20% of the Nuclear Power Plants are known as Boiling Water Reactors (BWRs). These BWRs have pumps that cool their reactor. In the design of new BWRs, ways to cool the core by a natural circulation flow, without pumps, also called natural circulation BWRs, are being considered. In these new systems, a chimney is installed on top of the core to increase natural circulation flow. A possible disadvantage of natural circulation BWRs might be their susceptibility to instabilities, which could then lead to both flow and power oscillations. The stability features of both natural circulation and forced circulation BWRs have been investigated thoroughly, using dedicated experimental setups, analytical models and numerical codes. We distinguish between pure thermal-hydraulic stability – where the fission power is assumed to be constant – and coupled thermalhydraulic-neutronic stability – where the two-phase mixture in the core influences the fission chain reaction. In conclusion, the dynamics and stability of the boiling two-phase flow have been investigated, both in the natural circulation flow with a chimney and in the forced circulation flow without a chimney. In addition to the thermal-hydraulic stability, coupled thermal-hydraulic-neutronic stability (both core-wide and regional stability) was studied experimentally and analytically for both natural circulation and forced circulation BWRs.
