Subcritical Reactivity Monitor Utilizing Prompt Self-Powered Incore Detectors

Abstract

A subcritical reactivity monitor that utilizes one or more primarily gamma sensitive (prompt responding) self-powered detector style radiation measurement devices located within the core of a nuclear reactor to determine the amount that the reactor multiplication factor (K eff ) is below the reactivity required to achieve or maintain a self-sustaining nuclear chain reaction. This invention utilizes measured changes in the self-powered detectors' current(s) to allow a reactor operator to measure the value of K eff at essentially any desired interval while the reactor is shutdown with a K eff value less than the critical value of 1.0. This invention will enable integration of the output of the value of K eff directly into the Reactor Protection System, which will enable the elimination of the operational and core design analysis constraint costs associated with the current Boron Dilution Accident prevention methodology and enable automatic control of the Chemical Volume Control System.

What is claimed is: 
     
         1 . A nuclear reactor system comprising:
 plurality of fuel assemblies housed within the nuclear core with at least some of the fuel assemblies having one or more primarily gamma sensitive, prompt responding self-powered neutron detectors located therein, responsive to the fission reaction activity within the core to provide an output indicative thereof;   a subcritical reactivity monitor that receives the output from the one or more of the primarily gamma sensitive, self-powered neutron detectors and is responsive thereto to generate a core average value of a gamma radiation distribution within the core;   a processing system configured to calculate a value of K eff  in accordance with the following equation 1;   
       
         
           
             
               
                 
                   
                     
                       
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                         eff 
                       
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       and output the value of K eff ; and
 a Chemical Volume Control System configured to receive a triggering signal to add Boron to the coolant in the event a predetermined, undesirable change in K eff  is detected. 
 
     
     
         2 . The nuclear reactor system of  claim 1  wherein the value of K eff  is monitored substantially continuously during a shutdown period of the nuclear reactor system. 
     
     
         3 . The nuclear reactor system of  claim 1  wherein the subcritical reactivity monitor is configured to determine the fraction of the output from the primarily gamma sensitive, self-powered neutron detectors caused by gamma radiation released by fission byproducts in the vicinity of the primarily gamma sensitive, self-powered neutron detectors and, the fraction of the measured output directly caused by subcritical fission events. 
     
     
         5 . The nuclear reactor system of  claim 1  wherein the fraction of the output of the gamma sensitive, self-powered neutron detectors resulting from fission byproducts is subtracted from the output of the primarily gamma sensitive, self-powered neutron detectors to determine a real-time value of K eff.    
     
     
         6 . The nuclear reactor system of  claim 1 , including a source range detector, for monitoring fission events within the core and providing an output from which the fission events can be determined, the sourced range detectors being positioned outside of the pressure vessel, wherein the value of K eff  is calculated without input from the source range detectors. 
     
     
         7 . The nuclear reactor system of  claim 1  wherein the Chemical Volume Control System automatically adds Boron to the coolant in the event a predetermined, undesirable change in K eff  is detected.