In this master thesis a small modular sodium-cooled metal-fuelled pool-type fast reactor design, called SPARC - Safe and Passive with Autonomous Reactivity control, has been designed. The long term reactivity changes in the SPARC are managed by implementation of the the Autonomous Reactivity Control (ARC) system, which is the novelty of the design. The overall design is mainly based on the Integral Fast Reactor project (IFR), which experimentally demonstrated the passive safety characteristics of a metal fuelled, sodium-cooled, pool-type reactor system. Whilst mimicking the passive safety features of the IFR, the vision of the SPARC design is a battery type reactor, which can operate with minimum interference from human actors. In this thesis, two reactor examples have been developed which operate using different fuel compositions. One reactor operates on recycled nuclear waste from today's nuclear power plants, and the other reactor operates on enriched uranium. Both reactors have a thermal power of 150 MW, and are meant to operate for 30 years without refuelling. The design was developed using the ADOPT software, and was simulated in Serpent. Using Serpent, criticality analyses were carried out which show that the ARC system is able to control the long term reactivity changes of the reactors.