A basic reactor control system is shown schematically in Fig. 7.1 During operation at power, the demand signal is an output of the plant control system. This demand signal is compared with the measured neutron level, and the reactivity is correspondingly adjusted by programming the control-rod actuators to increase or decrease reactor power.

In reactors that are inherently stable, it is possible, though not necessarily preferable, for an operator to keep the power at the demand level by manually adjusting the control-rod position. When a reactor is not inherently stable, a continuous feedback control system, usually a servo-controlled rod, is essential.

There are four distinct phases of reactor operation the approach to criticality, power increase or decrease, power operation, and shutdown Each phase imposes different requirements on the reactor control system.

7- 2.1 Approach to Criticality

The reactor is manually controlled by an operator during the approach to criticality. The control rods are

moved intermittently to add reactivity until the reactor is critical. At this juncture the reactivity is zero and the rate of neutron production from fission is just equal to the rate of neutron loss The magnitude and rate of rod motion is governed by the need for maintaining the reactor period longer or slower than some predetermined value

The following requirements are imposed on the control system in this phase of reactor operation

l. The control rods must be capable of motion in increments small enough to insert very low values of 5k. Rod drives subject to uneven motion, e g, because of fric­tion, must be avoided

2 The rod actuation system must be capable of measur­ing necessary reactor performance data during the initial start-up. Typical measurements required are total and in­cremental rod worths. (Ganging several control rods which are then driven by one actuator sometimes makes this measurement difficult.)

3. The control system must be able to insert reactivity at a maximum rate consistent with the start-up time requirements.

Nuclide A species of atom characterized b> its mass number (number of neutrons and protons m nucleus), atomic number (number of electrons m the neutral atom), and energy state of the nucleus, provided that the mean life in that state is long enough to be observable

Neution An elementary particle, electricallv neutral, whose mass is approximate!) equal to that of a hydrogen atom which with a half-life of about 1 1 7 min, decays, m the free state, into a proton and an electron

thermal muttons Neutrons essentially in thermal cqui librium with the medium in which they exist

last millions Neutrons of kinetic energy greater than some specified value In reactor physics the value is frequently chosen to be 0 1 MeV

Beta particle An electron, of either positive charge ((?) oi negative charge ((3 ), which has been emitted bv an atomic nucleus or neutron in the process of a transfor­mation

Radioai live dc < ay A spontaneous nuelear transformation m which the nucleus emits particles or gamma radia­tion, or undergoes spontaneous fission, or in which the atom emits x-radiation or Auger electrons following orbital electron capture or internal conversion

Decay constant (or disintegration constant) for a radio­active nuclide (radionuclide), the probability per unit time for the spontaneous radioactive decay of a nucleus It is given by

. 1 dN

N dt

m which N is the number of nuclei of concern existing at time t

( tine The special unit of activity (nuclear disintegration rate) One Curie equals 3.7 XIO10 disintegrations per second, exactly “Curie” is abbreviated as Ci

Half life (lad/oac live half life) for a single radioactive decay process, the time required for the activity (dN/dt or AN) to decrease to half its value by that process I he half-life is related to the dccav constant T^ = (logt 2)/A ~ 0 6931 5/Л

( mss section measure of the probability of a specified interaction between an incident radiation and a target particle or svstem of particles It is the reaction rate per target particle for a specified process divided by the particle-flux densitv of the incident radiation (шию scopic ci oss section) In reactor physics the term is sometimes applied to a specified group of taiget particles, eg, those pci unit volume (mac I osc opic cioss section) or per unit mass, or those in a specified bodv |of< Unless otherwise qualified the term “cross section ’ means “micioscopic cross section ”]

Ifuioscopic cioss section 1 he cross section per unit volume of a given material for a specified process It has the dimension of recipiocal length, for a pure nuclide, it is the product of the microscopic cross section and the number of target nuclei per unit volume, for a mixture of nuclides, it is the sum of such products М/с і osc opn cioss section 1 he cross section per target nucleus, atom, or molecule It has the dimension of area and tnav be visuali/cd as the area normal to the direction of an incident particle which has to be attributed to the target particle to account geometri­cally for the interaction with the incident particle Microscopic cross sections are often expressed in bains, where 1 barn = 10 24 cm2

Panicle Jlu density At a given point in space, the number of particles or photons incident per unit time on a small sphere centered at that point divided by the cross-sectional area of that sphere It is identical with the product of the particle density and the average particle speed The term is commonly called flux (Xe utrou flu density Particle-flux density for neutrons Also commonly called neution flu Often denoted by nv or 0

Particle finance At a given point in space, the number of particles or photons incident during a given time interval on a small sphere centered at that point divided by the cross-sectional area of that sphere. It is identical with the time integral of the particle-flux density Often denoted by nvt

Particle density At a given point in space, the number of particles or photons per unit volume in a small sphere centered at that point

Ionizing radiation. Any electromagnetic or particulate radiation capable of producing ions, directly or indi­rectly, by interaction with matter Indirectly ionizing particles Uncharged particles or pho­tons which can liberate directly ionizing particles or tan initiate a nuclear transformation Directly ionizing particles. Charged particles having suffi­cient kinetic energy to produce ionization by collision.

Exposure. A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electrical charges on all of the ions of one sign produced in air when all electrons liberated by photons in a volume element of air are completely stopped in the air, divided by the mass of the air in the volume element. The special unit of exposure is the roentgen.

Roentgen. The special unit of exposure. One roentgen = 1 R = 2.58 X 104 coulomb per kilogram of air.

Dose. A general term denoting the quantity of radiation or energy absorbed in a specified mass. For special purposes, its meaning should be appropriately stated, e. g., absorbed dose.

Absorbed dose. The energy imparted to matter in a volume element by ionizing radiation divided by the mass of irradiated material in that volume element. The special unit of absorbed dose is the rad. (Absorbed dose is often called dose.)

Rad. The special unit of absorbed dose. One rad equals 100 ergs/gram.

Dose equivalent (radiation protection). The product of absorbed dose, quality factor, dose distribution factor, and other modifying factors necessary to express on a common scale, for all ionizing radiations, the irradiation incurred by exposed persons. The special unit of dose equivalent is the rem.

Rem. The dose equivalent in rems is numerically equal to the absorbed dose in rads multiplied by the quality factor, the distribution factor, and any other necessary modifying factors.

Quality factor (radiation protection). A linear-energy — transfer-dependent factor by which absorbed doses are to be multiplied to obtain the dose equivalent. (Note: The term “RBE” should be used only in the field of radiobiology.)

Linear energy transfer (LET). The average energy locally imparted to a medium by a charged particle of specified energy per unit distance traversed. Noles: (1) The term “locally imparted” may refer either to a maximum distance from the track or to a maximum value of dis­crete energy loss by the particle beyond which losses are no longer considered as local. In either case, the limits chosen should be specified. (2) The concept of LET is different from that of stopping power. The former refers to energy imparted within a limited volume, the latter to loss of energy from the particle regardless of where this energy is absorbed.]

Dose distribution factor (radiation protection). A factor used in computing dose equivalent to account for the nonuniform distribution of internally deposited radio­nuclides.

Maximum permissible dose equivalent (MPD) (radiation protection). The largest dose equivalent received within a specified period which is permitted by a regulatory agency or other authoritative group on the assumption

that receipt of such dose equivalent creates no appre­ciable somatic or genetic injury. Different levels of MPD may be set for different groups within a population. (By popular usage, “maximum permissible dose” is an accepted synonym.)

Kerma (kinetic energy released in material). The ratio of the sum of the initial kinetic energies of all the charged particles liberated by indirectly ionizing particles in a volume element to the mass of the matter in the volume element.

L. J Csider, A. J. Hornfeck, D. Wurster, and R. W. Check

4- 1 INTRODUCTION

The term “process instrumentation” in nuclear power plants refers to all the out-of-core (exposed to <10’ 1 fast neutrons cm 2 sec 1) instrumentation except nuclear- radiation instrumentation.

Emphasis in this chapter is on the techniques used to measure the most important plant operating parameters— temperatures, pressures, flow rates, and coolant properties Many other process instruments are involved in nuclear power plant operation, ranging from those associated with auxiliary chemical processes (waste treatment, coolant purification, etc ) to those involved in maintaining the plant atmosphere (air filtration and conditioning, ventilation, etc )

Certain process instrumentation is so closely associated with specific plant components (e. g, control-rod position sensing) or with a particular reactor type (e. g, moisture detection in gas-cooled reactors) that it is more properly discussed in other chapters Cross references are provided in these cases

No single book (and certainly no single chapter in a book) can cover all significant aspects of even one category of process instrumentation m nuclear power plants

The circuits described in the previous section operate on the average-magnitude-squared (AMS) principle and are

therefore frequency-sensitive Adjustments must be made in the band-pass filter to make the system fit a particular reactor The lower cutoff at 4 kHz, indicated above, corresponds to about 25,000 radians/sec, this is below the

‘Neither system described allows final use as a current chamber for overpower monitoring

0/7 cutoff of a fast reactor (where the roll-off in the transfer function is at about 70,000 radians/sec.)

The principle involved is to use the lower frequency components of the band to assist in pulse overlap Thus there is an incentive to use a lower frequency cutoff (The high-frequency cutoff is selected for noise rejection.) As the low-frequency cutoff is raised in the AMS system, the overlap with the pulse-counting range decreases

In a test run in 1967, the AMS and true MSV systems were compared using the instrument setup shown in Fig. 5.24. The data shown in Fig 5.25 were obtained with a band pass of about 250 to 500 kHz The EBR-2 was put on a 59-sec period, and, because of its wide flux range before reaching power feedback conditions, this is constant over four decades or more. As shown by the data, the MSV system had a correct signal for nearly two decades before the AMS signal became correct. Had a lower band pass been used, this could have been corrected, but the low pass of
the band selected is not important to the true MSV system and bands above 250 kHz are used for other reasons

If on Fig 5.21 the blocks representing the band-pass filter and rectifier were replaced with a Hewlitt—Packard true RMS convertor, the signal would be as shown in Fig 5 25 At present, the use of this converter adds materially to the number of components in the system, and commercial suppliers are hesitant to supply such systems.

Fission gammas and neutrons interact with surrounding material in many ways Three interaction processes are of special interest m reactor power measurement, nuclear reactions, recoils or collisions, and ionization The inter­action of any single gamma photon or neutron may involve more than one of these three basic processes (For details on these interactions, see Refs 3 and 4 )

A nuclear reaction results from sufficiently energetic collisions of a specific radiation with the nuclei of a specific material The consequence of a nuclear reaction is a nuclear excitation or transmutation or the formation of a new

material The reaction may be described symbolically as A (a b) C where A is the target nucleus, the first symbol, a within the parentheses denotes the radiation causing the reaction, the second symbol, b, within the parentheses denotes the effect or secondary radiation, and C is the nucleus that remains after the secondary radiation, b has been emitted An example of this symbolism is the nuclear reaction 1 03Rh(w,7)’ 04Rh, in this reaction 103Rh has been converted to 104Rh by the capture of a neutron, n and the emission of a capture gamma, 7

If the principal interest lies not in 104Rh but in its decay product, the expression may be expanded to 103Rh (я,7)’ 04Rh — Д. 104Pd Here the arrow with j3 superposed indicates radioactive decay by beta particle emission to 104Pd

Usually there are other radiations associated with the nuclear reaction, but the symbolism is restricted to the principal reaction or the reaction of interest Reactions of interest m nuclear instrumentation are neutron induced and result in the emission of fission fragments and of alpha particles, namely, (n, f) and (n, a) reactions

Most fission-neutron interactions with the nuclei of material m and around a nuclear reactor core are capture reactions [(я,7) reactions] and elastic collisions [(и, я) reactions] Some interactions are transmutations of the (n p) type, and some are inelastic collisions {n n ) reac tions] The products of these interactions, 1 e, the neu trons, gammas, and protons, also interact with the nuclei of material in and around the reactor core

Fission gammas and the gammas produced in neutron capture or -scattering reactions interact with the electrons in surrounding material, usually creating energetic electrons (Compton or photo effect)

Energetic particulate radiations (protons or nuclei recoiling after a nuclear reaction), if not stopped in nuclear reactions or nuclear collisions, ultimately lose their energy by ionizing atoms Ionization results in a “cloud” of free electrons and positive 10ns The cloud or track of ionization is sharply defined by the trajectory of the initiating particle and usually has a definite length, or range, that depends on the particle energy and the density of the medium A heavy, highly charged particle, such as a fission fragment has a short, densely ionized range An electron, or beta, has a longer range with less dense ionization

The neutral radiations (neutrons and gamma rays) associated with fission travel much farther than the charged particles and have poorly defined ranges They can penetrate thick layers of matter Neutrons usually terminate in some nuclear reaction, gamma rays produce secondary electrons, which, in turn, are stopped by ionizing other atoms The difference in the penetrating power of neutron and gamma rays makes possible a partially selective detection process in monitoring nuclear reactors

All the interactions of the radiations accompanying fission produce heat Thus the heat generated is a direct measure of the power of a nuclear reactor (It is also a nuisance in instruments used to detect radiation ) Unfortunately, heat is a very slow indicator (because of thermal inertia) In the steady state, the reactor heat or thermal power can be accurately measured and used in calibrating the nuclear instrumentation

(a) Area Meters. In an area flowmeter the fluid flows upward, displacing an obstructing float or piston. The float or piston is arranged so that the unobstructed area increases with upward displacement, the float or piston then moves until the area is open enough to permit the flow to pass. The basic theory of area flowmeters is the same as that of a differential-pressure meter, but, since differential pressure is held constant, or reasonably so, square-root extraction is not required A measurement of float or piston position is a
measurement of unobstructed area, and thus of flow. Area meters are of two general types rotameters and piston-type meters.

A rotameter (Fig 4 38) consists of a float inside a tapered tube, the small end of the tube being at the bottom The force exerted in the tube by the flow moves the float upward until the area of the annular space between float and tube is sufficient to permit a flow — created differential pressure to balance the weight (less the buoyancy force) of the float. Differential pressure is determined by the weight of the float and its cross-sectional area. A scale is marked on the outside of the transparent, tapered tube so that float position can be read directly. Since the unobstructed area permitting flow is almost exactly proportional to float rise (for a slightly tapered tube), the scale markings indicating flow can be uniform. For high-pressure applications or for transmitters, the tube can be metal, and the float position can be sensed by a magnetic pickup.

In a piston-type area meter, upward motion of the piston or plug uncovers ports in the sleeve or cage, increasing the area of the opening in direct proportion to plug movement and to flow rate. A spring pulls downward on the plug to increase the differential pressure beyond that which might be obtained by the weight of the parts. The vertical position of the plug establishes the rotational position of a spindle, which extends through a packing gland to operate an indicating pointer and transmitting mechanism (either pneumatic or electric).

(b) Positive-Displacement Meters. The flowing fluid is divided into separate discrete volumetric portions that are counted by a mechanical register built into the meter Alternately, the rotation of the meter mechanism may be made to generate an electrical signal with frequency proportional to the rotational speed, the signal can then be transmitted to a remote register or recorder.

(c) Velocity Meters. A turbine meter is a line-mounted meter with a rotor having helical blades Rotation generates a series of electrical pulses, which are sensed by an externally mounted electrical pickup. The receiver may be arranged to display total flow by counting the pulses by digital techniques or to display rate of flow by measuring the pulse frequency. These meters are accurate through their recommended range from maximum flow rate to about 10% maximum. At lower flow rates friction tends to cause the meter to read low. Since the bearings are exposed to and lubricated by the flowing fluid, maintenance involves removal of the meter from the line for inspection or replacement of bearings. The turbine meter is sensitive to changes in fluid viscosity and is usually individually flow-calibrated. It was developed for and has gamed wide acceptance in the Aerospace industry.

A magnetic meter is an electrically insulated section of pipe with an imposed magnetic field, perpendicular to the pipe axis, through which a conductive fluid develops an electrical potential (perpendicular to magnetic field and
pipe axis) directly proportional to its average velocity through the pipe section Electrodes flush with the pipe wall are connected to a circuit for measuring the generated voltage The magnetic meter is accurate and linear through a wide range of flow rates and is available in a wide range of sizes Because it has no internal parts to trap sediment, it is widely used for slurries and dirty fluids, it can be recommended, however, for any flow of an electrically conductive fluid where minimizing the pressure drop is important The pressure drop is no greater than that of a straight pipe of the same length. This type flowmeter is used in sodium-cooled reactors (see Vol 2, Chap 17)

4- 4.3 Liquid-Metal Flowmeters

In power reactors that use liquid-sodium coolant, flow is usually measured with magnetic meters, as noted in the preceding section. Differential-pressure devices, however, have also been used to measure liquid-sodium flow rates. The principles of operation and the methods for correcting for thermal effects, wall effects, etc, in magnetic flow­meters are described in Vol. 2, Chap 17.

In reactor dynamics experiments the variable directly excited affects reactivity either directly, as in the motion of a control rod, or indirectly, as in the alteration of some system parameter (flow, pressure, etc.) Some ways in which reactivity can be varied are

1. By a specially installed rotary or reciprocating control rod.

2. By moving the normal control rod of the reactor through special switching or signal injection into the automatic control system.

3. By changing valve position, pump output, etc, usually by signal injection into its control system.

It is not surprising that 2 and 3 are more commonly encountered than 1, except perhaps in special-purpose experimental reactors, since they require relatively little modification of the existing system.

Among the specially installed rod oscillators, there is some preference for the rotary type over the reciprocating type, usually because of the higher frequencies attainable. Rotary types use the following methods to vary reactivity (see Fig 6.11) eccentric neutron absorbers rotating in a flux gradient,19,26 eccentric fuel rotating m a flux gradient,1 1 or neutron absorbers rotating past similar stationary absorbers which act as time varying shields 1 Usually neutron absorbers are oscillated with typical reactivity amplitudes being m the 0 5ф to 5$ range In zero-power fast reactors, fuel can be oscillated, however, this is unusual.

Regardless of the device used to perturb the reactor, there are a number of aspects to be considered, especially when high precision is important

1. Backlash and related effects causing phase un­certainty.

2. Random (long-term or short-term) variations in excitation frequency

3. Transfer function of the control device if only its input (rather than its output) is measured.

4. Unwanted harmonics when striving for a pure sinusoidal input

5. Reactor conditions affecting the excitation device.

In the last of these considerations, it should be remarked that more than just the integrity of materials is desired For example, the reactivity worth of an oscillator rod can depend on reactor power and flux distribution, for this

ECCENTRIC SOLID

ECCENTRIC SHEET

VARIABLE SELF SHIELDING (THE INNER RING HAVING VARYING ABSORBER AREAS)

ECCENTRIC VARIABLE SELF-SHIELDED SHEET

Fig. 6.11—Top view of some typical cylindrical rotary oscillator rods.

reason it is often desirable to have a sufficiently high frequency available to calibrate the oscillator rod against the zero-power transfer function in a frequency range where feedback is expected to be negligible.

Dry thimbles provide a common and convenient method of installing detectors in coolants The cost of installing dry thimbles is quite high, however, and, in addition, some installations may require cooling or cir­culating air to ensure dryness Then too, there is always some concern over possible leakage, particularly if the thimble is a penetration of a reactor

These difficulties tan be reduced by using detectors suitable for immersion m the particular medium involved Detectors with integral cable have been developed for immersion in water and hot gas, and a detector for immersion in liquid sodium is being developed

Direct immersion, while generally advantageous, has some disadvantages There must still be some sort of channel to restrict the path of motion of the detector Also, the difficulty of making adjustments in position is in creased Nevertheless, detectors for direct immersion are finding increased favor

5- 2.1 Introduction

A typical start-up channel, shown in block form in Fig 5 2, consists of the following major components (1) sensor, (2) pulse amplifier, (3) high-voltage power sup­ply, (4) amplifier—discriminator unit, (5) count-rate meter with control functions, and (6) readout equipment.

4- 2.2 Sensor

Because the neutron flux in a shutdown reactor is low, the output of the neutron detector (see Chap 2) is a series of pulses proportional to the neutron flux resulting from a neutron source in the reactor The detector must have a high neutron sensitivity with a very low sensitivity to gammas

The fission chamber is widely used because of its inherent ability to discriminate against gamma-generated signals A fission chamber 2 in. in diameter and 12 in long typically will have a neutron sensitivity of 0 7 (count/sec)/ (neutron cm 2 sec 1) with a gamma sensitivity of 4 X 1014 amp/(R/hr) The neutron signal generated m the fission — chamber circuit is of the order of 100 qV with a pulse width of approximately 0 5 qsec The gamma pulses produced in the chamber are smaller in amplitude A typical ratio of gamma to fission pulse height is of the order of 10 2 to 1 This ratio makes successful discrimination possible

Gamma pulses generated m the chamber can become a problem when the detector is located m a gamma field of 5 X 104 R/hr or more A pulse pileup in the discriminator circuit causes gamma pulses to be counted as neutrons Special techniques, such as pulse clipping or reshaping of the pulses, are used to minimize and protect against this Typical pulse shapes are shown in Fig 5 3

5- 2.3 Pulse Preamplifier

(a) Introduction. The pulse generated in the sensor must be amplified for transmission and for driving standard counting electronics The ideal pulse preamplifier must have high gain, wide bandwidth, stability, and low noise charac­teristics

Until recently fission sensor preamplifiers used vacuum tubes and were located as close to the sensor as possible In the last few years, good-quality solid-state preamplifiers have replaced the vacuum-tube units There are advantages and disadvantages with both types

A vacuum-tube preamplifier can be placed near the fission detector, і e, in the same radiation field as the sensor However, radiation damage to the preamplifier components decreases their life and increases spurious noise so that maintenance is required Vacuum-tube preamplifiers produce a large output pulse, ideal for counting equipment, but the vacuum tubes deteriorate with steady use and must be replaced regularly or suffer loss of gain. Generally, substantial maintenance is required per hour of successful operation

Solid-state preamplifiers, usually charge-sensitive, can be located at considerable distance (up to 90 ft) from the sensor but, in any event, must be out of the radiation field. This makes maintenance more convenient and increases channel availability However, this arrangement is suscepti­ble to noise pickup in the cable between the sensor and the preamplifier The output pulse from solid-state preampli­fiers, typically 10V, is generally smaller than that from a

vacuum-tube preamplifier Because of the lower signal output, the counting equipment must be capable of accepting a low-level signal and processing it for use in the readout and control equipment that follows it in the channel

(b) Vacuum-Tube Preamplifier. A vacuum-tube pre­amplifier is shown in Fig. 5.4. The amplification is provided by the four RCA 7586 (nuvistor) vacuum tubes The preamplifier has a gain of 30, a rise time of 5 X 10 8 sec, and a fall time of 2 X 10 7 sec Because vacuum tubes are used in this preamplifier, it can operate m a radiation field. The tube-filament heaters are d-c powered to minimize a-c noise pickup The capacitors are ceramic, both for im­proved temperature stability and for reduced susceptibility to radiation damage Capacitor Cl is used to isolate the detector high voltage from the preamplifier and to couple the sensor pulse to the preamplifier input circuit. The life expectancy of this preamplifier is approximately 600 hr in a 106 R/hr gamma flux (0 5- to 1 5-MeV gammas)

(c) Solid-State Preamplifier (Current Input). The

solid-state preamplifier shown in Fig 5 5 is a voltage amplifier, as opposed to the charge-sensitive amplifier to be discussed in the next subsection The active elements are transistors The input stage is a grounded-base transistor, which has low input impedance, high output impedance, wide bandwidth (high frequency), high bias stability, and good voltage gain.

The interconnecting coaxial cable between the sensor and the preamplifier is terminated into its characteristic impedance. A resistor in series with the emitter is selected to do this matching. This circuit eliminates cable ringing or reflections and provides a low impedance path for the current pulse generated in the sensor The pulse is capacitance-coupled to the input stage, and the capacitor also blocks the sensor d-c voltage The input current pulse from the detector is converted in transistor Q1 to a voltage pulse The remainder of the preamplifier is a high-gain standard operational amplifier with feedback. The pre­amplifier can amplify pulses at a repetition rate of 106 Hz or 106 neutrons/sec Typical preamplifier characteristics are

0 5 volt//iA 50 to 120 ohms ±3 volts into 50 ohms <5% for 200-nsec pulse width ±15 volts at 40 mA

As noted above preamplifiers mounted at a distance (20 to 40 ft) have noise problems associated with cable pickup Every effort must be made to shield against stray noise in the form of electrostatic or electromagnetic voltages be­tween the sensor and the preamplifier

In summary, the principal advantages of locating the preamplifier at a distance from the sensor are (1) sensor cooling is not so critical, (2) maintenance is simplified, and

(3) system availability is increased.

(d) Solid-State Preamplifier (Charge-Sensitive). A third type of preamplifier is a charge-sensitive unit (Fig. 5.6). This preamplifier is a fast-rise-time charge-sensitive pre­amplifier with dual-polarity output The input signal is coupled to the pulse-shaping and amplifier input module A1 by capacitor C4, which blocks the high voltage. A Shockley diode connected to ground at the input to amplifier A1 protects the input from momentary break­down of the detector or cable shorts. The output of A1 feeds a cable driver with dual-polarity output

Amplifier module A1 is a special fast-pulse amplifier connected in a charge-sensitive configuration. This is accomplished by connecting a stable small-value capacitor from the amplifier output back to its inverting input. This negative feedback will attempt to keep the amplifier input very near zero volts, thus making it a virtual ground.

Incoming charge is collected on the input plate side of the feedback capacitor, Cf This will cause some voltage shift at the input of A1 and, as a result of its inverting gain, a much larger inverted shift at the output Negative feedback action through Cf will restore the input to its normal zero-volt level. The magnitude of the output voltage shift is directly proportional to the amount of charge received, V = Q/Cf

It is very important that the amplifier have a very short rise time with respect to the incoming pulses, otherwise the virtual ground cannot be maintained, and charge may be diverted to ground through shunt protective devices or other circuit elements.

When only the feedback capacitor, Cf, is used, the circuit has become a charge-to-voltage converter. However, this configuration is limited by the ultimate saturation of its output as more and more charge is accumulated. For this reason the negative feedback resistor, Rf, has been added to discharge Cf between incoming input pulses The parallel combination of these two components, Cf and Rf, forms the clipping-time constant of the preamplifier, T = Rf X Cf This clipping time also serves to provide the best pulse shape required by subsequent circuits

The output of A1 is fed through C5 to Ql, which provides two outputs of opposite polarity by means of the output driver amplifiers. These driver amplifiers consist of parallel-connected Q2—Q3 and Q4—Q5 transistors that function as emitter followers to provide low output impedance suitable for driving any reasonable length of terminal coaxial cable

The power-increase phase is normally started by estab­lishing coolant flow, adjusting power and flow to equal the demanded values, and closing the automatic control loops As the coolant flow is then increased, the power level is also increased, and the reactor temperature rises to the operat­ing value. The reactor control system responds to the plant control demand by causing appropriate motion of the control rods. The coolant flow and power level may be
increased simultaneously or separately, with the flow reach­ing full value before power.

The primary requirements on the control system during this phase are

1 The control rods must smoothly adjust the reactivity in accordance with the plant controller demands. The amount of rod motion depends on the reactivity change associated with the incremental rod worth and the reactor temperature and pressure conditions. Particular attention must be given to the transient conditions during changes of reactor power and flow, since excessive overshoots in temperature or pressure might cause intolerable damage to the reactor core.

2. Since full-power conditions are being approached, the relative control-rod position becomes important to ensure uniform power distribution throughout the core.

6- 2.3 Power Operation Phase

During this phase the reactor must respond to the plant control-system demands to deliver the desired power, main­tain the reactor operating conditions, and remain within predetermined reactor parameter limits. The required dy­namic characteristics of the control system are different for each reactor type (PWR, BWR, and gas-cooled or liquid- metal-cooled reactors).

Some of the important factors that must be considered in the design of the control system for this phase of reactor operation are

1 Since the reactor in this phase is at or near full power, the control system must respond to the plant
power system demands rapidly enough to meet plant re­quirements and yet maintain the core temperatures within prescribed limits (such as the hot-spot temperature limit) To derive the maximum power, the designer faces a trade off between the desirability of operating close to the maximum temperature limits of the reactor and the as sociated requirements of more accurate temperature mea­surement and dynamic response of the control system

2 In addition to the requirement that the reactor rod-actuation system be accurately positioned m response to a demand, the banks of rods must also be positioned accurately relative to each other Inaccuracies in relative positioning of control rods causes local power increases in the region adjacent to those rods that are farthest with drawn Positioning inaccuracies can result in a smaller margin between the limiting temperatures and the normal operating conditions of the reactor core

3 Increments of control rod motion must be of such magnitude that thermal transients do not increase the temperatures or temperature gradients to undesirable values For example, the reactivity insertion steps resulting from unit rod motion in stepping-motor systems must be kept within allowable limits Excessive friction and control deadbands must also be considered in designing the overall control system

4 Some of the fission products produced during power operation absorb neutrons and necessitate the addition of reactivity by control-rod movement The most important of these are 13sXe and 149Sm (see Sec 1 3, Chap 1) The reactivity, 5k/k, to overcome the fission-product poisoning under equilibrium operating conditions is a function of reactor design but normally varies from 0 3 to 3 0% The control rods must be capable of compensating for the buildup of these neutron absorbers About 10 hr after shutdown, the 1 5 Xe poisoning increases to a value higher than the equilibrium value at full power The poisoning peaks at about 10 hr after shutdown and then diminishes This leads to the requirement that the control rods be capable of introducing sufficient neutron absorber at a rate that will maintain the reactor in a shutdown condition as the 1 35Xe poisoning is reduced

5 The reactivity must be compensated for the reduc tion of fissionable material attributable to burnup during power operation This compensation is normally provided by the control system, which automatically positions the control rods to maintain full power The control-rod worth designed into a reactor is a function of the amount of fuel depletion anticipated during the interval between reload ings In some reactors a burnable poison, such as 10B, is introduced into the core As neutrons are absorbed by the boron, the number of remaining boron atoms available for neutron absorption decreases The amount of burnable poison introduced into a reactor is governed by the desire to have this effect compensate for fuel depletion In this manner the total reactivity control requirement of the rod system is reduced

6- 2.4 Shutdown Phase

The shutdown phase of operation is usually accom plished by a controlled insertion of the control rods in response to a reduction in power demand A second type of shutdown is a scram[17] in which the reactivity and reactor power must be reduced in a short time to prevent exceeding the reactor or plant limits The control system must be designed such that sufficient negative reactivity is available to shut down the reactor under all conditions

To simplify the control rod drive system for normal power operation, the rate of control-rod motion is normally the same in either direction However, the rate of change of reactivity can still be varied significantly by using only one or a few rods at a time when increasing the reactivity and using all rods simultaneously when decreasing the reac­tivity