Dominique Gosset

Order-disorder phase transition induced by swift ions in MgAl2O4 and ZnAl2O4 spinels

Grazing X-ray diffraction was used to study in details the behaviour of two spinels, ZnAl2O4 and MgAl2O4 irradiated by swift ions. Such an irradiation allows to have an important irradiated depth and then accurate diffraction diagrams. Rietveld refinement done on these diagrams clearly show an order–disorder transition due to a melting of cations under irradiation in these two spinels. More especially, these results clearly exhibit that no new phase is created under irradiation in MgAl2O4. Raman spectroscopy, sensitive to the crystallographic space group, seems to confirm this analysis.

Determination of the helium diffusion coefficient in nuclear waste storage ceramics by a nuclear reaction analysis method

Abstract Host matrices for actinide immobilisation will undergo the formation of large helium quantities due to alpha decay. Helium diffusion rate has to be known in order to predict the long-term behaviour of the material, and particularly, the influence of helium accumulation on mechanical properties. A nuclear reaction analysis method, namely the 3He(d,xa0p)4He reaction, has been used to analyse the evolution of 3He profiles after ion implantations at 1 and 3 MeV in two materials, monoclinic ZrO2 (as a test material) and Ca9Nd(PO4)5(SiO4)F1.5(OH)0.5 britholite (envisaged for Am and Pu long-term storage). Two data processing methods are used: the classical excitation curve (proton yields versus deuteron energy) and second, the proton energy spectrum for a given deuteron energy. The characteristics of the 3He profiles (depth, width) obtained by both methods are compared to SRIM estimations. Their evolution during subsequent annealings allows an estimation of the helium diffusion rate in the britholite: D ( cm 2 / s )=(2.5±1.5)×10 −4 exp (−(1.07±0.03 eV )/ kT ) in the temperature range 200–400 °C, in agreement with previous results on similar materials. Moreover, the shape of the proton energy spectra suggests channeling effects in britholite.

Neutron irradiation effects in boron carbides: Evolution of microstructure and thermal properties

Abstract Different boron carbides, potentially used as control rods materials in fast neutron reactors, have been irradiated up to ≈ 25 × 10 20 captures/cm 3 in the French Phenix LMFB reactor. Microstructural and thermal characterizations show very different behaviors, depending on the density (from 70 to 98%), the composition (B 4 C or B 8 C) and the fabrication process (magnesothermal or carbothermal) of the materials. In spite of the low absorbed doses, all the materials show extended intergranular fracture, strong lattice distorsions and drastic thermal conductivity decrease.

Nuclear microprobe analysis of 7Li profile induced in HfB2 by a neutron irradiation

Abstract HfB2, a solid poor in boron, was irradiated by thermal neutrons in an experimental reactor. Using a nuclear microprobe, we have tracked lithium atoms produced by the 10 B ( n , α ) 7 Li reaction and compared the calculated and measured 7Li profiles in HfB2 irradiated samples. This comparison shows that Li atoms do not diffuse during irradiation (323 K). The comparison of non-annealed and annealed irradiated HfB2 plates clearly shows that lithium atoms do not migrate out of samples even at high temperatures (1273 K). These results associated to previous transmission electron microscopy (TEM) observations seem to show that lithium atoms are trapped by dislocation loops created by displacement cascades during neutron irradiation.