In practice, temperature effects are not entirely separa­ble from the gamma effect If it is assumed that there is no need for gamma discrimination, at elevated temperature the cable insulation resistance is seen to decrease as the temperature increases whereas the breakdown voltage, which creates noise or a mean deviation, is relatively constant Thus the effect of noise is minimal whereas the mean current attributable to neutrons can be completely obscured by direct-current leakage.

Quantitative results have not been reported. The first reported use of MSV methods was for in-core units of about 1 cm3 volume operating at about 600°F (316°C) The mean signal was obscured by leakage, but the MSV signal was still present The gross effect has been discussed by DuBridge et al (see the Bibliography at the end of this chapter).

From these observations it can be concluded that, if neutron-sensitive chambers and their associated cables are to be operated at elevated temperatures, the proper method
is to use MSV measurements instead of mean-current measurements.

5- 5.4 Wide-Range Neutron Monitoring Channels

As noted in Sec 5-1, the large range of neutron-flux values to be monitored m a power reactor requires the use of several separate channels with two decades of overlap between the channels With the MSV technique it has been possible to develop monitoring systems that cover 10 decades of reactor power. The signal from a single fixed-position fission chamber is used with what has come to be called counting-MSV or counting-Campbelhng cir­cuits. The combination of count rate and MSV can be read out as a linear indication of power, much as is done with a range-switched picoammeter (see Sec 5-4.4), or a log output can be taken from which period can be derived.

The wide-range instrumentation permits the use of fewer sensors, recorders, meters, cables, and sensor thim­bles. This represents a significant saving in power-reactor instrumentation costs.System Descriptions and Components

Figure 5 21 is a block diagram of an average-magni­tude-squared channel that covers 10 decades of reactor power The MSV portion of the channel uses the combina­tion pulse-charge or current preamplifier, band-pass ampli­fier, rectifier, log amplifier, and d-c amplifier The noise level for the Campbell part of the channel is reduced by special shielding of the 40-ft cable between the chamber and the double-shielded preamplifier. The impedance and signal levels out of the preamplifier are such that essentially any reasonable length of conventional coaxial cable to the rest of the circuit may be used with good results As shown in Fig. 5.21, the wide-range log power channel consists of an LCR circuit and a log-Campbell circuit working out of the same fission chamber and preamplifier Their output signals are combined to give a single output indication of log power and, at the same time, to eliminate normal limiting errors of each one.

The high-frequency components of the signal pass through a conventional LCR circuit using a discriminator, flip-flop, and multiple-diode log pump circuit This circuit gives an output voltage, Ei, that is proportional to the log of the count rate from about 0.3 to 300,000 counts/sec. A biased diode, Db limiter is used to cut off that portion of the response which is adversely affected by resolution counting loss (usually about 2 X 10s neutrons cm 2 sec 1) As is customary with LCR circuits, the log diode pump uses fixed low-temperature-coefficient components to obtain the log relation. The pump circuit output passes through a d-c operational amplifier with adjustable gain and bias, so output volts per decade and volt level for a particular flux level can be established.

I LEVEL I TRIP I_ .__
ADJ BIAS	ADJ VOLT
jcOUNT-RATE SIGNAL ■<> |rms signal
PREAMP
TEST AND CALIB
I———— 1 y. LIN I Чпт PWR |£HANNELj
BAND­ PASS AMP
LEVEL TRIP (LOW)
LOG COUNT RATE LOGA-C OUTPUT COMBINED OUTPUT
VOLT­ METER
HIGH- VOLTAGE SUPPLY
Fig. 5.21—Wide-range log channel
LCRM
INPUT

The low-frequency components governed by the Campbell band-pass amplifier pass through the linear rectifier, filter (T = 50 msec), and through a stable d-c log amplifier to give a log indication of power from 2 X 10s to 2 X 1010 neutrons cm 2 sec 1 The diode D2 allows only signals above zero to pass, therefore, if the output of the log amplifier is biased, the lower end of the response curve below 2 X 10s neutrons cm-2 sec 1 can be eliminated. This portion of the curve may be adversely affected by gamma and alpha background, noise, imperfect rectification, and lack of pulse overlap. A gain adjust at the output of the log amplifier allows the slope of E2 to be properly adjusted to match that of Ej Since the bias cutoff points can be adjusted to coincide, a smooth, fast, all-electronic transition is made from counting to Campbelling or vice versa

Since misalignment or drift might produce an offset at the crossover point (2 X 10s neutrons cm2 sec1) and cause an exaggerated false period indication m that region, great emphasis has been placed on stability of the circuits and on a built-in calibration and test circuit that will allow proper alignment without the use of a reactor High-gain solid-state operational amplifiers (some integrated circuits) with feedback through stable circuit elements were used throughout. The principal temperature problem, as usual, was associated with the log amplifier in the Campbell circuit, and this was solved by using two operational amplifiers and two sets of logging diodes thermally coupled to obtain a temperature-compensated log response. Temper­
ature tests were performed between 50°F and 150°F (10°C and 66°C) Temperature drift results in an output error of less than 15% from 50°F to 120°F and only a slight further degradation up to 150°F.

Another average-magnitude-squared channel is shown in Fig. 5.22. The circuit is divided into two portions, as shown in the figure The signal from the radiation sensor enters the instrument and is routed both to the log count rate circuit (LCRM) and the statistical level amplifier (MSV) Pulses within the range of 1 per second to 106 per second pass through the LCRM discriminator into a flip-flop, a driver network, and then to a Cooke—Yarborough diode pump log converter, where the pulse rate is converted to a d-c output At that point the signal is amplified to the desired output level and routed to the front panel meter and other instrument inputs When the input signal enters the MSV portion of the instrument, it is first applied to a filter that passes only the portion of the signal between 4 and 8 kHz This signal is then applied to a wide-range rectifier, thus providing a half-wave rectified signal to a smoothing filter, which then routes the d-c output to a log amplifier and hence to the output meter and other external devices The output of the MSV circuit is scaled to read two decades of output for each decade of input current to provide the square m the mean-square measuring technique

A third meter on the instrument measures the rate of change of both the LCRM and MSV channels alternately, switching from the LCRM channel to the MSV channel

automatically as the LCRM reaches near full scale and as the MSV channel begins to operate. Since this portion of the circuit is somewhat novel, it is detailed in Fig 5 23.

In operation the output levels from the LCRM and MSV circuits are each applied to a separate differentiator circuit that operates continuously. The two outputs are routed through a solid-state switching network that is controlled by the magnitude of the MSV input signal such that below a preset level of the MSV circuit the signal from the LCRM differentiator enters the output scaling amplifier and above this level the MSV differentiator controls the output scaling amplifier. The output of the scaling amplifier is fed to the rate-of-change meter, which reads from —1 to +9 decades per minute

Another feature of the rate-of-change circuit is the “LCRM suppress” feature This involves a circuit to suppress rate-of-change movements on the lower portion of the LCRM channel and thus has the effect of preventing rate-of-change meter movements where counting statistics cause an erratic signal

In addition to the rate-of-change circuits, the instru­ment generates pulses for calibrating the LCRM, ramp functions for checking the rate-of-change circuits, and a variable-level 5-kHz signal for calibrating the MSV circuit Furthermore, all meters have electronic level trip circuits that read the output level of a particular circuit and produce both an automatic resetting alarm and one that latches and must be reset manually after an alarm is
produced by an excessively high or low level reading on any one of the three front panel meters.*