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Nuclear Engineering and Design 239 (2009) 3132–3141

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

uclear plant temperature instrumentation

.M. Hashemiana,∗, Jin Jiangb,1

AMS Corporation, AMS Technology Center, 9111 Cross Park Drive, Knoxville, TN 37923, USAElectrical & Computer Engineering Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

r t i c l e i n f o

rticle history:eceived 26 March 2009eceived in revised form 14 July 2009ccepted 21 August 2009

a b s t r a c t

Most critical process temperatures in nuclear power plants are measured using resistance tempera-ture detectors (RTDs) and thermocouples. In addition to excellent reliability and accident survivability,nuclear safety-related RTDs are expected to have good calibration and fast dynamic response time, asthese characteristics are important to plant safety and economy. In plants where RTDs are installed inthermowells in the primary coolant pipes, response-time requirements have a range of 4.0–8.0 s versusthe direct-immersion RTDs installed in bypass loops which have a required response range of 1.0–3.0 s.The variety of problems that can affect the accuracy and response time of RTDs is extensive: dynamicresponse problems, failure of extension leads, low-insulation resistance, premature failure, wrong cali-bration tables, loose or bad connections, large EMF effects, open elements, thinning of the platinum wire,lead-wire imbalance, seeping of chemicals from the connection head into the thermowell, cracking ofthe thermowell, and erroneous indication. The causes of core-exit thermocouples failure can take theform of large calibration shifts, erratic and noisy output, saturated output, accidental reverse connec-

tions, and response-time degradation. Several effective methods for detecting RTD and thermocoupleperformance failure while the plant is operating are available. To detect accuracy problems, the cross-calibration technique is effective for both RTDs and core-exit thermocouples. It involves recording thereadings of redundant online RTDs, averaging these readings, and calculating the deviation of each RTDfrom the average, less any outliers. To detect response time degradation online, the loop current stepresponse (LCSR) test is the most accurate method. However, the noise analysis technique remains the

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. Introduction

Most critical process temperatures in nuclear power plants areeasured using resistance temperature detectors (RTDs) and ther-ocouples. In pressurized water reactor (PWR) plants, for example,

he primary coolant temperature and feedwater temperature areeasured using RTDs, and the temperature of the water that exits

he reactor core is measured using thermocouples. These ther-ocouples, called core-exit thermocouples, are mainly used for

emperature monitoring purposes and are therefore not generallyubject to very stringent requirements for accuracy and response-ime performance. In contrast, primary coolant RTDs typically feed

lant control and safety systems and must, therefore, be very accu-ate and have good dynamic performance.

RTDs have historically had sensing elements made out of plat-num, copper, nickel, and other metals or alloys in which electrical

∗ Corresponding author. Tel.: +1 865 691 1756; fax: +1 865 691 9344.E-mail addresses: hash@ams-corp.com (H.M. Hashemian), jjiang@uwo.ca

J. Jiang).1 Tel.: +1 519 661 2111x88320; fax: +1 519 850 2436.

029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2009.08.030

ponse time degradation in core-exit thermocouples.© 2009 Elsevier B.V. All rights reserved.

resistance is proportional to temperature. Today, the sensing ele-ment of industrial RTDs is almost always made from platinum wire.Early RTDs were often fragile and unstable because the platinum-sensing element became contaminated. Today, industrial RTDs arevery rugged and reliable, and can be used in applications as extremeas: (1) measuring brake temperatures of over 1000 ◦C in high-speedaircraft; (2) measuring oceanographic temperature under very highhydraulic pressures where high accuracy and quick response (lessthan 0.5 s) are important; and (3) measuring primary coolant tem-perature in PWRs at temperatures up to about 350 ◦C, flow rates ofover 10 m s−1, and pressures of about 150 bar (Plumb and Schooley,1982; NRC, 1990).

2. Nuclear-grade RTDs

Nearly 100 suppliers worldwide manufacture RTDs today, butfewer than 10 of them manufacture RTDs for safety-related applica-

tions in nuclear power plants. This is because the size of the marketfor this application is relatively small and the performance andreliability required of these RTDs are very stringent. Nuclear safety-related RTDs must pass environmental and seismic testing underInstitute of Electrical and Electronics Engineers (IEEE) standards

H.M. Hashemian, J. Jiang / Nuclear Engineering and Design 239 (2009) 3132–3141 3133

Table 1Partial listing of suppliers of nuclear-grade RTDs.

RTD manufacturer Model number RTD type

Conax 7N10 Thermowell-mounted7RB4 Direct-immersion7N13 Thermowell-mounted

RdF 21204 Direct-immersion21297 Direct-immersion21232 Thermowell-mounted21458 Thermowell-mounted21459 Thermowell-mounted21465 Thermowell-mounted

Rosemount 104AFC Thermowell-mounted176KF Direct-immersion177HW Thermowell-mounted177GY Direct-immersion

Sensycon 1703 Thermowell-mounted1717 Thermowell-mounted

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diverted from the primary coolant pipe to the location where itstemperature is measured.

In some PWR plants, direct-immersion RTDs are used in primarycoolant pipes as opposed to bypass loops. Fig. 3 shows photographs

Weed N9004 Thermowell-mountedN9007 Direct-immersionN9019 Direct-immersion

o demonstrate that they can survive a loss-of-coolant accidentLOCA), withstand a seismic event, and continue to provide reliableervice under postaccident conditions. The relevant IEEE standardsre (1) IEEE Standard 323, “IEEE Standard for Qualifying Class 1Equipment for Nuclear Power Generating Stations” (2004), and (2)EEE Standard 338, “IEEE Standard Criteria for the Periodic Surveil-ance Testing of Nuclear Power Generating Station Safety Systems”1988).

RTDs qualified for nuclear safety-related applications are mostlysed in the primary coolant system of PWR plants, normallyetween 16 and 32 RTD elements per plant. Some nuclear powerlants have more RTDs. For example, the Arkansas Nuclear One,nit 2 plant (ANO-2) in the United States has more RTDs to off-

et the impact of a temperature streaming problem in its hot-legipes that, while severe, does not pose a risk to the operation orafety of the plant. This temperature streaming, known as “hot-legnomaly,” is inherent in the hot-leg pipes of PWR plants, and itseverity depends on the specific design of each PWR plant. Sim-larly the Sizewell B nuclear power plant in Great Britain uses 60rimary coolant RTDs simply because this plant has a digital instru-entation and control (I&C) system with a complete analog backup

ystem and therefore needs twice as many sensors as most otherWR plants.

The number of RTDs used per PWR plant and the total number ofWR plants in operation worldwide allows us to estimate an inven-ory of fewer than 10,000 nuclear safety-related RTDs, includingpares, worldwide. Considering that the average life of these RTDss about 20 years, fewer than 1000 new RTDs per year are neededn any given year, underscoring the small size of the nuclear-gradeTD market. For that reason, the cost of nuclear safety-related RTDs

s generally very high compared with their commercial counter-arts, and only a handful of (mostly smaller) manufacturers are inhe business of producing them. Large companies cannot normallyustify the overhead, liability, and stringent quality assurance (QA)equired to supply equipment to nuclear power plants. Table 1 listsome of the suppliers of nuclear-grade RTDs.

. Calibration and response time

In addition to excellent reliability and accident survivability,uclear safety-related RTDs are expected to have good calibrationnd fast dynamic response time, as these characteristics are impor-ant to plant safety and economy. Fig. 1 shows a simplified diagramf a primary coolant loop for a PWR plant. In principle, the reac-

Fig. 1. Simplified diagram of a primary coolant loop of a PWR.

tor power (P) is the product of the temperature difference (�T)across the core and the mass flow rate (m) in the primary coolantsystem (i.e., P ≈ m�T). The �T is typically about 30 ◦C; thus, a one-degree error in measuring the �T corresponds to 3.33 percent inpower output. Therefore, calibrating RTDs accurately is very impor-tant to plant economy. For that reason, the primary coolant RTDsin PWR plants are typically calibrated to an accuracy of 0.3 ◦C orbetter before installation.

Fig. 2 illustrates the importance of response time for nuclear-grade RTDs. Fig. 2 shows the primary coolant temperature in aPWR plant experiencing a step change. To ensure safety, the RTDsof the plant are expected to promptly trigger a mitigating action,including a reactor scram if needed. This is why the response timerequirements of primary coolant RTDs in PWR plants are so strin-gent. These requirements differ from plant to plant, however. Forexample, in plants where RTDs are installed in thermowells in theprimary coolant pipes, typical response-time requirements have arange of 4.0–8.0 s. This contrasts with the 1.0–3.0 s that are requiredof the direct-immersion RTDs that are installed in bypass loops.Some plants use bypass lines to help sample the reactor water fromall coolant loops and mix it before it is used to measure the primarycoolant temperature. The RTDs in bypass loops therefore must befast to compensate for the time delay caused when the water is

Fig. 2. Illustration of RTD response to a step change in temperature in the reactor.

3134 H.M. Hashemian, J. Jiang / Nuclear Engineering and Design 239 (2009) 3132–3141

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ig. 3. Photograph and x-rays of direct-immersion Rosemount Model 177GY RTDs.

nd X-rays of two direct-immersion Rosemount Model 177GY RTDsnstalled directly in the primary coolant pipes of PWR plants byabcock and Wilcox (B&W). Fig. 4 shows the thermowell-mountedounterpart of this RTD, which is the Rosemount Model 177HW,lso used predominately in B&W plants.

. Problems with nuclear-grade RTDs

Nuclear-grade RTDs, like their commercial-grade counterparts,an suffer from a wide range of problems affecting both theirccuracy and their response time. These include calibration drift,esponse-time degradation, reduced insulation resistance, erraticutput, and wiring problems, among others. These problems occuress often in nuclear-grade RTDs than in commercial-grade RTDsecause the former are of much higher quality. A test in the

ate 1980s of nearly 100 nuclear-grade RTDs against comparableommercial-grade RTDs under both normal and harsh conditionshowed that nuclear-grade RTDs are generally twice as resilient and

mmune from performance problems as commercial-grade RTDsNRC, 1990).

In the early 1970s, at the height of nuclear power plant devel-pment in the United States, almost all nuclear safety-related RTDsor PWR plants were supplied by Rosemount. Over the years, other

Fig. 4. Photograph and x-ray of Rosemount Model 177HW RTD.

manufacturers have entered the market, and Rosemount has grad-ually reduced its presence. Rosemount RTDs are nevertheless stillused in some PWR plants and have provided excellent service.The experience of the nuclear industry with nuclear-grade RTDsfrom other manufacturers is also very good. However, as Rose-mount reduced its market presence, problems arose in the 1980sas new manufacturers of nuclear-grade RTDs entered the market(Hashemian, 2005). Some examples of these problems are:

1. Dynamic response problems2. Failure of extension leads3. Low-insulation resistance4. Premature failure5. Wrong calibration tables6. Loose or bad connections7. Large EMF effects8. Open element9. Thinning of platinum wire

10. Lead-wire imbalance11. Seeping of chemicals from connection head into thermowell12. Cracking of the thermowell13. Erroneous indication

Although these problems eventually became less frequent as

new manufacturers gained experience with design, development,and testing of nuclear-grade RTDs, they still occur. Nuclear plantpersonnel must have the training and tools to detect and mitigatethem.

H.M. Hashemian, J. Jiang / Nuclear Engineerin

Table 2Examples of problems encountered with response time of nuclear plant RTDs.

Plant Date of problem Response time (s)

Expected Measured Manufacturer

A 1978 5.4 21 XB 1984 4.5 37 Y

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he measured values are from in situ response-time testing performed using theCSR method while the plant was operating.

.1. Dynamic response problems

The dynamic response of an RTD is obtained by measuringts reaction to a step change in temperature outside the sensinglement (Wu et al., 1979). Table 2 provides three examples of RTDesponse-time failures in nuclear power plants. The RTDs involvedere from three different manufacturers, and these problems

ccurred in three different plants. Almost all cases involvingynamic response problems are caused by problems at the pointhere the RTD and its thermowell interface, at the sensing tip of

he assembly, specifically, dirty RTDs, dirty thermowells, residueeft from using thermal coupling compounds in the thermowell,nd dimensional tolerance issues involving the RTD and/orhermowell.

The best method for detecting dynamic response problems iso use an in situ response-time measurement technique called theoop current step response test (see Section 6.2.1).

.2. Failure of extension leads

In a typical RTD, an outer sheath and internal insulation hold inlace both the sensing element and extension lead wires that carryhe signal of the RTD from the sensing element to the transmitter.s new manufacturers of nuclear-grade RTDs were emerging in

he early 1980s, a number of RTD failures were caused by failure ofhe extension leads. Typically, the reason was that defective silveroldering had been used to attach the RTD leads to the extensionires protruding from the sensor.

.3. Low-insulation resistance

Insulation resistance (IR) is the electrical resistance betweenny extension lead that exits the RTD and earth ground. As theemperature increases, the insulation resistance decreases. A highnd stable insulation resistance is important for the accuracy of anTD. Most manufacturers accept that industrial RTDs must haven insulation resistance of at least 100 M� at room temperature20 ◦C) when measured with an applied voltage of 100 VDC. Mostuclear-grade RTDs readily meet this requirement, and their IRften reaches the giga-ohm range or higher. However, if moisturenters the RTD, the IR value can drop to as low as a few kilo-ohms.ften, even a very large drop in the IR is not apparent unless IR iseasured.To avoid this problem, before RTDs are installed in a plant

hey should be tested to ensure sufficient IR. Insulation resistances measured using an instrument called a megohm meter thatpplies 50–100 V DC across the insulation between any RTD wirend the sheath. Insulation resistance measurements are often dif-cult to make if there is much moisture in the RTD because theegohm meter will not remain stable enough to make a reliable

easurement. To overcome this problem, most procedures givespecific duration for the measurement. Minor insulation resis-

ance problems can be corrected by heating to drive the moistureut.

g and Design 239 (2009) 3132–3141 3135

4.4. Premature failure

Another consequence of the emergence of new RTD manu-facturers in the early 1980s was an unusually high failure rateafter installation. In fact, a batch of nuclear-grade RTDs fromone of the recognized manufacturers experienced a failure rateof about 50 percent early in their life. Although the failurerate of new nuclear-grade RTDs has been low; nuclear plantpersonnel need to be aware that “infant mortality” can be asource of component failure for RTDs and thermocouples (NRC,1990).

4.5. Wrong calibration tables

Nuclear plant RTDs are supplied with calibration tables gener-ated by the manufacturer. A few cases of significant temperatureerrors have been traced to improper factory calibration. In one case,a computer-aided calibration of several RTDs had not allowed theRTDs to come to equilibrium with the bath before measurementswere made. As a result, the RTDs indicated lower than the actualtemperatures. Cases have also occurred in which the calibrationcharts of different batches of RTDs were interchanged, leaving anuclear plant with a batch of RTDs but calibration charts belongingto a different batch.

4.6. Loose or bad connections

There are a number of transition points in an RTD circuit fromthe field to the instrument cabinets in the control room area. Alongthis path are terminal blocks, weld/solder joints, or splices whereloose or bad connections have frequently been found. LicenseeEvent Reports (LERs) of the Nuclear Regulatory Commission andthe Nuclear Plant Reliability Data System (NPRDS) of the nuclearindustry contain multiple examples of loose or bad connectionsrelating to RTDs. Of the 318 RTD failures reported in NPRDS overa period of about 10 years, for example, loose or bad connec-tions were the sixth-leading cause of RTD failure (accountingfor almost 10% of all failures). (Defective circuits, open circuits,normal and abnormal wear, miscalibration, and short/groundedfailures were, in descending order, the more common causes of RTDfailure.)

4.7. Large EMF errors

EMF, which stands for electromotive force, is a voltage sig-nal that may develop in an RTD circuit if an RTD is designedpoorly or contains dissimilar metals. EMF interferes with resistancemeasurement and can cause as much as 0.5 ◦C error depend-ing on the RTD, the application, and the temperature in whichit is used. If EMF occurs, the resistance of the RTD will dependon the measurement polarity. That is, if the resistance is mea-sured with one polarity, then the result will be slightly differentthan when the resistance measurement is repeated with reversepolarity.

Table 3 shows the results of a laboratory experiment thatinvolved six RTDs from two manufacturers of nuclear-grade RTDs.These RTDs were placed in an oil bath along with a standard RTDthat was used to measure the bath temperature. The output of eachRTD was measured with normal and reversed polarity. At the sametime, the open-circuit voltage at the RTD output was measured. ForManufacturer A, the three RTDs showed 80 �V of EMF voltage, and

polarity. For Manufacturer B, there was no EMF effect and almostno difference between the temperatures indicated by each RTD inthe normal or reverse polarity. In other words, the EMF effect, ifpresent, will cause temperature error. The error changes with time

3136 H.M. Hashemian, J. Jiang / Nuclear Engineering and Design 239 (2009) 3132–3141

Table 3Example of EMF problems with nuclear plant RTDs.

RTD I.D. Bath temp. (◦C) EMF (�V)

Standard RTD Normal polarity Reversed polarity

Manufacturer AA-1 285.33 285.59 285.45 80A-2 293.59 293.83 293.66 80A-3 300.36 300.62 300.41 80

Manufacturer BB-1 285.33 285.28 285.28 0B-2 293.59 293.56 293.58 0B-3 300.36 300.33 300.31 0

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nd temperature and is more pronounced at high temperatures andith shallow immersion depths.

To check for EMF, the RTD resistance is measured, the leadsre reversed, and the measurement repeated. If the results are dif-erent, thermoelectric effects are present in the RTD. When EMFccurs, the true temperature can still be obtained by averaginghe results of the two measurements. In precision thermometry,esistance measurements are made using AC bridges as opposedo DC bridges. This is because an AC bridge cancels any EMF effectn the circuit, yielding the true resistance of the RTD. In essence,n AC bridge works as if it measures the RTD resistance withorward and reverse polarities and displays the average of thewo.

To eliminate EMF errors, the extension wires in high qualityTDs are made of platinum to minimize thermoelectric effects.

.8. Open element

The platinum element in RTDs is very fragile and can crack orpen as a result of vibration, stress, and interaction with otheraterial in the RTD. Fig. 5 shows an electron microscope photo

ig. 6. Electron microscope photo of an open platinum-sensing element in auclear-grade RTD.

Fig. 7. Erratic behavior preceding the failure of a primary coolant RTD at a PWRplant.

of a nuclear-grade RTD element, and Fig. 6 shows an electronmicroscope photo of the platinum-sensing element of anothernuclear-grade RTD that has failed open. Usually, the weak pointswhere RTD elements fail are in weld points and places where theelement is bent.

RTD failures due to open elements are sometimes precededby erratic behavior whereby the RTD indication experiences largeswings, spikes, and random shift. Fig. 7 shows online monitoringdata of four hot-leg RTDs in a PWR plant (NRC, 1998). One of the fourRTDs exhibits erratic behavior. A month or so after this observation,this RTD failed open and was replaced. Interestingly, this behaviorwas not seen by the plant operators. In fact, during daily channelchecks, the bad RTD continued to pass as its indication agreed wellwith the other three RTDs and met the acceptance criteria of theplant.

4.9. Thinning of platinum wire

As noted, the sensing elements of industrial and nuclear-gradeRTDs are almost always made from fine platinum wire because plat-inum has superior properties. Platinum wire is more linear thanmetals like copper and nickel, and it has a much wider temperaturerange. The sensing elements of nuclear-grade RTDs have experi-enced corrosive thinning caused by the chemicals that were usedto clean the elements when they or the RTD were manufactured.This causes the cross section area of the sensing wire to decreaseand its resistance to increase. Thinning of the RTD element can alsoresult from the chemical interaction between the element and theRTD insulation material.

4.10. Lead-wire imbalance

Imbalanced lead wires are a potential problem in three-wireRTDs that are connected to three-wire Wheatstone bridges for mea-suring temperature. The two wires from across the RTD elementthat run from the sensing element to the resistance measuringequipment must have equal resistances.

Fig. 8 shows three-wire and four-wire RTD configurations. In afour-wire arrangement, the lead-wire resistances are completelycompensated, while in a three-wire arrangement, the resistance ofwire 3 (R3) must be equal to the resistance of wire 1 (R1) or wire2 (R2), depending on which wire is used as the common wire in

the three-wire bridge. If the resistance of wire 3 (R3) is not equalto the resistance of one of the remaining two wires, then an errorwill arise in temperature measurement. This lead-wire imbalanceerror can cause about 0.10 ◦C error, on average, at a temperature ofabout 300 ◦C.

H.M. Hashemian, J. Jiang / Nuclear Engineerin

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ture.Thermocouples can also suffer response-time degradation as

they age. Table 4 shows response-time results for core-exit ther-mocouples in four similar PWR plants with nearly identicalthermocouples. Note that the average response time of the ther-

Table 4Results of trending the performance of core-exit thermocouples in PWR plants.

Plant Years in service Thermocouple response time (s)

Fig. 8. HMH provide.

A similar problem can occur in RTDs that have dummy compen-ating leads.

.11. Seeping of chemicals into thermowell

The thermowell is the protective jacket or tube that protectsell-type RTDs from the process fluid and allow them to be easily

eplaced. The response time of well-type RTDs is extremely sensi-ive to installation in a thermowell. In some RTDs, the connectionead is filled with chemical foam to help it qualify for nuclear ser-ice. When the chemical seeps through into the RTD thermowell,s has occurred at some nuclear power plants, response time andther problems have resulted.

.12. Cracking of thermowell

Another issue involving well-type RTDs is a cracked thermowell.

TDs should extend into the reactor coolant piping to a depth that

s consistent with the fluid forces on the RTD and the mechanicaltrength of the RTD/thermowell assembly. Instances have occurredhere fluid forces have caused the assembly to bend and crack,

ncreasing the potential for a Loss of Coolant Accident (LOCA) orhearing-off of the entire assembly.

g and Design 239 (2009) 3132–3141 3137

4.13. Erroneous indication

Nuclear-grade RTDs have been found to suffer significant indi-cation problems, the causes of which can be quite varied. Amongthe worst RTD indication problems observed by the author in U.S.nuclear power plants are a 4 ◦C calibration shift within two yearsfor one RTD, a 2.7 ◦C difference between two elements of a dualRTD, a 3.3 ◦C indication error caused by dirty RTD contacts, a 1.1 ◦Cerror caused by low-insulation resistance, and a 0.6 ◦C indicationerror resulting from a wire-shielding problem.

5. Problems with core-exit thermocouples

PWR plants typically have between 50 and 60 core-exit ther-mocouples. Based on the author’s informal assessment of thesethermocouples in nearly 50 nuclear power plants, between 10 and20 percent of PWR core-exit thermocouples fail within the first20 years of plant operation. These failures take the form of largecalibration shifts (e.g., 10–30 ◦C errors at 300 ◦C), erratic and noisyoutput, or saturated output. Some core-exit thermocouples developcable problems while the thermocouple is still intact. Plants havebeen known to replace a core-exit thermocouple and later discoverthat the problem was not in the thermocouple but in the thermo-couple extension cables, connectors, or elsewhere in the circuit. Toavoid these problems, cable testing should be performed before athermocouple is replaced to distinguish cable/connector problemsfrom thermocouple problems.

Thermocouples can also be accidentally reverse-connected,meaning that the positive and negative thermocouple wires maybe crossed during the installation or wiring of the thermocouple.In such cases, at room temperature the thermocouple indicationcould appear to be normal, but as the temperature is increased,the thermocouple will show a negative reading. In one U.S. nuclearfuel facility (see NRC Information Notice IN-96-33), engineersattempted to correct for a reverse-connected thermocouple byreversing its extension leads at the indicator. Originally, the ther-mocouple appeared to read correctly at room temperature, butits indication became negative when the process began to heatup. Thus, the thermocouple leads were interchanged at the indi-cator, which made the reading positive and seemingly correct atlow temperatures. However, unbeknownst to the engineers, as thetemperature was increased, the thermocouple indication fell moreand more below the actual process temperature to the point that atabout 600 ◦C, the thermocouple showed 450 ◦C. This caused a fireat the nuclear fuel fabrication facility.

Thermocouples, even those that are properly connected, canhave good indication at room temperature but diverge from truetemperature as the plant heats up. Fig. 9 shows online monitoringresults for a group of core-exit thermocouples in a PWR plant. Thethermocouple readings are comparable at cold shutdown, but onedeviates significantly from the others at plant operating tempera-

Average High Low

A 30 2.01 2.9 0.6B 30 1.96 2.8 0.6C 10 0.97 1.5 0.5D 10 1.10 1.5 0.6

3138 H.M. Hashemian, J. Jiang / Nuclear Engineeri

Fig. 9. On-line monitoring results for a group of core-exit thermocouples.

Table 5Potential sources of error and their estimated values in industrial temperature mea-surements with thermocouples (for 50–500 ◦C range).

Source Abbreviation Range of error

Inherent thermocouple errors (TC) ±0.5–5 ◦CCold junction compensation error (CJ) ±0.1–0.5 ◦CA/D errors (AD) ±0.1–0.2 ◦CPrecision errors (Noise) ±0.1–0.5 ◦C

Total error = (TC) + (CJ)

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mismatch, grounding, etc.

ocouples after 10 years of service is about 1 s and after 20 yearsf service is about 2 s, a 100 percent increase. Although this increases not an operational or safety issue for the plant, it indicates that ashermocouples age, their response time increases. This and otherimilar observations have motivated some nuclear power plantso perform response time testing on thermocouples as a meansf aging management (Hashemian et al., 1990). In particular, it ismportant to establish the baseline response time of thermocou-les when the plant is new or the thermocouples are first installednd then repeat the measurements periodically (e.g., once everyve years) to determine if there is degradation from the nominalerformance.

In general, thermocouples are not as accurate as RTDs. This isartly because thermocouples are not normally calibrated individ-ally. Rather, thermocouple wires or a representative sample in a

arge batch of thermocouples are calibrated, and that calibration issed for all thermocouples in the batch. Table 5 shows estimatedemperature measurement accuracies of industrial thermocoupless a result of four error source types.

. Methods for detecting problems with RTDs and core-exithermocouples

Appropriate online testing can avoid the problems that affect theccuracy and response time of nuclear-grade RTDs and core-exithermocouples.

.1. Calibrating RTDs and core-exit thermocouples

In most PWR plants, the accuracy of RTDs such as the primaryoolant RTDs is verified periodically to ensure that any unaccept-ble drift or deviation is identified and corrected. For this purpose,he cross-calibration technique is used.

ng and Design 239 (2009) 3132–3141

6.1.1. Cross-calibration techniqueThe cross-calibration technique is a means for verifying online

that the calibration of a group of redundant sensors that measurethe same process parameters have not suffered a significant change.Its basic principle of operation is to record the reading of redun-dant sensors, average these readings, and calculate the deviationof each sensor from the average of the redundant sensors, less anyoutlier(s). The narrow-range RTDs are normally the most accuratetemperature sensors in a PWR plant and are therefore used to pro-vide the reference temperature for the cross-calibration test. Insome plants, only the narrow-range RTDs are cross-calibrated; inothers, the wide-range RTDs and/or core-exit thermocouples arealso included in the cross-calibration test. Typically, the outputsof plant sensors are sampled by the plant computer, convertedinto temperature, and stored. The sampling period depends on theplant and typically ranges from 1 to 10 s for RTDs and 10 to 60 s forcore-exit thermocouples.

Table 6 shows results from a typical cross-calibration test in anuclear power plant. The data for this test was collected at isother-mal conditions during plant heatup at approximately 280 ◦C. Thisprintout, referred to as a cross-calibration run, typically includesfour data-collection passes for each sensor. After data for a run iscollected, the results of the four measurements for each sensor areaveraged and recorded under a column marked “Average Temp.” Inthe example in Table 6, the average temperatures for all the narrow-range RTDs are averaged, and the result is subtracted from eachindividual average to identify the “deviation” of each sensor fromthe average of the narrow-range RTDs. The “Deviation” column isthe result of the cross-calibration run. These results are referred toas the preliminary results of the cross-calibration test. Typically, thedata is analyzed further to establish the final results of the cross-calibration test and to quantify the uncertainty of the deviationresults. The final results must include the necessary corrections forany significant instability and nonuniformity in the plant temper-ature when the cross-calibration data was collected (NRC, 1997).Also, test engineers must establish a means to verify that there isno common drift. This can be accomplished by, for example, cali-brating one of the RTDs that was involved in the cross-calibrationtest. Alternatively, test engineers can rely on historical drift data forthe type of RTDs that are cross-calibrated. An R&D effort in 1990found that most nuclear-grade RTDs showed random drift patternsrather than systematic drift patterns (NRC, 1990).

Any narrow-range RTD that deviates from the average by morethan a predetermined criterion (e.g., 0.3 ◦C) is excluded from theaverage. The criteria are different in different plants and usuallydepend on plant accuracy requirements for the primary coolanttemperature. Table 7 provides the criteria for RTD cross-calibrationused in seven nuclear plants and indicates both the deviation atwhich a narrow-range RTD is excluded from the average and thenumber of temperatures at which cross-calibration data is col-lected.

The cross-calibration process results in a preliminary analysis ofthe raw cross-calibration data and provides the preliminary resultsof the cross-calibration test. For more accurate results, the rawdata are corrected for any significant temperature fluctuations thatmight have been present when data was collected. This is referredto as instability correction. Also, the raw data should be correctedfor any significant temperature differences between the primarycoolant loops or the hot-leg and cold-leg temperatures. This isreferred to as nonuniformity correction.

Although simple, the cross-calibration process involves numer-

ous calculations. As such, it is prudent to automate the process tofacilitate the data-collection and data analysis tasks. Automatedsoftware can read and plot the raw data, perform the analysisincluding corrections for plant temperature instability and nonuni-formity, and print the results.

H.M. Hashemian, J. Jiang / Nuclear Engineering and Design 239 (2009) 3132–3141 3139

Table 6Results of a typical cross-calibration run.

RTD Temperature (◦C) Average temp. (◦C) Dev. temp. (◦C)

Pass 1 Pass 2 Pass 3 Pass 4

Narrow-range RTDs1 280.3278 280.3274 280.3087 280.2956 280.315 −0.0632 280.4091 280.3942 280.3853 280.3797 280.392 0.0143 280.3616 280.3621 280.3426 280.3305 280.349 −0.0294 280.3660 280.3655 280.344 280.3347 280.353 −0.0265 280.4729 280.4599 280.4608 280.4571 280.463 0.0846 280.3664 280.3329 280.3427 280.3274 280.342 −0.0367 280.3392 280.3276 280.3230 280.3178 280.327 −0.0518 280.4709 280.4574 280.4504 280.4453 280.456 0.0789 280.3308 280.3312 280.3047 280.3029 280.317 −0.061

10 280.4369 280.4355 280.4081 280.4118 280.423 0.04511 280.3765 280.3584 280.3477 280.3440 280.357 −0.02212 280.4593 280.4584 280.4375 280.4296 280.446 0.068

Wide-range RTDs13 280.0733 280.0612 280.0538 280.0352 280.056 −0.32214 280.6964 280.6871 280.6741 280.6602 280.679 0.30115 280.3290 280.3281 280.3067 280.3039 280.317 −0.06116 280.4881 280.4899 280.4704 280.4686 280.479 0.101

Core-exit thermocouples17 280.6723 280.6674 280.6261 280.6431 280.652 0.27418 280.6301 280.6082 280.5928 280.6025 280.608 0.23019 280.7786 280.7802 280.7640 280.7526 280.769 0.39020 280.5482 280.5660 280.5474 280.5474 280.552 0.17421 280.8232 280.8110 280.7940 280.7907 280.805 0.42622 280.8978 280.8588 280.8483 280.8248 280.857 0.47923 280.7680 280.7607 280.7445 280.7380 280.753 0.374

AN h sens

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TR

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24 281.1411 281.1394 281.139425 280.8037 280.7940 280.7510

verage temperature indicated by the narrow-range RTDs = 280.378 ◦C.ote: The deviation column in this table is equal to the average temperature of eac

The cross-calibration technique has been validated for bothTDs and thermocouples in a laboratory setting involving an oilath, an SPRT, and a set of cross-calibration test equipment (NRC,990).

able 7TD cross-calibration criteria in various PWRs.

Nuclear plant Temperature points Outlier criteria (◦C) Remarks

1 1 0.17 12 1 0.17 13 4 0.11 24 2 0.30 35 4 0.27 46 1 0.17 57 4 0.11 6

ootnotes: (1) Plant stability criteria for RTD cross-calibration are typically about0.15 to ±0.3 ◦C. (2) Temperature points: number of temperatures at which cross-

alibration data is collected.emarks:1) Cross-calibration data is taken for any number of plateaus. However, only theata for 292 ◦C is used to meet acceptance criteria and adjust the temperature trans-itters, as needed.

2) Data can be taken at a constant heatup rate. On 16 RTDs, data is taken as follows:TD number 1–16, reverse current 16–1, reverse current 1–16, etc. This presumablyorrects for both the ramping temperature and for EMF effects (reversing the cur-ent). Data is taken around 95, 170, 230 and 292 ◦C.3) Two plateaus: 170 and 292 ◦C. For deviations greater than 0.17 ◦C, the deviationst 170 and 292 ◦C are used to determine the error offset and the slope and to applyhe corrections to the temperature transmitter.4) Data is taken at four temperatures on 16 RTDs sequentially, 1–16, 16–1, etc. Theeatup rate is also measured.5) The 16 RTDs in this plant are tested one channel (four RTDs) at a time. Data isaken for 25 min at 5-min intervals. This is repeated for all four channels. The planttability requirement for the tests is 0.17 ◦C (i.e., the temperature cannot change byore than 0.17 ◦C from the beginning to the end of any test run).

6) Tests are performed at 120, 180, 230, and 275 ◦C. Data is taken on 16 RTDs, 1–16,6–1, 1–16, and 16–1. The four calibration points are used to determine a zero andslope for the correction to temperature transmitters.

281.1086 281.132 0.754280.7656 280.779 0.400

or minus the average temperature of the narrow-range RTDs.

6.1.2. Cross-calibrating core-exit thermocouplesThermocouples are also cross-calibrated in some plants. In PWR

plants, at isothermal conditions, the primary coolant RTDs andcore-exit thermocouples are at essentially the same temperature.For that reason, the core-exit thermocouples are cross-calibratedagainst the primary coolant RTDs because the latter are usuallymore accurate than thermocouples. Cross-calibrating thermo-couples involves subtracting the indication of each core-exitthermocouple from the average temperature indicated by thenarrow-range RTDs. The results are then tabulated, as shown inTable 8.

6.2. Response time testing of RTDs and core-exit thermocouples

Testing the response times of RTDs and thermocouples in situ(while the plant is operating) enables nuclear power plants to mea-sure the “in-service” response time of the sensor to meet technicalspecification requirements, regulatory regulations, or both. Plantscan also verify that plant sensors bottom out in their thermowellsand test for air gaps, dirt, and foreign objects in the thermowell.In situ response time testing also provides for predictive main-tenance, incipient failure detection, and aging management andenables plants to establish objective schedules for replacing sensors(IAEA, 2000). Finally, response time testing enables plants to distin-guish between sensor problems and cable or connector problemsand to diagnose sensor or process anomalies.

In almost all U.S.-made PWR plants, testing RTD response timesis required and is performed on one or more RTD channels onceevery operating cycle (NRC, 1997). For thermocouples, response-

time testing is not mandatory, but some plants perform the testsfor one or more of the reasons just noted.

Until 1977, testing the response times of temperature sensorsin nuclear power plants was almost always performed using theplunge test (Kerlin et al., 1980). This is defined as the time it takes

3140 H.M. Hashemian, J. Jiang / Nuclear Engineering and Design 239 (2009) 3132–3141

Table 8Results of thermocouple cross-calibration.

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he sensor output to achieve 63.2 percent of its final value after atep change in temperature is impressed on its surface. In nucleareactors, however, plunge testing is inconvenient because the sen-or must be removed from the reactor coolant piping and taken to aaboratory for testing. Nuclear reactor service conditions of 150 bar2,250 psi) and 300 ◦C (572 ◦F) are difficult to reproduce in the labo-atory. Therefore, all laboratory tests are performed at much milderonditions, and the results are extrapolated to service conditions.he combination of manipulating the sensor and extrapolating theesults to service conditions leads to significant errors in the mea-urement of sensor response times, sometimes by as much as aactor of three.

Because of these problems the nuclear industry therefore devel-ped the following three methods: the loop current step responseLCSR) test, the self-heating test, and the noise analysis technique.n the LCSR method, the sensing element is heated by an electricurrent, and the temperature transient in the element is recorded.rom this transient, the response time of the sensor to changes inxternal temperature is identified (Hashemian, 2005). In the self-eating test, as in the LCSR test, the sensing element is also heatedy an electric current. After the RTD output settles, the steady-state

ncrease in RTD resistance is measured as a function of the elec-ric power applied to the sensor. The result is referred to as theelf-heating index (SHI). Any significant change in SHI indicates ahange in RTD response time. Therefore, the SHI can be tracked toetermine the degradation of RTD response time. In the noise anal-sis technique, the natural fluctuations (noise) that normally occurt the sensor output during plant operation are recorded and ana-yzed to determine the response time of the sensor (Hashemian etl., 1988).

The LCSR method is the most commonly used technique forTDs. The self-heating test is applicable only to RTDs and is usedo identify only changes in response time; it does not measureesponse time itself. The noise analysis technique is useful in appli-ations where accuracy is not critical, for example, where a large

argin exists between the nominal response time of the sensor

nd the required response time of the plant. It is also the primaryethod used in the nuclear power industry for testing the response

ime of thermocouples, which are not held to the same stringent

Fig. 10. Comparison of Raw and Transformed LCSR Data.

response-time standards as RTDs. We will therefore discuss theLCSR method for response-time testing RTDs before discussing thenoise analysis technique for response-time testing of thermocou-ples.

6.2.1. The LCSR response-time testing method for RTDsThe LCSR method is the most commonly used technique for test-

ing the response time of RTDs because it can yield the response timeof a sensor with such accuracy. The LCSR test involves injectingthe RTD with an electrical current applied at the end of the sensorextension leads. The current causes Joule heating in the sensor andresults in a temperature transient inside the sensor. The time plot,of either the heating while the current is applied or the coolingafter the current is discontinued, is recorded. From this plot, theresponse time of the RTD is obtained by means of the LCSR trans-formation, which is a way of converting the LCSR transient fromthe internal heating of a sensor to yield the dynamic response ofthe sensor to a step change outside the sensor. Fig. 10 compares araw LCSR transient for an RTD, the converted LCSR transient, andthe response of the RTD to a step change in temperature outsidethe RTD, obtained using a plunge test. This data is derived from alaboratory test in which the RTD is placed in a rotating tank of waterat room temperature, ambient pressure, and a flow rate of 1 m s−1.

The LCSR test accounts for all the effects of installation and pro-cess conditions on response time and thereby provides the actual“in-service” response time of the sensor. It has been validated forseveral RTD models of the type used in nuclear power plants. Gen-erally, the accuracy of in situ response-time measurements madewith the LCSR test is ±10 percent. This is provided that: (1) the LCSRmethod has been validated for the sensor under test; (2) the LCSRdata is clean and smooth; and (3) optimum analysis parameters areused to process the data. Furthermore, the conditions of the plantmust be suitable for the LCSR test. This means that the plant tem-perature must be stable and undergo no significant fluctuations ordrift during the LCSR tests.

6.2.2. The noise analysis response-time testing method forthermocouples

Thermocouples are not routinely tested for response time innuclear power plants because response time is not as importantfor nuclear power plant thermocouples as it is for RTDs. When theresponse time of thermocouples is tested, nuclear power plants

prefer to use the noise analysis technique, because the LCSR testfor thermocouples, although more accurate, requires high heatingcurrents (>500 mA), which could degrade the seal of the thermo-couples and/or insulation material.

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U.S. Nuclear Regulatory Commission (NRC), 1998. Advanced instrumentation andmaintenance technologies for nuclear power plants. NUREG/CR-5501, Wash-

H.M. Hashemian, J. Jiang / Nuclear Eng

The noise analysis technique is based on the fact that theutput of all process sensors in nuclear power plants normallyontains fluctuations due to random flux, random heat transfer,urbulence, vibration, and other mechanical and thermal hydraulichenomenon. These fluctuations (noise) can be extracted from theensor output and analyzed to yield the response time of the sensor.he test involves three steps – data acquisition, data qualification,nd data analysis.

The normal output of a thermocouple is a DC signal on which therocess noise (AC signal) is superimposed. During the data acqui-ition step, the noise is extracted from the thermocouple outputy removing the DC component of the signal and amplifying theC component. This is accomplished simply by using commercialignal-conditioning equipment, including amplifiers, filters, andther components. The AC signal is then digitized using a highampling rate (e.g., 1 kHz) and stored for subsequent analysis.

During data qualification, the raw data must first be thor-ughly scanned and screened before any analysis can resume. Thiss normally accomplished by using data qualification algorithmsmbedded in software, which check for the stationary and linearityf the raw data and look for other abnormalities.

During data analysis, noise data is analyzed in the frequencyomain and/or time domain. For frequency domain analysis, theower Spectral Density (PSD) of the noise signal is first obtainedhrough a Fast Fourier Transform algorithm or its equivalent. Next,mathematical model of the thermocouple is fit to the PSD, fromhich the response time of the system is calculated. The PSDs ofuclear plant thermocouples have various shapes, depending onhe plant, the thermocouple installation and service, the processonditions, and other effects. For time domain analysis, the noiseata is processed using a univariate Autoregression (AR) model-

ng program. This provides the impulse response (i.e., response tonarrow pressure pulse) and the step response, from which the

esponse time of the system is calculated. Typically, the noise datas analyzed in both the frequency domain and time domain, and theesults are averaged to obtain the response time of the system.

. Conclusion

Nuclear-grade RTDs and core-exit thermocouples are made toigher specifications from higher quality parts than are commer-

g and Design 239 (2009) 3132–3141 3141

cial, industrial-grade temperature sensors. Nevertheless, they aresusceptible to a range of potential problems, affecting both theiraccuracy and their response time. The industry has developed arange of methods for detecting these problems while these sen-sors are online in operating nuclear plants. The cross-calibrationmethod (for accuracy) and the loop current step response test(for response time) are among the most effective and the mostwidely used RTDs. Potential sensor problems notwithstanding,nuclear-grade RTDs and thermocouples remain durable instru-ments. Unless specific problems arise that require more frequenttesting or plants have reason to suspect that their sensors are defi-cient in design, fabrication, or installation, the current industrypractice of verifying accuracy and response time at least once everyfuel cycle is reasonable. Barring these problems, the typical nuclear-grade RTDs can last 10–40 years depending on the conditions ofits use.

References

Hashemian, H.M., 2005. Sensor Performance and Reliability. ISA – Instrumentation,Systems, and Automation Society, Research Triangle Park, North Carolina.

Hashemian, H.M., Petersen, K.M., Mitchell, D.W., Hashemian, M., Beverly, D.D., 1990.In Situ Response Time Testing of Thermocouples, 29. ISA Transactions, p. 97+,no. 4.

Hashemian, H.M., Thie, J.A., Upadhyaya, B.R., Holbert, K.E., 1988. Sensor ResponseTime Monitoring Using Noise Analysis. Prog. Nucl. Energy 21, 583–592.

International Atomic Energy Agency, 2000. Management of Ageing of I&C Equipmentin Nuclear Power Plants. IAEA Publication TECDOC-II47, Vienna, Austria.

Kerlin, T.W., Miller, L.F., Hashemian, H.M., Poore, W.P., Shorska, M., Upadhyaya, B.R.,Cormault, P., Jacquot, J.P., 1980. Temperature sensor response and character-ization. EPRI Report No. NP-I486, Electric Power Research Institute, Palo Alto,California.

Plumb, H.H., Schooley, J.F., 1982. Temperature, its Measurement and Control in Sci-ence and Industry. Office of Ordnance Research, Instrument Society of America,National Bureau of Standards, American Institute of Physics.

U.S. Nuclear Regulatory Commission (NRC), 1990. Aging of Nuclear Plant ResistanceTemperature Detectors. NUREG/CR-5560, Washington, DC.

U.S. Nuclear Regulatory Commission (NRC), 1997. Standard review plan for thereview of safety analysis reports for nuclear power plants. NUREG-0800, Wash-ington, DC.

ington, DC.Wu, S.M., Hsu, M.C., Chow, M.C., 1979. Determination of time constants of reactor

pressure and temperature sensors: the dynamic data system method. Nucl. Sci.Eng. 72 (1), 84–96.