SOME QUESTIONS OF MATERIAL INELASTICITY AND FAILURE IN THE DESIGN OF CONCRETE STRUCTURES FOR NUCLEAR REACTORS.

The purpose of this paper is to analyze several aspects of inelasticity of concrete which are particularly important for the safety analysis and failure predictions of concrete pressure vessels and containments. An entirely new type of constitutive law is developed, in which the gradual build-up of inelastic strain, dilatancy, and damage to the microstructure is characterized by means of intrinsic time z. Furthermore, development of a mathematical model for coupled heat and moisture diffusion, based on irreversible thermodynamics, is outlined. It is shown that even when the constitutive law is known, the finite element method as currently used does not reliably predict failure loads, because it cannot model unstable strain localization (due to cracking and drop in tangent modulus) and the inherent size effect in strength and ductility. A method of approximate treatment is suggested. A new method of creep analysis in which creep is modeled as a two-dimensional random process in creep duration and age is outlined. Finally, the nonuniformity of shrinkage and creep throughout the thickness of a containment wall is shown to produce higher stresses than the sustained operating temperature gradient.