gravel fill.
• Precipitation of calcite and brucite (calcium carbonate and magnesium hydroxide).
• Alkaline degradation of cellulose in the waste (formation of complexing agents).
• Microbial and alkaline degradation of other organic material in the waste.
• Build-up of gas pressure in the waste packages and concrete enclosure.
• Influence of organic additives in cement on radionuclide sorption.
• Sorption properties of fresh and aged concrete.
• The importance of isotope exchanges and co-precipitation for retardation of low-sorbing
radionuclides.
Metal reactor internals comprise an appreciable portion of the waste. Some radionuclides are
present in crud on the surface of contaminated metal parts, but most of the activity derives from
induced activity. Then the metal must first corrode to render the radionuclides accessible. The
most common metal in waste with induced activity is stainless steel.
In contrast to the reactor internals, waste from research contains a large quantity of organic
material.
Radionuclides, as well as hazardous substances dissolved in water entering the waste packages,
can be transported out through the near-field barriers by diffusion and with water flowing
through the barriers. This outward transport is retarded by sorption in cement, concrete and
backfill gravel. Sorption is mainly dependent on the composition of the water, where pH, Eh
and the presence of organic complexing agents are important parameters.
Conclusions in RD&D 2001 and its review
SKI believes that better process models are needed that describe the long-term degradation of
concrete, and that should include a more detailed account of the influence of saline groundwater.
Newfound knowledge since RD&D 2001
Degradation of cellulose to ISA
Cellulose is broken down at high pHs and thereby forms a relatively large proportion of
isosaccharinic acid, ISA. This product is in turn a strong complexing agent at high pHs. Since
cellulose is a common material in LILW, this has been studied since it was noticed for the first
time more than ten years ago. The phenomenon was recently thoroughly investigated, with
support from SKB, at the Paul Scherrer Institute in Switzerland /25-5/ and previously in a
doctoral thesis at Linköping University /25-6/. ISA is formed with high yield (70–85 percent),
but the reaction ceases when approximately 20 percent of the cellulose has been broken down.
These reactions are influenced by the temperature. ISA decomposes in contact with calcium
hydroxide, one of the components in cement, and this reaction will be investigated more
thoroughly.
Sorption of radionuclides in concrete
Concrete comprises an important barrier to the dispersal of radionuclides in a repository with
LILW. Structural concrete is sufficiently dense to prevent the flow of water, but transport
of dissolved radionuclides by diffusion is possible. Nevertheless, concrete is an effective
barrier for most radionuclides in the waste, since radionuclides are sorbed on the concrete are
thereby retained. SKB is subsidizing researchers at Chalmers University of Technology and
the University of Linköping in their investigations of the influence of isosaccharinic acid and
other organic compounds on sorption of radionuclides in concrete /25-7, 25-8/. An important
332 RD&D-Programme 2004