Boiling of coolant must not occur at normal operation, anticipated opera­tional occurrences and design basis accidents. Even if a void is generated in the primary coolant, the inserted reactivity largely depends on the core size and core shape. Generally, the inserted positive reactivity becomes larger with larger core size. Although such an incident is beyond design basis and cannot occur from a technical viewpoint, an adequate safety level is ensured by the core design.

(5) Breeding performance

The required breeding performance for a fast reactor is determined based upon balance between the demand and supply of plutonium at the time of construction to avoid an excessive plutonium accumulation from the view­point of non proliferation.