Thermo-mechanical behavior of high level horizontal disposal cell during the period of exploitation under the heterogeneous load

Document available in extended abstract form only. The present work aims to study the evolution of the thermo-mechanical behavior of a horizontal disposal cell under the heterogeneous load during the period of exploitation of 100 years. This heterogeneity of load is due to the generated heat of seven high level waste packages of 1.6 m long which are intercalary disposed in 30 m of the horizontal cell following the concept in 2005 of ANDRA. The analyses have been conducted with different scenarios. The first one consists of modeling the whole length of the disposal cell but the lining is not taken into account for the sake of simplicity. In this case, the generated heat is heterogeneously distributed along the surface of cell. Precisely, at each position of 1.6 m of these seven disposal packages, the uniform heat flux calculated from the thermal power of a high level (HA-C5) waste package is imposed. The model 3D vertically extended from the natural surface to 1000 m of profound allows to account the Callovo-Oxfordian clay as well as its surrounding layers. Moreover, the length of model following the axial direction of the disposal cell is taken equal to 53 m which corresponds to the half distance between axes of the access drift. Following the other horizontal direction which is perpendicular to the axis of cell, a length of 6 m is considered by assuming the symmetrical condition between two disposal cells. This case of study using the coarse mesh is then completed by another model with only 2.35 m of length in the axial direction of disposal cell which allows to increase the precision of the results by using a finer mesh. This length represents a half of a waste package and a half of intercalary distance between two packages. Similarly to the first case, the uniform thermal flux of the half package is imposed on 0.8 m of the wall of the horizontal cell. The thermo-mechanical behavior of materials was used in all simulations by using the Flac3D (ITASCA) code based on the different element method. Concretely, a model of type of Lemaitre was implanted in this code to characterize the creep behavior of the Callovo-Oxfordian clay while other materials are supposed elastically isotropic. Otherwise, for the comparison purpose, a model 3D with 74 m of high between the center of cell and the superior surface of the Callovo-Oxfordian layer was studied in the Code-Aster (EDF). The boundary conditions applied to this superior surface of model are results obtained from the first case realized by code Flac3D to ensure the same conditions of these calculations. The second scenario deals with modeling the disposal cell at the presence of a steel lining. However, the interest in this case is dedicated to the heterogeneous load due to the apparition of a hypothetical break-out between the lining and the massif who can be arbitrary generated during the drilling of cell. For the precision purpose, we take into account in this model only 3.15 m of length which equal to the length of one section of lining in the construction of disposal cell. In order to represent the contact between the massif and the lining, one break-out is added in the mesh that its pessimistic position is carefully chosen. The heat flux in this model is uniformly applied on the wall of cell while the temperature obtained from a purely thermal analyze is imposed on the surface of lining. In addition, to simulate the possibility of relative displacement between the lining and the massif during heating, we use the available interface elements of the numerical code. The results extracted from these studies have highlighted the heterogeneous distribution of stress and strain field in the massif and the lining. The first scenario has demonstrated that the intercalary distribution of waste packages yields the fluctuation of the temperature fields and that of stress and strains around the wall of disposal cell. However, during the period of exploitation, the total (ortho-radial) stress in the massif is always in compression with the maximal value of 29 MPa after 20 years coinciding with the peak of temperature. The other scenario has shown the influence of the presence of break-out on the mechanical behavior of lining. This heterogeneous load has increased in the lining the maximum total (ortho-radial) stress up to 140 MPa in compression and to 86 MPa in traction after 50 years. These values, however, are inferior to the yield strength (210 MPa) of steel. (authors)