Irradiation-induced structural changes in surveillance material of VVER 440-type weld metal

The irradiation-induced microstructural changes in surveillance materials of the VVER
440-type weld metal Sv-10KhMFT were investigated by small angle neutron scattering
(SANS) and anomalous small angle X-ray scattering (SAXS). Due to the high fluence
a strong effect was found in the SANS experiment. No significant effect of the irradiationis detected by SAXS. The reason of this discrepancy is the different
scattering contrast of irradiation-induced defects for neutron and X-rays. An analysis
of the small angle X-ray scattering shows that the scattering intensity is mainly caused by vanadium-containing precipitates and grain boundaries. Both types of scattering defects are hardly changed by irradiation. Neutron irradiation rather produces additional scattering defects of few nanometers in size. Assuming these defects are clusters containing copper and other foreign atoms with a composition according to results of atom probe field ion microscopy investigations, both the high SANS and the low SAXS effect can be explained.