Detecting irradiation induced damage in RPV steels by SANS

It is well known that reactor pressure vessel steels suffer a degradation of the mechanical properties caused by neutron irradiation. Unfortunately, the ultra-fine microstructural features produced by irradiation cannot be directly detected by means of microscopic imaging techniques. Indirect evidence can, however, be obtained on the basis of Positron Annihilation Spectroscopy (PAS) and Small-Angle Neutron Scattering (SANS). The latter method has the advantage of high sensitivity to particle sizes from less than 1 to more than 10 nm in conjunction with integrating over a macroscopic volume (Fig. 1). Volume integration is important, because the difference between SANS intensities for neutron-irradiated material and unirradiated reference becomes independent of local fluctuations of both composition and microstructure. Furthermore, both size distribution and volume fraction of irradiation-induced defects can be calculated from measured SANS intensities. The ratio of SANS intensities for magnetic and nuclear scattering reveals also information on the composition of defects. In the Institute of Safety Research at the Forschungszentrum Rossendorf SANS experiments on Reactor Pressure Vessel (RPV) steels have been systematically and successfully designed, performed and analysed for a period of more than 10 years. In the present paper a short overview of the main results obtained by the Rossendorf group is given. Some open questions are summarized. On the other hand, one will not be able to obtain optimum information on the basis of a single experimental technique alone in most cases. From this viewpoint, supplementary experimental methods and numerical simulation techniques are considered.