An analysis of the transient behavior of a 255-Mw(e) advanced sodium graphite reactor, previously described in NAA-SR-3829, is presented. The reactor and its components are briefly described. Nuclear and thermal characteristics are presented as far as they affect reactor kinetics or are essential in interpreting the results. The study includes an investigation of the inherent kinetic characteristics of the reactor, as well as and analysis of its transient behavior for all conceivable conditions of abnormal operations. Assumed reactor excursions are analyzed with and without ensuing protective system action. It is shown that the reactor is dynamically stable and that power transients which are followed by normal protective system actions will not lead to potentially unsafe conditions. The conclusion is reached furthermore, that uncontrolled rod withdrawal accidents from source power will be terminated by coolant choking'' and fuel meltdown before extensive coolant boiling occurs, and that the large thermal capacity and long-time constant of the upper plenum will provide protection against pool boiling for other less serious accidents until the reactor can be shut down by external means. (auth).