Patent Number: 055241280
Section: summary

FIELD OF THE INVENTION This invention relates to control of boiling water type nuclear reactors. More particularly, this invention relates to a system for maintaining stable reactor operations during reactor maneuvering to avoid coupled neutronic-thermal hydraulic instabilities. BACKGROUND OF THE INVENTION Under certain conditions, Boiling Water Reactors (BWRs) are susceptible to coupled neutronic-thermal hydraulic instabilities. Such instabilities, which are characterized by periodic core power and hydraulic oscillations, can compromise established fuel safety limits. Industry events and analytical studies during the past several years have revealed that, in general, existing neutron monitoring features of the reactor protection system do not assure automatic protection against this class of events. Various organizations within the United States nuclear industry are actively pursuing permanent solutions to this problem that, in accordance with regulations governing the general design criteria for nuclear power plants, will automatically "assure that power oscillations which can result in conditions exceeding specified acceptable fuel design limits are not possible or can be reliably detected and suppressed." Development of these solutions is progressing, and modifications to reactor protection systems may occur within several years. One proposed solution is to provide instrumentation which can detect oscillations after they have commenced, and take appropriate control measures necessary to suppress them. An example of a system for doing so is shown in U.S. Pat. No. 5,174,946. However, implementing such a solution requires substantial modification to existing reactor instrumentation and control systems. Implementation of effective operational strategies, utilizing existing plant capabilities in the near-term and supplementing the permanent solutions, is critical for mitigation of the reactor instability phenomenon. SUMMARY OF THE INVENTION It is therefore a general subject of the invention to provide a BWR core stability control system for maintaining stable reactor operations and avoiding coupled neutronic-thermal hydraulic instabilities during controlled reactor maneuvering. It is another object of the invention to provide such a system which can be implemented using existing reactor instrumentation and control systems and without modification of the reactor. It is another object of the invention to provide such a system which does not require measurement of all variables which may be involved in core instability. In accordance with these objects, a simple, effective, control--limiting the core average bulk coolant saturation elevation above a predetermined axial plane--has been developed that provides the means to reliably influence the stability of a reactor. The stability control of the invention lends itself to use by operators during reactor maneuvering. By separating the instability phenomenon into fundamental constituent parts, consistent and direct control can be achieved. The control is designed for use during the quasi-steady-state conditions of controlled reactor maneuvering. Uncontrolled loss of stability margin resulting from transients is not generally amenable to operational response except by scram. This control, its phenomenological basis, sensitivity to relevant reactor parameters, and method of implementation as well as other objects and features of the invention are described below in the detailed description of the invention.