Stability of boiling water reactors - phenomena, occurrances and monitoring

Boiling water reactors are stable under their defined operating conditions. Instability can occur during start-up or shutdown operations if the known instability region is not avoided. The instability has a thermal-hydraulic component which is dependent on the parallel channel configuration and is mostly determined experimentally. The different feedback mechanisms in boiling water reactors and the applied methods to avoid instability are explained. The different incidents which occurred during the last ten years show the need for monitoring the stability of boiling water reactors. For this purpose an international standard has been developed which is outlined in this paper. (orig.)