ABSTRACT

ABSTRACT: In the framework of feasibility studies for nuclear waste disposal, numerical modelling of the underground facilities is necessary to study different physical processes occurring during the live-time of nuclear waste. In most cases, numerical computations for assessing the evolution of waste disposal cells are achieved under the assumption of constant gas pressure. However, after the introduction of the waste canisters, different potential gas sources due to chemical process exist in the storage cells. Induced gas overpressure may have an impact on the mechanical stability of host rock. This paper is devoted to study the gas-mechanical coupling effect, using a mechanical model and a two-phase flow model within the Biot?s poromechanical theory framework. To highlight the potential coupling effects of mechanical laws on gas transfers, two modelling cases are considered. First only flow problem is modelled and the medium is assumed infinitely stiff. In a second step, the couplings between fluid transfers and the mechanical behaviour are taken account. In this paper, one-dimensional simplified geometry is considered and two different numerical schemes are performed.

1. INTRODUCTION

Problem of gas production in underground waste storage disposal are one of key research items in its long term safety assessment. Gas production, principal hydrogen, is principally due to the corrosion of metallic components of waste disposal (over-package of canister, steel support of cell) [1,2]. Then it is important to predict the long term effect of this gas production on mechanical stability of the disposal cells. Gas transfer has been studied experimentally [2, 3], exhibiting complex behaviour especially for initially water saturated conditions. Tests showed that gas entry and breakthrough are often accompanied by the development of preferential pathways, which propagate through the sample [4]. Many facts are involved in the initiation of the gas pathways in a rock: gas pressure increasing rate, degree of saturation, percolation of pore networks, and mineralogical composition. Numerical simulation of preferential pathways is not under the scope of this paper. Regarding to the scope of gas-mechanical coupling effect at disposal cell scale, a continuum two-phase flow model within a Biot theory framework is used. In this paper we put the emphasis on phenomena induced mechanical effects by gas diffusion. A one dimensional simplified geometry will be studied. One hypothesis of these calculations is that hydrogen and liquid vapour are constitutive gas of the gas mixture. This hypothesis involved that air pressure is neglected. Moreover, thermal effect was not taking into account because the kinetic of heating and that of corrosion is shifted in time; the paroxysm of the first is about 10 to 20 years and the second one is about thousand years. First, we briefly describe hydromechanical model coupled with gas diffusion used for simulation of waste disposal in a deep argillaceous rock (called also argillites) investigated by Andra. Then two numerical schemes will be discussed. In the first one, all non linearity is taking into account. In the second scheme, fluids volumetric mass evolution is neglected regards to pressure evolution. This leads to consider fluids incompressible. Such hypothesis is usually used in case of petroleum production.

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