TEM characterization of irradiated beryllium

Abstract

It is suggested that beryllium will be used as a neutron multiplier material in the Helium-cooled Pebble Bed ({HCPB}) European concept of a breeding blanket of demonstration power reactor {DEMO}. Long-term irradiation tests in high-neutron-flux nuclear research reactors yield information about the evolution of the microstructure of beryllium pebbles under close-to-fusion conditions (temperature, damage dose, helium and tritium production) excluding 14 {MeV} neutrons which are not present in the neutron spectra of fission reactors. The previous microstructural characterisation of irradiated Be performed in the course of high dose irradiation program ({HIDOBE} I) show the temperature depended formation of cavities inside the materials [1,2]. In the recent study was investigated beryllium pebble which was irradiated at 750°C. The target preparation of {TEM} specimens was performed using focused ion beam ({FIB}) which enables {TEM} investigation of defined areas such as secondary phases or grain boundaries. Fig 1 shows a {TEM} image which demonstrates preferable formation of voids with sizes up to 1µm along the grain boundary. The edges of the voids correspond with crystallographic planes of the Be matrix, but because both halves of the bubble grow in two differently oriented grains, their shape is more irregular. The areas with high density of 30-70 nm large voids are located on the distance of 0.5 µm - 1.3 µm on both sides of the grain boundary. The areas of 0.5-1.5 µm thickness close to the grain boundary without any visible void can be named as void denuded zone. These zones formed because grain boundaries act as a sink for vacancies and interstitials that form in the nearest area. On the other hand, this effect promotes also the formation of large voids direct on the grain boundary. The similar effect was already observed in polycrystalline neutron irradiated tungsten. The voids in beryllium usually show shapes of flat discs which are located in the basalt plane of hexagonal Be [1]. In the Fig. 2 (a) and (b) the same area is imaged which was tilted to the angle of ≈60° (-29° till +30° alpha tilt of goniometer). It is clearly seen that shape of the voids changes from narrow strip at -29° to the hexagonal faceted void at +32° alphq tilt. These results enable determination of the void size and thickness distribution histogram. Imaging of lamella in an oxygen map (Fig. 3) reflects the topography of beryllium surface which has been oxidized by contact with air. The formation of this thin oxide layer enables imaging of voids which got contact to the foil surface during {FIB} preparation. In Fig. 3a {HAADF} image of an area with numerous voids is presented. Be is oriented with the c axis in the image plane as it marked in the image. The open voids got a surface decoration and are visible as narrow strips. The voids which do not have contact with a foil surface remain invisible in the oxygen map without any decoration of the void's surface with oxygen or other impurity elements. This imaging allows correct calculations of the size distribution of radiation induced voids in irradiated beryllium. Conclusion: Observation of preferable formation of voids on the grain boundary and a void-free zone in the area next to the grain boundary. {TEM} imaging of the voids depend on the orientation of the image plane that varies from a regular hexagon to an elongated shape as a line segment with an intermediate state as a rectangle.