K. Yasunaga

Microstructure evolution in tungsten during low-energy helium ion irradiation

In situ transmission electron microscopy (TEM) study was performed to investigate the microstructural changes in tungsten during low-energy He+ ion irradiations in an electron microscope linked with an ion accelerator. The irradiations were carried out with 8 and 0.25 keV He+ ions at 293, 873 and 1073 K. In the case of the 8 keV irradiation, irradiation-induced vacancies act as nucleation sites for dislocation loops and helium (He) bubbles. Accordingly, such defects were formed even at the higher temperatures. With increasing irradiation temperature, the growth rates of dislocation loops and He bubbles rise remarkably. Although no vacancies are produced during 0.25 keV irradiation, He platelets, interstitial loops and He bubbles were formed. Impurity atoms may act as trapping centers for He atoms, which form bubbles by ejecting W atoms from their lattice site.

Correlation between defect structures and hardness in tantalum irradiated by heavy ions

Abstract Pure tantalum specimens were irradiated with 2.4 MeV Cu2+ ions up to 3 dpa at temperatures between room temperature and 1073 K. Transmission electron microscope (TEM) observation and micro-indentation tests were carried out to correlate the microstructure and the hardness. Significant radiation hardening occurred at temperature ranging from 673 to 873 K. Isochronal annealing of a specimen irradiated at room temperature up to a dose of 0.3 dpa resulted in a rapid increase in hardening between 573 and 673 K and continued to increase up to 873 K. The microstructure showed that the formation of small defect clusters is the major reason for both the radiation hardening and the radiation-anneal hardening.

The influence of heat treatments on neutron irradiated Nb1Zr alloy

Abstract The microstructural evolution of neutron irradiated Nb1Zr alloy at 693–1003 K to doses as high as 47.2 dpa has been investigated. At all irradiation conditions, strong void swelling resistance was found. In the cold-worked and aged specimens, voids of about 50 nm diameter were observed at lower temperature (693 K) and higher temperature ranges (918–1003 K), but not at intermediate temperature ranges (744–842 K). The void density increased with dose. The swelling behavior obtained by density measurements was revealed to be rather complex, depending on irradiation temperature and dose. The complex role of starting condition on both microstructural evolution and density changes are presumably due to phase-related changes during irradiation.

Study on analysis of crystal structure in CeO2 doped with Er2O3 or Gd2O3

Abstract To simulate the effects of burnable poison doping in nuclear fuel UO 2 , Er 2 O 3 (or Gd 2 O 3 )-doped CeO 2 pellets were prepared. Changes in lattice constant and atomic disordering for CeO 2 due to the Er 2 O 3 and Gd 2 O 3 doping were measured by means of XRD and XAFS. By the Er 2 O 3 doping, the lattice constant decreased, and a disordering of lattice structure was induced in the samples. The doping with Er 2 O 3 also induced the disordering of atomic arrangement around Er atoms, which was observed through the change in XAFS spectra. In contrast, the effect of Gd 2 O 3 doping was smaller than that of Er 2 O 3 doping. The result was discussed in terms of ionic size of dopants in CeO 2 crystal.

Microstructure of tantalum irradiated with heavy ions

Abstract For the purpose of investigating the general response of tantalum to irradiation, microstructures have been observed after irradiation with 3.0 or 2.4 MeV Cu 2+ ions at 773 to 1546 K and to 20 dpa. Below 1073 K, the microstructures consisted mainly of high density of small dislocation loops and straight dislocations. The small dislocation loops observed were identified to be of vacancy-type. Voids were formed above 973 K, however, the swelling was negligibly small up to 20 dpa below ∼1100 K. High density of defects observed below 1073 K may cause significant change in mechanical properties. This may be a concern in applying tantalum to fusion high heat flux components.