Study of boron carbide evolution under neutron irradiation by Raman spectroscopy

Boron carbide, B12C3, is an absorbing material used to control the reactivity of nuclear reactors by taking advantage of nuclear reactions (e.g. B-10(n,alpha)Li-7), where neutrons are absorbed. During such reactions, radiation damages originating both from these nuclear reactions and from elastic collisions between neutrons and atoms lead to a partial destruction of this material, which gives the main limitation of its lifetime in nuclear reactors. In order to understand the evolution of B12C3 in nuclear plants, the effect of neutron irradiation in B12C3 has been investigated by Raman and nuclear magnetic resonance (NMR) spectroscopies. Comparisons of B12C3 samples irradiated by 1 MeV electrons, 180 keV helium ions and neutrons are used to study the microstructure evolution of this material by Raman scattering. The analysis of Raman spectra of different B12C3 samples irradiated by neutrons clearly shows that during the cascade displacements, the 385 and 527 cm(-1) modes disappear. These characteristic features of Raman spectra of the neutron irradiated samples are interpreted by a microscopic model. This model assumes that the CBC linear chain is destroyed whereas icosahedra are self-healed. B-10 atoms destroyed during the neutron irradiation are replaced in icosahedra by other boron and carbon atoms coming from the linear CBC chain. The B-11 NMR analysis performed on unirradiated and irradiated B4C samples shows the vanishing of a strong quadrupolar interaction associated to the CBC chain during the high neutron irradiation. The B-11 NMR spectroscopy confirms the previous Raman spectroscopy and the proposed microscopic model of B12C3 evolution under neutron irradiation. (C) 2000 Elsevier Science B.V. All rights reserved.