Instrument V4: Topic 1 - SANS investigation of irradiation-induced defects in RPV steels; Topic 2 - Characterization of nanoparticles in ODS Fe-Cr model alloys

Topic 1: Hardening combined with embrittlement of the reactor pressure vessel (RPV) due to the neutron irradiation during the operation of a nuclear power plant is one of the most safety-relevant phenomena. So far, design and safe operation have mainly relied on a predominantly empirical approach based on long-term irradiation programs in test reactors as well as on mechanical testing in specialized hot cells. However, the physical mechanism of radiation damage is not yet understood in detail. The microstructure that arise in RPV steels, primarily as a consequence of radiation-enhanced diffusion and defect clustering, are extremely fine-scaled, i.e. in magnitude of few nanometers. Among the experimental methods capable of detecting and characterizing irradiation-induced clusters small-angle neutron scattering is the only technique allowing a statistically representative size distribution of clusters to be calculated.
The measurements carried out on the instrument V4 have contributed to the understanding of the

• cluster formation under different irradiation conditions (temperature, neutron fluence and flux),
• effect of impurities and alloying elements on the formation of irradiation-induced clusters,
• cluster dissolution and/or coarsening after post-irradiation annealing treatments,
• reirradiation behaviour of materials.

The instrument V4 is particularly suitable for such kind of investigations because of the special equipment – an evacuated sample environment with electromagnet and a 14 position sample changer. This allows measurements with low background noise particularly important for the detection of very small scatterers such as irradiation-induced clusters.
Topic 2: Ferritic-martensitic chromium steels are candidate materials for future application in both Gen-IV fission and fusion technology. In order to improve irradiation-creep properties and to reduce detrimental helium effects oxide-dispersion strengthened (ODS) variants of these steels are of prime interest. Due to the complex microstructures of these ODS steels there is a multitude of hardening features, such as Y2O3 particles, carbides and alpha'-phase particles, that may also affect the brittle-to-ductile transition. Therefore, the fabrication and characterization of dedicated model alloys is an important means in order to separate individual effects and to improve mechanistic understanding.
The quality of the ODS material produced by means of a special procedure of powder metallurgy was checked and characteristics of ODS nanoparticles were determined by SANS.