The out-of-core system block diagram is shown in Figure 1-3; it closely parallels the in-corc cystem in many respects. The out-of-core detectors are much larger than their in-core counterparts, and are located in a fixed position near the core. The cable and eleetronico • portions of the out-of-core system are essentially identical,, and operate in an analogous fashion to the in-core system. ‘

SWITCH CONTROL

Figure 1-2. In-Core Log Count Rate Monitors and

Multi-range Mean Square Voliagё Monitor Systems

SWITCH CONTROL

Figure 1-3. Out-of-Core Log Count Rate Monitor and Multi-range Mean Square Voltage Monitor Systems

provides a means of identification for the test

Identification Detector Assembly 1 Detector Assembly 2 Detector Assembly 3 Detector Assembly 4 Detector Assembly 5 Detector Assembly 6 Detector Assembly 7 Log Count Rate Monitor

Multi-range MSV Monitor

Modified RG-6A/U Cable

Modified RG-114A/U Cable

RG-59A/U Cable Control Cable

Description.

In-core fission chamber (counter) • .

In-core fission chamber (counter)

In-core MSV fission chamber (Campbeller)

In-core MSV fission chamber (Campbeller)

Out-of-core MSV fission chamber (Campbeller)

Out-of-core fission chamber (counter) ,

Out-of-core fission — chamber (counter)

Counting electronics main chassis and remote amplifier

Campbell mean square voltage electronics main chassis and remote amplifier

Counting electronics signal cable with added shielding layers

Campbell electronics signal cable with added shielding layers

Used for many interconnection functions

Ten-wire control cable between, the Campbell remote amplifier and the main Campbell chassis.

5. 2. 1 Theoretical Treatment

For pulse counting, the form of the transmission line equation’ ‘ is

M)

2012 7Г b Z,

phase constant = 2 v f c ZQ,

resistivity of conductor material, in ohm-meters,

frequency, in cycles per second,

inner radius of outer conductor, in feet,

outer radius of inner conductor, in feet,

characteristic impedance of cable, in ohms,

capacitance, per foot, of cable,

length of cable, in feet,

voltage at sending end, and

voltage at receiving end.

Since we are interested in only the magnitude of the pulse, this can be written as

This equation is valid only if the cable is terminated in its characteristic impedance. To make it applicable for any termination, a term must be added to allow for the reflection factor. The complete equation, solved for I Vr! . is

where Z = termination impedance, in ohms.

GE АР-4 900

• • fi

The range of the out-of-core counting, system is 1.4 nv. to 1.4 x 10 nv (see Figure 8-11).

fi •

This system provides 1 count/sec at startup and 10° counts/sec at the upper end of the range.

Pulse overlap becomes large above 10 counts/sec, thus limiting the range of the system at this level. .

This particular chamber can be used up to300°C (aluminum limits the temperature). A

19

titanium fission counter of a similar design, with a specified life of 10 nvt, can be used up to 1000° F. The system uses the identical electronics as is used in the in-core system, except that a higher voltage polarizing power supply may be used if desired.

Consider the integrating circuit of time-constant т, labelled т in Figure 3-1, and con­sisting of a series resistor and a capacitor to ground. If a pulse of voltage of amplitude E and short time duration compared to т is applied to its input, its output is

s(t) — В e”t,/T. (3-17)

and the time-integral of this voltage must equal the time-integral of the input pulse; i. e.,

.00

(3-18)

so

and

s(t) = m e -‘/т

Also, if the circuit is suppliёd with a succession of short voltage pulses, not necessarily equal, its output is a linear superposition of terms like Equation (3-20) .

t/At

S(t) = ^ S^t-iAt),

І = -00

Or

2

Now the continuously-varying voltage, AV (t)., that is being applied across this circuit can be. considered as supplying a succession of short voltage pulses by dividing the time axis into

1L.

short equal intervals of length At. The amplitude of the iin pulse is •

E{ = Av (iAt),

Equation (3-4) shows that

00 00 .

V2(iAt) = ^ ^ 77t_a 77І-Ь у(аДt)y(bAt).

a = o b = о

(3-26)
і = — oo ■ a = о b=o
If we let
c “ ST "1’
(3-27)
Now the variance is obtained from
„ 2 .„2, 2 a = <S > — <S> ,
(3-28)
V t 71 t h t AT“c’a Ж “c_b £F “h“f
(3-29)

and this can be written as

where it is understood that no two subscripts of i] in any summation are equal.

Equation (3-30) is obtained from Equation (3-29) by performing the following operations:

a. Equation (3-29) is written as the sum of five separate summations. The first has the subscripts of all four it’s equal, i. e., four of a kind. The second has the subscripts of three uf the )j! o equal, i. e., three of a kind. The third has two pairs, the fourth has one pair, and the fifth has none equal. The second, fourth, and fifth summations will go to zero because of Equation (3-11), leaving only the first and third.-

b. All four subscripts are equal only if a = b, f = g, and c + b = h + g. For those terms

in which C>h, we write b for a and b + c — h for f and g, and for those terms in which c<h, we write g for f and g + h — c for a and b; the result is the first summation in Equation (3-30). . . .

c. There are three ways in which there can be two pairs. The first is for a = b and

f =. g. Then, if we write b for a and g for f, the result is the third summation in

Equation (3-30). .

d. The second way to obtain two pairs is for c + a = h + f and c + b = h + g, and the third-way is for c + a = h + g and c + b = h + f. For those terms in which c > h we write a + c — h for f and b + c — h for g to take care of the second way, and we write a + c — h for g and b + c — h for f to take care of the third way; for those terms in which c<h we write f + h — c for a and g + h — c for b to take care of the second way, and we write f + h — c for b and g + h — c for a to take care of the third way. The result is the second summation in Equation (3-30).

e. It is necessary to separate the terms in which c>h from those in which c<h to insure that the argument of v. is always positive [ cf. Equation (3-29)J.

If the average flux of neutrons is Ф and the counting efficienty of the detector is k, then the expected values of the 77 combinations are

<(t])4> = k$At

4n)2(n)2> = (»At)2

о

and the expected value of S (t) is

CAt hAt v(gAt + hAt — cAt) e ^ T (At)4

K) z z z z

c=ob=oh=og=o
g ф b + c — h

cAt hAt

v2(bAt)v2(gAt)e T T /A*’4

dx dy d£ (3-33)

By combining Equations (3-14), (3-28), and (3-33), we obtain the variance of the signal .00 ,00 .Л, л o-x/T

dx dy

v(?)v(y) e

‘o •’o

/(y+x-z)v(£+x-z)dz

+ I e~ ‘ v(y+z-x)v(£+z-x) dz x

dx dy d£. (3-34)

s(t) = — Vi V t v(aAt)v(bAt)Ate’CAt//‘ ■ т с = о а — о b — о ЭТ SC — c‘b

(3-27)

The monitor consists of a modularized discriminator, logarithmic integrator, level amplifier, period amplifier, and chamber polarizing voltage supply. In addition, there are trip circuits, power supplies, and filters. Modules are mounted in the chassis. These modules function as follows:

a. The discriminator comprises an input common base stage for cable termination, an integral pulse height discriminator, a shaper, a binary, and a driver.

b. The logarithmic-integrator is a passive integrator of the Cooke-Yarborough type. The breakpoints of the networks are spaced one decade apart. The time constant over the range is a function of the point on the range and follows a smooth curve. The analytic

, expression for the analog out is:

Vn (TP1) = — [ Log (count rate) + 1 ]

7 .

6-1

c. The level amplifier is a d-c amplifier which has a current gain of approximately 3. 5 amps/amp.

d. The period amplifier is a d-c amplifier with appropriate feedback components to provide a gain of approximately 200 volt/volt/sec. This corresponds to a full-scale period indication of 10 seconds.

e. The trip circuits are d-c difference amplifiers with two-state outputs.

f. The chamber polarizing voltage supply is a d-c to d-c converter which provides + 100 to +400 volts d-c to polarize the detector.

Several gamma background situations are now considered and plotted:

Case І: Ф = 10′ R/h.

= 0.110+ (3.0X10"9) (107 R/h) = 0. 140

and it can be seen from the graph that the count rate is less than 0. 01 sec* at a discriminator setting of 2. 5 volts,

Case II: At a count rate of 1 count/sec due to gamma, with discriminator set at

2. 5 volts,

c _ -5 •

4. 75

A plot of count-rate indication versus discriminator setting, at 1.45 x Ю4 nv

. ‘• П •

neutron flux and with both zero gamma and 2.5×10 R/h gamma, is shown in Figure 6-9 in Section VI. .

Case III: Conversely, if the area of the counter electrodes were larger, the count

rate due to gamma would increase. For a counter with 10 times the present

• • 7 •

electrode area the variance is found to be at 10 R/h, ‘a 2. (107 R/h) = 0.30,

thus
or
S

The large increase in pile-up count rate with increased electrode area shows that a larger counter would not be desirable.

The second possible effect that could limit the low end of the range is the electric potential distribution change in the gas volume of the chamber (space charge effects). When the flux levels are high enough, the electric field will go to zero in the vicinity of the positive electrode, and recombination will commence. As a result both the pulse height and the average current will reduce in magnitude. This effect, however, does not occur at flux levels that will be encountered by the in-core counter; for example, it would commence for Counter No. 1 at a gamma level of 4 x 108 R/h.

7. 1 GENERAL DESCRIPTION

The electronics for the Campbell subsystem[8] consist basically of five components: remote amplifier, local amplifier, inverter, mean square analog, and d-c amplifier.

7. 1.1 Remote Amplifier

The remote amplifier is constructed in such a manner that it can be located as near the reactor as possible while staying outside the biological shielding, the purpose of which is to minimize the distance between the remote amplifier and the detector. It receives its power from the main chassis, and acts as a junction between the detector and the detector polarizing supply located in the main chassis.

The remote amplifier is, in reality, two separate amplifiers housed in the same box, which differ in gain, input impedance, and frequency response. One is referred to as the low-range amplifier and the other as the high-range amplifier. The need for the two different ranges of amplifiers is dictated by the desire to monitor six decades of flux with one chamber.

a. The low-range amplifier consists of a high-gain common emitter stage and two com­plementary doublet stages. The over-all gain is a nominal 1200 before application of frequency breaks.

At the lower flux levels covered by the MMSVM, it is necessary to terminate the signal input cable in the high impedance in order to obtain sufficient usable signal, and to provide adequate pulse overlap so that the fractional standard deviation will fall within acceptable limits while maintaining a minimum averaging time constant. For these reasons, an input impedance of 5, 000 ohms was chosen for the low-range amplifier. Although not an extremely high input impedance, it is a factor of 27 greater than the 185-ohm characteristic impedance of the RQ-114/U cable used to couple the in-core assembly to the amplifier, and it is sufficiently high that the cable appears to be nearly open circuited.

When operating a cable in an unterminated mode, the cable capacity acts to degrade the signal. The cable capacity must therefore be kept to a minimum. It was for this reason that RG-114/U was chosen for the out-of-core signal cable since its capacity is only 6. 5 picofarads per foot.

The frequency breaks on the low-range amplifier were chosen to make best use of the signals from the unterminated cable and still maintain the response necessary for reactor control. The low-frequency break is 8 kc and the high frequency break is at 60 kc. With a detector and cable assembly of 2000 picofarads capacity connected to the input, the high frequency break is lowered to 16 kc.

b. The high-range amplifier consists of a common base input stage and two complementary doublet stages. The gain is adjustable from about 115 to 510 before application of frequency breaks. The adjustable gain makes it possible to align the two scales involved when switching from the low — to high-range amplifier.

At high flux levels, faster system response is necessary. For this reason, the band­width of the high-range amplifier is designed to be from 300 kc to 600 kc, and the averaging time constant is reduced by a factor of 10. It is also necessary to shorten the pulses into the amplifier to a length that will be compatible with the higher fre­quency response. To accomplish this, the input impedance of the high-range amplifier is matched to the characteristic impedance of the signal cable. This reduces the pulse width to a fraction of a microsecond, but pulse pile-up is adequate since the pulse count rate from the detector is high.

The development program utilized various test sites for evaluation of the prototype hardware. These test sites are described as. follows:

a. NEPS — Building W. Neutron Source Site ‘ •

This facility contains a small, neutron source from which a neutron flux of approx — ■ 4

imately 1. 5 x 10 nv is available. This flux was used to determine initial operability of the counting electronics hardware. In addition, measurements of counting system noise and in-core detector and cable temperature effects were conducted at this site.

b. Vallecitos — Cobalt-60 Facility

• , C —

The effect of gamma flux, up to a maximum value of 3 x 10° R/h. on the counting

and Campbell systems was investigated under ambient temperature conditions. .

Additional tests were performed at various gamma flux levels for both systems.

4

using a small neutron source of approximately 3 x 10 nv in close proximity to the detectors. .

c. Vallecitos — Nuclear Test. Reactor (NTR)

This facility was used to determine counting system range, linearity and approximate

system response during mechanical withdrawal movements of the detector, and

linearity of the Campbell system over the available flux range at NTR. This reactor ■ 4 ■ .

is easily controlled from a source level flux of approximately 10 nv to a maximum flux

of 6‘. 6i x 10s^ nv. The maximum’ gamma dose rate at full’ power is approximately

2 x 10 R/h for this reactor. A number of response measurements were also made

for both counting, and Campbell systems as a function of time following a reactor scram.

d. Vallecitos — General Electric Test Reactor (GETR)

A measurement of the Campbell system linearity up to a peak flux of approximately 14

10 nv was made in the Z trail cable. Campbell and counting system performances as a function of time following shutdown were recorded in order to make the sensitivity determination. .

SECTION II

The frequency to use in determining the attenuation constant, ot. can be chosen fairly well from the pulse width. For example, a rectangular pulse of width T seconds has an energy spectrum that has an upper half-power point at f = 1/(2. 2 T), goes to zero at f = 1/T, and has a maximum energy per decade of frequency at f = 1/(2. 5T). So an equivalent frequency of l/(2T) is a reasonable compromise, and this value of frequency produces good agreement with experimental values.

Table 5-1 shows calculated and experimental values of attenuation (! Vr I /I VI) for various cables. ,

It is useful to express the attenuation constant, о, in terms of independently variable quantities to facilitate its minimization:

2. 78 x 105 77 . b log 10 ‘ (b/a)

The out-of-core counting subsystem was tested for response to scrams from high power levels (900 watts and 300 watts), using both types of out-of-core fission chambers; i. e., the large 3-inch-diameter chamber (chamber No. 6) and the small 1-inch-diameter chamber (chamber No. 7). The output of the subsystem was monitored before and after the scram. Shown in Figure 8-12 is the decay curve for chamber No. 7 (sensitivity ~0. 1 counts/sec per nv). This curve indicates a constant slope after t = +4 minutes of 81 sec/ecade (186 sec/decade).

Shown in Figure 8-13 is the decay curve for chamber No. 6 (sensitivity ~0. 7 counts/sec per nv). This curve shows a constant slope after t = +3. 5 minutes of 78. 3 sec/ecade (180 sec/ decade). The response of the counting subsystem to rapid mechanical withdrawal at intermediate power levels (30 watts and 300 watts) was determined. The flux level at the detector was equiva­lent to an approximate full-scale reading on the subsystem. At this level the subsystem electronic response time is considered very short compared to the mechanical withdrawal time. ■

Chamber No. 7 was withdrawn four times at a. power level of 30 watts (see Figure 8-14 .

through 8-17). The average mechanical withdrawal time was 1. 5 seconds. The average time between the completion of the withdrawal and the time when the counting subsystem output reached its final value was 0. 75 second. Two withdrawals of chamber No. 7 were made at a power level of 300 watts (see Figures 8-18 and 8-19). The average mechanical withdrawal time for these withdrawals was 1. 22 seconds. The average time between the completion of the withdrawal and the time when the counting subsystem reached its final value in this case was 1. 42 seconds. These times are not known with great precision; the possible error includes a 1-second scaler integrating time.

Chamber No. 6 was withdrawn once at a power level of 100 watts (chamber No. 6 could not be reinserted with the reactor at power). The withdrawal time for this test (see Figure 8-20) was approximately і second, arid the counting subsystem reached its final value alter a delay of 1 second from the time that the withdrawal was completed.

In all cases above, no noise transient phenomena were evident, either on the withdrawals or the subsequent reinsertion. •

dr.