Tests were conducted at the General Electric Test Reactor (GETR) to determine the effects of nvt on the pulse height spectrum of an in-core detector. Detector assembly No. 2 was used for these tests. During the data-taking periods before and after high flux irradiation, the detector

О О

assembly was in a neutron flux of approximately 8 x 10 nv and a gamma flux of 4. 3 x 10 R/h.

13

The peak flux during irradiation was about 8 x 10 nv. The detector assembly was in this flux

20

for a period of 29 days, which resulted in an integrated exposure of 2 x 10 nvt. Compared in Figures 4-11 and 4-12 are the pulse height spectrums before and after the irradiation period.

No conclusions regarding changes in sensitivity can be drawn from this data since the exact values of neutron and gamma fluxes were not known. However, it appears that the high integrated flux had little effect on the pulse height spectrum.

POLARIZING POTENTIAL = 400V TEMPERATURE s 75°F COUNTING TIME = 10 MINUTES
_L
100	200 . CHANNEL NUMBER
300	400

со to
POLARIZING POTENTIAL = 400V TEMPERATURE — 100°F COUNTING TIME 10 MINUTES
320 280 сл 240 U
I
8 200 U_ 2 ібо I-
120 — 80 — 40 — 0 L
100	200 CHANNEL NUMBER
300	400

м •6.

300

200 CHANNEL NUMBER

100

400

яГ7nn"pDifnrfentfial AUlse H, eight sPectrurn, End of 300-Hour Temperature Run at 700 F — Detector Assembly No. 1 (Counter) at 250 V, 730°F

О 100 . 200 300 400 CHANNEL NUMBER

to СЛ

і СО 05

О 100 . 200

CHANNEL NUMBER

Figure 4-11. Neutron Differential Pulse Height Spectrum Before High nvt Exposure

20 Figare 4-12. Neutron Differential Pulse Height Spectrum After Exposure to 2 x 10 ‘nvt

N3 -J

The irradiation did cause a change in breakdown voltage from 850 V to about 400 V, but since a short portion of the in-core cable was also in the high flux region during the tests it is difficult to state accurately whether or not the breakdown actually occurred in the chamber or in the cable of the in-core detector assembly. .

4-28

SECTION V

The operability of the in-core counting subsystem was investigated at the NTR in several ways

a. Scrams were initiated at 30 kW of reactor power after various lengths of time at this

power level, and the output of the subsystem was monitored to determine if there were

any deleterious effects. Detector assembly No. 1 was operated at the core centerline 11 ft

Ф = 7 x 10 1 nv, = 2x 10 R/h) for a period of 5 minutes, after which the reactor was scrammed. Shown in Figure 8-5 is the decay curve with a 1/4-inch-thick lead shield on the detector. This curve has a final slope of 84 sec/ecade (193 sec/decade) at 4 minutes after the scram. Shown in Figure 8-6 is the decay curve after a similar time at the same power level but without the lead shield on the detector.

This curve also exhibits a final slope of 84 sec/ecade (193 sec/decade).

Counter No. 2 was operated in the same flux conditions without lead shielding for approximately 1 hour. The final slope of the decay curve after scram in this instance was 82.2 sec/ecade (189 sec/decade) (see Figure 8-7).

After fissioning has ceased, the decay curve of the delayed neutrons from the fission process contains five major groups of neutrons. The longest-lived group is from Krypton 87 (whose precursor is Bromine 87) with a 55.6-second half-life. The 55. 6-second half-life corresponds to a 80. 3 sec/ecade (185 sec/decade) slope on the decay curve. The close agreement of the experimentally determined slopes with the calculated slope demonstrates a proportionality between the neutron flux in the core and the output of the counting subsystem.

b. In addition, counter No. 1 was operated in its normal operation region prior to scram, and was immediately inserted to the core centerline upon indication of the scram

(see Figure 8-8). In this case, the indicated count rate followed the insertion and then continued to decay in a manner similar to that previously described. The slope of this decay curve, defined at 3 minutes after the scram, is approximately 65 sec/ ecade (150 sec/decade). Agreement of the slopes is not as good in this case as in the previous case. This is probably due to data not being available at long enough times after scram. However, the ability of the subsystem to follow the insertion is demonstrated.

c. A retract and insertion test was performed using detector No. 2 at the NTR facility.

The detector assembly was manually retracted from the core centerline at a reactor power level of 3 watts (7 x 10 nv at the detector) and then reinserted to the core centerline. The velocity of withdrawal and insertion was approximately 3 ft/min. The distance withdrawn was approximately 64 inches. No spurious counts or noise tran­sients were evident throughout the course of the experiment. The counting subsystem output as a function of time during this experiment is shown in Figure 8-9.

Figure 8-5 Count Rate Versus Time After Scram. Detector Assembly No. 1 (Counter) With 1/4-inch-thick Lead Shield.

This section derives the equations that can be used to design a Camptiell (MMSV) monitor for radiation measurement. The section is divided into four parts. The first part contains the derivations and results in the most general form; these results can be applied to any system.

The second part comprises the simpler results for a system that contains one high-pass filter and one low-pass filter. The third part consists of the derivation of the signal contribution due to reactor noise. The fourth part covers the derivation of the probability of false trips, with several optional maximum allowed average signal levels and a particular trip setting; a calcula­tion of trip probability as a function of operating level with a fixed trip point setting is also included.

The primary requirements on the cable insulation are that, at detector operating voltages, the d-c current drain is not excessive, and that the noise generated by the cable with potential applied is not large enough to cause spurious counts in the counting channel.

1 10 100 1000

APPLIED POTENTIAL DIFFERENCE IN VOLTS

Figure 5-11. Irradiation Effects on the Electrical Properties of Fiber Insulated Cable

СЛ I to. to

Figure 5-12. D-C Leakage Current Versus Neutron Exposure

The data demonstrate that the former requirement is met. Regarding the latter require­ment, it has been found that there is a correspondence between the amount of d-c leakage and the a-c noise generated. With another in-reactor test unit, which included a quartz insulated cable and an in-core counter, a-c noise was not a problem (see Section IV).

4. 1 DESCRIPTION.

4. 1. 1 In-Core Detector Assemblies

The in-core detector assemblies are designed for operation within power reactor cores in either a water or air ambience. Both the in-core Campbeller and the counter assemblies consist of a detector enclosure which has a 1/4-inch o. d. x 2-5/8-inch length, a 3/16-inch-o. d. radiation — and temperature-resistant cable, and a ceramic-to-metal cable seal termination. An adapter is mounted on the cable seal which mates with a standard HN connector. • Only Type-304 stainless steel is exposed to the reactor ambience. All exposed joints are inert-arc-welded.

The detectors proper are integral units 0. 160 o. d. x 1-3/4 inches long, contained within the detector enclosure of the probe. Chamber materials are titanium, fosterite ceramic, and argon filling gas. A uranium coating is electro-deposited on the inside diameter of the chamber shell. Careful attention is given to the outgassing and backfilling processes to prevent impurity contamination of the fill gas. All materials and processes used in the fabrication of these devices are similar to those used in the ceramic-metal electronic tube industry. The Campbell chamber differs from the counter in that the gas pressure-gap length product is smaller, and the chamber contains less uranium. . •

The coaxial cable consists of a Type-304 stainless steel center conductor, quartz fiber insulation, a stainless steel braid, a double braid of copper, and a Type-304 stainless steel sheath tube. The materials used in the cable were chosen to meet the requirements of signal transmission, physical compatibility, and tolerance to elevated temperature and radiation levels.

An HN type termination for the cable connector provides a high-quality, moisture-proof. connection to the interconnecting cable.

4. 1.2 Out-of-Core Detectors.

The out-of-core detector used in the tests was a standard General Electric fission counter, NA04, constructed mainly of aluminum and alumina ceramic. The argon gas gap length is 1/8 inch. External dimensions are 3 o. d, x 13-1/2 length. An HN termination providёs con­nection to the standard cable. Both the inner and outer surfaces of the active volume are coated with uranium.

A brief description of the pulse formation process in a fission ion chamber is contained in the following paragraphs of this section.

For a time constant versus input rate curve, as shown in Figure 6-16, there is a specific probability of obtaining a false safety signal for any particular trip point setting, the number of ‘ occurrences of the false signal being related to the time constant curve of the circuitry. * Table 2-3a shows the interrelationship between the point of operation (i. e., the particular input rate), a trip point count rate which has been set, and the resultant average number of spurious trips per unit time for this subsystem. Note that the functional variation is very rapid and therefore relatively small changes in time constant can greatly increase the average number of spurious trips. For this subsystem, operation up to an input rate of 3 x 10^ cps with a trip setting of 5 x 10^ will probably have no spurious trips from this cause during the life of a plant.

♦See Section II for a derivation of these’ relationships.

The various alternatives offered by the development instrumentation system are illustrated

in Figure 1-1. On the left, an application using an in-core detector (ion-chamber) would use the

-7-3

counting channel to monitor from 10 to 10 of rated PERCENT POWER, and by successive partial withdrawal movements of the detector could continue to monitor well into the intermediate

_4

range. The in-core Campbell system would overlap the counting channel at approximately 10 of rated PERCENT POWER, and provide coverage into the power range without withdrawal of the detector. Each counting and Campbelling channel has an associated mechanical drive subsystem to position each detector at any desired point between the initial in-core elevation and the storage elevation beneath the core. •

On the other hand, those applications not requiring the use of in-core detectors may attain. similar performance using larger out-of-core detectors, as shown on the right in Figure 1-1

The counting channel would function from 10 ^ to 10 ^ of rated PERCENT POWER, and the Campbell channel would overlap and continue coverage through full-rated output of the plant. These detectors remain in a fixed position at all times. ‘ .

Thus, the development instrumentation system offers the following significant advantages relative to conventional instrumentation: .

a. The power plant designer may choose between in-core and out-of-core detectors for

. his application, and yet retain a consistent electronics system.

b. Only two channels of electronics are required to monitor from source level through

.. rated plant output power. . . •

c. The Campbell system provides for. improved discrimination against gamma flux, and

■ enables monitoring under certain conditions where conventional instrumentation would be inoperable.

This section is concerned with the transmission of information from a detector (ionization or fission chamber) inside a reactor vessel to the electronic equipment outside the vessel.

At the present time, coaxial cable is used for this purpose; hence, the transmission of information through a coaxial cable is considered. Three modes of information transmission are studied:

a. The conventional mean-value system of instrumentation. This mode uses only the

d-c component of the current from the detector. For this system, only a theoretical treatment of the transmission of direct current is considered. • .

. • b. The counting system of instrumentation. This mode uses each individual pulse

generated by the detector. For this system, the transmission of pulses is studied theoretically, the theoretical pulse transmission data are compared with experimental data, the experimental frequency response and transient pulse characteristics of two. of the in-core cables are studied, and experimental temperature and radiation effects

are discussed.

c. The Campbell system of instrumentation. This mode uses the a-c component of cur­rent from the detector. Here the transmission of alternating current is considered.

In extensive tests that have been run up to 700°F (see Sections IV and V), temperature was

found not to appreciably affect the breakdown voltage of the detector assembly nor the pulse height

spectrum of the counter. An in-core counter has been successfully tested to 10 nvt, and

19

further tests are in progress. The specified life at this time is 10 nvt. The system is very well shielded and filtered to minimize noise susceptibility. Audio-frequency, radio-frequency, and transient-pulse-conducted and relay-radiated-interference tests have been conducted (less than 0. 3 counts/sec due to these sources).

12

The detector probe is retracted at 5 x 10 nv to avoid excessive neutron and gamma

exposure. The calculated worst case temperature in the dry tube is caused by neutron and gamma 14

heating at 1 x 10 nv,. and calculations based on the physical properties of the probe material show that this temperature will not destroy the integrity of the probe.

Consider the system shown in Figure 3-1. It consists of four components: a detector, an amplifier, a squaring circuit, and an averaging (or integrating) circuit of time-constant T.

The detector and amplifier are linear, and somewhere in the system before the squaring circuit (i. e., in the linear part of the system) there is a component to remove the d-c, Such as a series capacitor or a shunt inductor. The time-varying voltage at the output of the amplifier is designated as V(t). At the output of the squaring circuit the voltage is AV (t), where A is the squaring-circuit constant; i. e., the ratio between its output voltage and the square of its input voltage. The voltage at the output of the integrating circuit, which will be called the signal, is S(t), and is related to the output of the squaring circuit by the differential equation of an integrating circuit of time-constant j

dSW

. . dt

Due to the random nature of the input, the value of the voltage or current at any point in the system at time t cannot be predicted or expressed mathematically; however, if simultaneous observations were made at the same point and at the same time, t, in a large number of identical systems, the results could be averaged. This ensemble average is called the expected value of that voltage or current at time t, and can be predicted and expressed mathematically. Also, the square of the difference between each result and the expected value could be averaged to obtain the variance of that voltage or current at time t; this variance can also be predicted and expressed mathematically. .

We will use the brackets < > to indicate the expected value of a voltage or signal.

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It chould be noted that Equation (3-1) applies to the expected values of S(t)and AV (t) as well as to their instantaneous values; i. e.,

<*<S(t) > + <S(t) > = <AV2(t) > (3_2)

dt t t

Since the system is linear up to the input of the squaring circuit (output of the amplifier), the total effect at any point in this part of the circuit is just the sum of the effects caused by the detection of each neutron. Hence, if the detection of a neutron at time zero produces a voltage pulse v(t) at the output of the amplifier, then the voltage at time t due to the detection of a neutron at time t^ is v(t-t^), and the total voltage is the sum of the individual voltages produced by all previously-detected neutrons.

t

V(t) =53 v(t-tk), tk = — oo

where the kin particle arrives at t^..

This equation can be written in a form suitable for statistical analysis in the following man­ner. Divide the time axis into small equal intervals of length At, then the detection of a neutron during the nth interval of time will produce a voltage at time t of v(t-nAt), and the total voltage due to all of the previously-detected neutrons will be:

V(t) = ^3 v(t-n л 0, n

where the summation includes only those intervals during which a neutron is detected. If we define a random variable 77 that equals one if a neutron is detected during the n*1*1 time interval and equals zero if no neutron is detected during the n n time interval, then the total voltage at the output of the amplifier can be written as,

. t/д t •

V(t) = 23 ‘vn v(t’n At)’ . (3’.3)

. n = -00

where the summation now includes all the time intervals that precede t.