2-4.1 Proportional Counters

A proportional counter is simply an ionization chamber designed to operate using gas multiplication, a form of current amplification caused when the drift velocity of electrons and ions is sufficiently energetic to increase the total ionization 1 9-22

Gas multiplication requires a strong voltage gradient, which is difficult to produce in a parallel plate chamber One electrode is a surface and the other electrode is a thin wire The thin wire creates a strong electric field gradient near its surface Figure 2 17 shows a proportional counter The percentage of the total gas volume in which multiplica­tion takes place is adjusted by changing the spacing between electrodes The gam per pulse is most nearly constant when the detected particle is ionizing the non multiplying gas zone The collected ionization then passes entirely through the multiplying gas zone

Gas amplification is electrically adjusted by changing the applied high voltage The sharp gradient about the wire electrode permits operation at moderate voltage Thus, the high voltage supply may be interchangeable with the high voltage supply used for ionization chambers However, unlike an ionization chamber, the proportional counter is

extremely sensitive to voltage variation. A well-regulated high-voltage supply is essential.

The operating voltage of the proportional counter, like that of the ionization chamber, has a plateau.23 In any particular design there is a range of amplification in which the pulses have a fixed range of amplitudes for a given type

of radiation. When all pulses over some minimum voltage are counted (pulse height), the plateau is manifested as in Fig. 2.18. Operation on the plateau reduces sensitivity to voltage variation. However, the plateau is normally never as good as that for an ionization chamber.

When detecting neutrons, a proportional counter does not have a gamma tolerance as good as is possible with an
ionization chamber; it is typically of the order of 103 R/hr. Proportional counters for reactor use are usually BF3-filled or boron-coated. If boron-coated, a number of gas fills may be used. Figure 2.18 also shows the effect of gamma background. The gammas may be observed to have a deleterious effect on the plateau.

(a) Discussion of Applications. Electrical conductivity of a solution is a measure of all ions present. Pure water is a very poor conductor. The conductivity of a water solution is, in practice, almost exclusively due to ions other than the hydrogen (H+) and hydroxyl (OH ) ions. Figure 4.55 shows the conductivity of certain electrolytes as a function of their concentration in water.

Most practical applications of electrical-conductivity measurements fall into one of the following categories

1. Concentration in simple water solutions. Common examples are sodium chloride, sodium hydroxide, and sulfuric acid In such cases the concentration—conductivity curve must be known in advance, or the system must be experimentally calibrated.

2. Boiler steam quality detection. The exact nature of the electrolyte is usually less important than its magnitude. Nuclear installations require extremely pure feedwater

3. Measuring the extent of a reaction. Reactions such as precipitation, neutralization, and washing soluble electro­lyte from insoluble materials can be monitored by con­ductivity measurements. These procedures require calibra­tion or a comparison between conductivities of streams before and after the reaction.

4. Detecting contaminations. Leaks in heat exchanger with the resultant contamination. Any sudden change in conductivity of the heat-exchange medium is taken as leakage. Salt-water contamination of freshwater can be detected as well as breaks in condenser tubes.

(b) Measurement Methods. Measuring Circuits The a-c Wheatstone bridge circuit (Fig. 4.5 3) is the most widely used technique. It is sensitive, stable, and accurate. An ohmmeter circuit (Fig. 4.56) can also be used. The current is a function of cell resistance, the system is sensitive to

Fig. 4 56—Conductivity-measuring system using a simple ohmmeter circuit. (From D M. Considine, Process Instru­ments and Controls Handbook, p 6 163, McGraw Hill Book Company, Inc, New York, 1957 )

voltage variations In addition to these circuits, an a-c crossed-coil electrodynamometer can be used in a conduc­tivity-measuring circuit. One of the two crossed moving coils responds to the current flow in the conductivity cell circuit, the other responds to the source voltage. It is a relatively simple technique, but it is not as accurate or sensitive as the Wheatstone bridge.

Conductivity Cells. The first criterion m selecting a conductivity cell is that the cell constant must be such that the resistance of the solution under test falls within the limits of the cell range. When high electrolytic resistance is being measured, as in the determination of steam purity, a capacitative impedance in series with the cell has a negligible effect on bridge readings, on the other hand, capacitative impedance in parallel impairs the sharpness of bridge balance. Impedance varies inversely with frequency. Therefore, low bridge frequencies are desirable when measuring high resistance. A relatively low cell constant, such as К = 0.1, has large electrodes close together and is suitable for measuring high resistance systems. Spreading the plates apart and constricting the electrolyte cross section increases the cell constant.

The mechanical features of conductivity cells are illustrated in Fig. 4.57. There are four basic types: dip cells, designed for dipping or immersing in open vessels; screw-in cells, designed for permanent installation in pipelines and tanks; insertion cells with removal devices, designed to permit removal of the element without closing down the line in which they are installed; and flow cells, glass or plastic with internal electrodes close to the wall to offer little resistance to the flowing medium. (In small sizes, the flow-cell tubes are connected to the system with rubber or plastic tubing; in large sizes standard pipe flanges are used.)

Temperature, flow velocity, and presence of solids have significant effects on conductivity-cell performance. Tem­
perature should be held as nearly constant as possible. The conductivity of most solutions increases about 2.5% for each 1 C rise in temperature. The flow velocity should be sufficient to ensure circulation of liquid between the electrodes. Entrained solids and high velocity increase the scouring effect. Low velocity can result in the accumulation of solids and the plugging of the cell chamber.

Chemical considerations are important. Strong electro­lytes, such as hydrochloric acid, can slowly dissolve platinum electrodes. Tantalum or graphite electrodes should be used. Hydrofluoric acid measurement requires cells of tetrafluoroethylene and platinum. Conductivity measurements of condensed steam or demineralized water

can be made with borosilicate glass, dense ceramic, and most plastics. Types 304 and 316 stainless steel, nickel, gold, and platinum structural parts and electrodes are suitable. The use of fluxes in the fabrication of electrodes should be avoided so that there will be no subsequent contamination by leaching of electrolyte material. Avoid contamination by eliminating such items as pipe-joint compounds and dopes. Thoroughly wash items after chemi­cal cleansing or replatimzation.

Temperature compensation is particularly important. As shown in Fig. 4.58, the conductivity of sodium chloride solutions is temperature dependent. At uniform concentra­tion the conductivity increases about 2.5%/C. The most common means of temperature correction is the inclusion in one of the bridge arms of an adjustable resistor calibrated in temperature units. The calibration is based on an average temperature coefficient of conductance. This technique is used where variations in temperature are small. A knob is set to correspond to the temperature reading at the conductivity cell. It is best to avoid frequent knob settings by maintaining a constant sample temperature external to the cell by the use of throttling valves. Automatic tempera­ture-compensation methods can be used. A second conduc­tivity cell dipping into an isolated sample forms the variable-resistance arm of the bridge. Changes in tempera­ture will affect both the thermal cell and the measuring cell to the same extent, canceling out the temperature effect. Other automatic temperature compensators include bi­metallic strip electrodes, expanding or contracting metallic bellows coupled to the variable resistor, a rising mercury column in a special thermometer to shunt the standard arm of the bridge, a resistance thermometer that automatically adjusts the standard arm resistance, and thermistors of high negative temperature coefficient.

(c) Sources of Error. Errors m conductivity measure­ments may be attributable to

1. Insufficient circulation. Sluggish response is a symptom.

2. Contaminated cell. Sluggish response to great concen­tration changes.

3. Need of electrode revitalization. Characterized by broad null point or stepwise change in recorder.

4. Electrical leakage in conductivity cell, characterized by erratic results.

5. Leaching of electrolytes. Characterized by drift toward higher conductance.

6. Temperature errors. Characterized by drifting when concentration is known to be constant

7. Reference temperature. Characterized by inability to obtain check reading from two different instrument sys­tems even though each bridge and cell checks out against data.

8. Bridge calibration. Bridge will not check fixed resistor values. Check resistors in bridge circuit.

9. Change of cell constant. Characterized by inability to obtain correct instrument reading in known solution.

Walter H. Esselman, Robert L. Ramp, and Garold L. Hobmann

7- 1 INTRODUCTION

7- 1.1 Reactor Kinetics*

A nuclear reactor that is generating heat at a constant rate is a chain-reacting system in which the number of neutrons being produced in nuclear fission processes ex­actly balances the number of neutrons being absorbed m or escaping from the system. If it is desired to change the rate of heat generation (number of fissions per second), means must be provided to increase or decrease the absorption and escape of neutrons Once the heat-generation rate has reached the desired new level, means must be provided to restore the neutron balance so the system will once again generate heat at a constant rate The specific means used in present-day power reactors to alter the heat-generation rate upward or downward or to keep it constant are discussed in this chapter.

In the steady state (constant rate of generating heat), the reactor is critical when the effective multiplication constant к (sometimes written as keff) is just equal to 1 To increase or decrease the power level of the reactor requires that к be increased or decreased above or below unity during the interval when the power level is changing. Once the desired power level has been reached, к must be restored to unity so the reactor can again operate in a steady state, albeit at a new power level. The fractional deviation of the effective multiplication constant from unity is defined as the reactivity1

к — 1 1

Reactivity = p = ———— = 1 — j^-

or

5k

Reactivity = -— (with 5 k = к — 1)

К

The fundamentals of reactor kinetics are summarized in Chap. 1.

CHAPTER CONTENTS

7 1 Introduction……………………………………………………………………………………………….. 167

7-1.1 Reactor Kinetics…………………………………………………………………………… 167

7-1.2 Reactivity Variations During Operation…. 168

7-1.3 Methods of Reactivity Control…………………………………………………… 168

7-2 Reactor Control System……………………………………………………………………………….. 168

7-2.1 Approach to Criticality…. 168

7-2.2 Power-Increase Phase………………………………………………………………… 169

7-2.3 Power Operation Phase………………………………………………………………. 169

7-2.4 Shutdown Phase. . 170

7-3 Selection of Reactivity-Control Method………………………………………… 170

7-3.1 System Requirements. … . . 170

7-3.2 Means of Control… .. … . 173

7-3.3 Materials……………………………………………………………………………………….. 174

7-3.4 Rod Shape……………………………………………… . . . . 174

7-3.5 Rod Configuration. . . …………………………. 174

7-3.6 Types of Drives…………………………………………………………………………… 176

7-3.7 Rod-Position Indicators…………………………………………………………….. 178

7-4 Examples of Reactivity-Control Systems……………………………………… 180

7-4.1 PWR Power Plant at Shippingport…………………………………………….. 180

7-4.2 San Onofre Atomic Power Plant… … 183

7-4.3 Dresden Nuclear Power Plant…………………………………………………….. 185

7-4.4 Gas-Cooled Reactors………………………………………………………………….. 186

7-4.5 Fast Reactors……………………………………………………………………………….. 191

References………………………………………………………………………………………………………………… 191

Bibliography………………………………………………….. . . . … 191

Because к is very close to 1 at all times in an operating power reactor, the reactivity p or 5k/k is often abbreviated to Sk or “excess k” if 5k > 0. In terms of reactivity, the basic types of reactor performance are

p = 5k/k= 0 constant power level p = 5k/k > 0 power level increases p = 5k/k < 0 power level decreases

When the reactivity is not zero, the reactor power level increases or decreases with a characteristic time constant (reactor period) that is primarily dependent on the value of the reactivity, the prior operating history of the reactor, and the reactor configuration (arrangement and composi­tion of fuel, moderator, coolant, etc ) Period is the time required for the neutron level to increase (or decrease) by a factor of “e” (2.718) (see Chap. 1). The reactor period becomes too short for practical control if the reactivity is increased above zero by an amount equal to the delayed — neutron fraction, /3. For most presently operating power reactors, /3 ^0.007. This means that a positive p or 5k/k is always between zero and about 0 06% During operation at power, reactor control systems normally make adjustments at rates in the general range from 10 3/sec to 10 5/sec m 5k/k per second. Although the control adjustments during operation involve relatively small changes in reactivity, this is not necessarily true during reactor start-up and shut­down. The control system must be capable of “adding negative reactivity” to balance out the reactivity excess built into the reactor, and, if an emergency exists, it must do so rapidly. Under certain conditions reactivity changes

of—— 0 1/sec in Sk/k per second may be required The

excess reactivity built into a power reactor depends on many factors, it can be more than 10% in 5k/k For this reason control (and safety) systems must be capable of accomplishing large changes in reactivity during reactor start-up and shutdown. In addition, they must be capable of compensating for the effects of changing concentrations of the fission products 13sXe and 149Sm These can involve reactivity changes of several percent (see Sec 1-3.6 of Chap 1)

Nuclear power technology has reached the stage where there are “accepted practices” m many aspects of reactor design and construction The systems of instrumentation used in reactors of a specific type have more common features than differences Changes are gradual and evolu­tionary What is accepted practice today will be recognized as good practice for some years to come This does not mean that there will be no major changes in nuclear power technology in the future, it simply means that the rate of change will not be so rapid that what is learned today will have to be forgotten tomorrow

The instrumentation systems of today’s power reac­tors—including those on the drawing boards—are de­scribed in this book The performance and characteristics of the major components of power-reactor instrumentation systems are presented but with a minimum discussion of component design For example, in the chapters concerned with nuclear radiation sensors, sensor construction and performance are described m detail, but the data required by one who wishes to design a nuclear radiation sensor are not given This is in keeping with the basic intent to emphasize the systems aspect of power-reactor instrumenta­tion

The book is intended for the designers and operators of power-reactor instrumentation systems, 1 e, those con­cerned with the applications, not with the invention, of devices All systems aspects are discussed, including the problems associated with integrating individual components into subsystems and systems, the so-called “interface” problems The requirements (or design bases) to be satisfied by each system and subsystem are given, and current practices are outlined and evaluated

As the title indicates, systems associated with the nuclear power reactor are considered Systems associated with the electric power generated and with generator operation are not discussed In a sense the book is concerned with steam generation by nuclear reactors, although the fact that a turbogenerator is being driven by the steam does become involved in some of the instrumen­tation systems discussed

The book is organized into 18 chapters, divided into two volumes After an introductory chapter that sum­marizes basic definitions, reactor kinetics, and reactor types, Volume 1 continues with three chapters concerned with sensors The next chapter is concerned with the important electronics associated with neutron sensors Systems for determining the dynamic properties of nuclear reactors are then described Because control-rod drives and control-rod-position indicators have such a unique relation to the operation of nuclear power reactors and are so closely coupled to the neutron and position sensors of protection systems, these are briefly described in a chapter

The next four chapters are concerned with topics that are relevant to all reactor systems The increasing use of computers in data handling and process control in power reactors is described Systems for monitoring nuclear radiations and radioactive materials in nuclear plants are discussed Since power supplies are essential to the opera­tion of instrumentation systems, a chapter on the subject is included Many problems are the result of improper installation of the components of instrumentation systems, a chapter is devoted to this topic In the same manner, a chapter on quality assurance and reliability provides basic information needed by all reactor-instrumentation-systems designers and users

Volume 2 takes up the application of the material developed in Volume 1

The importance of reactor protection systems is such that one chapter is devoted to outlining the bases for their design and to describing current designs And a chapter describing radiation monitoring is included

A chapter summarizing the status of standards and codes on nuclear reactor instrumentation systems is then followed by the “big four ” These final four chapters summarize the current state of the art in instrumentation systems for the four major reactor types pressurized-water reactors, boiling-water reactors, sodium-cooled reactors, and gas-cooled reactors

Volume 2 concludes with one appendix a summary of in-core sensors in present-day reactors.

It is possible, but not practical, to use l0B-hned chambers for fixed in-core neutron-flux monitoring in power reactors They are not practical because the thermal — neutron cross section for 10 В is more than six times that for 23SU and results in too-rapid burn out For example, the neutron sensitivity of a fission chamber is reduced to 50% of its initial value after nine months of operation in a
thermal neutron flux of 4 X 1013 neutrons cm"2 sec1 (typical for most water reactors at full power) In contrast, a 1 0 B-lined ion chamber under the same conditions is down to 50% of its initial sensitivity in l’/2 months At the end of nine months of operation, the 10B lined ion chamber would have less than 2% of its initial sensitivity Figure 3 9 compares sensitivity vs time for the fission chamber[10] and the 10 В-lined ion chamber in a flux of 4 X 101 3 neutrons

-2 -i

cm sec

Boron-lined ion chambers can be used satisfactorily as neutron sensors on traveling in-core probes since the total time of exposure to the neutron flux is only a small fraction of the total operating time of the reactor The time required to run a complete core traverse seldom exceeds 3 mm, and the frequency of traversing is seldom more than once a month, thus many years of satisfactory operation can be obtained from a boron lined ion chamber used on a traveling in-core probe The characteristics of an in core ion chamber detector for traveling in-core probe service are described in Table 3 2

*The useful life of fission chambers can be extended by using a mixture of uranium isotopes eg 10% 23SU and 90% 2 3 4 U In such a mixture the 2 3 4 U is transmuted to 2 3 5 U and the useful life is thus extended beyond that achieved with a 2 3 5 U lined chamber

5- 5.1 Basis of Method

The MSV method depends on the fact that, if the time distribution of pulses from a nuclear radiation sensor is a Poisson distribution, the variance (mean of the squares of the deviations from the mean) is a direct measure of the mean. (See Table 5.1 for relevant definitions and formulas ) For all practical purposes this condition is met by boron and fission counting chambers

There are at least three advantages to be gained from using MSV methods increased gamma discrimination (com­pared to compensation), improved operation when cham­bers and cables are exposed to elevated temperatures, and more efficient use of chambers (sensors).

So that the advantages of the method can be realized, a measure of a quantity proportional to the square of the charge (or current) is made. One way to do this is to subtract the mean signal with a differentiator and measure the temperature rise in a resistor, as is done in a true root-mean-square meter commonly used in the shop or laboratory Another way is to pass the variable (a-c) signal through a half — or full-wave rectifier measuring, as a result, the average magnitude or average magnitude squared This latter technique is accurate at only one frequency for which a correction factor can be applied More generally, the pulses can be passed through an electronic squaring amplifier and the output read without correction as a linear measure of the mean square If a mean-square signal is sent to a log converter circuit, the output is again proportional to the log of the mean.

5- 5.2 Gamma Discrimination

Gamma discrimination must be compared with “com pensation” for gammas as accomplished in the CIC dis­cussed in Sec 2-2 2 of Chap 2. In a CIC a compensating signal generated in a volume not sensitive to neutrons is subtracted electrically from the gamma-plus-neutron signal In practice the compensating volume and the mass of material forming the neutron and compensating volumes cannot be matched exactly, through engineering com­promise the compensation is usually between 95 and 99% of the gamma signal Although m theory the compensation could be much better, it just cannot be achieved Commer­cial units use two concentric volumes that are adjusted so that overcompensation in some gamma range is avoided. Overcompensation would result in negative readings and confusion to control-system functions Manufacturers usu-

ally guarantee a gamma/neutron signal ratio of 1/20 and a maximum of 1/100, the latter to avoid overcompensation With MSV methods the compensation or discrimination is not dependent on the mechanical construction of the chamber Only one ionization volume is involved, arj advantage is taken of the charge ratio of a fission fragment to a gamma-scattered beta particle If the number of events is N per unit time and the charge collected per event is Q, then an average voltage Ej c is developed

Ed-c = NQ/0 h(t) dt (5.4)

where N is the mean number of events, Q is the mean charge per event, and h(t) is the circuit response to a single pulse of unit charge. By definition the mean-square voltage is

Ems = (Ed-c)2 +NQ2 /„ [h(t)l 2 dt (5 5)

The voltage E<j. c is made zero if h(t), the circuit response, is limited to acceptance of a-c signals alone In this case

Ems = N Q2 /о [h(t)] 2 dt (5 6)

It is not difficult, as indicated earlier, to make the circuit such that Ems is the lone acceptable result. A differentia­tion circuit at the input of the squaring circuit will do the job.

The relation between the resulting signals can be established if a chamber operating in the d-c mode is compared to a similar chamber operating in the MSV mode. If a subscript n is used for neutron events and a subscript (3 for gamma effects (since gammas scatter /3 particles or electrons into the chamber), the discrimination in the d-c mode for gammas is (from Eq. 5.4)

NnQn

КрЦз and the discrimination for the MSV mode is

NnQn

ЩЩ

The ratio of the discriminations is thus

Dms _ Qn Off ~ Qn (5

Dd-c Qn Q/3 0)3

The ratio Qn/Q| has been set equal to (Qn)2/(О/з)2 m deriving Eq 5.9 since it can be shown that in practical cases this is a good approximation.

The ratio Qn/Qd is about 103 for a fission chamber Compared to a compensated chamber, this means an assured 103 discrimination against gammas instead of the 20 to 100 In practice, experimental results show a nearly hundredfold improvement in gamma discrimination, mainly because the 1/20 ratio of gamma to neutron signal is more realistic for a CIC than the 1/100 ratio.

Nuclear power plants are categorized according to the type of nuclear reactor that is the primary heat source. In this book, reactor types are identified by the coolant used to extract heat from the nuclear fuel *

Bressunzed-water rent tors reactors cooled by water in the liquid state

Boiling water realtors reactors cooled by water in the liquid and gaseous states

Sodium-i ooled realtors reactors cooled by liquid sodium Gas-cooled reaetois reactors cooled by gas (helium in the United States)

‘Reactors can also be classified m other ways according to the energy spectrum of the neutron population (thermal, intermediate, and fast), according to use (research, development, test, plutonium production, and power), according to fuel arrangement (homo­geneous or heterogeneous), or according to whether the fuel fissioned is less than or greater than the fuel generated (breeder, nonbreeder, and converter).

Fvery nuclear power plant presently m operation in the United States derives its heat from a reactor in one of these four categories

Classification of nuclear power plants according to the primary reactor <oolant is particular^ appropriate to a consideration of power-reactor instrumentation systems since the coolant properties determine many aspects of instrumentation design. This is not surprising since the basic function of the nuclear reactor in a power plant is to generate the heat and to transfer it to a coolant that ean then transfer heat to the steam that drives a turbogenerator The coolant that extracts the heat from the nuclear fuel is the key link in the sequence of operations that converts nuclear energy to electrical energy. Moreover, because material constraints are critically important in any heat engine, the properties of the coolant have a strong influence on the plant design.

Figures 1.12 through 1.1 5 illustrate the basic configura­tions of the four categories of nuclear power plants It must be emphasized that the figures do not purport to show any actual plant configuration (see Chaps. 15 through 18) but rather show those features of each reactor type which are relevant to the design of the principal instrumentation systems

1-4.2 Sensed Variables

The nuclear chain reaction produces heat (primarily from the dissipation of the kinetic energy of the fission fragments) and nuclear radiations. Consequently, nuclear — power-reactor instrumentation depends primarily on ther­mal sensors and nuclear-radiation sensors The former

big. 1.14 —Sodium-cooled reactor. (From A. Pearson and C. G. Lennox, The Technology of Nuclear Reactor Safety, Vol 1, p. 289, The M I. T. Press, Cambridge, Mass., 1964.)

(thermocouples and resistance thermometers) are discussed m Chap 4 and the latter in Chaps. 2 and 3. Although a number of nuclear radiations are associated with the fission process, only neutrons can be unambiguously related to the occurrence of fissions, because of this, neutron sensors are the most important of the nuclear-radiation sensors. The neutron sensors are used to determine the rate of fissions, the time derivative of the fission rate, and the fission rate as
a function of position in the reactor. (The circuits required to convert the signals from neutron sensors into outputs that are directly related to nuclear reactor performance are described in Chap. 5 )

The primary coolant that transfers heat from the nuclear fuel to the turbogenerator (or to a heat exchanger coupled to the turbogenerator) must be examined by suitable sensors to determine such important parameters as

(1) the temperature of the coolant entering the reactor (T, in Figs. 1.12 through 1 15), (2) the temperature of the coolant leaving the reactor (To in the figures), (3) the temperature of the coolant at other positions in the reactor, (4) the rate of flow of coolant into and out of the reactor (Fc in the figures), (5) the rate of flow of coolant in various coolant channels in the reactor, (6) the radioactivity of the coolant after leaving reactor, (7) the purity of the coolant, and (8) the presence of water vapor in the coolant when the coolant is a gas To sense these parameters, there must be temperature sensors, flowmeters, humidity detectors, nu­clear-radiation (gamma in this case) sensors, etc

Reactor operation itself involves a number of parame­ters, including (l)the position of the control rods (La in Fig. 1.15), (2) the water level of the moderator (Lm in Fig. 1.12), (3) the water level in the reactor (Lc in Fig. 113), (4) the pressure in the primary system (Pp in Fig. 1.13), (5) the pressure at the coolant outlet (Pc in Figs. 1.12 and 1.15). and (6) the temperature ot the moderator (Tm in Fig. 1.15) Temperature sensors, position indicators, pressure transducers, etc., are required to take data on these parameters.

The steam system is characterized by such parameters as steam flow rate (Fs in the figures), steam pressures (Ps in the figures), steam quality, and feedwater flow (Ff in Fig. 1.13). Thermal sensors, pressure and differential-
pressure transducers, flowmeters, water-level indicators, and other sensors (Chap. 4, Sec. 4-6), must be used

In addition, there will be sensors associated with the important components of the plant. Thus, for example, tachometers to sense turbine rotation, meters to sense electrical generator output, thermal and mechanical devices to sense the performance of primary-coolant-pump drive motors, etc., must be installed

REFERENCES

1 American National Standards Institute, American National Stan dard Glossary of Terms tn Nuclear Science and Technology, N1 1 1967, American National Standards Institute, New York, New York

2. A. M. Weinberg and I P. Wigner, The Physical Theory oj Neutron Chain Reactors, University of Chicago Press, Chicago, 1958

3. S. Glasstone and M C Edlund, The / lements of Nuclear Reactor Theory, D. Van Nostrand Company, Inc., Princeton, N. J, 1952.

4. R V. Meghrebhan and D К Holmes, Reactor Analysis, McGraw Hill Book Company, Inc, New York, 1960

5. J. M Harrer, Nuclear Reactor Control engineering, D Van Nostrand Company, Inc., Princeton, N. J., 1963.

6. S Glasstone and A Sesonske, Nuclear Reactor engineering, D. Van Nostrand Company, Inc., Princeton, N. J, 1963

7. Reactor Physics Constants, USAEC Report ANI -5800(2nd I d ), Argonne National Laboratory, Superintendent of Documents, U. S. Government Printing Office, 1963

Sensor manufacturers subject their designs to a series of tests simulating actual operating conditions to determine the on-line operating characteristics. Standard definitions are given in the Scientific Apparatus Makers Association (SAMA) publication PMC-20, Measurement and Control Terminology.

A sample performance report on a motion-balance

sensor is given below

Description

Pressure-sensing mechanism Bourdon tube (316 stainless steel) Electric transmission Output-signal ranges

±10 volts d-c, ±50 mV d-c, 0 to 100 mV d-c Operating Conditions

Ambient temp, nominal, 75°F, reference, calibration ±5°Г, normal, 40 to 140° F, operative limits, -10 to 200° F Supply voltage nominal, 118 volts a-c, normal, 107 to 127 volts, operative limits, 100 to 1 35 volts Frequency nominal, 50 or 60 H7, normal, 48 to 62 Hz, operative limits, 45 to 75 Н/

Ambient temp, effect 7ero-shift error/100°F temp, change, — 1% range span, Range-shift error/100°F temp change + 1% range span

Reference Performance Characteristics (°0 range span)

Accuracy 0.5%

Dead band 0.2%

Hysteresis 0.5%

Linearity 0.25%

Repeatability 0.25%

Design Data

Source impedance a-c signal coil, 200 ohms, d-c signal demodulator, 180 ohms

Minimum external load a-c transmitted signal, 2000 ohms, d-c transmitted signal, 30,000 ohms Maximum ripple 0.15% a-c ripple

Case classification NEMA (National Electrical Manufacturers Association) type 2 or N1 MA type 7D Over-range protection ll/4 times max. scale measured pressure

Performance data on a force balance sensor is given as

Description

Pressure-sensing capsule 316 stainless steel Electric transmission 2 wire d-c Output signal range 10 to 50 mA d-c Operating Conditions

Power supply 63 to 85 volts d-c Supply voltage effect 0 25% per 10-volt variation Performance characteristics (% range span)

Accuracy 0 5%

Dead band 0.005%

Repeatability 0.15%

Design Data

Output load limits 600 ohms (+10%, -20%)

Case classification NEMA type 4, hazardous area Class I Group D, Div. 1

4- 3.7 Transmitting Devices

(a) Pressure Switches. These are widely used to actuate alarms or initiate sequential operations. A Bourdon tube or similar sensor is linked to a snap-acting mechanical switch.

(In some cases an enclosed mercury switch is used.) The switch may be indicating or nomndicating, have range­setting capability, and provide necessary logic at pre­determined pressures.

(b) Electric Modulating Transmitters. These produce an electrical output proportional to input pressure (or force) applied to the sensor. Either the motion-balance or the force-balance principle may be involved. The output may be a voltage or a current of suitable value and range for input to readout devices, such as recorders, indicators, computers, and control loops to action equipment. A sample circuit for the motion-balance example of Sec. 4-3.6 is shown in Fig. 4.23. Forms of the linear voltage differen­tial transformer (LVDT) mechanism and a sample output curve are shown in Fig. 4.24.

(c) Pneumatic Modulating Transmitters. Differential — pressure sensors installed with one side open to the atmosphere and the other side connected to a pressure source can be used The device shown in Fig. 4.35 can be used and the pneumatic force-balance principle applied to obtain a pneumatic output proportional to sensor gage pressures at connection H (or L, as desired).

When the various methods of obtaining information about the reactor transfer function are compared (from the standpoint of which might be the most appropriate to use), many factors appear

1 Structure, operating conditions, and physical limita­tions of the reactor system

2 Available time, personnel, equipment, and money

3 Type of information desired

4 Accuracy desired

5 Established operating and experimental policies of the plant

The various methods described in preceding sections are treated here with this set of determining factors in mind

A distinction is made between methods involving system excitation by external apparatus and those which rely on internally generated noise. In Table 6.13 an attempt is made to evaluate the methods in a general fashion, however, it should be noted that variations in the methods or special “ground rules” for comparison may lead to exceptions

Whether an input excitation signal is applied to a neutron absorber or to a plant control device will depend primarily on the information desired and secondarily on con­venience. When this selection has been made, the signal may then be chosen to be either sinusoidal or pseudorandom In recent years there has been some preference for pseudo­random signals which simultaneously measure all fre­quencies in the band of interest Pseudorandom excitation has been preferred107 because smaller perturbing ampli­tudes can be used and less reactor time is required for obtaining a given frequency resolution (see also Sec 6-7).

For the noise methods there is no input signal injection However, there must be sufficient internally generated noise in the frequency band of interest to excite the one or more variables being investigated. If this is the case, then selection of the appropriate data-acquisition and data — processing devices is the major consideration, as is also true for excitation experiments In the following sections the types of equipment are described and their relative merits are assessed

Whether one or more than one signal is used in noise analysis depends on the information desired Transfer functions in power reactors normally require two signals Furthermore, these must have sufficient coherence, Eq 6 8, in the frequency band covered to achieve the accuracy desired, і e, the effects of the same noise source—to a larger extent than separate independent noise sources— must be seen in both signals Besides giving the amplitude and phase of transfer functions, these multisignal experi­ments also may provide insight into the cause of the intiinsic noise

Self-powered detectors24 operate on the well — publicized principle of the nuclear battery. The incident — neutron flux activates a central electrode, which emits betas that are collected by a surrounding electrode. This type detector is usually designed for in-core neutron-flux sensing. It is discussed in Chap. 3, Sec. 3-3.3.

2-4.3 Activation Detectors

Neutron flux at a given position in a reactor can be measured by exposing a material object25 to the flux, removing it from the flux, and determining the activity that has been induced by exposure to the flux. From the exposure time and the known properties of the exposed material, the incident-neutron flux can be determined. This method can be used in in-core neutron-flux mapping. The exposed material can be in the form of wire, foil, ribbon, etc. Even liquids and gases26 can be used.

2-4.4 Solid-State and Scintillation Detectors

Solid-state and scintillation detectors2 7 30 can only be used where the neutron — and gamma-flux levels are low, for example, in radiation-monitoring systems. Solid-state de­tectors convert directly to an electrical signal, while scintillation detectors require an intermediate photoelectric stage. There is a wide variety of types. Most, however, are not applicable at neutron fluences above 101 5 neutrons/ cm2 and integrated gamma exposures of 1012 rad.

2-5 INSTALLATION

Once a nuclear radiation sensor has been selected, installation arrangements must be made (see also Chap. 10).