Gas Production and Liberation from Rock Salt Samples and Potential Consequences on the Disposal of High-Level Radioactiv
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GAS PRODUCTION AND LIBERATION FROM ROCK SALT SAMPLES AND POTENTIAL CONSEQUENCES ON THE DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE IN SALT DOMES
NORBERT JOCKWER Gesellschaft flr Strahlen- und Umweltforschung mbH, Institut fQr Tieflagerung, Theodor-HeuB-Str. 4, D-3300 Braunschweig, Federal Republic of Germany
1.
INTRODUCTION
Rock salt from the north German salt domes which are to be used for disposing high level radioactive waste consist, minor minerals anhydrite, such as carbonates,
along with the major mineral halite,
the
polyhalite and kieserite and of some trace minerals
clay, bitumencomponents.
Small amounts of free water - and
gas components either are adsorbed on the crystal boundaries or form inclusions with a size of less than one micrometer up to some millimeters. In
the surroundings of an emplacement borehole with high level radioactive
waste the rock salt will be heated to about 200 OC and the water of the hydrated minerals as well as the water and gas adsorbed on the crystal boundaries and present as inclusions will be liberated into the intergranular spaces. At the elevated temperature carbondioxide,different hydrocarbons,
hydrochloric acid
and sulphurdioxide may be generated by thermal decomposition of the carbonates, clay, bitumen and other trace minerals. As a result of
y-radiation gases such as hydrogen,
oxygen,
hydrochloric acid
and carbondioxide will be generated by radiolysis.
2.
RESULTS
2.1
GAS GENERATION AND LIBERATION AT ELEVATED TEMPERATURE
Some hundreds of rock salt samples from the Asse mine and about 50 from three other north German mines out of different depths and different stratigraphic layers have been investigated to determine the mineralogical composition and the water and gas contents. Table 1 shows the minerals which have been found in
136 salt samples from
statigraphic layers which might be suitable for disposing of high level radioactive waste. The amounts of these minerals,
in
addition to the halite,
range
468
from 0.1 to 5 wt %. In
some small layers the concentration of the minor minerals
reach 20 or even 30 wt %. The content of clay, carbonates and bitumen is much less than 0.1 wt %.
TABLE 1 Minerals being found within 136 salt samples from the Stassfurt and Leine horizon Mineral
being found in
halite anhydrite polyhalite kieserite sylvin
Primarily,
...
samples
136 73 106 19 3
as a result of the content of the hydrated minerals rock salt con-
tains small amounts of water.
The investigation of some hundreds of salt samples
has shown that 55 % of these samples have a water content less than 0.1 wt % and 75 % less than 0.2 wt %. (Jockwer 1;2;3). at the crystal boundaries and in
The amount of the water adsorbed
fluid inclusions is much less than the water
derived from the hydrated minerals. At elevated temperature in the range of 200
eC
this water will be liberated and then migrate on the crystal boundaries
and through microfissures as a result of a gradient of total pressure,
partial
pressure, concentration and temperature. To inv
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