THM modelling of the ALC1604 experiment

The report presents an interpretation of the full-scale HA-ALC1604 in situ heating test carried
out on Callovo-Oxfordian claystone (COx) in the Meuse/Haute-Marne underground research
laboratory (MHM-URL). The MHM-URL is a site-specific facility planned to study high-level
radioactive waste disposal in the Callovo-Oxfordian claystone. The main aims of ALC fullscale emplacement experiment performed within the context of in the EU project LUCOEX
(WP3) and consists in demonstrating the construction feasibility of a High-Level Waste (HLW)
cell representative of the 2009 benchmark concept and in determining the impact of thermal
loading on the overall behaviour of the cell. The experiment has also been used to acquire new
data on the THM behaviour of the surrounding rock and to compare them with those acquired
in previous small scale heating experiments.
The concept of the test consists of a horizontal micro-tunnel approximately 25 m long and 0.7
m in diameter, which excavated in the direction of the major horizontal stress. The excavation
rate of the micro-tunnel was around 0.3-0.5 mh-1, and the excavation was completed in seven
days. A non-alloy steel casing is placed in the cell body. The cell head has a metal sleeve called
the Insert. Five heaters (H1 to H5), each 3 m long and 0.5 m in diameter, have been installed in
the body section, continuously. The power applied in the deepest 15m was constant and equal
to 220 W/m, in order to reach around 85 ºC in two years. Heating has been applied in two
stages; A cooling stage (also applied in steps) completes the experiment. A comprehensive
instrumentation system has been installed to gather observations that define the THM behaviour of
the argillite. The test has been completed by the schedule envisaged.
Interpretation of the test has been assisted by the performance of a series of analyses simulating the
experiment reported in this document. Coupled 3D-THM numerical analyses have been carried
out to provide a structured framework for interpretation, and to enhance understanding of THM
behaviour of Callovo-Oxfordian claystone. Numerical analyses have been based on a coupled
theoretical formulation that incorporates a constitutive law especially developed for this type
of material. The law includes several features that are relevant for a satisfactory description of
the hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity
and occurrence of plastic strains before peak strength, significant softening after the peak, timedependent creep deformations and permeability increase due to damage. Particular attention
has been devoted to the modelling of the steel lining and the gap between it and the rock.
By performing the numerical analysis, it has been possible to incorporate anisotropy of material
parameters and of in situ stresses. The performance and analysis of the in situ tests have
significantly enhanced the understanding of a complex THM problem and have proved the
capability of the numerical formulation to provide adequate predictive capacity. The reference
analysis has achieved a satisfactory reproduction of the pore pressure observations in all the sensors.
Anisotropy effects during heating and cooling are adequately captured. They generally support the
values of the THM parameters used in the simulation of previous experiments.
More specifically, several thermal analyses have been performed to obtain the best estimated values
of anisotropic thermal conductivity that lead to the closest reproduction of the temperatures
recorded during the test. They differ by 2.5% from the reference values in both the direction parallel
and orthogonal to the bedding plane. As a general rule, an excellent reproduction of the temperature
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field and its variation with time during both heating and cooling has been achieved. Those optimised
anisotropic thermal conductivity parameters are subsequently used in the THM computations.
As foreseen, heating of the Callovo-Oxfordian argillite has resulted in significant increases of pore
pressures due to the differential thermal expansion of pore water and soil skeleton and solid phase.
The evolution of pore pressures is the result of the interplay of pore pressure generation by heating
and pore pressure dissipation. Observations of pore pressures show that the evolution of pore
pressures is affected by the anisotropic properties of thermal conductivity, stiffness and
permeability and the increase of permeability in the Excavated Damaged Zone. Numerical THM
model provide results in good agreement with all the pore pressure sensors.
A specific feature of ALC1604 experiment is the presence of a steel lining around the heater and a
gap between the steel lining and the rock, that are slightly eccentric with respect to the centreline of
the micro-tunnel because of the weight of both heater and steel lining. This peculiar feature has
been carefully reproduced in the 3D model and allows for a well-reproduction of lining deformation
during all the experiment. Aspects like lining ovalization during convergence of the rock on the
lateral side of the lining, ovalization stop when lining enters in contact with the vault of the microtunnel and further ovalization inversion is well-captured by the model.
The HA-ALC1604 experiment has yielded valuable data on the in situ behaviour of the CallovoOxfordian claystone subjected to heating and cooling around a HA disposal cell as well as on the
response of the steel lining. Interpretation, based on thermal and coupled THM analyses, has proved
satisfactory showing that the processes involved are well understood and adequately incorporated
in the formulation and associated computer code. It is worth noting that the numerical works
performed in this project have converged to a unique final three-dimensional model accounting for
most experiment elements (rock, steel lining, insert, gap, thermos-hydraulic effect of surrounding
galleries), based on THM parameters consistent with those used in other tests, and reproducing well
all measurements.