Shielding mass estimation model for gas-cooled space nuclear reactors

Abstract

The shielding is expected to be “mass-effective” for space nuclear power systems (SNPs) because it accounts for a significant portion of the total weight. Hence, the shield mass should be estimated rapidly but accurately especially in the early stage of SNPs’ design. In this paper, a simple mass estimation model was developed based on the preliminarily optimized shield design for the Jupiter Icy Moons Orbiter (JIMO) reactor. In the model, the reactor-induced 1 MeV equivalent silicon neutron fluence and photon absorbed dose at the unshielded payload were predicted by a circular surface source hypothesis at the core bottom, and the proportional coefficients were determined by the Monte Carlo calculation. Moreover, the attenuation factors of the shield were derived by solving the neutron/gamma coupled transport process within the shield body, and the correlations were established between the attenuation effect and the shield thicknesses. Afterwards, when given the radiation limits at the payload, the shield geometry and associated mass could be easily determined. Several detailed calculations corresponding to the variation of shield design were used to verify the estimation model. Those variations include the thickness of the Be-B4C layers, the effective density of the B4C and the number of the shielding layers. Additionally, the potential changes of the reactor such as the neutron energy spectrum and the structural materials in the core were also considered to evaluate the applicability of the model. Totally, the estimation model agrees with the detailed calculation within 15 % in a wide range, which suggests the model can be incorporated into the system code to help optimize the overall performance of the SNPs.