A model steam generator including an improved feedwater system for monitoring the conditions of the heat exchange tubes within a nuclear steam generator is disclosed herein. The feedwater system generally comprises a first conduit fluidly connected between the boiler vessel of the model steam generator and the feedwater of the nuclear steam generator via a first valve, and a second conduit which is fluidly connected between this boiler vessel and a feedwater reservoir by way of a second valve. The feedwater reservoir is in turn connected to a source of demineralized, deaerated water by means of a third conduit having a third valve. This reservoir is further fluidly connected to the feedwater of the nuclear steam generator by means of a fourth conduit having a fourth valve. The improved feedwater system of the invention gives the operator three running options. First, he may run the model steam generator directly off the feedwater used in the nuclear steam generator. Second, he may fill the reservoir with feedwater used in the nuclear steam generator and mix anti-corrosive or cleaning additives into this feedwater before introducing it into the model steam generator in order to test their efficacy. Finally, if anti-corrosive or cleaning additives are already in use in the nuclear steam generator, he may fill the reservoir with demineralized, deaerated water and test alternative additives. The improved feedwater system also includes a means for maintaining a turbulent flow through the feedwater inlet conduit so that particulate contaminants in the feedwater will not have an opportunity to settle out before the feedwater is introduced into the model steam generator.