ABSTRACT
Under certain conditions, both boiling water reactors (BWRs) and pressurized water reactors (PWRs) can exhibit local and global oscillations in their flow fields that are known as hydrodynamic instabilities. These instabilities occur in both the primary and secondary loops. This chapter explains how these instabilities develop and how they can be controlled. Ledinegg instabilities are static instabilities that develop in the flow field when the mass flow rate m˙ in a boiling channel transitions itself from one steady-state value to another steady-state value. In BWRs, hydrodynamic instabilities develop between the core inlet and the core outlet when the coolant is allowed to boil. PWRs are generally operated to the left of the single-phase stability line and at least during normal operation, instabilities in the flow field in PWRs are not a significant problem. BWRs can have several different types of hydrodynamic instabilities, and each of these instabilities can have a different vibrational mode.
