Abstract
Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the abrupt degradation of thermal conductivity of boron carbide (B4C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in B4C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed B4C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup.
Highlights
Boron carbide (B4C) is a promising neutron absorbing material which is extensively used as control rod and shielding material in nuclear reactor due to its high neutron absorption cross section accompanying low neutron induced radioactivity [1,2]
The irradiation-induced defects influencing the thermal conductivity of boron carbide generally consist of three types of defects: (1) the substitutional defects of boron atoms by lithium atoms; (2) the Frenkel defect pairs of boron and carbon atoms owing to the collisions with the fast neutrons; (3) the helium bubbles as well as the strain fields surrounding them [26,27] and the accompanying microcracking
It implies that the abrupt degradation of thermal conductivity at around 5×1026 cap/m3 should not be attributed to the helium bubbles and the surrounding strain fields, which is contrary to the assumption of Gosset et al [7]
Summary
Boron carbide (B4C) is a promising neutron absorbing material which is extensively used as control rod and shielding material in nuclear reactor due to its high neutron absorption cross section accompanying low neutron induced radioactivity [1,2]. Mahagin et al [4] inferred that the point defects induced by fast neutron irradiation as well as the plate-like pores/microcracks could serve as phonon scattering centers, while the crystal structure modifications may enhance the phonon–phonon scattering (Umklapp processes), both of which degrade the thermal conductivity significantly. They assessed the contributions of the defect scattering and Umklapp processes to the degradation, and came to a conclusion that the contribution of the former, especially the point defects, was remarkably higher than that of the latter. A theoretical model with an overall consideration of various factors was proposed and validated to predict the thermal conductivity variation of boron carbide under fast neutron irradiation
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