Strategies for the start-up procedure of natural circulation Boiling Water Reactors

The economy of existent NPP designs suffers for the complexity of the safety systems (which have to be redundant and diversified). Nuclear reactors can be made more competitive and reliable by adopting a simpler design and passive safety systems. Following this philosophy, the design of natural-circulation BWRs was developed. In this type of reactors the coolant circulation is gravity-driven and is ensured as long as heat is produced in the core (if a sufficient water level is maintained) without the need of any active system. The elimination of the recirculation systems (pumps and related piping and components) simplifies the design and excludes a whole series of transients and initiating events that can lead to accidents (pump trips, LOCAs due to failures of the recirculation piping, etc.). Unfortunately, natural circulation systems may exhibit thermal-hydraulic instabilities under low-power and low-pressure conditions. Hence, appropriate start-up procedures have to be planned in order to avoid flow oscillations. In the paper strategies for the start-up of natural circulation BWRs are discussed.