Abstract – The Pebble Bed Modular Reactor (PBMR) is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, inter-cooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the differences and similarities between the so-called Pebble Bed Micro Model (PBMM) and the actual PBMR. It provides the layout and overall specifications of the plant and describes the approach followed in the design of the start-up system. It gives the results of the actual start-up event including a comparison between the measured and simulated conditions.