The Effect of Point Defects Induced by Fast Neutron Irradiation on the Thermal Conductivity of Boron Carbide

Abstract

The thermal conduction behavior of the neutron absorbing ceramic boron carbide in the initial stage of the irradiation was analyzed and a classical thermal conduction model was used to estimate the variation of the thermal conductivity in this paper. The calculated thermal conductivity using the model shows a large degration in the initial stage of irradiation. As the burnup increases, the thermal conductivity turns to be free of temperature dependence. These calculated results are consistent well with the expermental data of thermal conductivity of the irradiated boron carbide, which may suggest that the variation of the thermal conductivity of boron carbide is predominantly determined by the point defects scattering in the initial stage of irradiation.

Thermal conductivity of boron carbide under fast neutron irradiation

“…Since the 10 B atoms are continually removed from the lattice due to the neutron captures, the resulting boron vacancies appear most feasible to accommodate the locally generated lithium atoms. These substitutional defects of boron atoms by lithium atoms were considered to be the primary factor leading to the degradation of thermal conductivity in our previous work [9]. In the present work, the contribution of these substitutional defects on the thermal conductivity was re-evaluated in terms of the phonon scattering theory.…”

“…) It is believed that the lithium atoms generated by this reaction are most likely to substitute for the 10 B atoms which have been removed from the lattice [10]. The substitution of boron atoms by locally generated lithium atoms has been considered to be one of the most important types of point defects to strongly scatter the phonons and significantly reduce the thermal conductivity, as discussed in our previous work [9]. On the other hand, the helium atoms associated with lithium atoms are virtually insoluble in the lattice of boron carbide [11].…”

“…The controversy [4,7,9] with regard to the determining factor in the degradation of thermal conductivity stems from the lack of quantitative assessment on the contribution of various irradiation-induced defects. So we attempted to re-evaluate the contributions of the three types of primary defects throughout the irradiation process.…”

“…This may offer an opportunity to evaluate the contributions of Frenkel defect pairs and the substitutional defects separately. In our previous work [9], it was natural to assume that the contribution of Frenkel defect pairs was negligible due to the limited recovery. However, it probably underestimated the degradation of thermal conductivity, because the influence of point defects on phonon scattering and thermal conductivity is nonlinear.…”

“…However, Gosset et al [7] assumed that the helium bubbles as well as the high micro-strains induced, rather than the irradiation defects, might serve as phonon scattering centers. In our previous work [9], we tried to quantitatively estimate the effect of irradiation-induced point defects on the thermal conductivity of boron carbide by a classical phonon scattering model. The model predicted the degradation of thermal conductivity at the initial stage of irradiation.…”