Neutron Monitoring Systems Including Gamma Thermometers And Methods Of Calibrating Nuclear Instruments Using Gamma Thermometers

Abstract

A neutron monitoring system uses a gamma thermometer to calibrate a nuclear instrument to ensure more accurate neutron flux measurements in a nuclear reactor core. The nuclear instrument generates a first signal proportional to a measured local neutron flux. The gamma thermometer generates a second signal proportional to a measured local gamma flux. A processor is used to compensate the second signal by filtering groups of delayed gamma sources to a reduced number of specific groups of delayed gamma sources. A third signal is calculated based upon yield fractions and time constants that are determined for the specific groups of delayed gamma sources. A gain of the nuclear instrument is calibrated based on a compensated second signal that is determined by subtracting the third signal from the second signal.

1 . A neutron monitoring system, comprising:
 a plurality of nuclear instruments located in a nuclear reactor core;   a plurality of gamma thermometers located in the nuclear reactor core;   a processor; and   a memory;   wherein each gamma thermometer is associated with one of the nuclear instruments,   wherein each nuclear instrument measures local neutron flux and generates a first signal proportional to the measured local neutron flux,   wherein each gamma thermometer measures local gamma flux and generates a second signal proportional to the measured local gamma flux,   wherein the processor is configured to perform the following for each gamma thermometer,
 filter groups of delayed gamma sources to a reduced number of specific groups of delayed gamma sources, 
 calculate selected yield fractions for the specific groups of delayed gamma sources, 
 calculate time constants for the specific groups of delayed gamma sources, 
 store the selected yield fractions and time constants for the specific groups of delayed gamma sources in the memory, 
 calculate a third signal that corresponds to delayed local gamma flux based on the selected yield fractions and time constants, 
 calculate a compensated second signal by subtracting the third signal from the second signal, 
   wherein a gain of each nuclear instrument is calibrated based on the compensated second signal for the associated gamma thermometer, and   wherein the filtered groups of delayed gamma sources include ensuring that the specific groups of delayed gamma sources have decay time constants greater than 5×10 −1  seconds and less than 5×10 5  seconds.   
     
     
         2 . The neutron monitoring system of  claim 1 , wherein the specific groups of delayed gamma sources are selected from a table or equivalent analytical representation in an American National Standards Institute/American Nuclear Society Standard for Decay Heat Power in Light Water Reactors. 
     
     
         3 . The neutron monitoring system of  claim 2 , wherein the table or equivalent analytical representation includes data related to thermal fission of Uranium-235. 
     
     
         4 . The neutron monitoring system of  claim 2 , wherein the specific groups of delayed gamma sources include thirteen specific groups of delayed gamma sources. 
     
     
         5 . The neutron monitoring system of  claim 1 , wherein the selected yield fractions for the specific groups of delayed gamma sources are calculated in advance,
 wherein the time constants for the specific groups of delayed gamma sources are calculated in advance, or   wherein the selected yield fractions for the specific groups of delayed gamma sources are calculated in advance and the time constants for the specific groups of delayed gamma sources are calculated in advance.   
     
     
         6 . The neutron monitoring system of  claim 1 , wherein the processor calculates the compensated second signal for each gamma thermometer in real-time. 
     
     
         7 . The neutron monitoring system of  claim 1 , wherein the gain of each nuclear instrument is automatically calibrated based on the compensated second signal for the associated gamma thermometer. 
     
     
         8 . A nuclear power plant, comprising the neutron monitoring system of  claim 1 .