An improved, compact model steam generator having multiple primary systems is described herein. The model steam generator of the invention is capable of simultaneously simulating a plurality of thermo-hydraulic conditions which may exist in various areas of a full-scale nuclear steam generator in order that the effect of these various conditions on the heat exchange tubes within the full-scale generator may be separately monitored. The model steam generator of the invention generally includes a boiler vessel having a primary side which houses a plurality of individually controllable primary systems, a tubesheet, a secondary side, and a plurality of sample heat exchange tubes for transferring heat between each of the individual primary systems and the secondary side of the boiler vessel. A heat flux control system connected to each of the heat sources within the primary systems allows the operator to separately adjust the heat fluxes of each of the ends of the sample tubes disposed within the secondary side of the boiler vessel. In order to reduce the longitudinal and diametrical dimensions of the primary side of the boiler vessel, the heat source used in each of the individual primary systems is preferably a single, high-intensity electrical heater formed from a coil or other high density configuration of electrical resistance wire. Moreover, each of these primary systems may be housed within the tube-receiving bores of the tube sheet of the boiler vessel in order to minimize the longitudinal dimensions of the primary side even further.