A Frequency Domain Control Perspective on Xenon Resistance for Load Following of Thermal Nuclear Reactors

Xenon (Xe) is a noble gas with an isotope (Xe-135) that has an extremely high absorption cross section of neutrons moving at thermal energies. Xe-135 is produced through a chain of decay of isotopes resulting from nuclear fission, in addition to directly resulting from fission. The decay chain results in a phase lag between Xe-135 concentration and the neutron flux or power in a nuclear reactor. The phase lag results in oscillations and several hours of delay between the set power and the response power of a reactor. To enable rapid load following of reactors to a maximum of one change per hour, or a frequency of 2.7 × 10^-4 Hz (1.74 × 10^-3 rad/s), a Bode-step controller is designed to expand the functional bandwidth of the reactor and to stabilize the control loop. The resulting controller is capable of projecting the Xe-135 concentrations and providing a phase lead to overcome the inherited phase lag. The controller was found to eliminate the power oscillation and enable the targeted “one change per hour” power set point change.