Study of B4C microstructure evolution under neutron irradiation by X-ray diffraction profiles analysis

During neutron irradiation of boron carbide, helium bubbles nucleate, inducing cracks and then reducing lifetime of control rods. The role of helium bubbles has been clearly idientified by Transmission Electron Microscopy (TEM) photographs. X-ray diffraction may be a complement to TEM studies of B4C microstructure evolution under irradiation. In this article, we show that X-ray profiles may be used to calculate a mean bubble density and a local strain value as a function of neutron irradiation. Both the data are useful to present a quantitative analysis of the mechanism responsible for the damage of irradiated B4C material. To observe an eventual solubility of helium atoms in the B4C matrix, we have performed different isochronal annealing on irradiated B4C samples. Results of X-ray diffraction experiments on both irradiated and annealed samples permit to confirm previous works on B4C behaviour under neutron irradiation and to present a quantitative analysis of irradiated B4C samples. The study of strain eta and Coherent Diffracting Domains (CDD) as a function of N(alpha), number of neutronic capture per unit volume, exhibits a saturation of eta near 1% and a constant increase of CDD up to 4 x 10(16) CDD/cm(3). This eta and CDD evolution can be explained by helium bubble growth up to a 'characteristic' size in the material during irradiation. Moreover, no second phase has been observed during both irradiation and annealing of B4C irradiated samples. (C) 1999 Elsevier Science B.V. All rights reserved.