Gas-cooled fast reactor (GFR) is one of the six recommended nuclear reactor types of Generation IV Forum (GIF) with the lowest technical maturity. Cooled by inert gas like helium and super critical carbon dioxide which performs not so good as water or liquid metal in heat transfer, GFR has been challenged by safety issues especially in Loss-Of-Coolant-Accident (LOCA) events. It has been considered to be an effective way to improve the core inherent safety of GFR by strengthening the temperature feedback on reactivity in the core. However, with no moderating materials and low neutron reaction rates which cause a harder neutron spectrum than other reactor types, GFR has very weak negative temperature feedback.

This study aims to optimize nuclear design of GFR core by increasing the negative temperature feedback.

Firstly, moderating materials were utilized in the fuel assemblies (FAs) in order to get a softer neutron spectrum in the core and increase both the doppler effect of the fuel and the temperature feedback on reactivity. Four moderators including graphite, beryllium oxide, zirconium carbide and zirconium hydride were used in the FA with different geometric structures such as uniformly mixing in the fuel pellets, separate rods distributed in the fuel rod bundles and thick layer outside the fuel rod bundles. Then, Monte Carlo (MC) calculation software RMC was employed to carry out neutronics analysis of the GFR core. Neutronics characteristics of these FA models was comparatively analyzed in details to find the best performance FA model. Finally, a 10-megawatt-power micro GFR core design was given based on the selected FA structure. Effects of the High-to-Diameter ratio (H/D) value as well as the uranium enrichment of fuel on the temperature feedback of the core were thoroughly studied and optimization of the GFR nuclear design was conducted.

The MC simulation results show that the optimized GFR core has a more than twice larger reactivity temperature coefficient value compared to the general core design, which greatly enhances the inherent safety of GFR core. Meanwhile, flat power distribution of the core has been demonstrated with the axial and radial power peaking factor of 1.14 and 1.23, respectively. Results of temperature field around the hottest fuel rod show sufficient safety margin of and that the core has the ability to automatically shutdown by negative temperature feedback solely.

FA model with a layer of beryllium oxide moderator has shown the best performance, and the effectiveness of the optimization methods for reactivity temperature feedback and core design of GFR is verified in this study, providing design experience for the future GFR nuclear design and optimization.