Acceleration of irradiation hardening of low-copper reactor pressure vessel steel observed by means of SANS and tensile testing

Abstract

Neutron irradiation of low-copper reactor pressure vessel steels containing manganese and nickel gives rise to microstructural changes and a deterioration of mechanical properties. This deterioration apparently progresses slower than in steels containing more than ∼0.1 wt% Cu. An acceleration of this process after the accumulation of a threshold fluence caused by the so-called late blooming phases is a matter of debate. We report results of small-angle neutron scattering experiments and tensile tests for two low-Cu model RPV steels irradiated at 255°C. Motivation is given for the use of the integrated magnetic scattering as a microstructural parameter combining the volume fraction and magnetic contrast of nanometer-sized irradiation-induced features. The results indicate one of the rare cases of acceleration of both a change of a microstructural parameter and a yield stress increase. The issues of the nature of the irradiation-induced features, the role of phosphorus, and the observed strong correlation of integrated magnetic scattering and yield stress increase are addressed.

Keywords:

• neutron irradiation
• SANS
• tensile testing
• pressure vessel steel

Acknowledgement

The SANS experiment was performed on the spectrometer D11 of ILL Grenoble with the kind assistance of Charles Dewhurst.