A hydrodynamics model has been developed to study extreme deformation of the space reactor system impacting on the ground with a high velocity. Two-dimensional geometry models for a monolithic core and a pinned core reactor have been developed with dynamic material models, including the material constitutive models and the equation-of-state models. Calculations have been performed for the reactor impacting onto dry sand at 230 and 150 m/s. A pinned core has a much larger fraction of gas volume in the reactor core and thus collapses faster than a monolithic core. The 150 m/s impact velocity case reveals that the gas coolant channels survive in a monolithic core even though the reactor is massively deformed. In a pinned core, however, most of the gas coolant region collapses with intact or partially collapsed fission product gas cores that are protected by solid UO2 fuel. The sand density varies as it is being compressed. Generally, sand beneath the impacting reactor has a higher density as it is compressed. In addition to consideration of global criticality, it is necessary to investigate local criticality. Because of nonuniform distribution of the gas coolant channels in a deformed monolithic core for the 230 m/s impact velocity case, it may be possible to induce criticality locally in those regions where collapse is more severe. It is not straightforward to make an engineering judgment based solely on impact analysis regarding which core concept is more susceptible to criticality events. The current impact study reveals that a pinned core reactor collapses faster than a monolithic core reactor. A reactor that collapses faster is thought to be more susceptible to producing a criticality. However, a monolithic core reactor with much higher mass and kinetic energy develops much higher compaction in the dry sand beneath the reactor. This means that it is expected to better reflect fast neutrons from the bottom boundary where the sand density for a monolithic core impact becomes much higher than for a pinned core impact. It is strongly recommended that neutronics calculations be performed to determine the susceptibility of criticality for the massively deformed nuclear reactors including appropriate reflecting boundary conditions.