Patent Number: 054815754
Section: claims

1. A method for detecting oscillations in the core of a boiling water nuclear reactor, comprising the steps of: detecting instability of the nuclear reactor on the basis of oscillations in the output signals of a plurality of neutron detectors mounted in said nuclear reactor;  generating for each neutron detector of said plurality of neutron detectors an oscillation output signal (SS) if the output signal from said each neutron detector fulfills the criterion that said oscillation output signal (SS) assumes at least one upper extreme value exceeding an upper limit, and at least one lower extreme value lower than a lower limit during an oscillation interval (T2) of a predetermined value; and  generating for each neutron detector of said plurality of neutron detectors at least one alarm signal (Ka) for remaining oscillations, if said oscillation signal (SS) remains during a delay interval (T3) of a predetermined duration.  means for detecting instability of the nuclear reactor on the basis of oscillations in a plurality of neutron detectors mounted in said nuclear reactor;  first means for generating for each neutron detector of said plurality of neutron detectors an oscillation output signal (SS) if the output signal from said each neutron detector fulfills the criterion that said oscillation output signal (SS) assumes at least one upper extreme value exceeding an upper limit, and at least one lower extreme value lower than a lower limit during an oscillation interval (T2) of a predetermined value; and  second means for generating for each neutron detector of said plurality of neutron detectors at least one alarm signal (Ka) for remaining oscillations, if said oscillation output signal (SS) remains during a delay interval (T3) of a predetermined duration. 2. A method for detecting oscillations in the core of a boiling water nuclear reactor according to claim 1, wherein in said step of generating an oscillation output signal (SS), said oscillation output signal (SS) exceeds said upper limit if the difference between the time-average value (TAV) of said oscillation output signal (SS) and the upper extreme value thereof is greater than a first predetermined value ( G1), and said output signal is lower than said lower limit if the difference between the time-average value (TAV) of said oscillation output signal (SS) and the lower extreme value thereof is greater than a second predetermined value (G2). 3. A method for detecting oscillations in the core of a boiling water nuclear reactor according to claim 1, wherein in said step of generating at least one alarm signal, a second alarm signal (LR) is generated if said oscillation output signal (SS) from a neutron detector fulfills at least one oscillation criterion during each of a predetermined number of consecutive alarm intervals (T1) of a predetermined duration. 4. A method of detecting oscillations in the core of a boiling water nuclear reactor according to claim 2, wherein in said step of generating at least one alarm signal, a second alarm signal (LR) is generated if said oscillation output signal (SS) completes at least one condition of oscillation during each of a predetermined number of consecutive alarm intervals (T1), each of a predetermined duration. 5. A method of detecting oscillations in a boiling water nuclear reactor according to claim 3, wherein said step of generating at least one alarm signal includes the step of generating an alarm signal (Ra) for intermittent oscillations, provided that said second alarm signal (LR) exists and that no alarm signal (Ka) for remaining oscillations exists. 6. A device for detecting oscillations in a boiling water nuclear reactor, comprising: 7. A device for detecting oscillations in a boiling water nuclear reactor according to claim 6, wherein said first means for generating includes means for determining the difference between the output signal of a neutron detector and the time average value (TAV) thereof and generating a signal (P) if said difference exceeds an upper limit and a signal (N) if said difference is below a lower limit. 8. A device for detecting oscillations in a boiling water nuclear reactor according to claim 7, further comprising means for filtering said oscillation output signal. 9. A device for detecting oscillations in a boiling water nuclear reactor according to claim 8, wherein said first means for generating further includes means for extending said signal (P) and said signal (N) by a predetermined duration corresponding to an oscillation interval (T2). 10. A device for detecting oscillations in a boiling water nuclear reactor according to claim 9, wherein said first means for generating further includes means for generating an oscillation output signal (SS) with the simultaneous presence of said extended signal (P) and said extended signal (N). 11. A device for detecting oscillations in a boiling water nuclear reactor according to claim 10, wherein said second means for generating includes a first delay member for generating an alarm signal (Ka)for remaining oscillations if said oscillation output signal (SS) remains during a time interval (T3) of a predetermined duration. 12. A device for detecting oscillations in a boiling water nuclear reactor according to claim 10, wherein said first means for generating further includes means for generating an extended oscillation signal (FSS) by extending said oscillation signal (SS) a predetermined duration corresponding to an alarm interval (T1); and a second delay member for generating a second alarm signal (LR) if said extended oscillation signal (FSS) remains during a delay interval (T4) corresponding to a predetermined number of alarm intervals (T1). 13. A device for detecting oscillations in a boiling water nuclear reactor according to claim 11, wherein said first means for generating further includes means for generating an alarm signal (Ra) for intermittent oscillations when said second alarm signal (LR) is present and said alarm signal (Ka) for remaining oscillations is absent.