 The European Nuclear Energy Agency, ENEA, has constructed a facility called STEAM to study the behavior of a once-through STEAM generator, OTSG, in both steady state and transient conditions. This OTSG was designed to scale down the dimensions of the demo OTSG while maintaining its height ratio. The design was supported by relap5 slash mod 3.3 thermal hydraulics calculations, computational fluid dynamics, CFD, and finite element analysis, FEA. This article was authored by Alessandro Venoni, Pierdomenico LaRusso, Marica Eboly, and others.