Abstract: Space power nuclear systems require reactors to have characteristics such as compactness, miniaturization and efficiency. The combination of helium xenon mixture gas and the Brayton cycle is a good choice. Existing research on helium-xenon gas cooled small reactors mostly focuses on the system level, with insufficient consideration for the refinement of individual components. The core is the most critical part of the entire Brayton cycle system, and the thermal safety characteristics of the core are the most important component of the safety characteristics of the reactor system. Therefore, mastering the thermal safety characteristics of the core is particularly important. There are not only two forms of heat transfer inside the core: convective heat transfer and heat conduction, but also radiative heat dissipation. In this paper, the three-dimensional simulation method was adopted to establish a 1∶1 model of the helium-xenon gas cooled small reactor core and analyzed the radiation heat dissipation characteristics inside the core. The Monte Carlo method was used to verify the proposed method of calculating radiation angle coefficients using three-dimensional simulation. The relative error between the two calculation results is less than 1%, proving the accuracy of the three-dimensional simulation method. On this basis, a study was conducted on the radiation heat dissipation characteristics of fuel rods in the core of helium-xenon gas cooled small reactors. The sensitivity analysis on geometric parameters such as fuel rod aspect ratio, fuel rod surface temperature and fuel rod length was conducted, and paid attention to their impact on the radiation heat dissipation characteristics of fuel rods. The results show that as the aspect ratio between fuel rods increases, the radiation angle coefficient also increases. When the length of the fuel rod is less than 100 mm, the length of the fuel rod has a significant impact on the radiation angle coefficient. When the length of the fuel rod is greater than 100 mm, the radiation angle coefficient is not affected by the length of the fuel rod. Finally, a universal empirical formula for the radiation angle coefficient of fuel rods in the core of a helium-xenon gas cooled small reactor was established based on the relationship between the radiation angle coefficient and the radial aspect ratio of fuel rods, the arrangement of fuel rods, and the length of fuel rods. The calculation relative error is less than 8.5%. This paper aims to study the radiation and heat dissipation characteristics of fuel rods and core radiation, understand the laws of internal radiation and heat dissipation, and provides technical support for subsequent research on core thermal safety characteristics.