Level Neutron Flux Monitor

At background or shutdown levels the Lov Level Nemtron Flux Monitor System (Sub-Crltlcal Monitor) is used This system is eoi prlsed of two separate and identical channels each composed of a chamber mounted on a screw drive mechanism and an amplifier-counting circuit indicating period and level  

The high level nuclear monitoring system is the Bechmans  

and K Reactors still have the RXG-2 Beckmans which have been out of production for about nine years. Engineering effort is currently underway to changeout the RXG 2 s for Model Vs. Readout information is in the fom of indicating meters and recorders, remote and adjacent to the instrument. 

The C and K Reeustors are equipped with an Octant Flux Monitoring System. The DR uid H Reactors have holes in the shielding for the chambers however, the instrumentation has never been Installed. The octant concept is similar to the Beckman system concept however, there are eight channels instead of four and 

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The Neutron Flux Monitor System comnonly referred to as the "Beckmans"I is composed of four picoammetersi ion chambers and controllers for setting safety circuit trip points. The four ion chambers are located in instrument risers beneath the reactor and are positioned for equilibrium power level calibration ... 

Ihe present Beckmai s will be retained as the bottom neutron flux monitors in the any two-out-of-four tripplng network. TOie level trips of the Intermediate Range Monitor will be in a two-out< of-three network at the B, D, DR, F, and H Reactors where three channels will be installed. An "any-two-out-of-four" tripping network should be used at C and the K Reactors. 

Because of the addition of the top neutron flux monitoring system in the form of the level trip of the Intermediate Range Monitor, adequate protection can be still maintained with a reduced Zone Temperature Monitor (especially when considering... 

Inadvertent dilution can in principle be detected using nuclear flux measurements and boron concentration monitoring. Neutron flux monitoring is difficult during reactor shutdown or startup phase, because neutron level in the ionization chambers is very low (10 to 10 n/ctn xs). The continuous boron concentration monitoring is based on boron meters which have limited sampling points in the RCS. 

Fission counters are used extensively for both out-of-core and in-core measurements of neutron flux in nuclear flux in nuclear reactors. In out-of-core situations, they monitor the neutron population during the early stages of power ascension when the neutron flux level is very low. For in-core measurements, fission counters are used for flux mapping (and consequently, determination of the core power distribution). They are manufactured as long thin q lindrical probes that can be driven in and out of the core with the reactor in power. Typical commercial fission counters for in-core use have diameters of about 1.5 mm (0.06 in), use uranium enriched to at least 90 percent in as the sensitive material, and can be used to measure neutron fluxes up to 10 neutrons/(m s) [10 neutrons/(cm s)]. 

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 primary functions of the Nuclear Boiler System (NBS) are (1) to deliver steam from the RPV to Ae turbine main steam systan (TMSS), (2) to deliver feedwater from the condensate and feedwater system (C FS) to the RPV, (3) to provide overpressure protection of the RCPB, (4) to provide automatic depressurization of the RPV in the event of a LOCA where the RPV does not depressurize rapidly, and (5) with the exception of monitoring the neutron flux, to provide the instrumentation necessary for monitoring conditions in the RPV such as RPV pressure, metal temperature, and water level instrumentation. 

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. 

NUclear Instrumentation measures the levels the distribution and the rate of change of neutron flux density in the reactor. Monitoring data and safety circuit trip signals are provided as appropriate over the full range of neutron flux levels from those existing during sub-critical shutdown conditions to those characteristic of full production level operation. 

Instrumentation needs for adeqrrate monitoring of the neutron flux for reactor power levels, including startup and shutdown conditions, shotdd be stated. These may... 

Answer Hie No. 1 Galvanometer indicates the total current which is proportional to the neutron flux, or power level. Through the use of a potentiometer, the No. 2 Galvanometer becomes a differential monitor that reads small deviations about a preset or operating range. 

Beckman - A micro-microammeter made by Beckman Instruments, Inc. These are used with an ionization chamber to monitor neutron flux level and/or other types of radiation in various parts of the reactor. 

Sub-Critical Monitor - An electronic system for monitoring the neutron flux level while the Keff is less than unity. The system consists of a fission chamber in the active zone of the reactor connected to an amplifier-scaler, etc. 

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

The greater the reliability of the individual components within an I C system, the greater the rehabihty of the overall system. There are, however, practical limits to the levels of rehabihty of individual components. Higher reliability is achieved by the use of redundancy or diversity. For example, it may be possible to monitor reactor power with multiple channels or by diverse means such as measurement of neutron flux or temperature and fluid flow or pressure. The use of redundancy provides protection against random failures. Use of diversity provides protection against certain common cause failures. 

The two safety rods and the coarse control rod are each worth approximately 1.3 percent reactivity. The fine control rod, fuel loaded, is worth about 0.3 percent. The neutron flux is monitored by three detectors two BF3 ionization chambers and one BF3 proportional counter, all of which are located in the water tank just outside the lead shield. The detectors are connected respectively to a logarithmic micromicroammeter, a linear micro-microammeter, and a pulse amplifier and count rate meter located on the reactor console. Each indicator is connected through a sensitrol relay to a scram circuit. Additional safety interlocks provide for reactor shutdown if the level of the shield water drops, if the reactor temperature falls below 16 C, or if an earthquake occurs. Sequential interlocks are also present to ensure that the proper operational method is followed. 

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