Mechanical and SCC Behavior of an API5L Steel Casing Within the Context of Deep Geological Repositories for Radioactive Waste

Abstract

Andra, the French national radioactive waste management agency, is in charge of studying the possibility of disposal of High Level activity Wastes (HLW) in deep geological repositories. The concept of HLW cells consists of horizontal micro-tunnels of about 0.7 m in diameter, equipped with a steel casing. In order to ensure the reliability of the casing, particularly with respect to Stress Corrosion Cracking (SCC), several in-situ experiments dedicated to the analysis of its short term mechanical and corrosion behavior have been performed at Andra’s Underground Research Laboratory (URL) as well as in surface laboratory. Reduced and full scale in-situ experiments consisting of equipping boreholes parallel to the major horizontal stress (σH) with instrumented steel tubing, have been performed to analyze the mechanisms involved in the casing/rock interface. The main characteristics of the short term mechanical load applied by the rock have been determined from local strain and convergence measurements. Although in-situ stress is isotropic in the section of boreholes parallel to σH, measurements exhibit a strongly anisotropic load. SCC experiments conducted on different steel grades, in contact with the clay host rock containing CO2/H2 revealed that general corrosion is the main type of corrosion expected for steel casings. However, the selected steel casing must have a sufficiently high yield strength (above 400 MPa) to reduce the risk of early stage plasticity due to host rock anisotropic convergence and thus to overcome SCC. API5LX65 steel seems to meet both mechanical and corrosion requirements, therefore being likely the appropriate material for the manufacture of the casing.

Assessment of the resistance to environmentally assisted cracking (EAC) of C-steel casing and overpack in the COx claystone

“…This ovalisation of the casing results from an anisotropic external load applied by the surrounding rock. This behaviour is directly related to the anisotropic extent of the excavation induced fractures network around the cell and is consistent with the convergence anisotropy measured on uncased full scale HLW cells having the same orientation with respect to the natural stress field [8,10] showing that horizontal convergence is higher than vertical convergence. This anisotropic loading results in a radial bending of the casing.…”

A full-scale in situ heating test in Callovo-Oxfordian claystone: observations, analysis and interpretation

“…According to HLW disposal cell reference concept, an API5L steel grade with a high yield strength (above 450 MPa) is selected for the casing to reduce the risk of Stress Corrosion Cracking (Bumbieler et al, 2015).…”

THM analysis of argillaceous rocks with application to nuclear waste underground storage

“…In order to ensure the structural reliability of the casing, several in situ experiments have been performed to analyse the mechanical and thermomechanical behaviour of steel casing and the mechanisms involved in the casing/rock interface at the MHM URL (Bumbieler et al, 2015). The measurements exhibit a strongly anisotropic load, in spite of isotropic in situ stress which was consistent with the convergence anisotropy measured on drifts (Armand et al, 2013) showing that horizontal convergence is twice the vertical convergence.…”

“…The selected steel casing has a sufficiently high yield strength (above 400 MPa) to reduce the risk of early-stage plasticity due to host rock anisotropic convergence and to overcome Stress Corrosion Cracking (SCC) (Bumbieler et al, 2015). The casing is considered to be linear elastic with parameters E, v and α, respectively, for Young's (elastic) modulus, Poisson's ratio and the coefficient of thermal expansion.…”