Nuclear fission. The division of a heavy nucleus into two (or, rarely, more) parts with masses of equal order of magnitude; usually accompanied by the emission of neutrons, gamma rays, and, rarely, small charged nuclear fragments.

Last fission. Fission caused by fast neutrons.

Thermal fission. Fission caused by thermal neutrons.

Spontaneous fission. Fission which occurs without the addition of particles or energy to the nucleus.

L’issionable. Of a nuclide, capable of undergoing fission by any process.

Eissile. Of a nuclide, capable of undergoing fission by interaction with slow neutrons. (In reactor physics, slow neutrons are frequently defined as those of kinetic energy less than 1 eV.)

Eertile. Of a nuclide, capable of being transformed, directly or indirectly, into a fissile nuclide by neutron capture. Of a material, containing one or more fertile nuclides.

Tission fragments. The nuclei resulting from fission and possessing kinetic energy acquired from that fission.

h’ission products. The nuclides produced either by fission or by the subsequent radioactive decay of the nuclides thus formed.

Eission yield. The fraction of fissions leading to fission products of a given type.

Prompt gamma radiation. Gamma radiation accompany­ing the fission process without measurable delay.

Prompt neutrons. Neutrons accompanying the fission process without measurable delay.

Delayed neutrons. Neutrons emitted by nuclei in excited states which have been formed in the process of beta decay. (The neutron emission itself is prompt, so that the observed delay is that of the preceding beta emission or emissions.)

4- 2.1 Thermocouples

(a) Basic Considerations. A thermocouple (Fig. 4.1) is a device that generates a small electromotive force (milli­volts) almost directly proportional to the temperature

difference between its “cold” and “hot” junctions (Seebeck effect) The degree of departure of the voltage—tempera­ture relation from linearity depends on the materials used in the thermocouple (Fig 4.2).

In a circuit made up of wires of dissimilar metals, the existence of emf’s other than those at the junctions was discovered by Thomson (Lord Kelvin). An examination of this effect led to the conclusion that in general an emf exists between any two regions at different temperatures in a single conductor. In other words, a thermal gradient in a homogeneous material creates an emf, the direction of the emf is reversed when the thermal gradient is reversed The effect is usually negligible in thermocouple applications since there are equal and opposite thermal gradients around the circuit Moreover, the practice of keeping both legs of

thermocouple circuits in the same thermal gradients ensures that the Thomson effect can be neglected

Of greater importance are changes in the properties of thermocouple materials after prolonged exposure to ele­vated temperatures and steep temperature gradients such as exist near the hot junction. The thermocouple wires become inhomogeneous along their length, the degree of mhomogeneity depending on temperature, time, and the environment (air, steam, water, etc ) If there is a thermal gradient along the region where the homogeneity varies, then an emf can be generated (as if there were a thermocouple composed of fresh and altered material). This
effect can be taken into account by thermocouple calibra tion or replacement However, the calibration must be made in the same thermal gradient as that which will exist when the thermocouple is used

(b) Thermocouple Lead (or Extension) Wire. The cold junction of a thermocouple circuit is almost always located at the indicating or controlling instrument itself The instrument usually has means for automatically compen­sating for variations of the cold junction caused by variations in the ambient temperature Indicating instru­ment and sensor are sometimes separated by substantial distances (as much as a few hundred feet) The thermo­couples should be made long enough to extend from the point of heat measurement to the indicators If the thermocouple were made of platinum—rhodium, it would be prohibitively expensive Therefore, lead (or extension) wires of cheaper metals that have thermoelectric character­istics matching the thermocouple over a limited tempera­ture range must be used This range is based on the ambient temperature expected at the point where the thermocouple extension wires connect to the thermocouple and where the greatest range of ambient temperature is expected

(c) Thermocouple Materials. Thermocouple materials are available for use within the approximate limits of —300 to +3200 F (—185 to +1760°C) No single thermocouple meets all application requirements, but each possesses characteristics desirable for selected applications Platinum is generally accepted as the standard reference material against which the thermoelectric characteristics are com­pared

Thermocouples are classified into two groups identified

as noble metals and base metals

Noble metals

1 Platinum vs platinum—10% rhodium is used for de­fining the International Temperature Scale from 630 5°C (1166 9°F) to 1063°C (945 4°F) It is chemically inert and stable at high temperatures in oxidizing atmospheres It is widely used as a standard for calibration of base-metal thermocouples This couple will match the standard refer­ence table to ±0 5% of the measured emf

2 Platinum vs platinum—13% rhodium is similar to 1 and produces a slightly greater emf for a given temperature.

3 Platinum—5% rhodium vs platinum—20% rhodium and platinum—6% rhodium vs rhodium are similar to 1 and 2 but show slightly greater mechanical strength

Base metals

1 Copper—constantan is used over the temperature range of -300 to +700°F (-185 to +370°C) The con­stantan is an alloy of approximately 55% copper and 45% nickel

2 Iron—constantan is used over the temperature range of -200 to +1400°F (-130 to +760°C) and exhibits good stability at 1400°F (760°C) in nonoxidizing atmospheres

3 Chromel—alumel is used over the temperature range of -200 to 2300°F and is more resistant to oxidation than other base-metal combinations Chromel-P is an alloy of approximately 90% nickel and 10% chromium. Alumel is approximately 94% nickel, 3% manganese, 2% aluminum, and 1% silicon This combination must be protected against reducing atmospheres Alternate cycling between oxidizing and reducing atmospheres is particularly destructive

4 Chromel—constantan produces the highest thermo­electric output of any conventional thermocouples It is used up to 1400°F (760°C) and exhibits a high degree of calibration stability at temperatures not exceeding 1000°F (5 38°C)

The upper temperature limits for the various thermo­couple materials depend on the wire sizes and the environ­ment in which the thermocouples are used Table 4.1 lists the recommended upper temperature limits for thermo­couples protected from corrosive or contaminating atmo­spheres

The ranges of applicability and limits of error for thermocouples and extension wires of standard sizes are given in Table 4.2. See National Bureau of Standards Circular 561 for expanded reference tables of these thermocouples, emf vs temperature, and temperature vs emf, °F, and °С

(d) Thermocouple Charts. Thermocouple data are usually presented as tables of emf (millivolts) vs. tempera­ture (°С or °F) with the 0°C or 0°F as the reference (cold junction) temperature. Tables 4.3 and 4.4 present the data

Table 4 1—Recommended Upper Temperature Limits (°F) for Protected Thermocouples*

•Does not include use or installation errors.

tFrom American Society of Mechanical Engineers, Power Test Code 19.3, p. 23, 1961.

for six thermocouples in Fahrenheit and Centigrade, respec­tively The use of these tables is described below

If the emf is measured, then the temperature can be determined For example, suppose an emf of 26 50 mV is observed for an iron—constantan thermocouple with a cold-junction temperature of 70°F From Table 4 3, 70°F is 1 68 mV (linear interpolation) with 0°F cold-junction reference temperature The unknown temperature thus corresponds to 26 50 + 1 68 = 28 18 mV with 0°F refer­ence temperature From Table 4 3, this is seen to cor­respond to 960°F

In some situations the temperature is assumed to be known and the emf is to be determined For example, a potentiometer (P) having cold-junction compensation (і e, the millivolt readings correspond to the cold junction at 32 F) and calibrated for a type К (chromel—alumel) thermocouple is to be checked at 1450°F by means of the output of a potentiometer (S) of known accuracy From Table 4 3, the temperature 1450°F for a type К thermo­couple corresponds to 33 48 mV with a cold junction temperature of 0°F The emf between 0°F and 32°F is also seen to be 0 69 mV (linear interpolation in the table), so the temperature 1450°F with a cold-junction temperature of 32°F corresponds to 33 48 — 0 69 = 32 79 mV This is the potential that should be observed on potentiometer “S” when potentiometer P is checked at 1450°F

In the charts usually supplied with thermocouple devices, the temperature interval is smaller than the 50 F (or 50°C) interval of Tables 4 3 and 4 4 This reduces the labor of interpolation

(e) Series-Connected Thermocouples. In one method for averaging temperatures, each thermocouple is connected in series with the others (Fig 4 3) using extension wires of the correct materials Note that the extension wires are connected at the instrument from each couple in the series This permits proper cold-junction compensation The emf developed at the terminal G is the sum of the emf’s

developed by all the thermocouples. Consequently, instru­ments used with this type of circuit must be calibrated for the total emf of all the thermocouples, and the cold — junction compensation must be adjusted to compensate for the greater milhvoltage change due to ambient temperature changes.

The advantages of this method are (1) a large emf is developed for a given temperature and (2) burnout of any single thermocouple is immediately apparent

The disadvantages are (1) a special calibration is required, (2) a short circuit, which might materially reduce the emf of one couple, might not be detected by observation of total emf, (3) on a multipoint installation the same number of thermocouples must be used in series at each point, and (4) grounded thermocouples cannot be used.

(f) Parallel-Connected Thermocouples. Temperatures can be averaged by connecting thermocouples in parallel (Fig. 4.4). Here the net emf developed at G is the average of the potential drops across each individual branch of the

circuit rather than an average of the emf’s. Since current circulates among the thermocouples when the temperatures Tj, T2, and T3 are different, the resistance of each individual thermocouple circuit must be equalized by resistors Rj, R2, and R3 (swamping resistors).

The resistance of the actual thermocouple will also vary with its temperature. The effect of this variation can be minimized by making the values of Rb R2 , and R3 high (500 ohms typical) in comparison with the resistance changes encountered Ri, R2 , and R3 are of equal value The resistances of the swamping resistors are limited by the sensitivity of the indicator (amount of current required to actuate it) and the number of thermocouples in the arrangement. Maximum sensitivity is achieved when the total impedance (resistance in this case) of the thermo­couple circuit (all thermocouples and swamping resistors) is equal to the internal impedance (resistance) of the indica­tor. The resistance of a thermocouple circuit can be measured with a millivoltmeter and rheostat (Fig. 4.5).

The advantages of parallel connection are (1) standard instrument calibrations and cold-junction compensation for a single couple can be used and (2) if one couple fails, operation can be continued.

The disadvantages are (1) failure of a single couple is not readily apparent and (2) grounded thermocouples cannot be used.

These thermocouples are used for measuring the effi­ciency of heat exchangers

(g) Differentially Connected Thermocouples. Two

typical arrangements for differentially connected thermo­couples are shown in Fig. 4.6. Figure 4.6(a) illustrates the basic circuit in which one couple senses one temperature and a second couple senses another temperature. Note that the similar metals are interconnected and that it is not necessary to refer the cold junction back to the galvanome ter. This is because the one couple constitutes a hot junction and the second the cold junction. Also, both leads to the galvanometer are of the same material, and each of these leads joins a third metal at a point where there is no temperature difference. As a result, even though two junctions of dissimilar metals are formed, they have no effect because there is no temperature difference. There are no unbalanced thermocouple emf’s created when the copper leads of the galvanometer are both connected to the constantan leads at a point where no temperature differ­ence exists.

COLD JUNCTION

Л

♦CONST CONST COPPER

NULL

INDICATOR

Figure 4.6(b) shows a circuit using differentially con nected couples in series to provide larger emf’s with small temperature difference Note that the basic differential circuit in Fig. 4 6(a) is duplicated in Fig. 4.6(b) Any number of couples may be connected similarly to produce a desired emf.

When differentially connected couples are used, cold — junction compensation is not required in the indicator or controller since, from the basic nature of the circuit, there is no cold junction located at the indicator as in all the other circuits discussed.

Applications of differentially connected couples include measurement of temperature differences across pumps and heat exchangers, the detection of temperature differences across large furnaces, etc.

(h) Indicators and Controllers. The two most popular types of measuring devices are the galvanometer and the potentiometer. The former finds application for control and indication in noncritical industrial processes The
potentiometer is more suited for critical processes that demand great stability in their control

In early potentiometer circuits a galvanometer that was sensitive to vibration was used. Comparatively complex devices were required to “feel” or determine the galva­nometer pointer position. These complex devices and relatively slow responding galvanometers limited instrument speed and required considerable maintenance

Figure 4.7 shows a basic galvanometer type instrument. The No. 6 dry cell provides a constant current through the slide-wire. This current is held constant by periodically shifting the standardizing switch to the “standard” position and comparing the voltage of the No 6 dry cell with that of the standard cell. If they are not equal (the galvanometer being deflected from its null position), the standardizing rheostat is adjusted until the galvanometer does not deflect in switching from the No 6 dry cell to the standard cell The “standard” switch is then set at the “run” position.

The thermocouple is now in the galvanometer circuit, and, by moving the slide-wire contactor along the slide-wire, a point will be reached at which the thermocouple voltage is matched by the voltage drop across the slide-wire. The scale located above the slide-wire is calibrated in terms of temperature in place of voltage, thus a conversion from voltage to temperature is avoided.

The circuit of Fig. 4.7 shows the essentials of the potentiometer type measuring instrument In practice the measuring slide-wire is one arm of a bridge circuit that includes temperature-compensation circuits and ratioing resistors as well as calibration resistors. A galvanometer was used in these circuits until about 1940, at which time an electronic amplifier replaced the galvanometer.

At the present time, temperature transmitters, which are solid-state high-gain amplifiers, have come into wide use. Having no slide-wires or other moving parts, they may, for all practical purposes, be considered maintenance-free devices They are very useful in a modern coordinated plant control system because their output (typically 4 to 20 mA), which is proportional to the input, can be used in many ways, e. g., in conversion to pneumatic signals and in driving recorders, indicators, and controllers. Temperature trans­mitters may also serve as one of the inputs to Btu-calcu-

lating devices and as inputs for temperature compensation in flow-measuring and — recording meters Figure 4.8 is typical of a 4- to 20-mA temperature transmitter.

(i) Thermocouple Response Time. The rate at which a thermocouple reaches the temperature of the medium in which it is immersed (or which it contacts) depends primarily on the rate of heat transfer (conduction) through its protecting sheath.

In Fig. 4.9 the time lag between the thermocouple temperature and the medium temperature is shown for three thermocouple arrangements. The medium is stirred water heated at a constant rate. The time lag is shown for a bare 20-gage thermocouple, a thermocouple embedded in a silver plug that is in contact with the bottom section of the thermocouple well, and a thermocouple (14-gage) butt-
welded and forced against the bottom of the well. With respect to the bare couple, the average time lags are about 20 sec for the well-and-plug arrangement and over 90 sec for the couple forced against the bottom of the well.

The silver-plug arrangement (so-called “high-speed couple”) reduces the response time considerably Further reduction could be achieved by reducing the wall thickness of the well, however, operating conditions may not permit such reduction.

In Sec. 4-2.3(c) the structure of thermometer wells is discussed in detail.

(j)
Testing Thermocouples. To realize the degree of accuracy obtainable with a modern industrial pyrometer, thermocouples are carefully manufactured to match pub­lished temperature—emf calibration tables within specific

tolerances. Consequently, there is seldom any need to check the calibration of a new thermocouple.

While an oxidizing atmosphere has a greater effect on the life and calibration of iron—constantan thermocouples, a reducing atmosphere has a more severe effect on chromel—alumel couples.

It is not recommended that a used thermocouple be removed from the installation for testing in a laboratory furnace since it is practically impossible to duplicate in the laboratory furnace the temperature gradients of the actual installation. It is advisable to test a used thermocouple
under the same conditions and in the same installation where it is normally used.

Two basic methods are used for checking the accuracy of thermocouples (1) the fixed-points method in which the emf is measured when the couple is immersed in standard liquid metals at their freezing points or in water at the boiling point or in subliming C02 or boiling liquid oxygen and (2) the comparison method in which the emf of the couple is compared with the emf of a standard couple Table 4.5 summarizes the methods used for testing the major types of thermocouples Note that in all except

high-precision laboratory work the comparison method is used to test and calibrate thermocouples.

To test a used thermocouple, you must have a reference standard that is known to be accurate, A new thermocouple whose calibration has been determined by comparison with a primary standard or directly against platinum may be used as a reference standard. It should then be labeled accordingly and reserved for this purpose. Since the original characteristics of the reference couple (like that of any used thermocouple) will change as it is used in testing, such a reference standard should be tested at intervals determined by the frequency of use and replaced when it is beyond the desired limits of accuracy

The method selected for testing used thermocouples depends on the type of installation. The success of each method depends primarily on the stability of the tempera­ture during tests. The following methods are recommended in the order in which they are listed.

1. Insert a reference standard into the same protecting tube the used thermocouple is in if the size of the tube permits. Connect each thermocouple to a portable potenti­ometer through a selector switch, and compare the emf’s developed

2. Install a reference standard adjacent to the fixed thermocouple. Drill a hole as close to the fixed installation as practicable, and install the standard in such a manner that the ends of rhe two protecting tubes are as close together as possible To ensure a fair comparison, use thermocouples of the same size and protecting tubes of the same size and type. Connect the couples to the instrument as in method 1, and compare readings.

3. Compare readings of successive installations of used and reference thermocouples. Test the used thermocouple first by reading the emf developed at a selected tempera­ture. Remove it from its protecting tube, and replace it with the reference standard. Note that the standard should always be inserted in the protecting tube to the same depth as the used thermocouple. Insert the assembly to the same depth as the one tested, wait for the reference thermo­couple to come to equilibrium, and then read the emf developed and compare it with that of the used thermo­couple to come to equilibrium, and then read the emf others because it requires that the temperature remain constant for a longer period.

Data taken on a commercial CIC were compared with commercial log-average-magnitude-squared (LAMS) units operating off fission counters in the EBR-2 shutdown gamma field of about 2 X 10s R/hr. The resulting curves, shown in Figs 5.26 and 5.27, are multivalued on the ordinate to emphasize the overlap As shown, the overlap for the CIC is only a factor of 2 compared to the overlap of

the Gulf General Atomic counters of 100 If a true LRMS system had been used, the overlap would be the same or greater The improvement over a CIC is evident

Had the gamma field been more intense, the CIC overlap would be ml but the LAMS or log mean square voltage would still be 150. Table 5 2 shows comparative values of the discrimination ratio for the various systems

BIBLIOGRAPHY

Campbell, N R, The Study of Discontinuous Phenomena, Proc Cambridge Phil Soc, 15: 117, 310, 513 (1909-1910).

Chase, R L, Nuclear Pulse Spectrometry, McGraw-Hill Book Company, Inc., New York, 1961.

DuBndge, R A, J. P. Neissel, L. R. Boyd, W. K. Green, and H. W. Pielage, Reactor Control Systems Based on Counting and Campbelling Techniques, Final Progress Report, USAEC Report GEAP-4900, General Electric Company, July 1965.

Elmore, W C, and M. Sands, Electronics Experimental Techniques, McGraw-Hill Book Company, Inc., New York, 1949

Fowler, E P, and R W. Levell, The Use of Current Fluctuations from a Neutron Detector with Logarithmic Amplifiers as a Measure of Neutron Flux Levels, British Report AEEW-R-375, June 1964.

Green, W К, Ten-Decade Wide Range Neutron Monitoring System, Final Test Report, USAEC Report GEAP-11094, General Electric Company, October 1970.

Gwinn, D A, andW. M. Trenholme, A Log-N and Period Amplifier Utilizing Statistical Fluctuation Signals from a Neutron Detec­tor, IEEE (Inst Elec Electron. Eng), Trans Nucl Set, NS-10(2) 1-9 (April 1963).

Harrer, J M, Nuclear Reactor Control Engineering, D Van Nostrand Company, Inc, Princeton, N J, 1963

Murphy, G, Elements of Nuclear Engineering, John Wiley & Sons, Inc., New York, 1961.

Popper, G F, Counting and Campbelling A New Approach to Neutron-Detection Systems, Reactor Fuel Process Technol, 10(3) 199 (Summer 1967).

___ , W C Lipinski, and J M Harrer, Ten Decades of Continuous

Neutron Flux Monitoring from a Single Fixed Position Detector, Trans Amer Nucl Soc, 9(1) 316-317 (June 1966)

Thomas, H A, and A C. McBride, Gamma Discriminations and Sensitivities of Averaging and RMS Type Detector Circuit for Campbelling Channels, Report GA-8035, Gulf General Atomic.

Trenholme, W M, and D J Keefe, A Neutron Flux Measuring Channel Covering Ten Decades of Reactor Power with a Single Fixed Position Detector, IEEE (Inst Elec Electron Eng), Trans Nucl Sci, NS-14(l) 253-260 (February 1967).

In principle, any fission neutron or gamma interaction with matter which produces measurable effects can be used for rcactor-power measurements Practical considerations, however, limit the choice to a few The commonly used detectors have evolved through a selection process based on considerations of signal strength, response time, and tolerance of interfering radiations

The radiation detector produces an electrical signal that overrides typical electronic-circuit noise levels If the detector is to be used in reactor protection systems, its signal must be reliable and its response time should be short In event of a reactor accident or major component failure, the detector may have to respond rapidly to initiate a shutdown before damage can occur I he time constants of signal conditioning circuits are longer than detector response and normally are limiting for protection system considerations In addition, a strong gamma background can obscure the neutron signal Fission product gamma radiation is always a large contributor to the background gammas in power reactors

Despite considerable research and development on other types most operating detectors depend on gas ionization Generally, gas ionization detectors5 can be made to have sufficient sensitivity without excessive size and to have a wide operating range, a fast response time, and adequate radiation selectivity

Most neutron sensors utilize gas ionization caused bv charged particles emitted in neutron-induced fission reactions in 23SU or m (n a) reactions in boron Gamma sensors detect the ionization caused by Compton electrons

A variety of sensors are used to locate the position of devices or liquid levels in vessels They are described in other chapters in connection with the devices or vessels with which they are usually mechanically integrated.

Techniques for sensing and indicating the positions of control rods are discussed in Chap. 7, Sec. 7-3.7 and in the examples of Sec. 7-4

There are many examples of level sensing in pressur — lzed-water and boiling-water reactors (Vol. 2, Chaps. 15 and 16), for example, sensing the water level in a boihng-water — reactor vessel Usually, the sensors are differential-pressure transducers or a series of pressure-actuated switches. In sodium-cooled reactors, level sensing can also be accom­plished with resistance or induction probes or with acoustic devices. These sensors are described in Vol. 2, Chap 17, Sec. 17-4.3

The steam systems of all nuclear power plants (and of fossil-fueled plants as well) include a variety of level sensors, ranging from simple sight tubes to pressure transducers.

4- 6 STEAM PROPERTIES SENSING

4- 6.1 Quality

(a) Definitions. Steam quality The percentage by weight of dry steam in a mixture of saturated steam and suspended droplets at the same temperature.

Moisture The percentage by weight of suspended droplets of water in a mixture of dry saturated steam and water droplets at the same temperature.

(b) Sample Collection. Sample collection is carried out according to ASTM D1066 or ASME Performance Test Code, Part II. A few salient points are extracted here. It is important to note that the ASME Performance Test Code does not recommend using the electrical conductivity method for determining the moisture content of steam. A recommended form of sampling nozzle is shown in Fig. 4.39.

The pipe or tube in the sampling nozzle extends across the pipe on a diameter to within 0.25 in. of the opposite wall. The drilled holes face upstream in the pipe and are spaced so that each port represents an equal area of pipe section. For a representative steam sample, the hole size in the sample tube must be chosen so the rate of sample flow is equal to the rate of steam flow. The shortest possible connection should be used between the sampling nozzle and the calorimeter or cooling coil.

(c) Moisture Determination. Throttling Calorime­ter. This is a simple device (Fig. 4.40). Its essential details are a throttling orifice admitting steam to an expansion chamber and a thermometer well entirely surrounded by the low-pressure steam from the throttling orifice. The principle of operation is the equality of initial and final enthalpies when steam passes through an orifice from higher to lower pressure, provided there is no heat loss and the difference between initial and final kinetic energies is negligible. Two conditions are necessary in the use of a throttling calorimeter: (1) there must be a significant pressure difference between steam in the sample and steam in the expansion chamber and (2) the quality of the sample must be high enough to produce a measurable degree of superheat (8°F) in the calorimeter.

For example, if the pressure in the expansion chamber is atmospheric, the temperature is 280°F, and the sample pressure is 135 psia, the constant enthalpy line on a Mollier chart shows the initial moisture to be 1%, or 99% quality.

Quality can be calculated from the following formula:

X = h2,~— X100 (4.16)

hfg

where X = the quality (%)

h2 = the enthalpy of superheated steam at the calo­rimeter pressure and temperature hf = the enthalpy of saturated liquid in the mixture prior to throttling

hfg = the enthalpy of vaporization of steam entering the calorimeter

Separating Calorimeter. The throttling calorimeter cannot be used when the enthalpy in the calorimeter chamber is equal to or less than the enthalpy of saturated steam. Either a separating or universal instrument must be used. In the separating calorimeter (Fig. 4.41), water is separated out from the steam and read in a graduated gage glass. The quality (%) is

X = w +Mxl0° (4Л7)

W + M

where M = the weight of dry steam condensed after passing through the calorimeter

W = the weight of water as read from the gage-glass scale

R = the weight of water corresponding to heat loss by radiation

With an insulated calorimeter, the radiation loss can be neglected.

The accuracy of the separating calorimeter is somewhat less than that of the throttling calorimeter.

Throttling Separating Calorimeter. The throttling sepa­rating calorimeter is made up of two calorimeters, a

throttling calorimeter and a sepaiating calorimeter of low and high range, respectively, in series The steam first passes through a throttling orifice and, if the moisture is not excessive, the quality is determined as m a throttling calorimeter If the moisture is outside the throttling — calorimeter range, the separating calorimeter is at once available and no delay is caused by its use

Separating Throttling Superheating Calorimeter. A throttling calorimeter can be connected to the exhaust of a separating calorimeter if the separation of moisture in the separating calorimeters is not complete. The quality of the original sample can be found by multiplying the qualities for each process.

Radioactive Tracers. Radioactive tracers can be used to determine steam quality from a boiling-water reactor. This method is generally limited to steam pressures under 1000 psi because of the occurrence of volatilized salts m the steam. Concentrations of specific radionuclides in condensed steam and boiler-water samples are determined with a multichannel analyzer Steam quality, in percent, is calculated from the tracer activities as follows

efficiency, or utility. Solids in the steam may be in different proportion to each other than the solids in the water from which the steam is derived.

(b) Gravimetric Determination. The steam is evapo­rated to dryness, and the residue is chemically analyzed in the classical manner. The method is described in ASTM D1069, Tentative Method of Test for Suspended and Dissolved Matter (Suspended and Dissolved Solids) in Industrial Water and Industrial Waste Water.

(c) Electrical Conductivity Method. The conductivity of a condensed steam sample is proportional to the concentration of lomzable constituents dissolved in the sample. The conductivity, expressed in micromhos, is meaningful only if it is compared with data from a gravimetric determination. The gravimetric-determination data are the primary standard. Usually the steam sample is given a preliminary treatment through degassers to remove most of the gaseous impurities that contribute to the measured conductivity. Unfortunately, degassers are not very effective m removing amines, hydrogen sulfide, and sulfur dioxide.

There are four sources of interference with respect to the calibration of electrical conductivity measurements

1. Dissolved volatile substances, such as ammonia, amines, H2S, II2, C02, and S02, increase conduc­tivity. Ammonia and hydrazine are commonly used in once-through boilers for pH adjustment and dissolved-oxygen scavenging.

2. Dissolved solids, such as the oxides of silicon, copper, and iron, ionize very little and therefore have little influence on conductivity.

3. The conductivity of pure water, as small as it is, must be subtracted from the combined conductivity.

4. The current-carrying capability of each ion species is different at any one temperature, and temperature coefficients are different, therefore, calibration de­pends on an assumed composition that may change in any one system and is almost certain to be different in different systems.

(d) Sodium Tracer Method for High-Purity Steam. The

method is described in ASTM D2186. Generally, this method assumes that the ratio of sodium concentration to impurity concentration in the steam is equal to the ratio of sodium concentration to impurity concentration in boiler water

4- 6.2 Purity

(a) Definition. Steam purity refers to the solid matter in steam. Solid matter is defined as materials in steam which are solids at room temperature and which are capable of deposition as solids m superheaters, steam lines, turbines, or other steam-utilizing apparatus so as to reduce capacity,

where St = concentration of impurities in steam Ss = concentration of sodium m steam Wt = concentration of total solids in boiler water Ws = concentration of sodium in boiler water

The values on the right-hand side of this formula are determined by ASTM methods

The principal advantages of this method are its freedom from interferences, its ability to measure extremely small concentrations of impurities, and its rapid response to transient conditions. Sample temperature control is not required in the method.

(e) Determination When Silica and Metal Oxides Are Present. Electrical-conductivity measurements are not always reliable when significant quantities of the oxides of metals or silicon are present. These oxides do not ionize significantly. Eliminating these impurities in the feedwater is the best precautionary measure. Metal oxides carried over into the turbine can plate out on the blades and impair efficiency. If these substances are present in significant quantities, determination should be made from one of the following ASTM methods D857, aluminum, D859, silicon, D1068, iron, D1687, chromium, D1688, copper, and D1886, nickel.

6- 5.1 Usage

Experimental data from transfer-function tests consist of records of signals from which transfer functions or related quantities are to be extracted. Tables 6.10 and 6.11 indicate that there are a variety of approaches to data acquisition and analysis This section treats those which operate on the signals in the time domain to obtain a transfer function Section 6-6, Frequency Analyzers, is devoted to analysis in the frequency domain.

The signals to be analyzed can consist of any or all of the following components a single frequency, a continuum
of frequencies having meaning to the test, and unwanted frequencies, either discrete or continuous. In general, the purpose of an analyzer is to separate these three with the primary purpose of picking out the first two in the presence of the third.

Results from time-domain analyzers can be expressed ultimately in the frequency domain. For tests where a single frequency is excited, the combined results from a number of sequential tests at various frequencies constitute a transfer function evaluated at those frequencies If a continuous frequency is used m external excitation or self-excitation, the correlation function obtained in the time domain may be subsequently Fourier analyzed, as indicated in Table 6.1, to obtain the desired frequency function.

6- 5.2 Null-Balance Analyzer

The principle behind the null-balance method of analyz­ing sinusoidal excitation experiments is one of nulling or bucking out the output signal with the input signal. Here the transfer function is simply the gain and phase adjust­ment used on one signal or the other to achieve this cancellation. A number of rod-oscillator tests have used this method successfully.1 4 л s’2 6 However, the method is not applicable to analysis of a continuum of frequencies, as in pseudorandom excitation

Figure 6.12 shows schematically how the oscillating component of the ion-chamber current, Ij sin(cot + 6), is nulled against a mechanical oscillating signal to a sine potentiometer from the excitation device By having a sinusoidal resistance variation m the potentiometer through which the ion-chamber current flows, you obtain a mixed
signal whose sin cot components are nulled by resistance and mechanical phase adjustments Usually the output (Fig 6 12) is sent through a band-pass filter (for fre­quencies near that of the oscillator and observed by an operator making the nulling adjustments Some skill is required for high precision.

6- 5.3 Synchronous Transfer-Function Analyzer

A specially constructed analyzer has been used in rod-oscillator experiments11,15 and has been found to give high-precision results The basic principle, as indicated in Fig. 6 13, is to multiply the ion-chamber signal (with its steady-state component bucked out) by sin cot or cos cot using a synchro-resolver whose mechanical input signal is precisely in phase with the rod-oscillator device Since

I] sin (cot + 0) = Ii sin cot cos в + Ii cos cot sin в (6 32)

integration of sin cot or cos cot times the right-hand side of

Eq 6 32 over an integral number of cycles gives a result proportional to Ij cos 0 or It sin0, respectively. The amplitude and phase of the ion-chamber current may then be obtained from these two integrated outputs

Amplitude = [(Ij cos0)2 + (Ij sin 0)2 і’"4 (6 33)

Tangent of phase = — 1 Sm ^ (6 34)

11 cos 8

As indicated in Fig. 6.13, the signal at the point of the modulator modulates a carrier (typically several hundred Hertz), which is later demodulated at the demodulator

Regarding the noise accompanying the signal I] sin(cot + 0), the analyzer acts as a sharply tuned filter at со. The noise near со will cause randomness in successive determinations of Ii and в, the randomness being proportional to the noise and inversely proportional to the integration time

Detector environment is largely determined by the type of reactor being instrumented In general, out-of core detectors can be placed in a mildei environment than in-core detectors, especially if out of vessel locations arc — suitable The various environments are described in Chaps 1 5 to 18

The most important single environmental condition is temperature, which can vary from near room level remote from the reactor core to very high in the vicinity of the core of a gas-cooled or sodium-cooled reactor Ionization chamber detectors that can operate in temperatures up to 750°F (400°C) are available and are suitable for most existing reactors Experimental ionization chambers have been operated with varying degrees of success at tempera tures as high as 1400°F (760°C) It is likely that the temperature ratings of commercially available detectors will increase in the future

Gamma background is also an important environmental condition for neutron detectors As reactor power densities are increased in the more sophisticated designs, there will be an increasing need for detectors that can operate in higher gamma backgrounds Improvements in gamma tolerance can be made only with great difficulty The successful use of neutron detectors in high gamma fields depends largely on the ingenuity and skill of the reactor and instrument designers and represents a source of continued difficulty Internal heating may be a problem in gamma fields exceeding 5 X 107 R/hr

High neutron backgrounds in neutron detectors do not normally cause difficulty, since neutrons are the principal instrumentation objective The trend, however, is toward power reactors that have a larger fast neutron fraction in their neutron spectrum Since the sensitivity of neutron detectors, which are primarily thermal neutron detectors, decreases as the energy of the neutrons increases, diffi culties are to be expected The main difficulty is assurance that the detector senses neutrons that truly represent reactor power, 1 e, that vary in a regular way with power variation

In addition, to produce an adequate signal in a fast neutron flux, the detector may require exposure to a high neutron flux This can adversely affect the life and physical characteristics of the detector

Adverse environmental conditions can be avoided by moving the detector For example, start-up detectors that do not have to operate when the reactor is at power can be moved to regions of lower radiation intensity once they are out of range Similarly, other sensors may be moved to a more favorable environment as reactor power is increased The use of detector withdrawal might enable reliable instrumentation where the background is otherwise too severe Safety instruments should not be moved unless they are no longer required Satisfactory operation in the new position must be assured

Excessive neutron flux and gamma gradients may contribute to instrumentation faults In general, it is desirable to operate with a large signal to extend the sensor range In a high flux gradient, a part of a sensor may be operating in excess of its rating This implies operation with a lower input signal than proper use would provide In addition, the most effective gamma compensation is ob tamed in a low gradient gamma field Thus, to the extent that the gamma gradient is related to the neutron gradient, it is likely that compensation may be less effective and there will be a loss of range

The log count-rate meter (LCRM) is a pulse-counting component used to convert input pulses from the detector and preamplifier analog signal for use in control compo­nents The LCRM has five basic functions (1) pulse-height discrimination, (2) count-rate indication, (3) period indica­tion, (4) scaler output signal, and (5) adjustable alarm output Each of these functions is discussed below.

Figure 5.7 is a block diagram of an LCRM. The unit has all the items listed above along with a built-in calibrator,

B-12P

Fig. 5.4—Vacuum-tube preamplifier.

• •

power supply, and test source. The unit uses all solid-state elements for improved reliability and low maintenance

(a) Pulse-Height Discriminator. The solid-state pulse — height discriminator shown in Fig 5.8 performs three functions (1) provides for pulse-height discrimination, (2) reshapes pulses for counting, and (3) provides a scaler output signal The heart of the pulse discriminator is the dual n—p—n transistor and the potentiometer R3. Resistor R3 is a 10-turn potentiometer mounted on the front panel of the LCRM. Resistor R3 provides a d-c bias on one-half of the dual n—p—n transistor. This turns this haf of the transistor on while the input half is off. A pulse applied to the input with amplitude greater than the d-c bias turns the input transistor on, forces the biased half off, and generates an output pulse for the counting circuits A pulse of height less than the d-c bias has no effect on the output and is thus uncounted. The discriminator then allows only pulses of amplitude larger than a set threshold to be counted, thus providing a convenient means for eliminating low-amplitude noise pulses generated in the sensor and cable.

The pulses from the discriminator are reshaped in a trigger circuit so that each pulse has the same height and width, essentially a square wave. Transistor Q9 provides a pulse output for a scaler, and transistor Q8 provides a pulse output for the LCRM counting circuitry.

(b) Count-Rate Indication. Figure 5.9 shows a Cook— Yarborough log circuit. The function of this circuit is to convert the constant-width and constant-height pulses into an analog signal. Since five or six decades of counts are to be covered by the LCRM, the circuit provides a logarithmic signal.

The diode pump is composed of CR5 to CR25, C7 to C27, and resistor RIO to R20. The purpose of the diode pump is to convert from a count rate to a d-c voltage. This is accomplished in the following manner. Consider compo­nents RIO, C7, CR4, CR5, and CR6. Component CR6 is a pulse-coupling capacitor Component CR4 is a positive — voltage clipper which prevents a positive voltage from appearing at the cathode of diode CR5 Diode CR5 and capacitor C7 form a low-impedance charge circuit for negative pulses. After the negative pulse has passed through CR5, the diode prevents C7 from discharging back through CR1 or the input circuit Hence the capacitor C5 must discharge through resistor RIO. The time constant is then RIO times C5. Should a second pulse follow the first pulse rapidly, the capacitor will not have time to discharge, and, as a result, the input to the d-c amplifier through RIO is essentially a d-c or rectified a-c voltage.

There are 11 such circuits with various time constants, varying from 40 sec to 8 msec. Resistors RIO to R20, C7 to C27, and CR5 to CR25 serve the same purpose.

The pulses are thus converted to a d-c voltage output proportional to different count rates. The d-c outputs for high count rates are provided for by the shorter time — constant circuits (C27 and R20), whereas the circuits with long time constants provide a voltage output for both low and high count rates.

The output of the diode pump is a d-c voltage amplified in the scaling amplifier. The output of the scaling is properly sized for meter operation and for a potentiometric recorder output The analog signal from the scaling ampli­fier is also fed to a differentiator circuit for period and to the level alarm circuits.

(c) Period Indication. The reactor period, T, is the reciprocal of the fractional change in the neutron popula­tion per unit time (see Chap 1, Sec 1-3.1)

_ dn/n _ dn/dt _ d(ln n)

T dt n dt *

where n is the neutron density, In is the natural logarithm, and t is time

Figures 5.10 and 5.11 show two circuits used to obtain a period signal from the rate of change of the log-count-rate (LCR) signal The circuit shown in Fig. 5.10 uses opera­tional amplifier A2 with feedback to achieve the period signal Operational amplifier A1 serves only as a circuit calibrator and to provide for a test ramp signal to A2. The output signal from the diode pump is supplied to A2. Amplifier A2 differentiates any input-signal changes in level and provides an output signal for the time rate of change of the input signal

The circuit shown in Fig. 5.11 is similar in function to that shown in Fig. 5.10. The circuit is essentially an operational amplifier with a high-impedance input, FET A8, a resistive feedback element, R29, and an input capacitor, Cl2. The output voltage is then of the form

e0=Rk = RC^1 (5 2)

dt

where dej/dt is a measure of the time rate of change of the neutron flux When dn/dt is constant, dT/dt is unity (assuming T and t to be measured in the same units). The output voltage produced by the period amplifier will be some base level to keep the meter reading up scale A change in dn/dt produces a change in period-amplifier output

The output from the period differentiator drives an operational amplifier for proper signal scaling for the meter, recorder, and period-trip (alarm) board

(d) Scaler Output. Associated with the discriminator in Fig. 5.8 is a pulse output stage used to drive a pulse scaler or counter. (The scaler itself is discussed in Sec. 5-2.6.) The pulse-generating equipment consists of the circuit described in Sec (a) above and also transistor Q9. Transis­tor Q9 is used to isolate the scaler output from the LCR circuit

(e) Alarm Unit. The alarm unit provides a signal at selected and adjustable values, such as low count rate, high

+15V

+ 15V

. R31 > 390

‘ 51 0 -Л |cs >R1 7 — cm ,R14 ■ R16 > Ж ЛК < ;5 ik

1 u- ;R29 co_L з. UT CN f 4 7K -L гм >язо J ;R32 f 1К CR13 Г 2K inj *

+ 15V

[Г] ONE SHOT

PULSE INPUT QQ

PULSE INPUT RETURN

ONE SHOT

I——I +15 V ■=“ 2K Q9 75

i l CR7

CR62‘

——і Cl 1 ± 2 2 flF

—|~R~| ONE SHOT (AUXILIARY) — ЦП ACCESSORY CONTROL -[ЇЇ] ACCESSORY PULSES — ГП CALIBRATE CONTROL —|~K~| CALIBRATE PULSES —|~J~| OPERATE CONTROL

DISCRIMINATOR [T]- DISCRIMINATOR fTTf-

Fig. 5.8—Pulse-height discriminator

count rate, period, or loss of chamber high voltage A solid-state alarm (trip) unit is shown in Fig 5.12 Relays R1 and R2 are deenergized when the set point is exceeded. Transistors Q3 and Q7 must change from the saturated to the unsaturated (off) condition. The output of Q1 or Q2 will change any time the input-signal voltage levels exceed the set-point voltage as determined by potentiometers R28 and R29.

Each alarm function has two identical trip amplifiers similar to the one shown. The redundancy is necessary to decrease the probability of a failure in the operation of the alarm unit This is very important for reactor protection since the output transistor Q3 or Q7 could short between collector and emitter, in which case the relay would not deenergize (fail to trip) when the set point was exceeded. The contacts of the trip relays are connected in series so that either relay deenergizing will cause an alarm

7- 3.1 System Requirements

The requirements that must be met by the reactor control system, including sensors and control rods, may be divided into the following categories

1 Amount of reactivity controlled and rod positioning accuracy

2 Rate of change of reactivity for normal operation, including initial start-up, planned shutdown, and restart

3 Emergency shutdown

4 Reliability

The excess or maximum reactivity requirements of a reactor are dependent on the planned rate of fuel depletion, fission-product buildup, inherent reactivity effects (e g, temperature), and control range desired Table 7 1 shows that the range of total rod reactivity for a number of typical power reactor plants varies from 6 to 25% in 5k/k

The control rod drive must be capable of making small changes in reactivity to maintain a flat neutron flux dis­tribution (uniform generation of heat throughout the core) and to be able to adjust the power level of the reactor with sufficient precision throughout full-power operation Typically, changes in 5k/k £ 10 s are required In terms of positioning accuracy, this means that the control rod drive system must be able to position a rod to an accuracy of about 0 02 in This value can vary widely, however, depend mg on the particular reactor design and the individual rod worth, values from 0 01 in to several inches are possible 2

The reactor designer determines the required rate of change of reactivity by examining how rapidly power must be changed to maintain proper operation during both

normal and emergency operation For conventional start up and power phases, periods of 30 sec or longer are normal This requires reactivity adjustments in the range of 10-3 to 10 5 Sk/sec, with an average value of 2 X 10 4 5k/sec * This same rate is usually satisfactory for steady-state con­trol For linear control rods, this corresponds to about 10 m /mm as an average, although it can vary2 (with reactor design) from 3 to 300 in /min For a directly coupled rotational system, where the control drum may rotate 180°, this rate corresponds to about 0 2°/sec During shutdown the reactivity (and thus the reactor power) is usually decreased more rapidly than its rate of increase during start up As noted earlier, this requirement is often satisfied by moving the control rods at the same rate used for start-up, but moving all rods simultaneously rather than a few at a time

For scram or emergency shutdown, the required rate of reactivity reduction normally exceeds the insertion rate for power control by a factor of 10 to 100 Higher rates of reactivity reduction do not yield significant benefits since, after the reactivity becomes about 1% below critical (p = 6k/k = — 0 01), the reactor power decreases as the delayed-neutrons decrease Of more importance in shut­down is the release time or turnaround time following receipt of a shutdown command Small delays in beginning the reduction of reactivity can result in significant power

•The expression 2 x 10 4 6k/sec means 2 x 104/sec = Sk/sec This notation is commonly used in nuclear engineering excursions The usual practice is to design for release times of about 10 to 50 msec When this rapid initiation is coupled with a reactivity insertion rate of about 5 X 10 2 5k/sec, the reactor can be shut down on a nominal negative period of 5 sec or less

It is common practice to satisfy both normal perfor­mance and safety requirements with one reactivity adjust ment mechanism It is also common practice to design the scram mechanism to operate in a fail-safe mode, і e, to operate in the event of loss of primary power (The primary-power loss may be inadvertent or it may be initiated by another part of the scram system ) There are many ways of designing the rod-drive mechanism to fail safe One general practice is to use gravitational force to store energy for scram A simple example of this practice is to place a coupling device between the control rod and its drive mechanism that has the same primary-power source as that which supplies the control-rod actuator If the primary power is lost, the control rod is released and gravity forces the rod into the reactor Springs and hydraulic devices can also be used to store energy and to release it on power failure If higher scram velocities are desired, a spring may be incorporated to increase the acceleration of the control rod into the reactor If springs are used, the actuator may be designed such that the rod is held against force, and, if the primary power is lost, the spring returns the control rod rapidly to the shutdown position The spring can also serve to eliminate backlash and thus reduce the deadband in the control loop

The reliability requirements for the control-rod drive system are influenced by considerations of safety and maintainability For safety reasons there must be a high level of confidence that the scram system will operate correctly, і e, it will be reliable (see Vol 2, Chap 12) The required confidence that this system will work is signifi­cantly higher than that required of the control system during normal operation In addition, the availability of the control-rod drive system is essential This means the system must be available for operation during all scheduled operational periods, except during normal preventive — maintenance periods when the reactor is shut down A failure in the control-rod drive system normally requires that the reactor be shut down in view of the possibility of unsafe operation (loss of control of output power or loss of scram capability) The reactor is then unavailable for the total time required to shut down, correct the failure, and restart Hence, unscheduled maintenance must be avoided Nuclear power plants are designed for a life of about 30 years, with scheduled shutdowns for maintenance at in­tervals of 6 to 18 months Since the control-rod drive systems can be serviced during the scheduled maintenance periods, their reliability requirements are correspondingly reduced In essence, control-rod drive systems must be highly reliable, but only for relatively short periods of time For an increase in overall reactor reliability, some systems are designed so a failure of one control-rod drive does not require shutdown In these systems failures may occur during operation, but corrective maintenance is not re­quired until the next scheduled maintenance period If unscheduled maintenance is required because a rod-drive mechanism fails and forces a shutdown, it is very desirable to minimize the time required for corrective maintenance This time can be reduced if the designer has considered this requirement during the initial design phase

The requirements discussed above result from nuclear design operational considerations and are applicable to reactors controlled through a primary control loop on nuclear power In establishing the requirements for a control-drive mechanism, the designer must first consider the reactivity span and the rate of change needed, these are used to calculate the desired period and steady-state operating conditions However, since an automatic control loop is normally used, the designer must also ensure that the control system can operate in a stable closed-loop manner The requirements on the control-drive mechanism that must be met to provide stable closed loop operation during all feasible transients and perturbations may be more severe than those for satisfactory period and steady-state operation These requirements are identified by dynamic control analysis of the complete reactor system

As shown in Fig 7 1, the control-rod position loop is usually an inner loop of the automatic power control loop Although the speed of response of the power-control loop is selected to provide the desired reactor performance, a dynamic control-systems analysis would indicate that the speed of response of the rod-position loop must be from 2 to 10 times more rapid to result in stable (nonoscillatory and nondiverging) operation when all control loops are closed

The basic power-control loop, as shown in Fig 7 1, is sometimes supplemented by a trim loop that controls the temperature of the primary-coolant flow This latter would be an outside loop on power control which compares a measured temperature with a demanded value and produces a supplemental power-demand signal More stringent re­quirements are placed on the drive mechanism when the reactor controllers are complicated by introducing coolant temperature, or variables from the steam side of the power plant loop, because of the interaction of these parameters and the normal dynamic requirement to make all inner loops respond about six times faster than outer loops

The requirements of the amount of reactivity con­trolled, the rod-positioning accuracy, reactivity rate of change under various situations, and the reliability and availability have been met in many power reactors by using low-maintenance, high-reliability a-c motors for the control-rod drive The coupling to the control rod is usually by a rack-and-pinion mechanism or a lead screw and nut However, other techniques have been used, including d-c motors, hydraulic cylinders and motors, linear induction motors, and magnetic jacks The designer must establish the requirements for a given reactor design and then review all available systems and components to select the most appropriate The advantages and disadvantages of a number of systems and typical applications are discussed in the following sections

7- 3.2 Means of Control

The control rods selected for water reactors are usually linear structures of a neutron-absorbing material designed to be moved vertically into and out of the core The amount of neutron absorber in the core is determined by the position of the rods with respect to the core At shutdown, the rods are positioned with the maximum amount of neutron absorbing material within the core As the rod is withdrawn from the core, the reactivity and neutron population increase by an amount generally pro portional to the amount of neutron-absorbing material removed

Although control rods of neutron-absorbing material are used in most reactor installations, in some instances neutron-reflecting and neutron-moderating or fuel-bearing rods are used Another type of control, a cylindrical device called a drum, has its surface comprised partly of neutron absorbing material and partly of neutron-reflecting ma tenal, or there can be combinations of absorber—fuel or absorber—moderator Several of these drums are located in a vertical position around the core periphery and have rotary control motion, the extent of drum rotation determining the core reactivity

Reactivity control by a neutron-absorbing liquid may be heterogeneous, with the liquid flow in a sealed pipe or pipe system adjacent to the reactor core, or homogeneous, with the liquid mixed with the reactor coolant water and extracted by ion-exchange equipment For example, mercury can be used in a heterogeneous system and boric acid can be introduced into the reactor coolant water system in a homogeneous system

Nuclear chain reaction. A series of nuclear reactions in which one of the agents necessary to the series is itself produced by the reactions so as to cause similar reactions. Depending on whether the number of reac­tions so caused directly by one reaction is on the average less than, equal to, or greater than unity, the

reaction is convergent (si/btntical) self-sustained (спи cal), or divergent (supercritical)

Nuclear rcactm A device in which a self-sustaining nuclear fission chain reaction can be maintained and controlled (fission reactor). The term is commonly called “reactor” or “pile ”

Iast reactor A reactor in which fission is induced predominantly by fast neutrons (Also called fast- neutron reactor.)

Thermal reactoi A reactoi in which fission is induced predominantly by thermal neutrons.

Multiplication factor The ratio of the total number of neutrons produced during a time interval (excluding neutrons produced by sources whose strengths are not a function of fission rate) to the total number of neutrons lost by absorption and leakage during the same interval When the quantity is evaluated for an infinite medium or for an infinitely repeating lattice, it is referred to as the infinite multiplication factor (k»,) When the quantity is evaluated for a finite medium, it is referred to as the effective multiplication factor (kepf) (The term is also called multiplication constant.)

Critical Eulfilhng the condition that a medium capable of sustaining a nuclear chain reaction has an effective mult’phcation factor equal to unity (A nuclear reactor is critical when the rate of neutron production, exclud­ing neutron sources whose strengths are not a function of fission rate, is equal to the rate of neutron loss )

Delayed critical Identical with critical, the term is used to emphasize that the delayed neutrons are necessary to achieve the critical state

Prompt critical fulfilling the condition that a nuclear chain-reacting medium is critical utilizing prompt neu­trons only.

Prompt neutron fraction. The ratio of the mean number of prompt neutrons per fission to the mean total number of neutrons (prompt plus delayed) per fission.

Delayed-neutron fraction. The ratio of the mean number of delayed neutrons per fission to the mean total number of neutrons (prompt plus delayed) per fission.

Effective delayed neutron fraction. The ratio of the mean number of fissions caused by delayed neutrons to the mean total number of fissions caused by delayed plus prompt neutrons. (Note The effective delayed-neutron fraction is generally larger than the actual delayed — neutron fraction )

Reactivity. A parameter, p, giving the deviation from criticality of a nuclear chain-reacting medium such that positive values correspond to a supercritical state and negative values to a subcritieal state Quantitatively, p = 1 — (l/keff), where keff is the effective multiplica­tion factor.

Excess reactivity. The maximum reactivity attainable at any time by adjustment of the control members.

Built-in reactivity The reactivity of a system as a function of design excluding the experimental and control inserts of the system.

Reactor control The intentional variation of the reaction rate in a reactor or adjustment of reactivity to maintain a desired state of operation.

Reactivity coefficient. The change in reactivity caused by inserting a small amount of a substance in a reactor The reactivity coefficient of a substance may depend upon the amount and distribution of the substance inserted, but is usually quoted as the reactivity change per unit mass of the substance at specific positions in the reactor or as a uniform distribution.

Void coefficient The partial derivative of reactivity with respect to a void (i. e, the removal of the material) at a specified location within a reactor. It is equal to the reactivity coefficient of the material removed.

Isothermal temperature coefficient of reactivity. The change of reactivity caused by a one-degree increase in the uniform temperature of a reactor at zero power

Powei coefficient of reactivity The change of reactivity per unit change of reactor thermal power when other variables are not independently changed