A dynamic one-dimensional model of a simple rectangular natural circulation loop with tube bundles in the two vertical legs has been simulated. A supporting experimental study using water at atmospheric pressure as the working fluid has been conducted to obtain data to verify the model. Good agreement was obtained between the predicted and measured mixed-mean fluid temperatures and surface temperatures of the tube bundle as well as the loop piping and insulation temperatures at several locations around the system. A number of different simulations and tests have been performed and are discussed. In general, at steady state there was less than 7% deviation between the model predictions and the test data, which indicates that the one-dimensional model is capable of predicting the transient temperature response of the loop structural components and of the circulating fluid.