Deuterium behavior in ceramics for Fusion Breeder Blanket after gamma-irradiation: a comparison between different characterization techniques

(1) CIEMAT - UNED,Foundation & Department of Energetic Engineering, Madrid, Spain
(2) CIEMAT
(3) UC3M

The ceramic breeders of an operating fusion blanket will be exposed to high levels of neutrons and gamma radiation. In ITER, it has been estimated a damage rate of about <10^-7 dpa/s and <10^3 Gy/s caused by the flux of high-energy neutron and ionizing radiation, respectively. The effects of ionizing radiation on insulators are quite marked due to the excitation of electrons from the valence to the conduction band, giving rise to charge transfer effects. On the other hand the comprehension of light ions behavior in solid breeder blanket (BB) candidates is of special interest for the optimization of the Tritium Blanket Module (TBM). In this contribution, the studies of processes taking place with temperature in irradiation damaged materials are analyzed with the support of several techniques.  Several experiments are performed on breeder candidate ceramics selected by Japan (Li2TiO3) and EU (Li4SiO4) for ITER TBM-testing. The aim of this experimental work is the understanding of Deuterium diffusion behavior in these materials before and after their γ-irradiation by a 60^Co source. Deuterium is implanted in selected disc-shaped ceramics with a 70 keV beam to a total fluence of 10^17 at/cm^2, and the depth profile as a function of thermal annealing treatments of as-received and irradiated samples are characterized by Nuclear Reaction Analysis (NRA). For comparison purposes, a different set of pellet samples are exposed to a Deuterium atmosphere at room temperature inside of pressurized (1 bar) steel capsule during 25 days. In this case, the sorption behavior of previously dehydrated and irradiated samples is observed by Thermal Induced Desorption (TID) technique during heating. Whereas the barrier effect of the surface is more evident in NRA analysis, it is possible to observe more clearly the contribution of the ionizing radiation on D2 outdiffusion by TID analysis.   

Secondary Ion Mass Spectrometry (SIMS) technique is used for analyzing both D-implanted and exposed to a D atmosphere samples. A SIMS HIDEN Workstation with a 6kV oxygen ion gun as the primary ion source is used for the Deuterium profile determination. The elemental analysis shows different depth profiles of D-implanted and D-sorbed samples. As expected, γ-irradiations influence the in-depth location of D inside the ceramic matrix, due to radiation-induced further internal D trapping and higher desorption activation energies. Finally, in an attempt to establish the nature of the defects associated to ionizing radiation acting as D trapping centers, two techniques of Positron Annihilation Spectroscopy (PAS) are used for investigating defects in the bulk of the irradiated materials: positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB). The creation of F+ defects due to γ-radiation is confirmed and better structural recovery in the case of a more pure material, after thermal treatments, is found.  This comparative study is especially interesting for the understanding of the processes taking place inside the material and the role of the surface in the thermal desorption of Deuterium at relevant operation conditions for the BB system here analyzed.