Power limitation represents a major issue within space applications aimed to human settlements on
solar system planets. Among these planets, Mars is considered the most attractive because of its
nearness to the Earth and the probable presence of minerals which can be used by the settlers to
live off the land. In this frame, small size nuclear power plants can be an interesting solution to
overcome the energy supply problem. This paper presents a preliminary feasibility study of a 100
kWe self-pressurized water space reactor, with the aim to design a system characterized by
compactness, intrinsic safety and simplicity of the main reactor control components. To this end an
innovative reactivity control system, based on the control of the primary coolant mass flow rate,
was adopted. The introduction of this system in the reactor design required a comprehensive core
neutronics analysis in order to properly quantify the effect of the coolant on the reactor behaviour
also as a function of the fuel burn-up. Here only the main results of this analysis, concerning
neutron flux profiles and multiplication factors, are discussed. Moreover preliminary results on
long term reactivity control are presented, showing the possibility to operate the reactor for as long
as 7 years with no need of human intervention
