A Lightweight, Multifunctional Hybrid Metal Matrix Composite for Neutron Shielding

Thermal neutron radiation poses a serious threat to both equipment and people in three aerospace environments: fission surface power (reactor-emitted neutrons can activate nearby regolith and harm nearby astronauts), extraterrestrial habitats (existing shielding can counter-intuitively increase neutron radiation), and high-altitude aviation (cosmic radiation-produced neutrons can induce a single event upset in critical electronics). Contemporary radiation shields developed for aerospace cannot readily stop such neutrons. Using terrestrial neutron radiation shielding would result in parasitic mass—that is, these shields would only protect from neutrons and thereby reduce available payload mass. A lightweight, multifunctional material with both neutron radiation shielding and structural capabilities would avoid parasitic mass while also reducing neutron radiation risks. Metal matrix composites (MMCs) can serve in these applications; often, these have an aluminum alloy matrix with boron-rich ceramic particles as reinforcement. Using a magnesium alloy instead of an aluminum one for the metal matrix could reduce mass by upwards of 25%. Naturally abundant boron-10 in the ceramic reinforcement readily absorbs thermal neutrons without producing harmful secondary radiations. However, this hard ceramic reinforcement drastically reduces ductility and machinability. Introducing another reinforcing component like softer ceramic particles should ameliorate the mechanical deficiencies of a hard-ceramic MMC by improving ductility and machinability. Such a three-constituent or Hybrid MMC, termed ‘Bormag’, would provide a lightweight, multifunctional neutron shield with good formability and structural characteristics. Prior to fabrication, computational modeling will predict both the optimal boron content for neutron attenuation and the expected formation of beneficial phases. One promising fabrication technique is stir casting, which is simple and thus cost-effective. Alternatively, induction melting would result in a cast with greater uniformity. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) will analyze bulk and specific phases. Scanning electron microscopy (SEM) will image the microstructure for evaluation. Energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) will provide additional compositional and crystallographic analysis, respectively. Mechanical (hardness, tensile strength, etc.) and neutron irradiation testing will verify shielding and structural functionalities. The mass savings thus provided will enable greater mass for useful equipment on airplanes and extraterrestrial missions. With improved neutron shielding, airline crews and passengers, astronauts on extraterrestrial surfaces, and FSP sources will experience fewer neutron-induced health and technical risks. Spin-off benefits include improved neutron radiation shielding for mobile nuclear power sources in disaster recovery efforts. The primary technology area at level two scoping is “Human Health, Life Support, and Habitation Systems: Radiation (TA 6.5)”. The most relevant level three element is “[Radiation] Protection Systems (TA 6.5.3)”. Ostensibly, Bormag structures would protect both humans and equipment from lower-energy, yet still harmful, thermal neutron and secondary radiations. The secondary technology area at level two scoping is “Materials, Structures, Mechanical Systems, and Manufacturing: Structures (TA 12.2)”. The two relevant level three elements are “Lightweight Concepts (TA 12.2.1)” and “Innovative, Multifunctional Concepts (TA 12.2.5)”. Bormag incorporates radiation shielding and structural functionalities while reducing mass as compared to existing shield technologies. Multifunctional radiation shields are currently at a low TRL, making their development ideal for NSTRF support.

The mass savings thus provided will enable greater mass for useful equipment on airplanes and extraterrestrial missions. With improved neutron shielding, airline crews and passengers, astronauts on extraterrestrial surfaces, and FSP sources will experience fewer neutron-induced health and technical risks. Spin-off benefits include improved neutron radiation shielding for mobile nuclear power sources in disaster recovery efforts.

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