Development of Aluminum Boron Carbide Neutron Absorber Structural Material

Neutron absorber is a material used as a divider to prevent the criticality between fuel assemblies in dry cask storage and/or transportation. Until recently, stainless steel/boron carbide neutron absorbers have been widely used as both neutron absorbers and structural material for casks due to stainless steel’s reliable tensile strength and proof stress. However, spent fuel nowadays tend to have higher burn-up than before, and this trend has led to higher requirements for heat removal that cannot always be achieved by traditional stainless steel structure. This has made aluminum/boron carbide neutron absorbers, which have high thermal conductivity, more desirable for dry cask storage.Up to now, the main aluminum/boron carbide neutron absorber used for both neutron absorption and structural material in dry casks has been based on Al-Mg-Si system alloy (6000 series aluminum alloy) since it has excellent extrudability and mechanical properties. However, it has been found that Al-Mg-Si system alloy’s tensile strength and proof stress dramatically deteriorate over time due to the over aging effect, which can limit its future applicability in casks with high burn-up spent fuel. To improve aluminum/boron carbide neutron absorber structural material, an extruded neutron absorber made by powder metallurgy was developed that uses a specially dedicated Al-Mn system alloy (3000 series aluminum alloy) combined with boron carbide, whose tensile strength and proof stress remain stable over time even after being exposed at high temperature for a long time. This neutron absorber structural material is a metal matrix composite (MMC) formed by extruded billets that are prepared by cold isostatic pressing (CIP) and sintering. In order to get the best possible results from the Al-Mn system alloy, its chemical composition was enhanced and the parameters of each manufacturing process, from CIP to extrusion, were fine tuned. This paper shall show how the Al-Mn system alloy is the better candidate over Al-Mg-Si system alloys for neutron absorber structural material.