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

During neutron irradiation of boron carbide, helium bubbles nucleate, induc ing cracks and then reducing lifetime of control rods. The role of helium b ubbles has been clearly idientified by Transmission Electron Microscopy (TE M) 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 s train value as a function of neutron irradiation. Both the data are useful to present a quantitative analysis of the mechanism responsible for the dam age 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 be haviour under neutron irradiation and to present a quantitative analysis of irradiated B4C samples. The study of strain eta and Coherent Diffracting D omains (CDD) as a function of N(alpha), number of neutronic capture per uni t volume, exhibits a saturation of eta near 1% and a constant increase of C DD 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 durin g irradiation. Moreover, no second phase has been observed during both irra diation and annealing of B4C irradiated samples. (C) 1999 Elsevier Science B.V. All rights reserved.