Type of neutron detector Proportional counter Type of signal cable Coaxial

Location of pulse preamplifier Source range detector thimble junction box Location of pulse amplifier Nuclear instrument cabinet Distance between pulse amplifier and preamplifier 1210 ft Distance between neutron detector and preamplifier 19 ft

Method of grounding A single point grounding system is used The grounding point is at the pulse-amplifier chassis. The pulse-amplifier chassis is grounded to the station grounding grid by bus bar and cable

Operating problems and modifications During preoperational checkout of the system, it was discovered that the majority of the coaxial connectors in the nuclear instrument system were assembled with poor workmanship The main problems were the preparation of the cable, the soldering of the pins, and the assembly of the connectors Many hours of rework were required to correct the problems When the system was put in operation, the source-range channels were very noisy, a check of the system revealed that induced voltage was causing ripple on the + 15-volt power supply to the preamplifier The leads carrying the + 15-volt power supply were changed from nonshielded to shielded conductors, which improved the condition but did not eliminate ripple sufficiently to not affect the operation of the source-range channels A RL filter was designed and installed at the preamplifier in the+15-volt supply This filter removed the ripple and corrected the noise problem

11- 3.1 Introduction

(a) Importance of Design Phase. Reliable instrumenta­tion systems can only be created during the design phase. No amount of quality control, field testing, or maintenance can adequately compensate for a lack of careful planning during the conceptual design. A few designers seem to have an intuitive sense of good design, and their products enjoy a reputation for reliability Most designers can acquire the art of producing reliable designs by adhering to some of the disciplines of reliability engineering

A designer’s most frequent failing is to be trapped into thinking only about how to design a system that will work and giving no thought as to how it might fail. Everyone is amused by the famous Rube Goldberg designs (Fig. 11 1) It is readily apparent how Rube’s systems work and equally apparent how prone they are to fail. Modern reactor instrumentation systems have become so sophisticated that the designer, in concentrating his efforts on making the system work, forgets to look for ways in which it may fail By applying the techniques of reliability analysis, a good designer should consistently turn out a product that not only works but also has a low incidence of failure

The attitude of the designer is all important He must have the desire to create a reliable system. He must not be lulled into the attitude that the design is adequate because the system passed a design audit or was granted an operating license. A trained reliability engineer can probe deeply and learn much about a system, but, unless he has full cooperation from the designer, the effort will fall short of complete success

System reliability and system capability must not be confused. For example, the nameplate rating on a power supply tells the designer about the load capacity it may be expected to accommodate. Choice of a power supply with a rating in excess of the load requirements assures capability. A reliability study assumes system capability and goes on to make an assessment of the probability that the system will actually be successful in performing a given task within its capability The two terms, capability and reliability, are related through the term “design margin,” and the designer should recognize the favorable influence excess capability may have on reliability through the application of appropri­ate derating factors.

Effort invested in systems reliability analysis is eco­nomically attractive The potential for cost saving lies in systematically selecting the higher reliability systems for detailed design, in choosing the simpler of two alternatives, in avoiding overdesign on portions of the system that do not contribute to reliability, in avoiding costly retrofits, and in gaining a reputation for a reliable product

(b) Reliability and Availability. The term “reliability” is frequently used in a qualitative sense to imply quality

and integrity As pointed out m Sec 11 1 of this chapter, reliability is a measure of the time stability of a product’s performance This concept can be expressed in a quantita tive sense in the definition of reliability approved by the IEFL 4 “ 1 he characteristic of an item expressed by the probability that it will perform a required function under stated conditions for a stated period of time ” In the general sense, this definition of reliability does not allow for failure and repair during the stated period of time For example, consider a pressure switch monitoring the reactor pressure located where it is totally inaccessible during reactor operation An assessment of the numerical reliabil­ity of the switch to survive one fuel cycle is a meaningful measure of its integrity

In many other cases, inspection, test, and repair during operation are permitted, and this is, in fact, the preferred mode of operation In this event, the most meaningful measure of integrity is called “availability” and is defined as4 “The characteristic of an item expressed by the probability that it will be operational at a randomly selected future instant in time ” Stated another way, availability is the fraction of the time the system is operational In the long-term steady-state situation, avail­ability is given by the equation

Up time

Up time + Down time where the up time is approximately equal to the average time between failures and the down time is the average time to repair and restore the system to service Since the down time is the average time to repair the system, it must include the elapsed time between the failure and its discovery, a term that may be of primary significance especially in standby systems

Availability, not reliability, is the term that is most applicable to the usual reactor instrumentation system, where redundancy, repair during operation, and a para­mount concern for detecting and eliminating unsafe failures are prominent characteristics

The control, display, and recording functions of any system constitute the major portion of the instrumentation Man is linked to the machine by the displays and controls, and he uses the recorded data to analyze and improve operation.

10-6.1 Control—Display Relation

Proper relation of the control function to the display is the basic purpose of the system. The function of the display system is to provide information to enable the operations personnel to make decisions regarding plant and system operation. The grouping of control—display com­ponents should always reflect the use of human factors engineering The maximum efficiency with which data can be perceived and the control action initiated indicates the effectiveness of the control—display design

10-6.2 Installation of Hand Controls

The installation of hand controls should utilize human — factors engineering Considerable thought must be given to aspects such as type and placement of controls as related to function Likewise, operator qualifications and limitations should be considered. All controls should be mounted to withstand the rigors of normal use and possible abuse Controls affecting plant or system safety should be pro­tected from accidental actuation Controls that are subject to extreme environments should be protected, including the marking and identification of the control.

10-6.3 Installation of Specific Controls

Nuclear reactor instrumentation systems use some specific controls that are unique, not necessarily in the components used, but rather with application and interac­tions of the controls When components are being selected, the function of components being controlled as well as the function of the controlling component should be consid­ered. Such components as push buttons, selector switches, and level switches have specific applications Factors such as speed of response and resolution of control should also be considered in selecting specific components. Control for such components as control and safety rods should be selected and installed in the prime control areas of the main control console, with primary and secondary loop and auxiliary controls being installed according to frequency of use and importance to the system.

10-6.4 Installation of Visual Display and Recording Equipment

The location of all equipment that must be visually monitored should be given priority consideration. The physical abilities and limitations, as well as the psychologi­cal characteristics, of the operating personnel should be taken into account Viewing distance and angle as well as illumination must be considered, along with character size, configuration, and background, to avoid eye strain, inaccu­rate perception, and glare Audible display should be considered in areas where there is a possibility of failure to give the necessary visual cognizance for critical and semi — cntical parameters

Compatibility with related controls and possible percep­tual interaction with other displayed information are important factors in the installation of effective displays Recording equipment also used for displaying information should adhere to the criteria for any display equipment. In addition, recording equipment must be installed so that routine operational maintenance, such as inking and chart — paper replacement, can be accomplished with maximum efficiency

10-6.5 Installation Recommendations

The installation of an instrumentation system involves the use of a combination of practices to achieve the desired results, both from a functional and an aesthetic standpoint (see Fig 10.25) Instruments and components should be grouped and mounted so that they are aesthetically neat and orderly as well as logically functional. Instruments with varying front panel dimensions should be mounted so that the instrument case tops are level Switches and pilot lights should be mounted with the center lines even Instruments with hinged doors should be mounted so that there is no interference between adjacent instruments. In general,

controls, displays, and recorders should be placed in the prime functional areas of the panels in which they are mounted. Components that require visual readout should be as close as possible to eye level. All control actuation components should be kept within easy reach (at least 18 in. above floor level).

If, for economic reasons, the reactor instrumentation and control system is designed for use on a single d-c source, then the power-source system is simple If a single d-c power source is not satisfactory, the choices of power-source configurations increase greatly. Factors to be considered include alternating source voltage and fre­quency stability, harmonic distortion, plant auxiliary-power outage time and allowable transfer time, maximum allow­able rate of change of voltage and frequency, and load power factor. A specification that is overly restrictive, just to be safe, results in an unnecessarily high cost for the power supply and should be avoided.

Momentary loss and surges and dips of voltage are relatively frequent in plant auxiliary-power systems Voltage dips and surges are caused by switching, failure of equipment remote from the critical bus, and starting large motors on the same power system The duration and intensity of the undesirable transient depend on its proxim­ity and the clearing time of the protective equipment ahead of the faulted section. A single cycle can be severely distorted without a power interruption. (Such distortion might occur when heavy loads are placed or pulsed on a radial feeder remote from the critical bus ) Wave-shape distortion can also result from momentary faults that, m fact, become equivalent phase shifts Large banks of capacitors and intermittent reactive loads can give rise to wave-shape distortion

The most widely accepted method of avoiding or minimizing these effects is to insert a noninterruptible power source to act as a filter between the plant auxiliary — power supply and the critical instrumentation load In addition, the system cable routing and installing must be designed to segregate the redundant systems and methods so that the clean output from the noninterruptible power source will not be contaminated by induced noise from cables in other noncntical systems The imposed static and dynamic seismic loading postulated for the specific area within the nuclear plant in which the noninterruptible power supply is to be located must be taken into account The seismic criteria should be incorporated within the equipment specification. In addition, proper seismic design of equipment foundations and anchors is essential, electric cables and conduit between all critical items of equipment must be flexible.

The reliability of a noninterruptible power system is only as good as the weakest part of the total installed system. This point should be emphasized not only during the design and installation phases but also throughout the life of the system. A continuous and conscientious mainte­nance and testing program is essential

7- 4.1 PWR Power Plant at Shippingport

The Shippingport reactor is a pressurized-water reactor with 32 vertical hafnium control rods, both manually and automatically controlled, to adjust the power level (Figs. 7.9 and 7.10). Since the reactor coolant system is completely sealed, the control rod and the driving element must be within the pressure barrier of the coolant system. For this application the “canned” motor shown in Fig. 7.6 is used. The rod-drive mechanism is a roller nut attached directly to the motor rotor; it operates on the lead-screw portion of a control-rod extension as shown in Fig. 7.11. Since the coupling between the rotor and roller nut is direct, i. e., no reduction gearing is used, a special very slow (22 rpm) motor is required.

The motor torque is applied by magnetic coupling through the pressure barrier (the thin “can” between the rotor and stator). For the control-rod lead screw to disengage quickly from the roller-nut mechanism, the nut is split into halves that are held together by a magnetic flux generated in the stator winding. Thus cutting the power to the motor also scrams the rod. The split roller nut must be kept from reengaging until the rod has fully inserted to prevent damage to the mechanism.

The motor winding is designed for a three-phase power supply to produce a rotating field similar to that of the ordinary 60-Hz induction motor, except that it operates on alternating current with a frequency variable from zero (direct current) to a few hertz. The d-c power (zero frequency) is required, as pointed out earlier, to keep the nut engaged to the lead screw and to maintain the rod at the selected position. Even though the frequency can be reduced to zero to stop rod motion, the voltage must be kept applied to maintain the latch of the roller nut to the rod. This implies the requirement that the alternating current be changed to direct current without any change in value. Another requirement is for phase sequence to reverse the direction of rod motion.

A low-frequency d-c to a-c converter is used to meet these unusual power supply requirements (see Fig. 7.12). Basically, this converter is a circuit of series-connected resistors formed into a closed ring and mounted on an insulated commutator disk. Each junction point of the resistors is connected to one of the commutator segments arranged in a ring. Two diametrically opposite points on the ring of resistors are permanently connected to a d-c power supply. A rotating brush structure, with three insulated brush segments 120° apart, picks off a three-phase voltage from the commutator segments. Shorting-type brushes are used to prevent circuit interruption as the brushes move from one segment to the next. The a-c voltage is taken from the brushes through slip rings on the rotating brush structure. The frequency of the rod-motor voltage is determined by the speed of a small d-c motor that drives the brush structure. The peak a-c voltage is determined by the value of the d-c voltage applied to the series-resistor rings. Stopping the brush rotation automatically results in applying a d-c voltage to the motor stator fields so that the motor is held stationary.

In the Shippingport reactor two sets of brushes are incorporated into each resistor-commutator assembly. Each set of brushes supplies power to two rod-drive motors in
parallel. With this arrangement four control rods are moved simultaneously whenever a commutator assembly is ro­tated. If four rods from different portions of the core are selected to operate off each commutator assembly, the core power and the desired symmetrical distribution of neutron flux can be more easily maintained at the various power levels. For the 32 rods 8 inverters are used; 2 spare inverters are available.

In addition to four rods being controlled by one inverter, the rod programming plan for this reactor divides the rods into two or more groups, each having subgroups of four rods. Group 1 is necessary to bring the reactor from shutdown to initial criticality. Sixteen rods were selected for this group. They are moved individually in multiples of four in sequence such that any subgroup is never more than 3 in. beyond the position of the remainder of the group. This procedure, together with a maximum limit on rod speed, meets the criteria for the maximum rate of insertion of reactivity. The remaining 16 rods are programmed in groups of 8, 4, and 4. The rod-programming equipment is designed to place 16, 20, 24, 28, or 32 rods in the first group, leaving any remaining rods grouped in multiples of

4.

Versatile combinations of rod speed and grouping are necessary to provide for a rapid and uniform burnout of 13SXe after a scram or a power reduction. Although a scram causes all rods to drop, there are some situations demanding a fast or “safety” insertion in which power must be rapidly reduced a relatively small amount without a full-scale scram. (The latter, incidentally, also demands a correlative reduction or shutdown of the power-plant output.)

Each rod position is displayed on the control console by an individual column of indicator lights. Each light is connected to a small transformer on the control-rod — extension housing. As the magnetic material of the control — rod extension passes through each transformer, the coupling between the windings increases, and the lamp is lit.

Automatic control of reactor power is provided by a power and temperature control system. The reactor is inherently self-regulating through a negative temperature coefficient of reactivity that compensates for reactor coolant-temperature variations caused by variations in steam load at the turbine. The accumulation of fission — product poisons makes gradual adjustment necessary. As in

most reactors, rod movements must be fairly frequent immediately after start-up or a load change.

9- 3.1 Instrument-Rack Structure

The instrument-rack structure is defined, for the pur poses of this chapter, as a structure in which the compo­nents of an instrument system are mounted This structure mav be a completely enclosed cabinet with the components mounted inside the enclosure or a panel with the compo­nents mounted in a cutout in the face of the panel It may be a rack used to mount and support the equipment but which does not function as an enclosure for the compo­nents

(a) Structural Materials 1 he structural materials used in an instrument-rack structure must have sufficient strength to support all equipment mounted in the struc ture Steel and aluminum, the materials most used for panel structures, are easily worked and readily available The thickness of the structural material used for the front panel depends on the type and weight of the instruments being installed, the structural material used, and the panel design Stiffening members may be required, however, they must not limit access to instruments and terminal points at the rear of the panels

Since instrument systems are generally assembled at a vendor’s plant rather than at the location where the system is to be used, the instrument rack structure must have sufficient strength to allow the handling of the structure with all the equipment mounted Lifting eyes should be provided to permit moving completed rack assemblies

(b) Standard Modular Enclosures Modular enclosures are made by several manufacturers to meet the require­ments of both stationary and mobile instrumentation systems They are designed to conform to an industry

standard, such as the Electronic Industries Association[20] (EIA) Mounting Standards, thus eliminating by the stan­dardization of parts the need for many special mounting devices. Because of the modular design, a series arrange­ment of almost any configuration can be developed. Manufacturers of modular enclosures also make many special features and accessories for the enclosures, such as radio-frequency interference (RFI) shielding, equipment­cooling blowers, and special enclosure trims.

Modular enclosures allow flexibility in panel configura­tion and equipment layout (see Fig. 10.3).

(c) Mounting Practices. Instrument cabinets are gener­ally designed to be free-standing structures which may or may not be fully enclosed. Instrument cabinets that are stationary are mounted on bases or curbs. The bases or curbs are either steel or concrete and are designed so that the instrument structure with all the equipment installed
can be placed on them and attached and held in place by bolts or clamps.

A general practice is to provide each rack group with a fabricated, 3- to 5-in. channel iron base. This allows for easy mounting to curbs with bolts and clamps sufficiently strong to withstand nominal seismic forces.

It is unlikely that complete plant automation will come about in big jumps It is interesting to contemplate and discuss an imagined power station with everything from control rods to switchgear under computer control, but that stage will be reached in small steps over a long period of time, just as analog control technology has developed

Lacking a charter to automate the whole plant, the engineer must select the type of control (analog, hybrid, or digital) to be applied to the constituent processes and the degree and scope of automation appropriate to those under the command of the computer Every facility has one or more process elements for which conventional control devices are inadequate, these are the prime targets for computer control 14 Next come those elements for which computer control promises a substantial cost savings either in capital investment or in greater operating efficiency Finally, there are those functions where the advantages of either digital or analog control are comparable The scope of computer applications will depend heavily on the strength and completeness of the control engineer’s system analysis Examples of the results of this stage of planning can be found in recent and current reactor plant designs, several of which are described in Sec 8-7

Up to now this section has dealt with computer control, but from here on our discussion of control equipment and software must include their other functions data acquisi tion and display. All new reactor plants will have a computer-based data-handling and data-display system, the control functions can be considered as an extended use of equipment already planned A small percentage in hardware cost will buy computer-control capability However, adding control programs raises the level of complexity of the system software in proportion to the number of functions provided

8- 5.3 Program Description

The programs discussed in this section are those necessary to perform data handling, control, and routine self checking They are called process programs and are recognized by their being always in the computer system and routinely or potentially operative when the computer is on line as a part of the operating plant A second kind, called systems programs, are those used to write and debug the process programs These include the programmers console routines, loaders, assemblers, compilers, and editors They are discussed in the section on equipment specifications because they are always part of the computer procurement

For an analog control system, the engineer can develop a set of functional specifications, most of which can be translated into equipment requirements based on a one-to — one correspondence among the set of input-data points, control-system channels, and control-signal outputs This simple relation between function and hardware is shown in Fig 8 2 However, there is no such correspondence in a computer-based system As seen m Fig 8 4, only the process interfaces can be sized according to the system functions, the rest of the hardware must be specified on the basis of the computer programs needed to perform the data acquisition and display and control tasks

So the computer-system designer starts by compiling the usual functional specification, setting forth what the system must do, and the process and operator inputs that will be needed to implement them The next step is to develop a detailed description of the computer programs required to make the hardware function as specified Th з task is the most demanding part of the design procedure If it is not done well, an adequate system procurement specification cannot be written

The program description can be developed in three stages for convenience in planning and carrying out the separate activities

1 The process software is divided into major programs The level of the division is dictated by the need for each program to have a distinct and identifiable function The number of programs on the list should be such as to make it easy to assign them to different engineers on the pro­gramming staff Too few program elements will overload the programmers, too many will make it difficult to combine them into a working whole

2 The memory required for each program is estimated An accurate estimate requires a detailed knowledge of process requirements in terms of precision and response times, the programs that provide the control functions, and how they are to be accomplished by computer-program
instructions The engineer may have to write trial routines in a typical process assembly language

3. Each program is labeled as to whether it will normally reside in core, in auxiliary memory, or partly in each The results of this step determine the proportion of computer memory which must be provided by high — and low-speed devices

Table 8.1—Major Control Programs

handled manually by the control-room staff A second reason is that a high plant-availability factor is not essential, therefore the reactor safety instrumentation can take care of computer failure. These are the cautious approaches to computer control which place little reliance on the com­puter part of the system They are becoming less evident since experience and familiarity with digital systems have

The results of the preceding three activities are sum­marized in a deceptively brief compilation similar to that shown in Table 8 1 These data, along with roughly estimated execution times, form the basis for specifying memory sizes, computation speed, and capabilities of peripherals, such as line printers and high-speed operator displays

It is important that one recognize the d’sparity in level of the different programs on the list The one titled “executive-monitor,” for example, is of the highest level It is the master program that ties all the others into a coordinated system. Its development requires the talents of the best process programmer On the other hand, programs such as the calibration routines can be handled by more junior personnel

All the power-supply systems discussed in this chapter are comprised of individual components, or building blocks

The degree of power continuity and overall system re­liability and cost achieved m any system depends on how the basic building blocks are combined The following sections describe the building-block components of the system. Some necessary aspects of properly specifying the components as part of the overall system are also covered.

9- 4.1 Static Inverters

Static inverters become an essential part of a high — reliability power system if short transfer time and an a-c output are required. These inverters are usually used in conjunction with batteries and a static rectifier to provide, on failure of the plant auxiliary-power source, instanta­neous transfer of power from the plant auxiliary-power source to the battery system.

Static inverters consist of three basic parts a low-power oscillator, a power-switching section, and (usually) an output ferroresonant transformer. The low-power oscillator determines the operating frequency of the inverter and can be independent of or synchronized with the plant auxiliary — power system. Once the inverter has been running for several hours, an output-frequency stability within +0.25% of the desired frequency is obtained The major factors tending to change the frequency are long-term drifts in components and variations in the ambient temperature

The power switching section is probably the most critical portion of the overall inverter and usually consists of bridge-connected silicon controlled rectifiers (SCR) The four main SCR’s are alternately switched in pairs, which converts the input d-c source into a square-wave alternating voltage that is applied across the primary of the output transformer The peak amplitude of the alternating voltage is essentially equal to the direct input voltage At the end of each half cycle, the two conducting SCR’s are shut off by momentarily providing a reverse voltage bias This process, called commutation, is an extremely critical function in the proper operation of the inverter If during any half cycle the commutation should fail, the system would be left with more than two SCR’s in the conducting state This would result in an effective short circuit across the battery and would shut down the unit Ensuring proper commutation even under the most adverse conditions is essential in designing reliable inverters In addition to proper commuta tion, the associated circuit must limit the SCR rate of rise of current (di/dt) or voltage (dv/dt) and the peak forward and reverse voltages that appear across the SCR’s

In most applications the desired output is a regulated low-distortion sine wave. Usually both regulation and filtering are provided by the ferroresonant output trans­former, a passive magnetic system similar to the commonly used constant-voltage transformer. An important feature of the ferroresonant transformer is that, as the load current is increased, the output voltage remains essentially constant up to a point in excess of rated load. Above this point the characteristic becomes a very nearly constant current mode As a consequence of this, the inverter can be operated continuously into any overload, up to and including a short circuit, without affecting the square-wave switching portion of the inverter A sine-wave inverter of this type can therefore satisfactorily handle load transients that might otherwise cause misoperation or lack of commutation in the square-wave section

The normal regulation that can be expected is ±3% for all conditions of input voltage and for loads between zero and rated maximum at unity power factor Loads at other than unity power factor have an additional effect on the output Generally, inductive loads reduce the output voltage whereas capacitive loads increase the output volt­age. Loads with a power factor below 0 8 lagging increase the harmonic distortion in the output For these reasons it is preferable to operate with a load having a power factor as near unity as practical, unity power factor also corresponds to minimum d-c input dram. Where the load has an inherent low power factor, the designer must provide suitable correction either at the load or the inverter

In applications where other critical loads are also fed from the battery system, it is desirable to use a filter on the input to the inverter With a sine-wave output from the inverter, the input current resembles a half-wave rectified sine wave Superimposed on this direct current is a large a-c component that may modulate the battery voltage suf­ficiently to cause an undesirable hum in the input of other equipment fed from the battery. The filter eliminates this problem. A second and probably more important function of an input filter is the elimination of spikes that are generated across the battery by other equipment, such as d-c motors and solenoids. Such equipment, commonly used in nuclear power plants, is notorious in generating large voltage transients during operation. Inverter input protec­tion should be provided for short-term transients, in the order of 100/isec up to 4000 volts, when fed from large-station battery systems

Inverters are readily available in a variety of standard output and input ratings, the most common of which are listed in Table 9.1. The typical standard ratings in Table 9.1 do not, of course, represent the limits of the manufacturers’ capabilities. Nonstandard inverters of different output and input voltage, kilovolt-ampere rating, and output quality are available for a premium price on request. The following is an outline of the major areas of importance which should receive attention when specifying a static inverter

The range of input voltage over which the required output must be maintained for a stated time during normal and emergency operation is most important In most cases the inverter supplier does not have control over the input source, which is usually the nuclear-power-plant station battery. It is not sufficient just to determine the normal long-term variations of input voltage. Transient input voltages are also important A common design error is to focus on the large-magnitude short-time transients and neglect the higher energy transients of low frequency (0 5 to several Hz). Input voltage transients due to starting motors may not show up on either a long-time source voltage recorder or an oscilloscope set up to detect switching transients Comprehensive knowledge of the characteristics of the input source is prerequisite to proper inverter application and protection

The precise definition of source impedance is not always necessary. However, it is relevant to note the length and size of conductors between the inverter and the d-c source and between the inverter and any switching or protective devices in the incoming lines Because of input-current pulsing dunng the SCR switching, an input filter may be desirable to remove unwanted modulation of the d-c source. This adds to the inverter cost, and, the actual need for it should be determined before specifying an input filter

The load characteristic should be carefully defined Of specific importance are the load power factor, the variation of load, and the maximum load to be switched at one time. Static inverters have definite limits to their momentary overload capacity, and the limits cannot be exceeded. This characteristic is different from the characteristic of a rotating inverter that has inertia and becomes very im­portant where motor loads or other high inrush loads are present. The possibility of load short circuits should also be considered. It may be concluded that a current-limited output is desirable, and, if so, this should be specified

The efficiency of static inverters may be defined in two ways For the amount of heat to be removed due to losses in the inverter, efficiency may be expressed as the ratio (in percent) of output power losses to rated output power losses However, if the purpose is to determine the source current, efficiency is expressed as the ratio of rated output a-c power to rated input d-c power (in percent). It is important to indicate which definition is to be submitted by the manufacturer in his bid proposal This is particularly important for load power factors, which differ significantly from unity.

The output of the static inverter may be synchronized with another source or with a frequency standard. It is important to indicate the impedance, potential variation, and transient noise capability of the synchronizing signal to be used.

In most cases an extremely low harmonic output distortion level is not necessary. If the a-c output is to be rectified and filtered, a square-wave output would even be desirable. Should a square-wave inverter output be used in conjunction with external transformer loads, the trans­formers must be capable of handling the additional 11% swing in flux without overheating.* In general, a wider harmonic distortion tolerance in the specification results in reduced size, cost, and weight of the inverter.

The San Onofre Atomic Power Plant in San Clemente, Calif., has a pressurized-water reactor similar to that at Shippingport except for the method of rod drive and control. Each of the 45 control rods for this reactor (Fig. 7.13) consists of a 5-in.-square spider-like cluster of 16 absorbing rods, 126-in. long, each fabricated from an alloy of silver, indium, and cadmium and hermetically sealed within a stainless-steel sheath. The cluster of absorber rods fits into guide thimbles in a fuel-rod assembly (Figs. 7.14 and 7.15) and is attached to a driven vertical shaft extending through the top of the reactor to the rod-drive mechanism. Boric acid is added to the coolant water to control reactivity during both operating and shutdown periods to reduce the required number of control rods and to achieve uniform neutron absorption throughout the core.

Whereas each rod drive of the Shippingport reactor uses a canned motor to rotate the nut mechanism attached to a lead-screw extension of the control rod, the San Onofre reactor rod-drive mechanism uses a form of the magnetic jack. There is a similarity between the two in that motion is produced magnetically through the pressure barrier. The motion is rotary in the Shippingport reactor; it is linear with the magnetic jack.

The rod-drive mechanism (Fig. 7.16) consists of a latch assembly and a rod-drive assembly that operate within a thimble, all at reactor pressure. A stack of operating coils surrounds the rod-drive portion of the thimble. A 120-in.- long coil stack over the upper part of the thimble, into which the control-rod extension travels as the control is raised, is used for rod-position indication. The reactor coolant water fills the pressure-containing parts of the mechanism and cools and lubricates the moving parts of the drive.

The drive shaft has circular grooves, spaced in. apart, machined into its surface along its entire length. Mag­netically operated gripper latches lock into the grooves to hold the drive shaft stationary with the drive. The operating coil stack consists of the lift coil, movable gripper coil, and stationary gripper coil. These are energized in a fixed sequence by cam switches actuated by a rotating cam shaft. The coils induce magnetic flux through the pressure housing and operate the latch components. Within the pressure housing, two sets of latches lift or lower the grooved drive shaft. Turning the cam shaft one revolution causes the lifting mechanism to cycle once and quickly moves the rod out in one %-in. step. Reversing the rotation of the cam shaft reverses the direction of rod motion. Latch actuation is produced by pole piece motion for the three magnets.

Since the magnetic jack develops a lifting force of 400 lb and the total drive shaft weight is 144 lb, there is

SNAP ACTION CAM SWITCH

— CONTACT OF ROD TRANSFER DEVICE

TO GROUP 1 PROGRAMMING CIRCUITRY

BRUSHES MOUNTED ON ROTATING PLATE FACE-PLATE COMMUTATOR

Fig. 7.12—Shippingport Pressurized Water Reactor, schematic diagram of rod-control d-c to a-c power inverter. (From The Shippingport Pressurized Water Reactor, p. 282, Addison—Wesley Publishing Company, Inc., Reading, Mass., 1958.)

ample capacity to overcome friction in the system. For rod insertion, gravity furnishes the driving force and overcomes the friction load.

The upper set of latches operates to raise or lower the drive shaft in %-m. steps. After one lift step the lower latches are engaged in a shaft groove and raise the rod У32 in. to unload the upper latches so they may be reset to lift the rod another increment. The latches are actually linked to sliding armature or pole pieces that move when their respective magnet coils are energized or deenergized.

Table 7.3 gives a detailed description of the sequence of steps in control-rod actuation at the San Onofre plant.

(a) Mounting Major System Components. Each instru­ment and piece of equipment in the instrument-rack structure should be mounted, wired, or piped, where possible, so it can be removed without interruption of service to adjacent instruments and equipment. The instru­ments and equipment should be located and mounted so all wiring terminals and piping connections are readily accessi­ble (see Fig. 10.4).

(b) Mounting Electrical Equipment and Hardware. The

electrical equipment and hardware, as well as the installa­tion of these items, should conform to some standard, such as the National Electrical Code. The ambient conditions at

the location where the system is to operate and the type of system will determine which sections of the electrical code are applicable.

Terminal Blocks. Terminal blocks are arranged so that one side of each block is reserved for field connections (see Fig. 10.5). Spare terminals (10% minimum) should be provided for future use and should be distributed through­out the terminal blocks. Where a field cable may fill all the spaces on a terminal block, the spares requirement should be overruled to prevent the need for splitting the cable between two terminal blocks. Terminal blocks should be located in the panels and cabinets to facilitate maintenance and testing without impairing access to other equipment mounted in the structure. Terminal blocks and terminal points should be identified and labeled.

Wiring Installation in Panels and Cabinets. The wire used for power distribution in the instrument cabinets should be adequate to carry the current used by the circuit. Design and installation engineers must adhere to the National Electrical Code in sizing wire for power-distribu­tion circuits. No wire smaller than No. 14 American Wire Gauge (AWG) should be used in power-distribution circuits. Control circuits with less than 5 amp of maximum operating current may use No. 16 AWG copper wire. Wire sizes smaller than No. 16 AWG could handle the current requirement of most control circuits; however, mechanical strength becomes an overriding consideration, and these wires are not recommended for installation in panels and cabinets.

Architect—engineers and reactor designers have speci­fied both stranded and solid wires for instrumentation and

control applications in nuclear plants. For power and control circuits, stranded wire is preferred and is much more widely used than solid wire. The flexibility of stranded wire facilitates installation and maintenance. Either stranded or solid wire can be used without affecting the electrical characteristics or performance of the circuit.

All wiring should have a minimum of 600-volt insula­tion and should be resistant to heat, oil, moisture, flame, and corrosive vapors. Insulation materials have been devel­oped which meet the requirements for switchboard and control-panel wiring without requiring braid or fibrous coverings. This results in a smaller overall diameter with fewer stripping and terminating problems.

All wiring connections in the instrument panels and cabinets should be made with preinsulated compression — type terminals unless a solder connection is required. For solder connections, insulated sleeves should be used to snugly cover the finished solder joint.

Wires entering or leaving the instrument cabinet should be terminated in terminal boxes to facilitate maintenance. However, some wires, such as coaxial, triaxial, and thermo­couple lead wires, should be terminated through appropri­ate connectors directly to the instruments or thermocouple junction boxes.

Multiconductor or twisted-pair shielded cable should be used for analog signals (low-level, millivolt or milliampere) in instrumentation circuits. Wire no smaller than No. 18 AWG is recommended to minimize wire breakage during installation. Each conductor and the outer jacket or sheath
of the shielded cable should have a flame-resistant insula­tion. The shield is carried as a separate conductor at all cable junction points.

The signal wires are run in wireways separate from the control and power wiring to minimize noise pickup in signal wiring. Separate terminal boxes are recommended for the signal and the power wiring. Lacing of low-level signal cables into bundles with power or control wiring should be avoided. Wiring in the instrument racks should not be spliced; each wire should run unbroken from terminal to terminal.

Wiring between panel-mounted instruments and termi­nal boxes should be grouped in a neat and orderly manner and run in enclosed metal wireways. Exposed wiring should be laced or bundled together with lacing, tie straps, or similar means.

Each wire should be properly identified. There are several coding or identification methods, such as noncon­ductive markers, color-coding the wires, and providing label identifications on the terminal blocks. Proper identification facilitates testing and maintenance. Wiring identification should correspond to that shown on the elementary and connection diagrams.

Terminations. The termination of conductors, whether they carry low-current signals or high-current power, is an important part of installation. Whether the termination is made by a simple “crimp-on” lug or a complex triaxial connector, good workmanship is of the utmost importance. Careful adherence to the manufacturer’s mounting instruc­
tions, including the use of proper tools, can save many hours of troubleshooting and wire tracing

Instructions and procedures for installing lugs and connectors on wire and signal cables are shown in Figs 10 6 to 10.9 The most widely used hardware for terminating

wiring and cabling is shown in these drawings Recommen­dations are included on how to avoid common problem areas in the installation of connectors

Crimp-on lugs Figure 10 6 shows the proper proce­dure for installing crimp-on lugs. To avoid installation problems, it is essential that

1 1 he proper type of lug (insulated or nomnsulated, ring or spade, etc ) be used.

2. The proper si/e lug for the wire and terminal be used

3. The insulation be stripped to the proper length (refer to “a” in Fig 10.6). The conductor should be inserted completely through the lug with the insulation butted up against the shoulder and the conductor cut so that it protrudes just past the crimped portion of the lug

4. The lug be properh crimped, using the proper crimping tool, with the wire conductor and insulator, where applicable, completely compressed to the lug It is recom­mended that a fixed-release crimping tool be used. This tool assures proper crimping of the lug evert time by not allowing release of the lug until the full amount of crimping pressure has been applied

Standard Coaxial (BNC) Connector I igure 10.7 show’s the proper procedure for installing standard coaxial (BNC) connectors. ‘I о avoid installation problems, it is essential that

1 All strands of the shield be free of the center conductor

2. All strands of the shield make a good contact with the connector shell.

3 All dimensions on the assemble drawing be followed precisely so that the connector will fit together properh.

4. A good solder connection be made between the contact tip and the center conductor

5. The connector and cable be cleaned properly with an appropriate cleaning agent

Crimp-On Coaxial (BNC) Connector Figure 10.8 shows the proper procedure for installing crimp-on coaxial (BNC) connectors To avoid installation problems, it is essential that

1. All strands of the shield make good contact with the connector shell

2 All assembly instructions and dimensions be followed precisely.

3. The connector and cable be cleaned properly with an appropriate cleaning agent

1 riaxial Connector. Figure 10.9 shews the proper procedure for installing triaxial connectors To avoid installation problems, it is essential that

1 All strands of the two shields make good contact with their conductor Strands left out of the conductor have been a source of noise problems, particularly with pulse circuits having fast rise times in the microsecond and nanosecond range

2. None of the shield strands from either shield touch each other or the center conductor

3 All assembly instructions and dimensions be followed precisely.

4. The connector and cable be cleaned properly with an appropriate cleaning agent.

(c) Mounting Pneumatic Equipment and Hard­ware. The pneumatic instrumentation system uses com­pressed air for the operation of the measuring devices, indicators and controllers, and final control elements

Relatively trouble-free operation can be realized. In systems requiring 100% availability, a backup or dual system as shown in Figs. 10.4 and 10.12 should be used. The installation of an instrument air sy’stem should con­form to a standard of the industry, such as the American Standard Association Code for Pressure Piping, ASA B31.1.*

Instrument Air Supply. In a pneumatic system the air is supplied by7 a compressor to a storage tank, and the system is supplied from the tank. The compressor and storage tank are sized so that the air usage of the system does not require continuous operation of the compressor. These items are generally located in a service equipment area 1 he air is then piped to the instrumentation-rack structure.

Condensation in a compressed-air piping system must be limited because moisture can damage instruments and make the system inoperative Several air-dry ing techniques are available to remove the moisture from compressed air.

Desiccant dryer A desiccant dryer is located in the piping between the compressor storage tank and the filter—regulator station. The dryer consists of two identical units, each unit has a desiccant chamber, check valve with a reduced-area bypass, and a solenoid valve, connected as shown in Fig. 10.10. Part of the dried air from the chamber

•See Vol 2, Chap. 14, for a discussion of standards and the addresses of standards organizations.

Fray shield and strip inner dielectric ^32 in. Tin center conductor.

Taper braid and slide nut, washer, gasket, and clamp over braid. Clamp is inserted so that its inner shoulder fits squarely against end of cable jacket.

With clamp in place, comb out braid, fold back smooth as shown, and trim З/32 in. from end.

Slip contact in place, butt against dielectric, and solder. Remove excess solder from outside of contact. Be sure cable dielectric is not heated excessively and swollen so as to prevent dielectric from entering into connector body.

Fig. 10.7—Assembly of standard coaxial connector.

in service is used to regenerate the other chamber; this

amounts to about one-third of the dried air produced.

Because additional air is required for regeneration of the system, the compressors must be sized to supply the

regeneration air in addition to the air required by the

instrument system. As the chamber drying the air becomes saturated, the chamber on the regeneration cycle is dried out. An electric timer controls the solenoid valves and periodically switches them, reversing the operating cycle of the system.

After-condensers, After-condenser air dryers are also located in the air line between the compressor-storage tank and the filter—regulator station. The after-condenser con­sists of a heat exchanger that uses water as a cooling agent. Figure 10.11 is a simplified diagram of a water-cooled moisture condenser. The use of chilled water for cooling increases the capacity of the condenser.

Filter—Regulator Station. A filter is located upstream of the pressure regulator and is used to remove foreign matter or contaminants from the air stream. The pressure regulator reduces the air pressure to the level required by the instrument system.

Where instrument air must be available to the system 100% of the time, a dual filter—regulator station is used. A typical dual station is shown in Fig. 10.12 in schematic form, and an actual installation, in Fig. 10,4. Each of the parallel filter—regulator stations is sized to handle the total requirements of the system. Isolation valves in each of the parallel piping arrangements allow either of the filter — regulator stations to be isolated from the system for repair and maintenance without shutting down the entire system. Each instrument using air is connected to the instrument air header through an isolation valve. Each instrument air header should have spare air takeoff points (10% mini-

Strip cable jacket, braid, and dielectirc to dimensions shown in table All cuts are to be sharp and square Important Do not nick braid, dielectric, and center conductor Tinning of center conductor is not necessary if contact is to be crimped For solder method, tin center conductor avoiding excessive heat Slide outer ferrule onto cable as shown

mum) The spare takeoff points should be equipped with isolation valves to allow the addition of new instruments to the system without requiring system shutdown. The header is sloped to the output, and so any condensation collects at the dram cock (see Fig 10.4)

Pneumatic Signal Lines The air supply and signal lines downstream of the instrument air header are plastic or copper tubing Runs of this tubing should be straight, parallel, accessible, and logical with vertical runs plumb and horizontal runs dropping away slightly from the instru­ments Tubing runs must be rigidly supported and fastened to the instrument structure or supporting braces These installation requirements ensure that the tubing installation
will not only have a pleasing appearance but also will be easily maintained.

bach instrument signal input line should be equipped with an isolation valve and a test tee with a shutoff valve This arrangement allows maintenance or testing without shutting down the whole system.

Pneumatic Input-Output Terminal Panel The fact that instrumentation systems are generallv assembled and tested at the vendor’s plant and not at the location where the system will be used requires that provisions be made for terminating the pneumatic input—output lines One method of providing a terminal for both instrument structure lines and field-installed lines is to use a bulkhead tubing

Slide nut, washer, and gasket over cable. Cut off outside jacket (using razor blade or wire strippers) to dimension a. Make a clean cut, being very careful not to nick braid. Cut first braid to dimension b.

Slide first braid clamp over braid up to jacket of cable. Fold first braid back over clamp, making sure braid is evenly distributed over the sur­face of the clamp. Trim second jacket to dimension c, again being very careful not to nick braid.

Trim second braid to dimension d. Slide on outer ground washer. Insulator, and second braid clamp. Fold second braid back over braid clamp, again making sure that braid is evenly distributed over surface of clamp.

Plug only: Place front insulator and outer contact assembly into back of connector body and push into proper place. Insert cable contact assembly into body. Screw nut into body with wrench until moderately tight.

Tin the inside hole of the contact. Tin wire and insert into contact and solder. Remove any excess solder. Be sure cable dielectric is not heated excessively and swollen so as to prevent dielectric from entering body of fitting.

NOTE: "a" thru "f" dimension depends on cable and connector type

Fig. 10.9—Assembly of triaxial connector.

connector (see Fig. 10.13). The bulkhead connectors are mounted on the enclosure surface or on a mounting plate in a panel. The connectors are located in an area accessible to the field lines.

After the pneumatic systems have been installed, each system must be pressure-tested to be certain that leakage in the system does not affect the operation of the system. The Instrument Society of America (ISA) Pneu-

matic Control Circuit Pressure Test, ISA RP7 1, is one test procedure for verifying the leakage in pneumatic systems and establishing the criteria for acceptance of the work