Abstract: The nuclear fuel exists as solution in the process of spent fuel reprocessing, and it may result in nuclear criticality accident to occur. It is very important to study the mechanism and source term of the criticality accident for preventing accident, consequence mitigation, emergency response and medical diagnostic treatment. A transient analysis code GETAC was developed based on the cylindrical vessel with the point reactor kinetics equation and 2-D thermal conduction equation. Calculation was conducted with this code to simulate the power transient during criticality accident of the SILENE transient experimental facility of France. The power oscillation and its changes under conditions with different values and modes of reactivity insertion, and with and without external neutron source were obtained. Simulation results are consistent with the transient experimental data and that of other code calculations.