Abstract. For a repository of heat generating radioactive waste, the thermal behaviour
of the host rock and the impact of temperature increase on rock properties is
of general importance. In the German Site Selection Act (2017), the maximum
temperature of the container surface is preliminarily limited to
100 ∘C but this limit might change in the future based on
scientific and technological findings. Rock salt, as one of the possible host rocks, consists predominantly of halite
with varying amounts of accessory minerals (e.g., Hudec and Jackson, 2007);
however, some lithological units within a salt deposit, e.g. potash seams,
show a different mineralogical composition with high amounts of potash
minerals. Most of them are not very stable regarding temperature resistance
and stress, contain water in the crystal lattice, and therefore react
sensitively to changes in the environment. The melting point of most evaporated minerals is higher than the expected
temperatures in a repository but dehydration and partial melting might occur
at relevant temperatures, depending on the confining pressure. For example,
the temperature of dehydration of carnallite is ca. 80 ∘C at
0.1 MPa confining pressure but increases to ca. 145 ∘C
at 10 MPa confining pressure (Kern and Franke, 1986). The melting
point of carnallite increases from ca. 145∘C/8MPa to
ca. 167∘C/24MPa, which corresponds to a depth of
ca. 1000 m. Depending on the mineral paragenesis and composition of saline solutions,
different minerals develop with increasing temperature. For instance, a salt
rock with an initial composition of
kieserite + kainite + carnallite + solution R
(25 ∘C) reacts solely to kieserite and solution R, when the
temperature increases to 78 ∘C. A rock with a composition of
kieserite + carnallite + bischofite + solution Z
(25 ∘C) reacts to kieserite + carnallite from
25 to 50 ∘C, from 50 to
73 ∘C only kieserite is stable, and at temperatures
>73 ∘C kieserite and bischofite develop (Usdowski and
Dietzel, 1998). For the construction of an underground repository, the mineralogical
composition of the host rocks and fluids have to be evaluated carefully and
play an important role for the site selection and design of the underground
facility.
Mineral stability, brine development and rock-fluid reaction at repository-relevant temperatures (<i>T</i> < 200 °C) in the potential host rock rock salt
