Power feedback of the IBR-2 pulsed reactor and its modernized version IBR-2M in comparison

Based on the analysis of the power transient processes caused by the intentional square-wave reactivity in the IBR-2 and IBR-2M reactors with burnup of 800-900 MW · day, the fast power feedback structure is substantiated. Due to the differences in the geometry of the reactor cores and fuel structure, symmetrical oscillations of reactivity induced asymmetrical power fluctuations in the IBR-2 reactor and symmetrical ones in the IBR-2M. The analysis of the measured pulse response of the reactor feedback at a nominal power of 2 MW shows that the IBR-2M is more stable in comparison with the IBR-2. A margin of the perturbation amplitude to the stability boundary of the IBR-2M is almost twice that at the IBR-2. A margin on the phase shift between the perturbing reactivity and power is large enough for both reactors. It can be expected that the operation time of the IBR-2M in the existing nominal mode with a power of 2 MW and a flow of coolant through the core of 100 m³/h before the oscillatory instability is longer than at the IBR-2.