An analytical representation is formulated for a nuclear reactor containing in-core thermionic devices suitable for transient studies. The resulting model is applicable to situations involving substantial changes in system operating conditions, as would be experienced during startup transients or during large changes in the electrical load requirements while at power. Neutron kinetics and heat transfer are represented by nodal descriptions. Contributions from all important system regions are retained to produce realistic transient response. The resulting set of equations is coupled to a digital computer integration routine to solve for the dynamic response. Thermionic converter physics is described by a complex iterative numerical scheme based on a diffusion approximation to the plasma processes. Other thermionic processes included are surface and Schottky effects, and an accounting of the electrostatic sheaths present. The analysis includes general application to thermionic diodes undergoing transients. Digital representation of the reactor model is tested against a comparable analog computer simulation and is shown to yield better accuracy. The complex thermionic analysis is compared to a simpler converter physics description and found to be far superior in predicting electrical characteristics of the converter for large changes in operating conditions. The thermionic analysis is also compared with transient experimental diode data over wide ranges of converter operations and produces excellent agreement. Application of the model to system startup is described for two postulated startup approaches encompassing either constant diode voltage or constant emitter temperature. This thermionic reactor model is very useful in obtaining insight and understanding of the overall system dynamic behavior during large changes in system operating conditions. Furthermore, since the thermionic analysis can be easily decoupled from the system model, separate application to studies involving only transient diode operations may be accomplished. An important finding of these analytical studies is that, under certain conditions, results obtained assuming an average and uniform description of the temperature distributions, especially for the emitter surface, may not be sufficiently accurate to represent all the important aspects of diode transient behavior. Analytical studies involving the complete reactor model demonstrate that simple control methods may be adequate to produce very reasonable response during system transients.