Patent Number: 
Section: claims

1. A digital rod position indication system for measuring drop time of a control rod within a reactor following a reactor trip, the control rod being coupled to a drive rod, the reactor including a trip breaker, the control rod adapted to drop to core bottom when the reactor trip occurs, said digital rod position indication system comprising:a coil stack comprising a plurality of detection coils, each of said plurality of detection coils sensing the presence of the drive rod and producing an output signal in response to rod movement, each of said output signals having identifiable characteristics representative of positions of said control rod within a corresponding detection coil;a data acquisition unit selectively electrically connected to a group of detection coils selected from said plurality of detection coils, said data acquisition unit sampling each of said output signals from said group of detection coils and producing a separate sampled output signal for each detection coil of said group of said plurality of detection coils; anda processing unit in communication with said data acquisition unit and the reactor trip breaker, said processing unit establishing a start time from the reactor trip breaker when the reactor trip occurs, said processing unit deriving the position of the control rod within the corresponding detection coil by analyzing said sampled output signals for said identifiable characteristics, said processing unit determining when the control rod reaches core bottom from said identifiable characteristics to establish a stop time, said processing unit providing a drop time for the control rod when a the reactor trip occurs, said drop time being the time period between said start time and said stop time. 2. The digital rod position indication system of claim 1 wherein said ac source operates at line frequency, said processing unit filters said sampled output signals to isolate said identifiable characteristics, said identifiable characteristics being drop signals induced in the detection coils by the drive rod moving through the detector coils, said drop signals occurring at frequencies lower than line frequency, said drop signals establishing said drop time. 3. The digital rod position indication system of claim 2 further comprising a low-pass filter, said processing unit applying said low-pass filter to said sampled output signals to obtain filtered signals, said low-pass filter having a cut-off frequency below line frequency. 4. The digital rod position indication system of claim 1, wherein an RMS voltage is obtained from said sampled output signals, said processing unit taking a derivative of said RMS voltage to obtain a rod drop velocity, said identifiable characteristics being peaks in said rod drop velocity. 5. The digital rod position indication system of claim 1, wherein said drop time is obtained from a combination of the voltage obtained from each detector coil of the plurality of detector coils. 6. The digital rod position indication system of claim 1, wherein said drop time is obtained from a composite voltage measured across the plurality of detector coils. 7. The digital rod position indication system of claim 3, said processing unit determining said stop time as the time when each amplitude of said filtered signals is substantially zero. 8. The digital rod position indication system of claim 1, said processing unit determining said stop time as the time when position information indicates that the control rod reaches core bottom. 9. A system for determining a drop time to core bottom for a control rod following a reactor trip triggering a reactor trip breaker, the control rod being coupled to a drive rod passing through a plurality of detector coils in a coil stack, said system comprising:a data acquisition unit having a plurality of inputs selectively in electrical communication with a group of detection coils selected from the plurality of detection coils, said data acquisition unit collecting a set of sampled data from each detection coil in the group of the detection coils, each said set of sampled data representing electrical signals indicative of positions of the control rod within a corresponding detection coil produced by the corresponding detector coil of the group of the detection coils in response to rod movement; anda processing unit in communication with said data acquisition unit and in selective communication with the reactor trip breaker, said processing unit setting a start time upon triggering of the reactor trip breaker, said processing unit setting a stop time when each said set of sampled data indicates the control rod has reached core bottom following triggering of the reactor trip breaker, and said processing unit indicating said drop time following triggering of the reactor trip breaker based on said set of sampled data. 10. The system of claim 9, wherein the plurality of detector coils are energized by an ac source operating at line frequency, said processing unit filtering said sampled data to remove any signals at or above approximately line frequency to produce filtered data, said filtered data corresponding to drop signals induced in the plurality of detection coils by the drive rod moving through the plurality of detector coils, said drop signals falling to amplitudes of approximately zero when the control rod reaches core bottom. 11. The system of claim 1, further comprising a low-pass filter, said processing unit applying said low-pass filter to said sampled output signals to obtain said filtered data, said low-pass filter having a cut-off frequency below the line frequency. 12. The digital rod position indication system of claim 9, said processing unit determining that the control rod has reached core bottom based upon position information obtained from the plurality of detector coils. 13. The digital rod position indication system of claim 9, wherein an RMS voltage is obtained from said sampled output signals, said processing unit taking a derivative of said RMS voltage to obtain a rod drop velocity. 14. The digital rod position indication system of claim 9, wherein said drop time is obtained from a combination of the voltage obtained from each detector coil of the plurality of detector coils. 15. The digital rod position indication system of claim 9, wherein said drop time is obtained from a composite voltage measured across the plurality of detector coils.