On the regional oscillation phenomenon in BWR's.

In the last 20 years many papers have been devoted to the study of BWR stab ility phenomenon. While the physical mechanisms of global power oscillation s are well-known, the regional power oscillation phenomenon is not understa ndable in all details. Our paper should be a contribution to the understand ing of conditions under which regional power oscillations can be expected. With this aim we have analyzed, with the system code RAMONA3-12, some stabi lity experiments which were conducted on the NPP Leibstadt. To test the abi lity of the monitoring system to cope with demanding operation situations, the power oscillations during the experiments were deliberately transformed from the in-phase into the out-of-phase mode, via changing some control ro d positions. Hence, we have been able to study the "real world" of a BWR co re in the regional oscillation mode. We focused our work on the analysis of the higher mode feedback reactivities (dynamical reactivities) and the cal culation of some spatial indices. The feedback reactivities have been calcu lated with the code LAMBDA-REAC. From the results obtained we conclude that it is in the case of certain specific types of power distribution that a p articular mode coupling mechanism can cause regional oscillations to occur. (C) 2000 Elsevier Science Ltd. All rights reserved.