Patent Number: 
Section: claims

1. An apparatus for monitoring nuclear thermal hydraulic stability of a nuclear reactor, comprising:a calculation unit configured to calculate a stability index of a nuclear thermal hydraulic phenomenon based on nuclear instrumentation signals, the signals being outputted by a plurality of nuclear instrumentation detectors placed at regular intervals in a reactor core;a simulation unit configured to simulate the nuclear thermal hydraulic phenomenon based on a physical model by using information on an operating state of the nuclear reactor as an input condition;a limit value updating unit configured to update a limit value of the nuclear thermal hydraulic phenomenon based on a result of the simulation; anda determination unit configured to determine, based on the stability index and the limit value, whether or not to activate a power oscillation suppressing device. 2. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 1, whereinat least one model is employed as the physical model from among:a three-dimensional reactor core simulator that simulates a three-dimensional distribution of the nuclear thermal hydraulic phenomenon inside the reactor core;a plant heat balance model that simulates heat balance of a plant;a plant transition analysis code that simulates a transient characteristic of the plant; anda stability analysis code that analyzes stability of the nuclear thermal hydraulic phenomenon in an arbitrary operating state based on results of these simulations,the apparatus further comprising:a data interface unit configured to transmit the input condition and data between the respective codes; anda man machine interface unit configured to display or output an analysis result based on an instruction from an operator. 3. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 1, whereindetermination based on the stability index and the limit value is performed for at least one object among:a core stability decay ratio;a regional stability decay ratio;a decay ratio of the nuclear instrumentation signal that is representative of those grouped by characteristics of fuel assemblies placed in the reactor core;a decay ratio of the nuclear instrumentation signal that reflects a thermal hydraulic phenomenon of a most thermally severe fuel assembly; andnatural frequencies of these nuclear instrumentation signals. 4. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 1, whereinthe stability index is calculated by extracting a frequency component corresponding to power oscillations from the nuclear instrumentation signal. 5. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 4, whereinwhen a maximum amplitude that is observed or that may possibly be observed in a stage prior to shift from a stable state to an unstable state is regarded as an oscillation determination amplitude, and an average value of standard deviations of the nuclear instrumentation signals during normal operation is regarded as a background noise amplitude in the determination unit, an allowable growth rate of the amplitude defined as a ratio of the background noise amplitude to the oscillation determination amplitude is employed as a determination criterion. 6. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 4, whereina criterion for determining occurrence of the power oscillations is set in consideration of a detection delay of the nuclear instrumentation detector, an activation delay of power oscillation suppression operation, and a delay until the power oscillation suppression operation becomes effective. 7. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 6, whereinthe delays are taken into consideration based on a ratio between an oscillation period derived from the physical model and an oscillation period derived from the nuclear instrumentation signals. 8. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 1, whereinthe determination unit performs determination in consideration of a result of evaluating uncertainty of the simulation result and a result of evaluating uncertainty at an operating point where stability evaluation is performed. 9. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 8, whereinthe determination is performed with the evaluation result estimated conservatively. 10. The apparatus for monitoring nuclear thermal hydraulic stability of the nuclear reactor according to claim 1, whereina standard deviation that indicates a variation of each nuclear instrumentation signal is calculated, a standard deviation and an average value of a plurality of nuclear instrumentation signals are calculated based on the calculated standard deviation of each nuclear instrumentation signal, and the determination unit is operated based on increasing rates of these calculated results. 11. A method for monitoring nuclear thermal hydraulic stability of a nuclear reactor, comprising the steps of:calculating a stability index of a nuclear thermal hydraulic phenomenon based on nuclear instrumentation signals, the signals being outputted by a plurality of nuclear instrumentation detectors placed at regular intervals in a reactor core;simulating the nuclear thermal hydraulic phenomenon based on a physical model by using information on an operating state of the reactor as an input condition;updating a limit value of the nuclear thermal hydraulic phenomenon based on a result of the simulation; anddetermining, based on the stability index and the limit value, whether or not to activate a power oscillation suppressing device. 12. A program for monitoring nuclear thermal hydraulic stability of a reactor to be executed by a computer that performs functions of:calculating a stability index of a nuclear thermal hydraulic phenomenon based on nuclear instrumentation signals, the signals being outputted by a plurality of nuclear instrumentation detectors placed at regular intervals in a reactor core;simulating the nuclear thermal hydraulic phenomenon based on a physical model by using information on an operating state of the reactor as an input condition;updating a limit value of the nuclear thermal hydraulic phenomenon based on a result of the simulation; anddetermining, based on the stability index and the limit value, whether or not to activate a power oscillation suppressing device.