Shielding Analysis for the Lower Reflector Block of a PGSFR Fuel Assembly

The purpose of this paper is to analyze the stability of the EBR-I core meltdown accident using the NuSTAB code. The result of NuSTAB analysis is compared with previous stability analysis by Sandmeier using the root locus method. The PGSFR uses single enriched uranium metal-alloy fuel, U-10%Zr, initially and it will be gradually converted into a TRU metal-alloy (UTRU- 10%Zr) fueled core. The fuel assemblies are arranged into a 7-hexagonal-ring configuration, surrounded by stainless steel reflectors and B₄C shields. Each fuel assembly consists of a fuel rod bundle with 217 fuel pins and a lower/upper reflector block. A lower and an upper reflector blocks are installed inside the fuel assembly to reduce the radiation damage on the core support plate and the radioactivity on head access area respectively. This design can satisfy the radiation damage limit on permanent structure, but it has a high coolant pressure drop caused by the warped coolant path when analyzed using detailed CFD. To address this issue, three reflector block designs which have a lower pressure drop were proposed. In this paper, we analyzed the shielding capability of the three new reflector blocks. The shielding evaluations were performed for the new different reflector block models. In order to describe the complex internal/external structure of the reflector design in detailed, Serpent 2 code was employed instead of MCNP6. The code comparison between MCNP6 and Serpent 2 was examined and confirmed good agreement within a few percent relative error. By the 3-D whole core modeling, the DPA analyses for 3 new reflector block models were conducted and all models satisfied the DPA limit on SS316 support grid plate. The NuSTAB code has an advantage of analyzing space-dependent fast reactors and predicting regional oscillations compared to the point kinetics. Also, NuSTAB can be useful when the coupled neutronic-thermal-hydraulic codes cannot be used for stability analysis