Carbon Aerogel Material for Impact Energy Absorption and Neutron Radiation Shielding in Used Nuclear Fuel Transportation Package Design

Radiation shielding and impact protection are major elements of used nuclear fuel transportation package development. The package structure uses high-Z materials such as steel or lead. These materials provide gamma attenuation, however they do little for shielding neutrons. Depending on source characteristics, shielding only the gamma radiation can be sufficient. It is prudent, or sometimes necessary, to add neutron shielding. Minimizing overall package weight is typically required to comply with transportation route regulations. To support this, efficiency can be realized with neutron shielding that also acts as an impact limiter. The Nuclear Waste Management Organization (NWMO), in collaboration with Lawrence Livermore National Laboratory (LLNL), is investigating alternative impact limiter core materials and has identified carbon aerogels as a potential option. The low density of aerogels make them an appealing possibility. Furthermore, the synthesis of aerogels is highly flexible and customizable. Compositional modification (e.g. incorporation of dopants) has the potential to increase the neutron shielding characteristics of these materials. Two main methods are to be examined: the addition of hydrogen (low-Z atom) to provide neutron scattering, and covering the outer surfaces with a layer containing Boron-10 (high neutron cross-section) to provide neutron capture. The NWMO has engaged in an initial experimental program to assess the impact energy absorption of a standard carbon aerogel material developed by LLNL. The objective is to compare the performance to a commonly used material, polyurethane foam. Specimens were subjected to quasi-static compression and moderate strain rate impact tests to obtain stress-strain curves. An MTS 500kN compression frame was employed for quasi-static compression to ASTM D1621 (modified to compress up to ~245kN). An Instron CEAST 9350 drop tower was employed for impact testing to a laboratory-prepared test procedure. Initial results show that the carbon aerogel has a low crush strength in comparison with a similar density polyurethane foam. It is postulated that addition of carbon nanotubes could enhance crush strength. This future work is required to determine whether carbon aerogels should be studied further for this application. Development of a hydrogen-doped carbon aerogel, and subsequent neutron shielding testing, will also comprise future work.