Abstract: A typical megawatt-class heat-pipe reactor nuclear power system was investigated in this study. To address the challenges associated with high integration, strong multiphysics coupling, and the inherent difficulties in predicting safety margins in modern megawatt-class nuclear power systems, a comprehensive system-level digital prototype was constructed using the object-oriented modelica modeling language. To achieve high-fidelity transient simulations, a modular modeling approach was adopted to couple the core neutronics, transient thermal conduction of fuel elements, high-temperature vapor cavity heat-pipe performance, and the components of the power conversion loop. Based on the developed digital prototype, comprehensive numerical simulations were carried out to evaluate the transient behavior of the nuclear power system under conditions of a linear power ramp and active power stability regulations. The simulation results demonstrate that, by relying on the active reactivity control system, the fluctuations of critical thermal-hydraulic and nuclear parameters, including the total reactor power, peak fuel temperature, and individual heat-pipe heat absorption capacity, can be strictly constrained within a narrow margin of less than 4%, exhibiting excellent stability and load-following capabilities. Furthermore, the inherent safety characteristics and passive safety margins of the heat pipe reactor system were thoroughly analyzed under two typical design-basis accident scenarios: a single localized heat pipe structural failure and a partial loss of reactor cooling accident. The numerical calculations indicate that even under the highly conservative assumption where no emergency reactor scram by control rods or active reactivity adjustment strategies is implemented, the transient peaks of the fuel temperature, cladding temperature, and the maximum heat transfer power of the neighboring operational heat pipes spontaneously stabilize and remain well below the predefined design safety limits. This autonomous stabilization behavior is primarily driven by the strong negative temperature reactivity feedback mechanisms inherent in the reactor core design. The quantitative findings effectively demonstrate that the proposed megawatt-class heat-pipe reactor power system possesses substantial thermal inertia and robust passive safety performance, preventing catastrophic core degradation under extreme accidental transients. The digital prototype developed in this work establishes a reliable, flexible computational platform for the future optimization of control systems, safety system architecture, and digital twin deployment for advanced space and terrestrial micro-reactors.