Nuclear reactors and their related systems have to be operated stably against various perturbations. Fluctuations in reactor power due to fluid flow characteristics and reactivity feedback, or some factors related to characteristics of reactor control systems can obstruct stable operation of the reactors. Returning to the original stable state is essential against those perturbations. As kinds of stability required in BWR core design, there are channel stability (thermal-hydraulic stability) against flow vibration caused by a pressure drop feedback in the flow path, core and regional stability against power oscillation caused by the nuclear feedback of thermal — hydraulic properties and void reactivity, plant stability for sufficiently stable control of plants, and Xe stability (Xe spatial oscillation stability) against power oscillation caused by the accumulation and destruction of FP Xe. The channel stability, core stability, and regional stability are particularly important for BWRs with a large variation in coolant density in the core.

As an indicator of stability evaluation, the decay ratio, which is defined as the ratio between the initial and the next oscillation amplitudes, charac­terizes the extent of oscillation amplitude of a watched parameter by a perturbation. The fundamental design criterion is decay ratio <1 for chan­nel stability, core stability, regional stability, and plant stability in all reactor operating conditions. A decay ratio less than 1 (e. g., 0.25) is set for core stability and plant stability during normal operation for a margin to the design criterion.

Characteristics such as pressure drop in the core flow path and void reactivity feedback have a large effect on the above stabilities. Overall, the stability against perturbations can be secured by setting these charac­teristics as a fixed range. The Xe spatial oscillation can be suppressed by designing a negative power coefficient less than a fixed value.

In relation to the mechanical design of reactor core elements other than the main criteria of the BWR core design mentioned above, the endurance of core structure materials is necessary against high-level radiation doses and high temperatures and pressures. The reactor pressure vessel should

also be able to withstand high pressures, high temperatures and radiation doses during the reactor operating period (e. g., 30-60 years). Mechanical design criteria are set for the reactor core elements.