RD&D-Programme 2004 - SKB

out when the buffer swells, but our understanding of the course of events and thereby the heat
conduction properties in the gap is fraught with uncertainties.
In KBS-3H and KBS-3V, there may also be an air-filled gap between buffer and rock. This gap
is of importance for heat transport in an early stage of the repository’s evolution.
Conclusions in RD&D 2001 and its review
In the calculations of the heat transport in the rock, SKB uses a model with simplified assumptions
regarding the geometry of the near-field. In the authorities’ view, SKB should shed light on
the impact of this simplification when presenting the results. The authorities also think that the
couplings between the thermal, mechanical, chemical and hydrological processes, particularly in
the buffer, need to be studied further.
Newfound knowledge since RD&D 2001
A report has been published describing how the canister’s heat output at deposition, the rock’s
transport properties, the spacing between canisters and between deposition tunnels, the heat
transport conditions in the deposition holes and the rock’s undisturbed geothermal temperature
together determine the maximum temperature on the canister surface /17-5/. The results are
based on combinations of numerical and analytical methods, which means that a large number
of combinations of parameter values have been analyzed for KBS-3V and KBS-3H. In the
analytical method there are correction factors that have been determined by calibration calculations,
making it possible to take into account canister geometry and the heat flow distribution
around the canisters.
Altogether, the maximum canister temperature has been calculated for around 1,000 different
cases. The results are presented in nomogram form so that the canister spacing required to meet
the temperature requirements (a maximum temperature of 100°C on the canister surface) can be
read off directly from given assumptions regarding decay heat, heat conduction conditions and
safety margins. For some of the cases, independent, numerically calculated results are available
for comparison, for example the temperature calculations performed prior to SR 97 for Aberg
/17-6/. Independent analytical solutions are also available /17-7/ for use in comparison and
verification. All comparisons show good agreement.
The largest uncertainty concerns what temperature margins should be posited. In SR 97 and
earlier, it has been assumed that effects of unfilled gaps, in particular between canister and
buffer, can give rise to temperature offsets of 10°C. Furthermore, uncertainties in data are
judged to warrant an additional margin of 10°C, so that the calculated maximum temperature
should be 80°C or less. The size of the estimated temperature offset across the canister-bentonite
gap is based on assumptions regarding the emissivity of the copper surface, i.e. its capacity to
emit radiative heat.
The measurements currently being performed in the Prototype Repository indicate a temperature
offset of at most around 19°C between canister and buffer. This figure applies to the most
recently installed deposition hole categorized as dry during boring and before installation.
Converted to a standard decay heat output of 1,700 W, and allowing for the fact that this heat
output has declined by around ten percent when the temperature peak occurs, this means that the
margin for open gaps may need to be 17°C. The buffer-rock gap has been filled with bentonite
in the Prototype Repository. In the most recently installed hole, the heat transport across the
pellet-filled gap around six months after installation is roughly equal to that through the buffer
blocks.
Programme
The results that have been obtained from the site investigations in terms of variations in the
thermal properties of the rock need to be evaluated so that inhomogeneities on different scales
202 RD&D-Programme 2004