Because of the integral nature of the cable and the chamber in the form of a detector assembly, the experimental determination of the breakdown characteristics was made on the entire assembly. The-results of these tests have been presented in Section TV.

5.6 SUSCEPTIBILITY TO EXTRANEOUS SIGNALS

Because of the. cable processing requirements mentioned earlier in this section, the transmission line that provided the best over-all performance was the stainless steel — quartz — stainless steel configuration, listed as the last item of Table 5-1. Of considerable concern in information transmission systems, especially those information transmission systems associated with the transmission of information about a nuclear device, is the susceptibility of the trans­mission system to extraneous signals and noise. In an attempt to minimize the susceptibility of the transmission system to extraneous electric, magnetic, and electromagnetic fields, the basic cable configuration mentioned above was evaluated with and without additional shields of various’ materials and in various modes of connection. The results obtained on a small sample indicated that the configuration for the best over-all performance was a stainless steel — quartz — stainless steel cable, to which was added two additional copper braids of about 20 mils total thickness.

This cable is then placed in the usual solid-sheath, stainless steel jacket and the resultant cable is called the "prototype” cable. This cable was mentioned previously in Subsection 5. 2.

SECTION VI

N.

When the detector is exposed to neutron flux, fission is induced in the uranium coating, U235^, F) fp. Two energetic, highly-charged fission particles travel in opposite directions from their original site. Since the uranium coating is extremely thin, it is highly probable that one of the fission particles will traverse the gas gap in the chamber.

The shape of both the ionic and electronic current pulses from a well saturated chamber have the form:

where

PQ = the charge density along the fission particle track, d = electrode spacing,

t = time from instant of track formation, and

Te = transit time of electrons (or ions) across gas gap.

The typical charged particle track is one that extends across the entire gas gap in the chamber since the electrode spacing is small relative to the range of the fission particle. After passage of a fission particle, the resultant ions and electrons drift respectively toward the electrode of opposite polarity. .

Assume that the rise of the pulse is instantaneous since it is associated with the transit time of the fission particle and the time required for the charged particle to reach terminal velocity. Then, the extent in space of the electrons due to the passage of a single charged particle at time, t, after pulse initiation is as shown in Figure 4-1.

An energy balance equation is written relating the work done by the battery or power supply to work done on gas by the movement of the charged particles. The work required to collect the remainder of the pulse is

[0(d) -0(x)] PQdx = Vp q, (4-2)

where Vp is the battery potential and q is the charge yet to flow at time, t.

But

and

where pe is the electron mobility (assumed to be a linear function of electric field). Therefore,

The charge collected, q’, is given as

q’ = qT — q = qT — p0

At t = o, q’ =o, hence

Differentiate q’ with respect to time to obtain the instantaneous current:

„ VP "e* V‘ e d d3

V / V t

= P0 ^ f f 1 —

The total time interval of the pulse due to the electrons is equal to the transit time of the electrons and is given by

Substitution into Equation (4-8) yields

(4-1)

which is the expression for the current pulse. The time interval of the pulse was measured to be 0.1 дsec.

Because the derivative of the level signal is being taken, the fluctuations on the period circuitry are very severe. For period circuitry of the form shown below,

°—ЛЛАЛЛЛ/—°

r2

And, if A » 1, then Equation (6-1) reduces to

Eq(S) _ _ r2C1S____________

Ein(s) " " (RiCjS+i) (r2c2s+i)

For the period portion of the electronic subsystem,

Therefore.

Although an exact analysis of the fluctuation on the period meter is not included in this report, it is seen from Equation (6-4) that the period output will change approximately 200 times the rate of change of the level output.

The essential elements of the development in-core reactor instrumentation system are. shown in Figure І-2. Dry tubes are provided in the reactor core, and the detectors are positioned vertically within the tubes by the mechanical drive hardware. At startup of the power plant, the counting and Campbell detectors are located at an initial in-core position and are connected to their respective electronics assemblies by means of specially shielded cables. The counting electronics provides a seven-decade log count-rate meter indication, an analog recorder output voltage for the log count rate (level), and a meter indication and analog recorder output voltage for the reactor period. The Campbell electronics provides a recorder and meter indica­tion output in linear switched increments over six decades. Auxiliary functions such as trip. signals are also provided by the Campbell system. As the reactor is brought to full-power conditions, the in-core detectors are retracted. from the high-flux region and ultimately stored several feet below the core bottom.

For direct current, the transmission line equations^ are:

(5-1)

and

(5-2)

5- 1

• 8 1Я

The range of the in-core Campbelling system is approximately 2 x 10 to 3 x 10 nv

7 11

(see Figure 8-10). This assumes a constant gamma level of 2. 5 x 10 up to 3 x 10 nv, above which the level of the gamma flux increases as the neutron flux increases. The gamma flux reaches 1×10 R/h when the neutron flux is 1 x 10 nv. The lower limit of the operating range with no gamma flux is determined by amplifier noise; the lower limit of the operative range for a hot restart is determined by the gamma flux and amplifier noise. The neutron/gamma output signal ratios can be found from the product of the neutron sensitivity/gamma sensitivity ratio and the ratio of neutron flux to gamma flux.

The upper limit of measurable neutron flux for this system is determined by ion chamber

saturation. The detectprs have passed breakdown tests at 700°F (Figure 4-5). The results of

the in-core counter life test mentioned in Section IV demonstrate that the Campbell chamber has 20 19

a life of at least 10 nvt. At this time the specified life is 10 nvt. Life tests are continuing.

The system is well shielded and filtered to minimize noise susceptibility. Audio-frequency,

radio-frequency, and transient-pulse-conducted and relay-radiated-interference tests have been

conducted (<• 1 percent added to signal due to these sources). The detector is retracted at.

12

5 x 10 nv to avoid excessive neutron and gamma exposures and a large. temperature rise in a dry tube.

a " — ST "n

by separating the terms in which a = b from those in which а ф b, and the expected value of V (t). is

■ 00 ‘

<v2(t)> = <(vt f > y2<aAt>

a = о V Ж ‘ а/

‘ 00 00

+ <7? t 7) t >v(aAt) v(bAt), (3-5)

a = о b = о Ж " a Ж ‘ b а Ф b

since the expectation of the sum of a number of random variables is equal to the sum of their expectations.

If the average flux of neutrons is Ф and the counting efficiency of the detector is k,

then the probability of detecting a neutron during the ntb time interval is кФДі, the probability

th

of detecting no neutron during the n time interval is l-кФді, and the expected values of the Tj combinations are

< h7t 2> = [l2 x k<I>At] + [o2 x (l-k^At)]

Ы — a]

= кФді,

<rjt 77 t > = [l2x (кФдО2] + [l x k$At x 0 x (1-кФдо]

+ [o x (l-k$At) x 1 x кФді] + [о2 х (1-кФд02]

= (кФді) ,

so

We have postulated a d-c removing component, such as a series capacitor or a shunt inductor, in the linear part of the circuit and it is easy to show that under this condition

.oo. ‘■ . ‘ • .

v(x) dx = 0. (3-11)

’o.

So the expected value of V^(t) is

(3-13)

Also, the output of the integrating circuit, т, satisfies the differential equations

dS(t) + S(t) = AV^(t)

dt t

d<S(t)> + <S(t) > = <AVz(t)> dt t t

and since <AV^(t)> is constant in time, then

If there are two different types of particles present, arguments similar to the above will

yield

(3-15)

where the subscripts 1 and 2 refer to the two types of particles. Because of Equation (3-11); this becomes

5. 1 GENERAL DESCRIPTION

The counting channel electronic subsystem is composed of the following parts: a remotely — mounted pulse amplifier, a locally-mounted monitor, a cable to connect the remote amplifier to the detector assembly (or the detector, in the case of the out-of-core system), and a set of three cables — one each for chamber polarizing voltage, signal, and power connections to the remote amplifier from the monitor.

6. 1. 1 Remote Amplifier ‘

The remote amplifier is a radio frequency pulse amplifier operating in the current ampli­fication mode. The amplifier contains the following stages: an input common base stage, a low level doublet amplifier, an attenuator, a buffer stage, a common collector amplifier stage, and. a high gain stage. These stages function in the following manner: The input common base stage provides a low input impedance for cable termination; the low level complementary doublet amplifier stage provides a current gain of approximately 40; the attenuator network provides attenuation of 1:1, 2:1, 4.1, and 8:1; the common base buffer stage acts as a buffer between low and high level stages; the common collector stage provides current gain of approximately 40 without phase inversion; and the high-gain complementary doublet provides a current gain of approximately 100. In addition to the amplification stages, the remote amplifier includes filters for the chamber polarizing voltage from the monitor and the power voltage from the monitor.

4.3.1 Counter.

The operating range can be considered as that range of the measured parameter throughout which the system. indication bears a stated relationship to the true value of the measured parameter, within the boundaries of other constraints on the system such as response time and allowable fractional variation.

The upper end of the range of the counting channel is limited by counting losses in the electro­nic subsystem at high count rates. The properties of the electronic subsystem are discussed in Section VI.

The lower end of the range of the counting channel is limited by the allowable error due to counting statistics coupled with the counting sensitivity of the chamber. For example, if a mini-

_3

mum of 1 count per second is required and the counting sensitivity is 1 x 10 counts per second.

3 .

per nv, then the lower end of the range is 10 nv.

There are other effects that could, in principle, limit the. lower end of the range to a different value than that determined by statistics and counting efficiency, but they are not realized in practice. At higher gamma levels than those normally encountered at startup, gamma-pulse pile-up would be more severe, requiring a higher discriminator setting and hence resulting in a lower counting sensi­tivity than at the normal gamma level. Some examples of this effect are worked out below. The rate, r^, that a pulse pile-up will exceed a certain, amplitude, per Second, is: .

(4-10)

(4-11)

where

rate that pulse pile-up exceeds the level Vj-> ,

critical voltage level.(proportional to discriminator setting),

voltage profile of a single pulse at point of interest in system,

n = pulse rate,

"o

f = mean square of spectral distribution of pulse power at point of interest

in system, and

0

a = the variance of the signal.

The above expression assumes background pulses of uniform size which is an approximation, of course; however, an appreciation of the functional dependence is afforded by the above expres­sion.

If the logarithm is taken of Equation (4-10) for the number of positive crossings due to pulse pile-up, the following is obtained: .

This expression can be applied to the integral bias curve to obtain

Figure 4-2 is a plot of integral bias data taken at different gamma levels. If the equivalent count rate is plotted on semi-log paper versus the square of the discriminator setting, Figure 4-3 results. From this figure the following is obtained:

The slope of each curve, S.

a.

1

(4-13)

(4-14)

■ •U Figure 4-2. Integral Bias Curves, Detector Assembly No. 1 (Counter) Versus Gamma Flux

2 2 ‘

The terms ct„ and a are obtained from the slopes S — below as:

• ©•/ I.

= = — 4.61 (a 2 + a j2)

-1.96 Vе 7 ‘

1

S2 (з x 106 R/h^ _1* 83

But since

then,

0.111 = о 2 + oy2

and

„ 2 ^ 30 „ 2

ae + 7 ayl.

or subtracting

0.008

or

■ Oy 2 = 1.2 X10’3

Thus,

and

a 2 = 0.111 — 0.0012 = 0.110

ІЛ

Figure 4-3. Count Rate Versus Square of Critical Voltage Level

4-9

Both the in-core and out-of-core subsystems are identical regarding protection from electric, magnetic, and e-m fields. The signal cable from the detector or detector assembly to the remote amplifier is constructed with additional shields and guards. The remote amplifier itself is packaged in a manner consistent with a minimally susceptible subsystem; i. e., a shielded, gasketed, and decoupled enclosure, and the signal cable connecting the remote amplifier to the monitor is double shielded and guarded.

For the in-core subsystem, the in-core cable must be protected against these fields.

Copper braids have been introduced on top of the stainless steel — quartz — stainless steel structure to provide this protection.

Minimalization of susceptibility to line-conducted interference is accomplished in both subsystems by filtering and shielding the input power leads to the monitor and to the remote amplifier.

SECTION VII