Irradiation-Induced Dimensional Change in Boronated Graphite

Boronated graphites containing 22 to 43 wt% boron as B4C and representing current warm-pressing or extrusion technology were irradiated at 650 ± 100°C to fast-neutron fluences up to 7 × 1021 n/cm2 (E > 0.18 MeV). The irradiation caused an increase in thermal expansivity, a decrease in thermal conductivity, and an anisotropic dimensional change related to the preferred orientation of the graphite crystallites in the graphite matrix.Although irradiation-induced B4C swelling has been reported, the dimensional change in boronated graphite is not significantly influenced by that effect. Dimensional change in boronated graphite is controlled by the fast-neutron and 10B fission damage irradiation properties of the graphite-binder matrix. At a constant 10B-isotope concentration, the irradiation-induced dimensional change of the boronated graphite increases with increasing 10B-isotope enrichment of the boron in the B4C addition. This effect, which may result from more severe 10B fission fragment damage in the matrix surrounding 10B-enriched B4C, leads to distortion directly related to the 10B burnup gradient. The use of natural boron in the fabrication of boronated graphite results in dimensional changes independent of the 10B burnup gradient and correlated with fast-neutron fluence under HTGR service conditions.