Range Channel

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. 

Figure 35 shows a typical source range channel in functional form. 

Figure 36 shows a typical intermediate-range channel. 

The output of each power range channel is directly proportional to reactor power and typically covers a range from 0% to 125% of full power, but varies with each reactor. The output of each channel is displayed on a meter in terms of power level in percent of full rated power. The gain of each instrument is adjustable which provides a means for calibrating the output. This adjustment is normally determined by using a plant heat balance. Protective actions may be initiated by high power level on any two channels this is termed coincidence operation. 

Figure 37 shows a typical power range channel. 

The nuclear instrumentation must be operable prior to reactor startup. The automatic rod control during startup will not operate if more than one of the three ex-vessel wide-range channels is out of service. The Safety Protection Subsystem requires at least three of its four nuclear input channels operating. The power range neutron flux control will not operate automatically with more than two of the six input channels out of service. 

Excessive wide range channel flux deviation... 

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. 

Method III Note Use both power range channels. 

The original instrumentation design proposed by GA (fig.3) was considerably modified to meet the stringent requirements of the licensing authorities, the final design is shown in fig.6. Although one wide- range channel is microprocessor controlled, it is only considered as an operational channel without safety relevance. 

Presence of features having dimensions both in the millimeter as well as in the sub-millimeter range (channel length of 2.88mm, reservoir chamber with diameter of 3mm) ... 

---