Effect of Neutron Flux on an Irradiation-Induced Microstructure and Hardening of Reactor Pressure Vessel Steels

Andreas Ulbricht,Elvira Oñorbe,Frank Bergner,Bertrand Radiguet,Andreas Wagner,Hieronymus Hein,Eric Hirschmann,Auriane Etienne,Mercedes Hernández-Mayoral

doi:10.3390/met12030369

Abstract

The existing knowledge about the effect of neutron irradiation on the mechanical properties of reactor pressure vessel steels under reactor service conditions relies to a large extent on accelerated irradiations realized by exposing steel samples to a higher neutron flux. A deep understanding of flux effects is, therefore, vital for gaining service-relevant insight into the mechanical property degradation. The existing studies on flux effects often suffer from incomplete descriptions of the irradiation-induced microstructure. Our study aims to give a detailed picture of irradiation-induced nanofeatures by applying complementary methods using atom probe tomography, positron annihilation, small-angle neutron scattering and transmission electron microscopy. The characteristics of the irradiation-induced nanofeatures and the dominant factors responsible for the observed increase of Vickers hardness are identified. Microstructural changes due to high flux conditions are smaller nm-sized solute atom clusters with almost the same volume fraction and a higher concentration of vacancies and sub-nm vacancy clusters compared to low flux conditions. The results rationalize why pronounced flux effects on the nanofeatures, in particular on solute atom clusters, only give rise to small or moderate flux effects on hardening.

Highlights

The irradiation of reactor pressure vessel (RPV) steels with energetic neutrons gives rise to the formation of several kinds of nanofeatures, including vacancy clusters, dislocation loops, Cu-rich clusters and Mn-Ni-Si-rich solute atom clusters [1,2,3,4,5]
To reach a complete description of the irradiated microstructure, it is obviously necessary to apply a multitude of complementary techniques, such as atom probe tomography (APT) [10,11], positron annihilation spectroscopy (PAS) [12,13], small-angle neutron scattering (SANS) [14,15] and transmission electron microscopy (TEM) [16,17]
The TEM results in this study indicate a smaller number density of dislocation loops for the RPV base material irradiated at the higher flux

Summary

Introduction

The irradiation of reactor pressure vessel (RPV) steels with energetic neutrons gives rise to the formation of several kinds of nanofeatures, including vacancy clusters, dislocation loops, Cu-rich clusters and Mn-Ni-Si-rich solute atom clusters [1,2,3,4,5]. These nanofeatures impede the dislocation slip resulting in a degradation of the mechanical properties, i.e., irradiation hardening and embrittlement. All reported results agree on the operation of a pronounced flux effect on the size of irradiation-induced solute atom clusters, while, for the cluster volume fraction, cases of both no flux effect [19] and significant flux effect [23]

Methods

Discussion

Conclusion