Phosphorous content and distribution in the NPP Kozloduy RPV Unit 1 Weld 4 metal

Phosphorous content and in depth distribution in the NPP Kozloduy Unit 1 Weld 4 metal were investigated. Wet chemical analysis was carried out on chips cut layer by layer at five levels reaching 7.1 mm distance from the inner RPV wall. The measured average concentration of P was 0.046% which is considerably lower than expected. No typical intercrystal fracture was found in 1 mm 2 specimen impact broken at −120°C under vacuum. Auger electron spectroscopy was applied in order to investigate the distribution of phosphorous on grain boundaries in the weld metal. Single regions of increased P content were found mainly on the periphery of secondary cracks or in zones of small size facets. In depth distribution of P in these regions was obtained by argon sputtering. The character of the in depth profile was more typical for the case of P-containing precipitations than of P segregation. The measured low P concentration and the character of P distribution could be regarded as less harmful to the mechanical properties, especially for the toughness of the weld metal, compared to the case of grain boundary segregation.