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.