Patent Number: 054815754
Section: summary

TECHNICAL FIELD The present invention relates to a method and a device for detecting oscillations in the core of a boiling-water nuclear reactor (BWR) comprising a plurality of neutron detectors, and wherein instability is detected on the basis of oscillations in the output signals of the neutron detectors. BACKGROUND ART A core in a nuclear reactor comprises a plurality of fuel assemblies. These are arranged vertically in the core in spaced relationship to each other. A fuel assembly comprises a plurality of vertical fuel rods, each of which contains a stack of pellets of a nuclear fuel, arranged in a cladding tube. The core is surrounded by water which serves both as coolant and as neutron moderator. The reactor core also comprises a plurality of control rods which, by being inserted into and withdrawn from the core, control the reactivity of the core and hence its output power. Thermohydraulic core instability is a well-known problem which may arise in a boiling-water nuclear reactor which is run with a high power and a low coolant flow. When cooling water is pumped upwards through the reactor core, steam is generated. The steam bubbles are in motion all the time, which produces variations in the water/steam ratio in the core. If these variations in the water flow are not damped in a natural way by friction, they may grow into sustained oscillations, that is, the reactor core has become unstable. Since water is a good moderator whereas steam is a poor moderator, these flow oscillations will also induce oscillations in the neutron flux. The oscillations may be of varying types and have different appearances and propagation. Certain oscillations start with a low amplitude and slowly grow larger and larger, others may be initiated by a temporary event, for example by a cooling pump stopping, and may then almost immediately assume its maximum amplitude and then continue to oscillate with a constant amplitude. The propagation of the oscillation may vary from global in-phase oscillations to local counter-phase and phase-shifted oscillations when some part of the core oscillates out of phase to the rest of the core. One form of intermittent oscillations have also been detected, that is, oscillations which move around in the core with a certain frequency. Oscillations in the neutron flux and the coolant flow may result in fixed margins for the fuel being exceeded, which in turn may lead to fuel damage. Instability in the reactor core must, therefore, be detected and acted upon. For measuring the neutron flux in boiling-water nuclear reactors (BWRs), neutron-sensitive detectors, so-called LPRM (Local Power Range Monitor) detectors, are used. A plurality of neutron detector tubes are arranged in spaced relationship to each other in the core, each of these tubes including four LPRM detectors located at four levels separated from each other in the vertical direction. The detectors form a regular lattice in the core. If the core becomes unstable and the neutron flux starts oscillating, this means that the output signals of the LPRM detectors start oscillating. U.S. Pat. No. 5,174,946 discloses a device and a method for determining whether the core is unstable, starting from the output signals from the LPRM detectors. The method and device described in the US patent specification determine whether instability exists starting from a combination of the output signals from a plurality of detectors. A disadvantage of using a combination of a plurality of output signals is that it may be difficult to detect local oscillations where only few (1-3) detectors oscillate. If only few detectors oscillate and these detectors are combined with other detectors which do not oscillate, the risk is great that the resultant signal does not oscillate to a sufficient extent to reveal the instability. The method and device described in the US patent specification detect only oscillations whose amplitude is growing, which means that an instability which causes an oscillation with a constant or slightly decreasing amplitude cannot be detected. The described method and device do not comprise any special detection of intermittent oscillations. SUMMARY OF THE INVENTION The invention aims to provide a device and a method for detecting oscillations in a reactor core. What characterizes the method and the device according to the invention will become clear from the accompanying claims.