Development of a novel neutron-gamma spectrum-based composite material optimization process for the shielding design of a space reactor

In this work, a Neutron and Gamma Spectrum-based Composite-gradient-shielding-material Optimization Process (NG-SCOP) has been developed to optimize the shadow shield design of a typical unmanned lithium-cooled small space reactor. This process has been accomplished by coupling an intelligently designed computational algorithm with NECP-MCX through a MATLAB program in an automated manner. The NG-SCOP algorithm designs an optimized shadow shield by adjusting its materials composition based on the relative contributions of neutrons and gamma dose rates in small incremental thicknesses along the entire shield length. Additionally, at each step, this algorithm homogenizes the optimized material composition from all preceding mesh cells up to that point and designs a single homogeneous composite material for the shadow shield structure. This process is repeated across all mesh cells until the fast neutron flux and silicon gamma dose limits for unmanned space reactors are met. The NG-SCOP technique is initially applied to four composite materials, C₂H₄-B₄C-W, C₂H₄-B₄C-Pb, LiH-B₄C-W, and LiH-B₄C-Pb by incorporating them in the shadow shield structure and calculating the thicknesses and masses required to attenuate neutron and gamma fluxes and doses to the required limits. The mass and thickness of LiH-B₄C-W has been further optimized by replacing tungsten, the dense material used for gamma attenuation, with a mixture of different weight fractions of W, B₄C, and LiH. The minimum masses obtained for the cell-wise optimized gradient composite and the homogeneous composite shields for LiH-B₄C-W are 628 kg and 592 kg, respectively. The results show that the NG-SCOP algorithm can design a more compact and lightweight shadow shield and can be adopted to design optimized shielding for any nuclear facility, including space and other portable nuclear reactors, using any combination of constituent materials and adjustments against any shielding design criteria.