The flow transients as initiated by rapid gas release are studied both experimentally and analytically. The mathematical model developed considers a multiple pin failure in a fast-reactor subassembly. In formulating the model, it is assumed that the released gas fills the subassembly cross section uniformly and that the coolant flow is incompressible. The model considers the inertial contribution of the liquid columns beyond the pin assembly, as well as the three-dimensional flow effects in the inlet and outlet plenums. In the application of the model to out-of-pile simulation loops, or in-pile test loops, points of departure in hydraulic simulation of the actual reactor conditions can be taken into account. A quantitative criterion for valid application of the model is obtained in terms of breach size, number of pins ruptured, initial gas plenum pressure and temperature, and subassembly operating conditions. The predictions of the flow transients obtained by means of the model agree well with the experimental data. An example of the application of the model to a reactor configuration is given using an FFTF fuel subassembly.