Patent Number: 052251497
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear power reactors, and more particularly to a method and apparatus for detecting thermal hydraulic oscillations in the core of a boiling water reactor. By the nature of its design, a boiling water reactor (BWR) operates at or near the nucleate boiling point of the coolant which flows through the reactor core. During certain startup conditions, before the main coolant pumps are started, but while operating in the power range under natural circulation flow, voids (steam) can appear within the active fuel region of the core. These local voids are less dense than the surrounding coolant and this results in a decrease in the local moderation of neutrons. The decrease in moderation and thermalization of neutrons, results in a decrease in the nuclear fission rate in the nearby fuel. Such localized depression of the thermal neutron flux can, under certain conditions, cause a power unbalance and thermal-hydraulic instability in the reactor. These can rapidly progress to large thermal hydraulic oscillations. Boiling water nuclear reactors typically contain a variety of instrumentation, including a plurality of local power range monitors (LPRM) distributed throughout the core. The LPRM is typically responsive to thermal-neutron flux and thus responds to localized flux depressions resulting from localized voiding. One known approach for detecting thermal hydraulic oscillations, uses the LPRMs and monitors the peak-to-peak level between selected quadrant symmetric LPRMs. When the peak-to-peak flux levels exceed a predetermined limit, an alarm or automatic corrective action is initiated. This prior art technique relies on stochastic signal noise analysis. Because noise analysis is computationally time-consuming, this conventional technique provides a relatively late indication of the occurrence of a problem. In other words, the oscillation is well underway before the detection system of the prior art generates an alarm or initiates corrective action. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for detecting the occurrence of thermal hydraulic oscillations in a nuclear reactor core, by overcoming deficiencies in current techniques that rely solely on neutron flux monitoring. More particularly, it is an object of the present invention to monitor parameters indicative of core thermal hydraulic behavior, with sufficient speed and sensitivity to detect incipient core thermal hydraulic oscillations. It is a further object of the invention to provide a method and apparatus by which improvement can be made to the performance of neutron-flux sensitive thermal hydraulic oscillation detection systems, without the need to replace or modify the fuel assemblies or core support structure in the reactor. These objects are accomplished with the present invention by exploiting the difference in the effect of an increase or a decrease in localized voiding, on the local thermal neutron flux and the local gamma radiation flux. Whereas thermal neutron flux is significantly affected by local voiding, the gamma flux is not significantly affected. More generally, a first type of sensor is provided which is representative of localized power generation in the core (and is significantly affected by localized coolant density, i.e., voiding) and correlated to a second type of sensor which is representative of the power in a region of the core (and does not vary significantly with localized density, or voiding). The approach of the present invention is to not only sense the local void condition, but also to sense the collapse of the local void. The void dynamics are measured and correlated regionally, compared with an acceptance reference criteria, and an alarm or other output is generated to indicate that the core is approaching an unstable condition. In a preferred form of the invention, a plurality of neutron flux sensors and a corresponding plurality of gamma flux sensors are distributed regionally at the same or closely proximate locations. A steady state comparison, or bias, between the signals of a given neutron/gamma sensor pair are monitored, and a divergence (or convergence) can readily be detected through simple signal filtering, to detect a shift in the bias. The pattern of significant convergence and divergence throughout the core, is indicative of the thermal hydraulic stability of the core. An instable pattern can be recognized, in one embodiment, by assigning to each neutron/gamma sensor pair, a coded value of, for example, plus 1, 0, or minus 1, representing void collapse, stable and void increase conditions, respectively. The assignment of a code is based on a given phase shift between the two signals. A straight forward information code calculation such as a gray code can then be made symbolic of the oscillation precursor pattern. The number of plus 1's versus the number of minus 1's, are the operatives and need not be quadrant related to point to a high probability of instability. Thus, the method of the present invention comprises the steps of sensing at each of a plurality of locations within the core, changes in the local neutron flux and gamma flux. From the sensed changes, output data indicative of the spatial distribution and time dependence of voids in the core, are generated. A system embodiment of the present invention, comprises a plurality of a first type of sensor spatially distributed in the core and sensitive to localized power fluctuations. A plurality of a second type of sensor is spatially distributed in the core, and relatively insensitive to localized power fluctuations. First means are provided for associating each of the first type of sensor with one of the second type of sensor, to define a plurality of paired measurement values. Second means are provided for generating first output data signals commensurate with a quantitative relationship among the paired measurement values. Third means, responsive to the second means, are provided for generating second output data indicative of the spatial distribution of voids in the core. The present invention is an improvement over known techniques for detecting void induced-thermal hydraulic oscillations, because a stochastic process analysis need not be developed through Fourier or other techniques. This avoids the need for large amounts of sampled data to develop statistical validity. The limitation in the inventive system is the time response of the sensors. The detection of incipient stability can be achieved in less than about five seconds. These and other objects and advantages of the invention may be better understood from the following description of the preferred embodiment.