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IEEE Transactions on Nuclear Science, Vol. 33, No. 1, February 1986 ELECTROMAGNETIC COMPATIBILITY IN NUCLEAR POWER PLANTS James Cirillo Michael Prussel Sanders Associates, Inc. 95 Canal Street, Nashua, NH typical of many power plant environments. EMC (electromagnetic compatibility) is being largely ignored in the design of nuclear power instrumentation and control systems. As a result, EMI (electromagnetic interference) is causing costly startup delays and spurious reactor trips. This paper describes existing problems, basic causes, and approaches to their solutions. Summary EMC (electromagnetic compatibility) is defined as the ability of a system and its component parts to perform its intended function in its operational electromagnetic environment. Many instrumentation and control (I&C) systems not designed with EMC considerations in mind have experienced severe upsets, resulting in spurious reactor trips, pretrip alarms, and unacceptably high noise on key monitor- ing and protection equipments. These problems have never presented a direct safety hazard, but they have adversely affected nuclear power projects in the form of increased operating costs, startup integration problems, and schedule slips. With the increasing use of sophisticated I&C equipments in nuclear power plants, the necessity for EMC control is becoming mandatory from a cost and schedule viewpoint. The electromagnetic environment in a typical nuclear installation may include such noise sources as hand-held transceivers, electrical contactors, SCR control circuits, digital circuits, and switch- ing power supplies. Victim equipments include low-level analog devices, digital circuitry, charge- coupled devices, and so on. The noise enters the system via coupling paths such as wiring and cables, powerlines, signal and shield grounds, and direct penetration through equipment cases. Controlling the interference sources and their effects is relatively easily accomplished by incorporating proven EMC design principles into the initial design of an equipment or system. This paper describes actual electromagnetic interference problems experienced in several instal- lations, explains the causes of these problems, and recommends an overall approach for solutions to present and future problems of this type. Historical Background In recent years, investigations have been conducted at reactor sites throughout the country to determine typical electromagnetic environments, to find the causes of various EMC-related operational problems, and to implement workable solutions. Given below are a few examples of interference prob- lems encountered at various sites: * In one installation, hand-held transceivers used for plant communications produced logic errors in the plant "reactor protection computer" (RPC). To gain further insight into the problem, the RPC was taken to an EMI test laboratory. Here the unit was subjected to RF fields while monitoring it for abnormal operation. The unit was susceptible to field strengths of less than 1 volt/meter in the 50 to 400 MHz frequency range, levels which are * The Excore neutron flux monitoring systems also have a history of noise problems. During the startup phases at several plants, these units have registered neutron activity (in the form of high meter readings and alarm indications) in the absence of nuclear fuel. On-site investigations revealed various causes and contributors to these problems, including high levels of interference on AC powerlines (vital buses), inadequate powerline filtering on the equipment, improper cable shielding and termination practices (from an RF viewpoint), and incorrect (albeit well-intentioned) philosophy regarding equipment ground bus (EGB) and instrument ground bus (IGB) isolation. Since plant wiring, grounding, and equipment layouts were relatively fixed at the time of these investigations, it was not possible to affect the root causes of the interference. Rather, the Excore monitor symptoms were alleviated by applying "patchwork" fixes. In one instance, the problem was fixed by replacing the coaxial signal cables with triaxial cable. In another, normal operation resumed only after apply- ing passive filters in the affected signal paths. * Several nuclear generating sites have experienced spurious reactor trips or pretrip alarms while on line. Although many of these trips were valid (attributable to radiation values exceeding preset thresholds), a large proportion of these actuations were seemingly random in nature and unrelated to nuclear radiation. Transient currents and voltages (due to switching of heavy equipments such as HPSI pumps) were strongly suspected to be the cause. These problems are the most difficult to correct, because of the short duration of the interference, and the difficulty involved in performing troubleshooting. To find the cause of any interference problem, individual equipments or subsystems must be switched on and off, and critical lines must be monitored with specialized test equipment to determine the coupling paths. These investigative techniques are generally not permitted during normal plant operations. Thus, nuclear generating sites experiencing spurious trips often have no choice but to "live with it", at least until a period of scheduled downtime. Further studies were prompted by these and other problems. These studies were intended to empirically predict and fix potential noise problems before equip- ments were actually installed in the field. Two basic approaches were involved: 1) definition of actual operational electromagnetic environments at various locations inside the installation, and 2) evaluation of the electromagnetic emissions and susceptibility characteristics of individual equipments in a labora- tory setting. The rationale behind this approach was straightforward. If an equipment's emission/suscep- tibility characteristics were measured, and if the equipment's operational environment were known, then a statement could be made regarding the equipment's ability to operate in its intended installation without upset or degradation. "Quick-look" surveys of electromagnetic environ- ments were conducted at several nuclear installations in various degrees of completion. These studies were typically confined to two general areas: measurement 0018-9499/86/0200-1015$01.00© 1986 IEEE Abstract 1015

Electromagnetic Compatibility in Nuclear Power Plants

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Page 1: Electromagnetic Compatibility in Nuclear Power Plants

IEEE Transactions on Nuclear Science, Vol. 33, No. 1, February 1986

ELECTROMAGNETIC COMPATIBILITY INNUCLEAR POWER PLANTS

James CirilloMichael Prussel

Sanders Associates, Inc.95 Canal Street, Nashua, NH

typical of many power plant environments.

EMC (electromagnetic compatibility) is beinglargely ignored in the design of nuclear powerinstrumentation and control systems. As a result,EMI (electromagnetic interference) is causing costlystartup delays and spurious reactor trips. Thispaper describes existing problems, basic causes, andapproaches to their solutions.

Summary

EMC (electromagnetic compatibility) is definedas the ability of a system and its component partsto perform its intended function in its operationalelectromagnetic environment. Many instrumentationand control (I&C) systems not designed with EMCconsiderations in mind have experienced severe

upsets, resulting in spurious reactor trips, pretripalarms, and unacceptably high noise on key monitor-ing and protection equipments. These problems havenever presented a direct safety hazard, but theyhave adversely affected nuclear power projects inthe form of increased operating costs, startupintegration problems, and schedule slips. With theincreasing use of sophisticated I&C equipments innuclear power plants, the necessity for EMC controlis becoming mandatory from a cost and scheduleviewpoint.

The electromagnetic environment in a typicalnuclear installation may include such noise sourcesas hand-held transceivers, electrical contactors,SCR control circuits, digital circuits, and switch-ing power supplies. Victim equipments includelow-level analog devices, digital circuitry, charge-coupled devices, and so on. The noise enters thesystem via coupling paths such as wiring and cables,powerlines, signal and shield grounds, and directpenetration through equipment cases. Controllingthe interference sources and their effects isrelatively easily accomplished by incorporatingproven EMC design principles into the initial designof an equipment or system.

This paper describes actual electromagneticinterference problems experienced in several instal-lations, explains the causes of these problems, andrecommends an overall approach for solutions topresent and future problems of this type.

Historical Background

In recent years, investigations have beenconducted at reactor sites throughout the country todetermine typical electromagnetic environments, tofind the causes of various EMC-related operationalproblems, and to implement workable solutions.Given below are a few examples of interference prob-lems encountered at various sites:

* In one installation, hand-held transceivers usedfor plant communications produced logic errors inthe plant "reactor protection computer" (RPC). Togain further insight into the problem, the RPC was

taken to an EMI test laboratory. Here the unitwas subjected to RF fields while monitoring it forabnormal operation. The unit was susceptible tofield strengths of less than 1 volt/meter in the50 to 400 MHz frequency range, levels which are

* The Excore neutron flux monitoring systems also havea history of noise problems. During the startupphases at several plants, these units haveregistered neutron activity (in the form of highmeter readings and alarm indications) in the absenceof nuclear fuel. On-site investigations revealedvarious causes and contributors to these problems,including high levels of interference on ACpowerlines (vital buses), inadequate powerlinefiltering on the equipment, improper cable shieldingand termination practices (from an RF viewpoint),and incorrect (albeit well-intentioned) philosophyregarding equipment ground bus (EGB) and instrumentground bus (IGB) isolation. Since plant wiring,grounding, and equipment layouts were relativelyfixed at the time of these investigations, it wasnot possible to affect the root causes of theinterference. Rather, the Excore monitor symptomswere alleviated by applying "patchwork" fixes. Inone instance, the problem was fixed by replacing thecoaxial signal cables with triaxial cable. Inanother, normal operation resumed only after apply-ing passive filters in the affected signal paths.

* Several nuclear generating sites have experiencedspurious reactor trips or pretrip alarms while online. Although many of these trips were valid(attributable to radiation values exceeding presetthresholds), a large proportion of these actuationswere seemingly random in nature and unrelated tonuclear radiation. Transient currents and voltages(due to switching of heavy equipments such as HPSIpumps) were strongly suspected to be the cause.These problems are the most difficult to correct,because of the short duration of the interference,and the difficulty involved in performingtroubleshooting. To find the cause of anyinterference problem, individual equipments or

subsystems must be switched on and off, and criticallines must be monitored with specialized testequipment to determine the coupling paths. Theseinvestigative techniques are generally not permittedduring normal plant operations. Thus, nucleargenerating sites experiencing spurious trips oftenhave no choice but to "live with it", at least untila period of scheduled downtime.

Further studies were prompted by these and otherproblems. These studies were intended to empiricallypredict and fix potential noise problems before equip-ments were actually installed in the field. Two basicapproaches were involved: 1) definition of actualoperational electromagnetic environments at variouslocations inside the installation, and 2) evaluationof the electromagnetic emissions and susceptibilitycharacteristics of individual equipments in a labora-tory setting. The rationale behind this approach was

straightforward. If an equipment's emission/suscep-tibility characteristics were measured, and if theequipment's operational environment were known, then a

statement could be made regarding the equipment'sability to operate in its intended installationwithout upset or degradation.

"Quick-look" surveys of electromagnetic environ-ments were conducted at several nuclear installationsin various degrees of completion. These studies were

typically confined to two general areas: measurement

0018-9499/86/0200-1015$01.00© 1986 IEEE

Abstract

1015

Page 2: Electromagnetic Compatibility in Nuclear Power Plants

of ambient electric fields in the 15 kHz - 1 GHz

frequency ranqe, and measurement of noise currents

on critical powerlines in the 60 Hz to 50 MHz range.

Laboratory evaluations were performed on

several instrumentation equipments and systems,

including a RPC, an Excore system, an annunciator

system, and a power inverter, among others. Theseequipments were tested in accordance with accepted

standards, such as MIL-STD-461 [1 ] and SAMA

Standard PMC 33.1[2]. Tests required by 'thesedocuments are classed into four groups: conductedsusceptibility, conducted emissions, radiatedemissions, and radiated susceptibility. Results of

the laboratory tests indicated that, in general,equipments were susceptible to relatively low levelsof electromagnetic energy. False indications or up-

sets occurred at field intensities as low as 1 V/m.Injection of low levels of RF current (severalmilliamps) on equipment power and signal linescaused similar malfunctions. Many equipments ex-

hibited high levels of conducted emissions on theirpower lines, caused primarily by voltage rectifica-tion and current pulse switching within their power

supplies. These emissions, while not a particularlysignificant problem taken alone, add up to excessivenoise levels in the installation when many equip-ments are tied to the same power line. Radiatedemissions for most equipments were not severe.

However, they have the potential to contribute to an

undesirable overall emissions signature.

The results of the laboratory testing and elec-tromagnetic environment surveys are summarized inthe table below. The data indicate that some

equipments cannot operate in the vicinity of thenoise-generating sources typical of most nuclearpower installations.

System-Level Causes

Groundinq: System grounding philosophy andpractice is one of the most basic factors underlying

EMC, but is very often misunderstood and misapplied.Most nuclear installations employ a combinationsafety/reference ground commonly known as EGB. Thisis typically in the form of a grid network at each

floor level, constructed of 4/0 copper run around the

periphery of the major equipment areas and rooms, with

ground pads facilitating connections to equipments and

cabinets. All metal which can potentially carry fault

current is tied to the EGB grid. The grid is extendedvertically by more 4/0 risers which tie in at each

floor level. At earth level, the network is tied to

the earth ground grid and to the building structuralsteel. In addition to the EGB network, one or more

separate, isolated grounding networks are typicallyspecified for the instrumentation systems, known as

IGB. Separation of instrumentation grounds from thoseof other less critical equipments is, in principle, a

desirable goal from the EMC point of view.

However, the EGB/IGB setup as implemented actual-ly leads to increased noise problems in many installa-tions. Figure 1 shows a representative groundingscheme employed for I&C systems. Typically, the IGBis isolated from EGB at all points except at thesingle point near the earth grid. Ties to the IGBconsist primarily of cable shields (and not the"control common" which is typically floated). Theproblem with the scheme is that noise currents on theshields cannot be effectively shunted, due to theextremely long lengths of shield pigtails and IGBwires. A 50 foot length of 4/0 wire has a distributedinductance of approximately 24 microhenries. Thistranslates to an effective RF impedance to earth of

150 ohms at 1 MHz. Because of this relatively high

Table 1

Interfering Source Levels vs. Measured SusceptibilityThreshold of Typical I&C Equipments

Source Typical Measured Victim SusceptibilitySource Level Level, Typical

Hand-Held Transceiver 2.5 Volts/meter (at 16 ft. 1-3 Volts/meter (from 350-450 MHz)distance)

Powerline Transients 400V, maximum (from Ref. [7]) 40V, 10 uS spike (single occurrence,injected on AC neutral lead)

Powerline RF Currents 2 mA from 100 kHz through 1 mA5 MHz

Magnetic Induction, 12 volt spikes (estimated) 0.4 volts, 10 uS spikes on an adjacentAdjacent Cables cable

Causes of Electromagnetic Interference

As indicated previously, site investigationshave found many causes and factors of EMC problems.Problems have been traced to inadequate RF grounds,poor cabling practices, poor shield terminations,inadequate RF shielding and powerline filtering, andeven (from the EMC viewpoint) poor circuit designand layout. All of these designs, however, complywith existing NRC guidelines and regulations. Thissection discusses specific causes and contributorsof interference.

impedance, the shields have essentially no ground, and

their energy is recoupled onto critical signal lines

and circuitry, instead of being shunted to earth.

The noise problems resulting from this condition

were ultimately cured by transferring all cable

shields to EGB, and abandoning the IGB bus. This

concept is shown in Figure 2. This scheme, although

more noise-immunf than the original groundingconfiguration, may present possibilities for undesired

"ground-loop" effects, since IGB and EGB are no longer

isolated. An approach which is effective against

radio-frequency interference, yet maintains isolation

between IGB and EGB, is shown in Figure 3.

1016

Page 3: Electromagnetic Compatibility in Nuclear Power Plants

1017

Figure 1 -- Typic

Figure 2 -- Bet

Eqt

Figure 3 -- RecommE

Shielding and Shield Termination Practices: It isrecognized in most nuclear installations thatprotective shields are required for many signals,

I&C especially low-level instrumentation signals.uipment However, the shielding techniques employed are oftenabinets not as ef fective as they could be. If a shield is to

protect a signal line from external radio frequencyenergy, it is necessary to reference the shield toground at both ends of the cable run, and preferablyat as many points as possible in between. Inaddition, maximum shielding effectiveness is obtainedwhen the shield is bonded circumferentially to themodule where the protected signal line(s) enter. Thisconcept is often violated in practice. Protectiveshields are tied to ground at only one end, or

IGB neither, thus rendering them totally ineffective atradio frequencies. Furthermore, even when shields aregrounded at both ends, they are typically terminatedby means of very long pigtails, a condition which also

Earth Grid reduces their effectiveness. Unfortunately, many

nuclear industry standards adhere to the single-pointgrounding concept, without mentioning that this

:al Grounding Scheme concept is ineffective as protection againstradio-frequency interference. Some installationspecifications do not permit a protective shield to begrounded at both ends, since such multiple-pointgrounding may violate ground-fault requirements. Whenthis is the case, the shield's RF integrity can be

I&C maintained by inserting a capacitor in series with theuipment shield, thus also maintaining the DC ground-faultabinets isolation.

Coaxial cable is often used for criticalinstrumentation signal runs. The outer shield is thereturn path, and is typically floated, or referencedto signal ground at only one point. In this case, ifthe coaxial line has no protective shield over it, thecable will pick up noise from adjacent cables and theenvironment, and carry this noise into the modulecase, thereby affecting the sensitive circuitryinside. This condition negates any shielding effec-

IGB tiveness afforded by the case. It is often mistakenlyassumed that the coaxial outer shield automatically

Earth Grid protects against any noise pickup. This assumption

~~not true, especially when coaxial cables are

left floated over runs of several hundred feet.

.ter Grounding Scheme Cabling Practices: In most nuclear installa-tions, cables are routed from one place to anotherwith apparently little attention given to proper

segregation techniques. Although in general, power

cables and signal cables are run in separate conduitor cable trays, there is no attempt made to furtherseparate signal types. Digital signal lines, which

I&C are notorious emitters of radiated energy, are in manyuipment instances routed close to cables carrying low-levelabinets analog signals, of which extremely vulnerable

to cable crosstalk effects.

\z 8Equipment Level Causes

Powerline Filtering: The AC buses of I&C equipmentsare often carriers of high levels of electromagnetic

0 !U energy. Much' of the AC bus noise is generated bypower inverters and equipment power supplies. Other

0 noise sources exist in the form of current and voltage

IGB spikes caused by on-off switching of equipments on the

_IGB lm bus. Often the neutral wire is far noisier than thehot leads. This conducted noise has been proven to be

_ Earth Grid a direct cause of equipment degradation. Individual

equipments, however, are rarely equipped with

powerline filters or transient suappressors to mitigate

endedGoundingSchemethe ef fects of such noise. It has been necessary toended Grounding Scheme incorporate powerline filters in the field to fix

certain I&C system problems.

Equipment Grounding and Referencing: Many

Page 4: Electromagnetic Compatibility in Nuclear Power Plants

1018

equipments and subsystems are required to have sig-nal commons totally isolated from the plant IGB/EGB.Floating of circuits in this manner can contributesignificantly to noise pickup. In some cases

equipment chassis are connected to EGB only by a

wire (a poor RF ground). Protective shields insideequipment cases are often terminated by means oflong pigtails, and are isolated from the equipmentchassis, thus providing poor or nonexistent drainpaths for shield currents.

Circuit Design and Layout: In some equipments,the circuit design itself rendered the equipmentextremely vulnerable to noise pickup. Inspection ofone module revealed that the op-amps were run open-

loop (highest gain mode), without external compen-

sation or filtering, thus permitting saturation atlow interfering signal levels. Amplifiers were notdesigned for common-mode rejection. Inside thechassis, power and signal wires were indiscriminate-ly bundled together, increasing the possibilitiesfor cross-coupling. Long narrow PC traces forsignal ground were observed, which may have contri-buted to the module's noise problems.

Approaches for Solutions

The approach and overall solution to a particu-lar problem depends upon the nature of the problemand the operational state of the plant. As indicat-ed throughout the above discussions, solutions toEMC problems have been successfully implemented forplants in the startup phases. These solutions haveinvolved "patchwork" fixes. Unfortunately, patch-work fixes treat only symptoms, and may fail as new

equipments are installed or seemingly unrelatedplant modifications are made. The process of deter-mining correct fixes is time-consuming, expensive,and often impossible once the plant is on-line.

In recent years, however, the power industry has

begun to gain awareness of the advantages of a

systems approach to EMC, an approach which has beenimplemented successfully for numerous militarysystems and platforms (aircraft, ships, and groundfacilities). The shortcominqs and pitfalls of

patchwork approaches are avoided by addressing EMCconsiderations well in advance of equipment or

wiring installation.

At the present time, there does not exist a

specification or guide which describes a trulyintegrated systems approach to EMC for nuclear powerprojects. Many existing nuclear industry specifica-tions allude to EMC control techniques, but only in

a general open-ended way. Requirements are often

vague and unverifiable. For example, one subcon-

tractor's specification to the equipment vendor

stated that the equipment shall be "insensitive to

stray electromagnetic or electrostatic fields".

Imposition of this requirement is not likely to

achieve the desired result, because it does not

quantify the "stray fields" or the verification

testing method. Other common industry standards,such as IEEE-Std-384[31 and IEEE-Std-420[41mention that electromagnetic interference shall be

considered, but give no insights into the details of

EMC control practices or the means of verifyinginterference immunity. IEEE-Std-518[51 is one

reasonably detailed design guide for EMC control,but it does not outline the systems approach to EXCwhich is needed for the nuclear industry. One

study[6] has recommended EMC design and testingprograms for I&C equipments, but these recommenda-tions have not yet been integrated into systems-level EMC programs. Another studyE7], based on

experience with electrical interference problems in

United Kingdom nuclear sites, points out the advan-tages of a coordinated EMC approach.

The systems EMC approach should integrate allaspects of EMC design, testing, and planning, andconcentrate the authority for administering the EMCrequirements upon the top-level contractor or systemintegrator. The EMC requirements must then be tiereddownward to systems designers and equipmentmanufacturers, who are responsible for details of EMCdesign and testing compliance. A systems EMC programcan be applied equally effectively to construction ofa new plant, an upgrade of an existing plant, ordevelopment of a new I&C system. Such a programshould apply EMC controls at all levels, and includethe following elements:

* Assignment of authority for supervision of EMCdesign, testing, and inspection tasks to variousfunctional groups (quality control, I&C engineering,startup groups, subcontractors, etc.)

* Documentation of radiated and conducted emissionsprofiles at locations where the new equipments/systems are to be installed. This includes ACpowerline noise characteristics (both steady-stateand transient phenomena), and radiated emissionsdata measured at planned module and cable locations.

* Documentation of susceptibility characteristics ofequipments/systems operating from the same powerlineor colocated with the planned system.

* Establishment of safety margins for new equipments/systems. The safety margin is the differencebetween the interfering signal level which theequipment must withstand in the laboratory and theactual signal level expected in the installation.

* Establishment of EMC qualification test require-ments, methods, and appropriate pass/fail criteria.

* Construction of an EMC design plan specificallygeared to the project at hand. The design plan neednot be excessively detailed, but give generalproject-specific guidance for shielding, wiring, andgrounding configurations. This plan might useportions of existing applicable specifications todirect design efforts towards "EMC-critical" areas.

Establishment of EMC Program Plan

The overall EXC approach as described above isbest communicated to subcontractors and cognizantpersonnel by means of a single document -- the EMCProgram Plan. The EMC Program Plan defines the overalltechnical and organizational framework for a givenproject's EMC effort. It is initiated in the earlieststages of system development or procurement, so thatmaximum EMC benefits are obtained. Many advantagesare gained by use of the EMC Program Plan:

1. It assures efficient integration of management,engineering, and quality assurance tasks relativeto EMC.

2. It assures continuous traceability of EMCrequirements and design changes throughout theproject. In this way, the impact of designchanges or equipment deficiencies are accuratelyidentified, properly communicated, and quicklycorrected.

3. It defines lines of responsibility early in theproject. This avoids the "finger-pointing" sotypical of present EMC-related problems.

Page 5: Electromagnetic Compatibility in Nuclear Power Plants

1019

4. It defines verifiable EMC test requirements sothat equipment designers have concrete designgoals.

Benefits of the Systems EMC Approach

The system approach, when implemented in earlystages of the project, serves to ensure that EMCcontrols are an inherent part of the equipment andsystem designs. As a results, when systems areinstalled and integrated, each performs its intendedfunction with no degradation. Patchwork fixes ofnoise problems during the final critical programstages no longer will be necessary, thus importantprogram milestones are not excessively delayed, norbudgets strained. Less downtime during plantoperations will occur, because spurious actuationsare significantly reduced, and because extensivestudies and troubleshooting efforts are unnecessary.In the long run, the small initial expense forimplementation of a planned systems EMC program iswell worth the investment.

References

[1] MIL-STD-461 B, "Electromagnetic Emissions andSusceptibility Requirements for the Control ofElectromagnetic Interference", April 1980.

[2] SAMA Standard PMC 33.1 "ElectromagneticSusceptibility of Process ControlInstrumentation, 1978.

[3] IEEE-STD-384, "IEEE Standard Criteria forIndependence of Class 1E Equipment andCircuits", June 30, 1977.

[4] IEEE-STD-420, "IEEE Standard for the Design andQualification of Class 1E Control Boards,Panels, and Racks used in Nuclear PowerGenerating Stations", Dec 7, 1982.

[5] IEEE-STD-518, "IEEE Guide for the Installationof Electrical Equipment to Minimize ElectricalNoise Inputs to Controllers from ExternalSources", April 29, 1977.

[6] J.R. Oranchak et al, "RFI Effects on PowerPlant I&C Equipment", IEEE Transactions onNuclear Science, Vol. NS-27, P 863-865, Feb1980.

[7] E.P. Fowler and I. Wilson, "Recent Experiencein Commissioning Neutron Flux Instrumentationwith Particular Reference to the Avoidance ofElectrical Interference Problems", IAEA NuclearPower Plant Control and Instrumentation, 1978,p 241-250.