Characterization of radiation damage in inert matrices for transmutation of nuclear waste

The radiation damage produced by energetic heavy ions in several ceramic oxides was characterized by different methods: thermoluminescence, optical absorption, fluorescence and electron paramagnetic resonance. Following ion irradiation, the thermoluminescence intensity of all the samples is shown to decrease. This can be related to the observed rise of the optical absorption in the whole wavelength range. Moreover, optical absorption and electron paramagnetic resonance measurements highlight the appearance of induced defects. The ones observed in the paramagnetic resonance spectra of irradiated pellets seem to be holes trapped on oxygen-ions. The concentration of these induced defects increases with ion fluence and fluorescence measurements indicate that some pre-existing defects such as F and F2 centers in alumina follow the same trend. INTRODUCTION The transmutation of highly radiotoxic minor actinides Np, Am and Cm and living fission products such as Tc appears as an effective waste management solution for the future. To develop this technique, the knowledge of the behavior of transmutation targets in reactor is necessary. These targets are composed of nuclear waste embedded in an inert matrix, which is designed for containing the different elements produced by the transmutation and withstanding the energy generated. Therefore, the material selected as possible inert matrix has to fulfill a certain number of criteria such as high melting point, good thermal conductivity, suitable mechanical properties, low neutron crosssection, compatibility with the coolant and the cladding and good behavior under radiation effects [1]. Since they respond to a certain number of these criteria, several ceramic oxides are good candidates. The purpose of this paper is to study the stability of three oxides under irradiation by fission products: alumina α-Al2O3, one of the standard materials for the science of ceramics, magnesium aluminate spinel MgAl2O4 used as inert matrix for the majority of irradiation tests in reactor and magnesia MgO chosen for the future tests in reactor (e.g. experiments ECRIX, MATINA, CAMIX-COCHIX and FUTURIX-TA in Phenix [2], and EFTTRA T5 in HFR [3]). To simulate the impact of fission products, several samples of these materials have been irradiated with highly energetic heavy ions. The radiation damage, especially point defects, was investigated using different characterization techniques namely thermoluminescence (TL), optical absorption, fluorescence and electron paramagnetic resonance (EPR). EXPERIMENTAL METHODS Irradiation of the Samples The investigated samples were disks of about 1mm height cut off from cylindrical sintered pellets of α-Al2O3, MgAl2O4 and MgO (diameter = 5 mm for magnesia and 8 mm for alumina and magnesium aluminate spinel). These disks were polished on one face (down to 1μm diamond powder) before annealing at 950°C during four hours, and they were irradiated at CIRIL-GANIL (Centre Interdisciplinaire de Recherche Ions Lasers Grand Accélérateur National d’Ions Lourds, Caen, France) with energetic heavy ions (365 MeV Kr and 540 MeV Kr). TABLE I. Irradiation conditions of the samples irradiated with 540 MeV Kr Fluence (ions/cm2) (dE/dx)e (keV/nm) 5×10 1×10 1×10 1.65×10 14.6 for Al2O3 13.5 for MgO 13.2 for MgAl2O4