TRIGA Reactor Dynamics: Frequency Response Tests

Frequency domain techniques proved to be very useful in the study of the linear dynamics of systems, e.g. the system stability and feedback effects can be evaluated. Nuclear reactors can be treated as a linear system with small reactivity perturbations as input with subsquent power changes as output. In this work, which was motivated by lack of any reported tests on TRIGA cores in the open literature, frequency response method was applied to ITU TRIGA reactor using a reactivity oscillator. The amplitude of the reactivity perturbation was such that it induced power oscillations with an amplitude of 10% in the prompt jump frequency range. Measurements were conducted in the frequency interval of 0.002-2 Hz. at 0.02, 4 and 200 kW reactor power levels. As for the evaluation of the results of the experiments the following observations were made: - Results at 0.02 kW closely follow the anticipated zero power reactor response which derives from point reactor kinetic equations with six delayed neutron precursor groups; - At 4 kW, within the studied frequency range, the effects of feedback are not clearly visible on the gain curve; - At 200 kW, DC gain of the reactivity feedback effect is measured as 1.5 c/%P; - Nyquist analysis of the measure transfer fucntion clearly confirmed the absolute stability of the reactor with respect to power-reactivity feedback effects; The power-reactivity feedback effect is proposed to be a first order model with a time lag of 0.16 sn.