Neutron irradiation damage to boron carbide: X-ray diffraction analysis

B4C boron carbide is a ceramic used as a neutron absorber in the control ro ds of nuclear reactors. The irradiation induces the formation of large quan tities of helium, coming from the neutron capture reactions by B-10. The he lium atoms agglomerate under very high pressure in parallel, penny-shape, n anometric bubbles. The resulting anisotropic deformations lead to an extens ive damage of the material (swelling, cracking). The Hall-Williamson method has been used to estimate the density of the bub bles and the associated strains on highly irradiated B4C samples (total bor on burnup from 1 to 10%). Before this, we have analysed the diffraction lin es profiles in order to take into account the large distorsions induced by the transparency effect. At last, the cell parameters have been determined by the Cohen's method. The density of defects (from 10(15) to 2. 10(16)/cm(3)) and the anisotropy of strains (around 0.2% in the plane of the bubbles, 1% in the perpendicula r direction) are in good agreement with the results of TEM examinations and theoretical estimations. A low irradiated sample (1% boron burnup) have th en be submitted to isochronal annealing from the irradiation temperature (a round 500 degreesC) to 2200 degreesC. We have then observed different recov ery stages of the cell parameters and of the strains, the main stage locate d around 1200 degreesC. This can be associated with the brittle/ductile tra nsition of B4C : at higher temperature, the plasticity of the material allo ws a relaxation of the internal pressure in the bubbles.