Reactivity Effects of External Void Insertion in Lead-cooled Fast Reactor

The utilization of liquid lead as a coolant within the active zone of the ALFRED reactor offers additional reactivity control possibilities, leveraging neutron leakage phenomena. With its high density and atomic mass, lead effectively serves as a neutron shielding; thus, manipulating its displacement allows for modulation of the neutron population within the core, enabling adjustments of reactivity levels, especially close to the core boundary. In this study, we investigate the possibility of employment of void insertion, which is applied within the channels of the reactor's safety system for reactivity control purposes. Utilizing the OpenMC Monte Carlo neutron transport simulation code, we analyze the effects caused by void insertions in the channels of III and VII rings of the reactor core's assemblies. Our analysis unveils non-linear trends in the relationship between reactivity insertion and introduced void volume. Furthermore, this work explicates the obtained results and considers the potential applications of void insertion.