The focus of this work is to perform an initial neutronic assessment of a Sodium- Cooled High-Temperature Reactor (HTR-Na cooled). This involves the development of a deterministic calculation schemes, followed by a detailed physical analysis and the formulation of some design proposals. The concept of the HTR-Na cooled draws inspiration from the Japanese High Temperature Test Reactor (HTTR), a High-Temperature Gas Reactors (HTGRs). The key objective is to downsize the reactor by employing liquid sodium as a coolant to replace the helium, while simultaneously arising the power core to collecting thermal energy for industrial application as the hydrogen generation. The work is divided into three main parts: first the construction of the assembly calculation scheme, then a comprehensive physical analysis at lattice level and finally the core calculation scheme.