Patent Number: 052290662
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a method and apparatus for determining the degree of insertion or the axial position of any and all control rods in a nuclear reactor core by analyzing the output signals of a number of strings of fixed location, neutron or gamma ray sensitive incore detectors. 2. Description of the Related Art Knowing the position of control rods in a nuclear core of a nuclear reactor is essential to safe operation of a nuclear power plant as well as a legal requirement for continuation of an operating license. Currently control rod position, where control rods include safety rods, grey rods, shim rods and displacer rods, as well as specifically "control" rods, can be determined in two different ways. The first uses a coil stack that sits on top of the reactor vessel in which the control rod drive shaft moves up and down. A magnetic impedance produced voltage generated by the coil stack is proportional to the length of the rod drive shaft residing in the coil stack, thereby allowing rod position to be inferred through the voltage output of each coil stack. Occasionally the rod position indicated by this system can be in question. Conventionally, the rod position at startup, as indicated by the coil stack indicators, is checked against the step demand counters to verify that the indicators are valid. If a rod position cannot be verified, because, for example, a coil stack is inoperable due to a mechanical or electrical failure, flux mapping can be performed to indicate rod position. If the position of the target rod cannot be verified, the rod position indicator must be considered inoperable. A reactor shutdown is usually necessary if more than one rod is considered inoperable. The second method of determining rod position determines enthalpy rise deviations in core power distribution using core exit thermocouples and an inlet temperature detector to determine control rod position change relative to a rod reference position as described in U.S. Pat. No. 4,927,594. Because enthalpy deviation changes with respect to rod position, the change in rod position can be determined from the magnitude of the deviation. By adding the change to the reference rod position, the actual rod position can thereby be determined. The accuracy of the rod position determined by a system in accordance with this second method, needs to be enhanced and supplemented to provide the operator with the most reliable indication of rod position possible, so that unnecessary reactor shutdowns are avoided. SUMMARY OF THE INVENTION It is an object of the present invention to provide an enhanced method of determining control rod axial position in a nuclear reactor core. It is another object of the present invention to supplement conventional rod position indication systems to provide plant operators with a diverse confirmation of rod position. It is also an object of the present invention to provide power distribution signatures that allow rod position and distribution abnormalities to be detected. It is also an object of the present invention to determine rod position by performing signature analysis on changes (deviations) in fixed incore detector responses. It is a further object of the present invention to determine rod position during reactor changes when not at full power or in a steady state. It is another object of the present invention to provide a protection grade rod position indication system, as defined by IEEE or ANSI standards. The above objects can be attained by a system which determines rod position by first producing a fixed incore detector response signature database appropriate to current core conditions. When a core thermal neutron or gamma ray flux distribution perturbation is detected the approximate rod configuration is determined by scanning the database for a match. If a match is found the rod position is known. If a match is not found the closest rod configuration is used as a reference position from which a search is conducted to determine rod position. These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.