Mitigation of an Anticipated Transient Without Scram Event in A Simplified Boiling Water Reactor by the Insertion of Fine-Motion Control Rods

Analysis has been performed for an anticipated transient without scram (ATWS) event in a simplified boiling water reactor. Loss of feedwater heating initiated this ATWS, which was mitigated by the insertion of fine-motion control rods. Calculations with RAMONA-4B code show that reactivity insertion, through increase in subcooling to the core, results in a rise in the core power to a new equilibrium condition. Insertion of fine-motion control rods from the bottom of the core is found to shift the peak of the axial power distribution to the upper part of the core until eventual shutdown of the reactor occurs.