RD&D-Programme 2004 - SKB

and the rock matrix, is also crucial for radionuclide transport. Matrix diffusion and sorption are
the two most important retention processes for radionuclides in the geosphere. Another factor
that can be of importance for retention is sorption on colloidal particles and transport with them.
The chemical environment in the water determines which speciation (chemical form) the radionuclides
will have, which is crucial particularly for the sorption phenomena. Certain nuclides
can be transported in the gas phase. Radioactive decay influences the groundwater’s content of
radionuclides and must therefore be included in the description of transport phenomena.
The research programme for the different processes in the geosphere is discussed in the
following sections.
19.2.2 Heat transport
Conclusions in RD&D 2001 and its review
In RD&D 2001, the viewpoint is expressed that heat is transported in the geosphere by conduction
in the intact rock, and that existing models and calculation methods are sufficiently verified
to enable the design criterion to be met.
In the opinion of the authorities, the maximum temperature nearest the canister is underestimated
in large-scale models with widespread heat loads, and the influence of simplified
calculation assumptions should be examined more closely. The THMC modellings that are
done in the future should be updated with the latest temperature calculations.
Newfound knowledge since RD&D 2001
The problem of the temperature evolution in the rock has been thoroughly addressed since
RD&D 2001, above all with a view to the maximum temperature nearest the canisters /19-1/.
The studies reveal relationships between the maximum temperature of the canister, the heat
transport properties of the rock, the heat output of the canisters at deposition, the deposition
geometry and the conditions in the buffer. The geometry of the individual canister is taken
into account so that the temperature evolution in the near-field, above all at the hottest part at
canister mid-height, is correctly calculated. Figure 19-1 shows an example. In the calculations,
which are performed with different combinations of analytical and numerical methods, the heat
transport problem is dealt with on all scales. The study also shows how sensitive the calculated
maximum temperature on the canister surface is to errors in input data, such as errors in the
thermal conductivity and heat capacity of the rock. The impacts of the simplified assumptions
that are generally made, for example the approximation to disregard the temperature dependence
of the thermal conductivity and the influence of the different heat transport properties of
the tunnel backfill, are analyzed and quantified.
Different laboratory-based methods for prediction of the thermal conductivity of the rock
nearest a borehole have been compared with a large-scale method for direct measurement in
boreholes, the thermal response test /19-2/. The thermal response test overestimates the thermal
conductivity by about 25 percent compared with the results obtained using laboratory methods
to determine the properties of the individual mapped rock types and calculate the effective
thermal conductivity based on the mapped rock type distribution. The interpretation is that
the full-scale method needs to be developed to compensate for the effects of water movements
in the borehole during measurement.
A systematic evaluation has been carried out of several methods for determination of thermal
conductivity and heat capacity in the laboratory /19-3/. The methods yield on average differences
on the order of a percent or so for a number of measured samples, while the variation
between individual samples can be considerably greater.
244 RD&D-Programme 2004