Neutron flux detectors, location

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. 

Neutron flux detectors positioned by hangers are placed in fuel channels, the reflector and the annular water-filled biological shield tank. They are located as follows ... 

A number of other devices are located in the main vessel the fuel transfer arm, the six control rods, neutron flux detectors, thermocouples, feiled fuel detection and location devices, the core acoustic detection system components, etc. 

In the correlation method the neutron flux density of the specified region is determined from the ratio of pulse measmement channel count rate to detector sensitivity s(E). If we assume that the neutron spectrum in the area of neutron detector location does not depend on the type of SRP being stored in the pit, then for all the measurements s(E) is a constant value. In the next calculations it can be assumed as 1 and thus, the coimt rate being measured can be assumed equal to the value of proportional neutron flux density. 

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. 

The ex-vessel neutron detection equipment consists of fission chamber neutron detectors mounted in six equally spaced vertical wells located just outside the reactor vessel as illustrated in Figure 4.3-4. The signals from these detectors are supplied to the nuclear instrumentation cabinet and Safety Protection Subsystem equipment located primarily in the reactor building. These data are used by the automatic control systems to operate the control rod drives or the reserve shutdown equipment, thereby changing the neutron flux levels within the reactor core. 

The need for startup neutron detectors in-vessel is dictated by the low neutron flux at the ex-vessel detector location at startup and to ensure a controlled startup. Therefore, in-vessel startup detectors are used for flux monitoring while the reactor is brought to a critical configuration and during reactor shutdown periods. Three startup neutron detectors are installed to ensure adequate neutron flux measurements during these low power intervals. 

Cladding encapsulation is completed by seal welding the end plugs. The specific core location of the sources is determined during final design of the core to assure adequate neutron flux at the source range detectors at all times. 

Neutron flux monitoring To cover the whole range of flux from shutdown to full power, 3 sets of detectors with overlapping ranges are provided. One set is placed within the core and is effective from mW to 100 W. The other two sets are located below the reactor. While the second set responds to the range from tens of watts to few MW, the third set covers the range from a few hundred kW to beyond rated power. 

Four control and safety rods containing fuel are inserted from the bottom of the core. The neutron flux is monitored by three BF3 detectors located in the shielding water, A very useful characteristic of the system is its high analytical sensitivity. 

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. 

It is easily seen from this expression that the counting rate obtained by the detector is proportional to the neutron flux at the detector location. The B °F3 detector used in this experiment has an active volume of approximately 12 cm and a sensitivity of one count per second per unit flux. The counter is placed in a fixed position at the end of an aluminum tube which is about 6 ft long. The tube is graduated in equal increments of length. The counter is positioned in the pile by inserting the tube into the vertical or horizontal holes provided. The electronics associated with the detector consist of a preamplifier and a MPC-1 linear amplifier, high-voltage supply, and scaler. 

In this experinnent, measurements are made of the fast, resonance, and thermal-neutron fluxes at the same location in the Argonaut core by means of induced-activation detectors. Thoroughly discussed are the selection and preparation of the foil materials (detectors), the calibration of the counting apparatus, the determination of the induced activity from the foil-counting data, and, finally, the calculation of the particular neutron flux from the activity data. 

The level is therefore proportional to the reactor integral flux, or the integral power. Hence a detector that is located near a reactor coolant discharge line can be used to monitor the activity level, and hence the reactor power. This approach provides a useful way for corroborating the signals obtained from neutron detectors and calorimeters (heat balances). However, it should be recognized that the power signal from a N-16 detector will be delayed relative to the true core power by the time required for the primary coolant to flow from the reactor core to the detector location. 

Neutron flux counting channels have to be incorporated in all research reactors. In order to avoid noise pick-up, the initial processing electronics (preamplifiers) have to be located at a short distance from the detector. However, preamplifiers are not located directly on the counters, to avoid radiation damage to its components. Three types or preamplifiers are available for use with detectors operating in the pulse counting mode ... 

The protection system was designed to avoid any unsafe condition. It was subdivided into two subsystems, the nuclear detection subsystem and the interlock subsystem. The nuclear detection subsystem is used to monitor neutron flux level and period. It is composed of 8 nuclear channels to monitor the neutron flux from start up to 100% of full power (100 watts), including comparators and isolation ampliBers. Three channels are used in the start-up region, and the others in the intermediate and power regions. In each region we have three measurements of the neutron flux (power) and three measurements of the period. The nuclear channels are complemented by two linear channels, used (alternatively) to control the reactor in automatic mode. Figure 5 shows the relative location of the detectors, and the operational interval of them. 

A Kaman pulsed neutron source was used, located in the reflector adjacent to the core,- with the target positioned at the center of one face. Scintillators of b" + ZnS(Ag) detected the leakage flux, and the pulse output was oteerved with a 256-channel analyzer. Some measurements were made with miniature BFj counters inserted in the core proper. Detectors were positioned to achieve maximum siqspresslan of higher mode contamination. Neutron bursts were about 10 ftsec long, and only data taken more than about 1.3 msec from the burst time were considered. 

If the neutron detector is in a location where the flux shape is significantly affected by control element motion, what effect will this have on the positive reactivity measurements On the shutdown measurements ... 

The flux-monitoring devices for the AGN-201 reactor consist of one BF3 proportional counter and two BF3 ionization chambers. These neutron detectors are located in the shielding water tank just outside the... 

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