Study of boron carbide evolution under neutron irradiation by Raman spectroscopy

Boron carbide, B12C3, is an absorbing material used to control the reactivi ty of nuclear reactors by taking advantage of nuclear reactions (e.g. B-10( n,alpha)Li-7), where neutrons are absorbed. During such reactions, radiatio n damages originating both from these nuclear reactions and from elastic co llisions between neutrons and atoms lead to a partial destruction of this m aterial, which gives the main limitation of its lifetime in nuclear reactor s. In order to understand the evolution of B12C3 in nuclear plants, the eff ect of neutron irradiation in B12C3 has been investigated by Raman and nucl ear magnetic resonance (NMR) spectroscopies. Comparisons of B12C3 samples i rradiated by 1 MeV electrons, 180 keV helium ions and neutrons are used to study the microstructure evolution of this material by Raman scattering. Th e analysis of Raman spectra of different B12C3 samples irradiated by neutro ns 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 mod el assumes that the CBC linear chain is destroyed whereas icosahedra are se lf-healed. B-10 atoms destroyed during the neutron irradiation are replaced in icosahedra by other boron and carbon atoms coming from the linear CBC c hain. The B-11 NMR analysis performed on unirradiated and irradiated B4C sa mples shows the vanishing of a strong quadrupolar interaction associated to the CBC chain during the high neutron irradiation. The B-11 NMR spectrosco py confirms the previous Raman spectroscopy and the proposed microscopic mo del of B12C3 evolution under neutron irradiation. (C) 2000 Elsevier Science B.V. All rights reserved.