Patent Number: 043269176
Section: summary

TECHNICAL FIELD The present invention generally relates to nuclear reactor control methods and particularly to such methods wherein the control is accomplished in response to a variable temperature set point which is a function of load demand. BACKGROUND ART Nuclear control systems for pressurized water reactors are known which are based on average reactor coolant temperature as a set point. These known control systems utilize either a constant average temperature set point or a set point which increases with increasing reactor power. Control of the average temperature to the set point is by reactor control rod movement and/or changes in the boron concentration in the reactor coolant. Rapid power changes have historically depended upon full length control rod motion for reactivity addition and upon partial length control rod motion for power distribution control within the reactor. However, the large changes in linear heat rate experienced by reactor fuel rods in the vicinity of the control rods, particularly those near the partial length control rods, have led to recent concerns about fuel cladding integrity in light of a phenomenon known as pellet-clad interaction. It is well-known that reactor operation with the partial length control rods totally removed from the reactor reduces the risk for pellet-clad interaction but at the same time restricts the maneuvering capability of the reactor based on full length control rods because there is no readily available method of controlling large power imbalances within the core. Rapid power changes utilizing changes in boron concentration, while possible would require very expensive hardware modifications to existing reactor designs and would substantially increase the radioactive waste processing requirements. Because the reactor coolant has an inherent negative moderator coefficient, methods of achieving positive reactivity addition and fast power increases by decreasing the average reactor coolant temperature are known. However, all the known methods employ a temperature drop below a set point which normally increases with power which is the standard control mode for nuclear plants with recirculating steam generators. What was needed was a control system which would allow large power load changes at rates up to five percent full power per minute and which could be implemented at a reasonable cost in conjunction with deletion of the partial length control rods from the existing design of a nuclear reactor system utilizing once-through steam generators. SUMMARY OF THE INVENTION The present invention is a specific method of combining known temperature set point control concepts employing constant average temperature, dropping average temperature, and constant reactor coolant outlet temperature into a single control concept which yields significantly greater benefits on plants employing once-through steam generators than any of the individually known temperature control methods. The invention solves the problems associated with the prior art devices as well as others by providing a method and apparatus for nuclear reactor control which allows reactor power changes at ramp rates up to five percent full power per minute while minimizing waste bleed volumes and pellet-clad interaction concerns for fuel integrity. To accomplish this, a variable average reactor coolant temperature set point is provided which is made a function of the power load demand requirements. This variable set point is made to have a constant temperature set point over the middle portion of the power load range and provides a gradually decreasing temperature set point at the high power end of the power load range. The control of the reactor in response to this variable set point is automatic and done by control rod motion. The programmed average reactor coolant temperature provides moderator temperature reactivity benefits which significantly reduce control rod motion and the need to change boron concentration during maneuvering at high power levels. To meet large power change requirements at rates up to five percent full power per minute, a manual "droop mode" of control is provided wherein the temperature of the reactor coolant is allowed to drop below the forementioned set point within predetermined limits. During this manually-actuated control function, the power requirements are met by varying the feedwater flow to the steam generator to drop the reactor coolant temperature and thereby increase the reactivity of the reactor. Utilizing this mode of control further minimizes control rod motion and boron concentration change requirements while meeting power load demands at rates up to five percent full power per minute. In view of the foregoing, it will be seen that one aspect of the present invention is to provide a method and apparatus for nuclear reactor control utilizing a variable set point which is a function of power load demand and has a constant temperature at the mid range of power load and a decreasing temperature at the high range of power load. Another aspect of the present invention is to provide a manual nuclear reactor control which is based exclusively on feedwater flow and allows high power demand changes with less boron concentration change than would otherwise be required. These and other aspects of the present invention will be more clearly understood upon a review of the foregoing description of the preferred embodiment when considered with the accompanying drawings.