Source range monitor

In the source range, the neutron flux is monitored by fission coimters, which are inserted to about the mid-plane of the core by the drive mechanisms, which move each chamber into the core through inverted thimbles. A range from below the source level to 10 nv is covered. 

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Indicate the criticality point (5E+4 cps for Source Range Monitor) ... 

After a reactor has been operational for an entire cycle, the startup sources are no longer necessary and they are removed from the core. There is a sufficient natural neutron population to maintain adequate count rates on source range monitoring instruments and to permit startup. 

In a cold clean core there is an indicated count rate on a Source Range Monitor of iO cpm,... 

GE SIL 409R2 (02/08/02) Inspection of SRM/IRM Dry Tubes Provides information and recommendations on the cracks found in Intermediate Range Monitor (IRM) and Source Range Monitor (SRM) dry tubes. [6.201... 

Required level of redundancy for neutron flux source range monitoring system. 

The BF3 proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced. 

Three ranges are used to monitor the power level of a reactor throughout the full range of reactor operation. The source range makes use of a proportional counter. 

Monitoring of core power level during power operation is provided by ex-vessel neutron flux detectors. Flux monitoring at lower powers and at shutdown conditions is provided by source range detectors which are located in the side reflector. The reactor core and reflectors rest vertically on a support structure below the core and are restrained by a core lateral restraint structure located between the outer side reflector and the reactor vessel. 

Signals to the Plant Protection and Instrumentation System (PPIS) and the NSSS Control Subsystem (NCS) are supplied by neutron detectors. During power operation, the neutron flux levels are monitored by detectors located in wells between the reactor vessel and the concrete cavity wall. These detectors are distributed symmetrically around the reactor vessel at about the core midplane. During low power operation, starting up, shutting down, and while shut down, the neutron flux levels are monitored by source-range detectors, located in selected side reflector elements near the bottom of the active core. 

In the refueling mode, the reactor vessel is depressurized. All control rods in the inner and outer reflectors are fully inserted except for two inner and two outer rods which may be removed for refueling a 60 degree sector of the core. The neutron flux level is continuously monitored by the source range detectors. 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

The original concept for neutron monitoring includes three systems. Source range. Intermediate range and Power range. During start-up the operator has to select and operate... 

Because of the unique conditions that exist during initial fuel loading, temporary neutron detectors may be used in the reactor pressure vessel to provide additional reactivity monitoring. Credit for the use of temporary detectors may be taken in meeting Tech-Specs requirements on the number of operable source range channels. 

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