Neutron flux oscillations at German BWRs

Neutron flux oscillations are a well known phenomenon for boiling water reactors (BWRs). The oscillations can occur in a specific region of power and core flow map. The oscillations themselves are caused by interdependencies between thermo-hydraulic parameters and the reactivity feedback. In the instability region these interdependencies can result in an insufficiently damped oscillation of the neutron flux with increasing amplitudes. It cannot be excluded that fuel rod cladding limits may be exceeded if these oscillations are not timely suppressed. During the specified normal operation a BWR should run in stable regions with adequate margins to the instability region. But, during transients the point of operation may enter the instability region. Worldwide several events occurred since the 1980ies related to neutron flux oscillations. Countermeasures have been derived from these events. In the last years, neutron flux oscillations occurred again in some BWRs, especially in Sweden and Germany. The events showed that the combination of advanced fuel assembly design with high burn-up, spectral shift operation, and low leakage core configuration could result in fast increasing neutron flux oscillations following transients. GRS has evaluated these new events under specific consideration of potential out-of-phase oscillations and derived recommendations to prevent, detect and suppress these oscillations. (orig.)