Thermomechanical In-Situ Experiments and Finite Element Computations
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THERMOMECHANICAL IN-SITU EXPERIMENTS AND FINITE ELEMENT COMPUTATIONS
ALEXANDRA PUDEWILLS,
ROLAND MULLER,
EKKEHARD KORTHAUS,
Institut fur Nukleare Entsorgungstechnik, Federal Republic of Germany
RAINER KOSTER
Kernforschungszentrum Karlsruhe,
INTRODUCTION In the ultimate storage of high level wastes in rock salt thermomechanical phenomena caused by the temperature rises play an important role for elaborating the storage concept.
Important factors are both large space
effects concerning the repository and the geological formation and small space phenomena influencing to a considerable extent the interaction between waste blocks and the host rock. While the large space aspects are largely inaccessible to an experimental investigation, the small space effects, holes,
especially the convergence behavior of individual heated bore-
are eligible for a systematic experimental and computational study
allowing verification of available computational methods and thermomechanical material laws for rock salt. As has been shown by previous theoretical studies,
the closure of heated
boreholes depends on the creep behavior of salt and on the stress boundary conditions prevailing at the place of the experiment.
Since we know from
experience that both parameters cannot be determined accurately enough, it
is
reasonable to perform experiments repeatedly at different test pla-
ces in order to obtain a realistic evaluation of the possible accuracy of computational predictions of the overall behavior of the heated salt, especially of the borehole closure. ments were performed in ASSE,
For this purpose three in-situ experi-
using a convergence measuring probe speci-
fically developed for such investigations,
and evaluated with the help of
thermomechanical model computations.
DESCRIPTION OF THE EXPERIMENTS The experiments were carried out in ASSE at 775 m depth in older halite. The novel mobile convergence measuring probe ("Standard probe") sively installed in three boreholes of 185 mm diameter, about 5 m depth from a tunnel,
was succes-
drilled down to
and the closure of the borehole taking
place during a heating phase of about 100 days was observed.
478
unheated body inner electrode outer electrode
centering joint
unheated body
Fig.1 Schematic drawing of the convergence measuring probe
479
The main components of the probe are an electric heater and an in-line convergence measuring device based on the measurement of the electric capacitance between annular electrodes placed on the probe body and at the borehole wall (Fig. 1). A number of thermocouples are installed on the probe body and on the external annular electrodes for temperature monitoring. The probe electronic system consists of a controlled power supply to the heater and an automatic data acquisition system for five convergence probes,
12 temperature measurement points,
as well as for heater
power and voltage. The convergence measuring technique applied here was chosen because other usual techniques of length or distance measurement are no
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