Fission chambers are most commonly used as in-core neutron flux sensors They are the backbone of the in-core neutron detection systems in the majority of boiling-water reactors (see Vol 2, Chap 16, Sec 16 2) The fission chamber is used as the neutron sensor in most traveling m core probe systems for both pressurized water reactors and boiling water reactors

Fission chambers feature relatively slow burnup of the uranium liner They provide satisfactory operation in all three basic modes (1) the pulse-counting mode, (2) the mean square-voltage mode, and (3) the mean-current (d-c) mode (see Chap 5, Sec 5 5, and Chap 2, Sec 2 2) In core fission chambers are thus suitable for use in source-range channels (where pulse counting is required because of the low value of the neutron flux) in the intermediate-range channels (where mean square voltage techniques extend the operating range), and in the power-range channels (where mean current techniques provide accurate power measure ments for both fixed sensors and traveling probes) For each of these modes of operation, the optimum design is different with respect to size, materials, fill gas pressure, emitter—collector gap dimensions, neutron sensitivity, etc

Just as m out-of core neutron detectors, both the neutron and the gamma fluxes contribute to the total output of an in core fission chamber Many of the design parameters which may be changed to achieve a specific neutron-sensitivity characteristic also affect the gamma sensitivity Since the output signal attributable to incident gamma radiation is not unambiguously related to the reactor power level, the design of an in-core fission chamber is optimum if at the end of detector life the ratio of the output current due to neutron flux to the output current due to gamma radiation is still acceptable

The mam design parameters that can be varied to meet the specific requirements for an in-core fission chamber are (1) the physical form of the uranium used, (2) the enrich­ment of the uranium, (3) the surface area and thickness of the uranium, (4) the type of fill gas used, (5) the fill-gas pressure, (6) the dimensions of the gap between the emitter and the collector, and (7) the dimensional tolerances Each of these is discussed below

(a) Uranium Form Two basic types of in-core fission chambers have been developed and manufactured One type incorporates an enriched uranium oxide layer plated on the inside of the detector housing that forms the outside wall of the sensitive volume (see Fig 3 1) The second type

TITANIUM TITANIUM Fig 3.1—Uranium oxide coated in core fission chamber

contains a machined sleeve of enriched uranium—aluminum alloy at the outer surface of the sensitive volume (see Fig 3 2) The more carefully the weight and thickness of the uranium coating or uranium—aluminum sleeve are controlled, the more accurately the neutron sensitivity of the detector can be controlled The majority of commercial detectors are manufactured with a ±20% tolerance on initial neutron sensitivity Under very special circumstances a ±5% tolerance on initial neutron sensitivity can be achieved by carefully controlling the uranium plating process or the uranium—aluminum alloy machining

(b) Uranium Enrichment. Because the enrichment of the uranium in a neutron sensor has no effect on the gamma sensitivity (total mass of uranium is constant), the best way to increase the signal-to-noise ratio is to increase the enrichment of the uranium used in the sensor Fully enriched uranium provides the maximum neutron sensi­tivity while maintaining the same gamma sensitivity.

(c)

Uranium Surface Area. For a given total mass of the enriched uranium layer, the neutron sensitivity of an in-core fission chamber depends on the surface area of the uranium. Surface area is varied by changing the chamber diameter and length. The gamma sensitivity also varies when the chamber geometry is changed. Consequently, there is a combination of sensor diameter and length which yields the highest signal-to-noise ratio.

(d) Type of Fill Gas. Argon is the most commonly used fill gas. It has all the desired properties (chemically inert, good thermal conductivity, low thermal-neutron cross section, and suitable ionization properties). Other com­monly used fill gases are helium and nitrogen or mixtures of argon and nitrogen.

Chemical inertness is desirable since a gas that is not inert may combine with chamber materials (particularly in presence of an intense nuclear radiation field) and thus reduce the gas available for ionization. High thermal conductivity is desirable to remove heat developed in the chamber by the signal-generating processes. If the thermal — neutron cross section is high, the fill gas will be depleted by nuclear transformation; in addition, neutrons absorbed by the fill gas are not available for reaction with the uranium.

(e) Fill-Gas Pressure. Neutron and gamma sensitivities of an in-core fission chamber are directly proportional to the fill-gas pressure as long as the range of fission fragments and gamma photons is greater than the gap between the emitter and collector. Most in-core fission chambers operate at several atmospheres pressure to achieve higher neutron and gamma sensitivities. Because both the gamma and neutron sensitivities are similarly increased, detector life is not appreciably affected by varying the fill-gas pressure.

(f) Emitter—Collector Gap. Of all the factors involved in in-core fission-chamber design, the most critical is the sizing of the gap between the neutron-sensitive emitter and the positively charged collector. Since the ionization current is a function of the number of fill-gas atoms, a large gap produces a large detector current. This characteristic is especially important at low flux levels, such as those existing in the source range. At higher flux levels the gap must be reduced to ensure detector saturation [Sec. 3-3.1(h)] up to and exceeding the highest neutron flux the detector is designed for.

(g) Dimensional Tolerances. As noted earlier, the accuracy of initial detector sensitivity is directly related to the dimensional tolerances applied to the neutron-sensitive material and to the emitter—collector gap. Because so many of the characteristics of in-core fission chambers are related directly to dimensions, the effects of tolerance accumula­tion are extremely important and must be carefully considered.

(h) Operating Characteristics. In-core fission chambers exhibit most of the operating characteristics of out-of-core neutron sensors. As pointed out in Chap. 2, Sec. 2-2.1, variation of chamber voltage provides three regions of detector performance: the low-voltage (presaturation) re­gion, the plateau (saturation) region, and the multiplication region. The exact shape of the current—voltage curve depends on chamber construction parameters.

In view of this characteristic behavior of fission chambers, it follows that the voltage applied to the chamber should be high enough to keep it on the plateau region at or above the highest radiation flux in which it is expected to operate. If there is any question about the chamber voltage required to achieve saturated operation, it is preferable to err on the high side, since the chamber current is proportional to the incident radiation flux in both the saturation and multiplication regions but not in the presaturation region. For most in-core fission chambers being used in power reactors today, the neutron flux never exceeds 2 X 1014 neutrons cm"2 sec 1 , so an operating voltage of 125 volts d-c is sufficient to guarantee saturated operation. Figure 3.3 shows saturation curves for a typical in-core fission chamber.

The ideal detector plateau would be flat, but this is never achieved. Below 1013 neutrons cm"2 sec the plateau has a slope that is generally attributable to detector-cable leakage current. As the neutron flux in­creases, the plateau starting voltage (the low end of the plateau) increases and the multiplication starting voltage (the low end of the multiplication region) decreases. When the neutron flux increases to the point where the plateau starting voltage equals the multiplication starting voltage, the plateau disappears. This point is generally defined as the upper flux limit of the detector.

Fig 3 3—Typical in core fission chamber saturation char acteristics

It is more important for an in core fission chamber operating in the mean square voltage mode[4] to stay in the plateau region than for one operating in the pulse counting mode or the mean current (d c) mode This results from the fact that the signal is a function of chamber current squared rather than chamber current alone Figure 3 4 shows that as a result the plateau starting voltage of a mean square voltage chamber is somewhat higher and the multiplication starting voltage somewhat lower than a d-c chamber

In the mean square voltage mode of operation, adjust ment of the chamber voltage must be related to the band pass of the signal amplifier into which the chamber operates Pulses from the chamber are distributed in energy in accordance with the power frequency spectrum curves in Fig 3 5 The breaks in the curves occur at the frequencies corresponding to the ion collection time, T, and the electron collection time, Te Reducing the chamber voltage shifts the entire power frequency spectrum curve to lower frequencies because the time required to collect the ions and electrons in the chamber increases

The power frequency spectrum curve is divided into two distinct regions, the low-power range and the high power range At low reactor power the low pulse frequency

Fig 3 4—Saturation curves for d c chambers vs mean square voltage chambers

allows ample time for both the ions and electrons to be collected in the chamber At high reactor power the pulse frequency is so high that the ions are not collected, only electrons are collected The break in the curve occurs at the point where the transition from the collection of ions plus electrons to the collection of electrons alone is made

T (RV) T (SV) Te(RV) Te(SV) FREQUENCY

Fig 3 5—Mean square voltage chamber performance char acteristics T, = ion collection time Te = electron collection time SV = saturation voltage and RV = reduced voltage

The best plateau characteristics in both the low power and high power ranges are obtained when the band pass of the amplifier is designed to be within the flat portion of the power frequency spectrum curve at saturated chamber voltage Improper setting of the chamber voltage shifts the frequency of the power frequency spectrum curve and drives the amplifier response off the flat portion of the curve Typical breakpoints in the low power and high power band-pass amplifier are defined in Vol 2, Chap 18, Sec 18-2 3(c)

There are three major factors creating nonlinear opera­tion of in core fission chambers (1) gas migration between the active and the inactive volumes of the chamber owing to temperature differences, (2) operation in the presatura­tion region, and (3) operation in the multiplication region A change in reactor power level always causes gas migration in an in-core fission chamber Gas migration decreases the chamber sensitivity as reactor power increases and vice fission chambers used to monitor reactor power from below source level to above the overpower trip level

Operating Range of Pulse Counting Fission Cham hers I he normal operating range of an in core pulse­counting fission chamber is from 101 to 109 neutrons cm [5] [6] sec 1 The lower limit is determined bv the statistics of pulse counting The variations in the neutron counting

Table З 1—Operating Characterisncs of Typical In-Core I ission Chambers Pulse-counting fission chamber Mean-square-voltage fission chamber Mean-current (d-c) fission chamber Electrode coating U3 08 enriched to U3 08 enriched to U3 Ob enriched to >90% 2 3 5 U >90% 2 3 5 U >90% 2 3 5 U Neutron sensitivity Pulse counting 0 5 to 2 5 x 10 [7] counts/sec/nv Mean current (d c) 7 x 10 ‘ 8 2 15 x 10 1 7 + 20% amp/neutrons cm 2 sec 1 Mean square voltage 1 5 to 4 5 x 10 3 ‘ amp[8] [9]/neutrons cm-2 sec 1 Gamma sensitivity Mean current (d c) amp/(R/hr) Mean square voltage amp2/(R/hr) 2 5 x 10 1 4 4 20 о 1 5 x 10 30 2 0 x 10 1 4 r ЗО о Neutron flux (max ) 1 x 1010 1 5 x 10’3 1 8 x 10’4 neutrons cm-2 sec 1 Gamma flux (max ) R/hr 2 5 x 107 1 68 x 10s Operating voltage volts (d c) 200 to 700 100 to 200 100 to 200 Temperature (max ) °С 540 540 315 1 ill gas A rgo n Argon Argon Dimensions Sensitive length in 1 00 1 00 1 00 Case diameter in 0 250 0 250 0 230 Case material Titanium Titanium Stainless steel Collector material Titanium 1 itanium St unless steel Collector to emitter Alumina Alumina Alumina insulator End sea! Titanium and 1 itanium and Titanium and Fosterite Fosterite I osterite Lifetime neutrons/cm2 1019 3 8 x 1021

versa When the sensor is operated in the presaturation region, a further decrease in sensitnity occurs because the chamber current is not linear with neutron flux

In the saturation region the chamber current is linear with neutron flux, so the only source of nonlinearity is gas migration In the multiplication region the decrease in current due to gas migration is counteracted b the increase in multiplication due to the gas migration It mav be desirable to set the chamber voltage somewhere in the multiplication region rather than in the saturation region to obtain maximum linearity on chambers with large gas migration sensitivity of a pulse counting fission chamber noted in Table З 1 result from anations m gamma flux at the detector The integral bias curves Fig 3 7, show the reduction in neutron counting sensitivity as the gamma exposure rate increases from 10s to 2 5 X 107 R/hr while the neutron flux remains constant Figure 3 7 also demon strates the importance of the proper discriminator setting Any discriminator setting less than 3 would result in operation to the left of the plateau at higher gamma leyels yvith attendant errors in count rate information At a neutron flux of 5 X 1()2 neutrons cm 2 sec 1 the chamber indicates approximately 1 count/sec, someyvhat less than is desirable for statistical confidence Accordingly, the neu tron source m the reactor should be sized to provide from 2 to 10 counts/sec

The upper limit of the pulse counting chamber is determined by the 1 MHz bandwidth of the signal amplifier and the 300-nsec rise time and collection time of the

MEAN- PULSE- SQUARE — D-C COUNTING VOLTAGE CURRENT CHAMBER CHAMBER CHAMBER OPERATION

Fig. 3.6- —Ranges of in-core fission chamber.

chamber pulses. At a true random input of 106 counts/sec, the counting loss is 23% of the true count rate. Since the pulse-counting chamber has a neutron counting sensitivity of greater than 103 counts/sec per unit flux (1 neutron cm-2 sec 1 ) in a 105 R/hr gamma field, the upper limit of
the neutron flux is 109 neutrons cm”2 sec 1 at 106 counts/sec.

When the in-core neutron flux exceeds 101 0 neutrons cm"2 sec 1 , the pulse-counting chamber should be removed from the core to prevent depletion of the fissionable material.

Operating Range of Mean-Square-Voltage Fission Cham­bers. The operating range of the mean-square-voltage fission chamber is 108 to 1013 neutrons cm"2 sec"1. The lower limit is set by the detector noise resulting from alpha emission from the uranium coating, prompt gamma radia­tion, and delayed neutrons at low reactor start-up levels. When the neutron flux is 108 neutrons cm"2 sec 1 or greater, the neutron-flux signal exceeds the noise signal.

The upper limit of the range of a mean-square-voltage chamber is reached when the plateau starting voltage becomes equal to the multiplication starting voltage, as described in Sec. 3-3.1(h). Because the plateau is shorter for chambers operating in the mean-square-voltage mode, the upper limit of their range is lower than the same chamber operating in the mean-current (d-c) mode.

When the in-core flux exceeds 1011 neutrons cm"2 sec 1 , the mean-square-voltage chamber should be removed from the core to prevent depletion of the uranium.

Operating Range of Mean-Current (d-c) Fission Cham­bers. The operating range of in-core fission chambers used in the mean-current mode is from less than 1012 neutrons cm”2 sec 1 to greater than 1014 neutrons cm”2 sec"1. The lower limit is set primarily by the leakage current in the ceramic-insulated cable. The upper limit is reached when the plateau starting voltage is equal to the multiplication starting voltage. Because mean-current (d-c) chambers normally provide the signals that initiate reactor overpower trip and are calibrated against a reactor heat balance, they

must have good linearity over at least one decade of their operating range

(j) Traveling In-Core Fission Chambers. The design requirements for traveling in-core fission chambers are the same as those for fixed in-core mean-current chambers except that the traveling chambers must withstand the rigors of periodic insertion into and withdrawal from the core The traveling in core fission chambers have the same requirements for operating range and linearity as the fixed in-core mean-current chambers Table 3 2 summarizes the characteristics of traveling in-core fission chambers com­monly used in power reactors Figure 3 8 shows a typical traveling m-core probe The helically wound outer sheath of the drive cable engages the drive gears to move and position the sensor