The SP-100 reactor will be ground tested at the SP-100 Ground Engineering System (GES) Test Site on the US Department of Energy (DOE) Hanford Site near Richland, Washington. Project direction and the flight system design evolution have resulted in a smaller reactor size and the consequential revision to Test Site features to accommodate the design changes and reduce Test Site costs. The significant design events since the completion of the Conceptual Design are discussed in this paper