1. Introduction

The purpose of this document is to describe the development and numerical implementation of a law of behavior under irradiation of the materials constituting the internal structure of nuclear reactor vessels: 316 and 316L steels worked for screws, as well as hyperhardened 304 and 304L steels for the partitions, reinforcements and the core shell. This law of behavior is formally the same for both types of steel, only the parameters varying from one material to another.

The developed model must be able to account (in addition to thermoelasticity) of the plasticity induced by the tightening of the screws, of the well known radiation creep for metallic materials, and of any swelling induced by irradiation. The effects of creep under irradiation and swelling differ in that deformation occurs at a constant volume for creep, which is not the case for swelling. The various phenomena mentioned above are listed exhaustively in the document [ref] and will therefore not be the subject of an additional description in what follows.

This document is structured as follows: the law of behavior chosen to describe the response of hyperhardened 304/304L and 316/316L steels is first presented, as well as the equations allowing its implementation in the form of implicit integration. Each mechanism is detailed in general terms. The dependence of the material parameters on the control variables that are irradiation and temperature is explained. The numerical values of these parameters are presented in note EDF R&D HT-26/05/045/A. The limitations of the model and its application are then presented in Code_Aster.

It should be noted that the formalism presented here, if it has been implemented in the context of a very specific application, can be used to describe the behavior of any type of material with characteristics of plasticity, creep under irradiation and swelling. In this case, it will be important for the Code_Aster user to obtain the material coefficients necessary to use the model.