Conventional transmission electron microscopy has been used to investigate the radiation-induced changes in the microstructure of VVER-type RPV steels. Specimens irradiated at ≈ 270 °C temperature in both the power light-water reactor (surveillance specimens) and the research reactor have been studied. In addition, the microstructure of the thermally aged specimens was analyzed too. Microstructural analysis has been supplemented by some preliminary results of the Doppler broadening and lifetime positron annihilation measurements.
Radiation-induced matrix defects visible by TEM in power reactor surveillance specimens consist of black dots, small dislocation loops and fine precipitates distributed heterogeneously with a dislocation substructure. Evolution of the radiation damage structure depends on the neutron fluence and the neutron flux. Only hardly discernible black dots were observed in specimens irradiated in the research reactor in NRI Řež. However, also in this case, the presence of a vacancy-rich component in the damage structure was suggested by positron lifetime measurements.