A Two-Range System for Ex-Core Neutron Flux Monitoring

In a reactor supplied power generating plant, one of the means conventionally used to monitor reactor power is a neutron flux monitoring system. The pressurized-water-reactor instrumentation systems have included ex-core neutron (leakage neutrons) monitors and have traditionally used three-range systems. The three ranges have been required because the flux values covered approximately 11 decades and the dynamic range of the available instruments was limited. The C-E neutron flux monitoring system applies the concept of wide and narrow range (two groups of instruments) monitoring of the neutron flux instead of using three ranges or groups of instruments. The system uses a total of ten channels which are subdivided into four wide-range logarithmic channels monitoring the neutron flux from source level to full reactor power level, and six linear power range channels monitoring the neutron flux from approximately 0.1 percent to full reactor power level. The four wide-range channels operate from a combination of proportional counters and fission chambers. A single output signal from each channel is produced by pulse counting techniques for the first five decades of flux values, and by mean square variation techniques up to full reactor power. This method reduces errors due to gamma background as well as resolution counting loss errors normally associated with high rate counting. The four wide-range logarithmic channels provide rate-of-change-of-power information to four reactor protective channels. The narrow (linear power) range channels operate from dual-section uncompensated ionization chambers, each section covering one half (axially) of the reactor core.