development chemistry control system secondary candu reactors

Development of an automatedchemistry control system forsecondary coolant circuits inCANDU nuclear power reactorsJ.R. Dean * and R.B. StewartAnalytical Science Branch, Atomic Energy of Canada Limited,lChiteshell Nuclear Research Establishment, Pinawa, Manitoba ROE 1LO, Canada.

IntroductionThe CANada Deuterium Uranium-Pressurized Heavy Waternuclear power reactor (CANDU-PHW) has two separate heattransport systems, with heavy water circulating in theprimary heat transport side and natural water in thesecondary side. Both systems require precise chemicalcontrols, the ultilnate aim of which is to minimize metalcorrosion. Corrosion on the secondary side of the steamgenerator could lead to boiler-tube failure and breakdown ofthe barrier between the two systems. While failures of boilertubes in CANDU power reactors have been very rare, thechance of such corrosion may be greater in those reactorswhich will make use of saline water for condenser cooling.There is also evidence in the industry that the probability offailures increases with increasing unit size and rating.CANDU 600 boilers of the future will be larger, operate athigher temperatures and higher heat fluxes and use somewhat

* Present Address: Powell Analytical Consulting & Service Ltd.,2023 2nd Avenue, S.E., Calgary, Alberta T2E 6K1

Table 1. Chemical specifications for feedwater and boiler waterduring leaking and non-leaking condenser conditions

Feedwater in systems using high nickel alloys for boiler tubes andcopper alloys in the feedtrain or condenser:

Parameter Permissible Range Desired ValuepH at 25C 8.8 9.2 9.1Dissolved 02,/.tg/kg < 5 < 5Hydrazine, tg/kg 10 20 10Iran, #g/kg < 10 < 10Copper, #g/kg < 10 < 10

Boiler water during normal operation:

Parameter Permissible Range Desired ValuepH at 25C depends on amine solution usedFree caustic, mg/kg < 0.1 < 0.1Dissolved 02, #g/kg < 5 < 5Silica, mg/kg < 5 < 5Total dissolved solids, mg/kg < 10 < 10

Boiler water during condenser leak"

ParameterpH at 25CPhosphate, mg/kg[Na]/CPO4 molar ratioFree caustic, mg/kgDissolved 02, tg/kgFluoride & chloride, mg/kgTotal dissolved solids, mg/kg

Permissible Range8.5 10.62-10

2.2 2.6zero

<5<75< 40

Desired Value9.462.3zero<5

as low as possible< 40

different construction materials than the boilers on whichmost of the Atomic Energy of Canada Ltd’s (AECL)experience is based. Therefore, analytical chemists of theWhiteshell Nuclear Research Establishment (WNRE)undertook to devise a system which could sample, analyzeand control the chemistry of a steam generator, automatic-ally. Its development and demonstration are summarized inthis article.

Theory of boiler-tube corrosionand treatmentThe two fundamental requirements for accelerated corrosionof boiler-tubes are (1) impurities in the boiler water whichbecome corrosive when concentrated, and (2) an impurity-concentrating mechanism. Water chemistry conditions incertain regions of a steam generator can deviate substant-ially from the bulk-water chemistry specifications. Forexample, should boiling occur within a porous deposit on atube, a concentrating mechanism is provided by evaporationand retardation of diffusion. In such situations, the presence

mol/kg

I0"5

I0"4 I0"a I0

"z I0" 1.013,OI-

12.01-2.2

I1.01-

.- 2.0I0.01

9.0! Temperature 25C

7.0J,1.0 I0 I00 I000 I0000 I00000

Tolal Phosphate Concentration g/g

Figure 1. Relationshi ofpH, orthophosphate concentrat-ion and Zphos, at 25-C.

252 Journal of Automatic Chemistry

Dean and Stewart Automated chemistry control system

BOILER

BOILER

2

BOI3LER .BOI LER

4

BLOWDOWN

TURBINE

’=’iPHOSPHATEpH

I,SOD. IUM,ICONDENSER

CONDENSER

FERROMAGNETIC DETECTORTURBIDITY, pH

OXYGENHYDRAZINE

SODIUM /

L,P,

HEATERS

DEAERATOR

EXTRACTIONPUMPS

BOILERFEED PUMPS

H.P.

HEATERS

Figure 2. Schematic of automatic chemistry control, system proposed for a CANDU 600 steam generator. Solid linesindicate fluid flowlines, broken lines indicate electrical signals.

of strong solutions of sodium hydroxide has been found tocause ’caustic embrittlement’. Acidic conditions are equallyundesirable.

Minimization of boiler-tube corrosion is achieved, accord-ing to one school of thought, by maintaining high purity offeedwater and boiler water. Maintenance of high purityreduces formation of deposits on the tubes or support platesand eliminates species that become corrosive when concen-trated. According to a second view, it is impractical to totallyprevent potentially corrosive species from entering the boilerwater, and it is thus feasible only to counteract the effects ofsuch species by adding a non-corrosive buffering chemicalsuch as sodium phosphate. This is beneficial in that it causescalcium or magnesium ions to precipitate as phosphatesrather than to deposit on the tubes as sulphate or silicatescales. The phosphates are less adherent than sulphates andsilicates, and can be more easily removed by blowdown.

AECL approach to steam-generatorchemistry controlBoth of the above approaches have merit, and AECL havedecided to incorporate both into the chemistry control ofthe CANDU 600 secondary water. In the absence of acondenser leak, the chemical control will be by the ’zerosolids’, or ’all volatile treatment (AVT)’ mode (Table 1).During this treatment, the pH of the feedwater is adjusted byaddition of a volatile amine such as cyclohexylamine. A lowlevel of dissolved oxygen in t-he feedwater is ensured by the

addition of hydrazine which is also volatile.During AVT, the boiler is unprotected if ingress of potent-

ially corrosive chemicals should occur. For example, if aseawater-cooled condenser should leak, the system would beincapable of altering the tendency of the boiler watertoward acidic conditions. In this situation the alternative toIn simplest terms, this is defined as the ratio of moles ofsodium associated with the orthophosphate species andOH-ions, to moles of orthophosphate. Laboratory experi-ments carried out in the Soviet Union and the United Statesindicate that a Zphos value of between 2.2 and 2.85 isdesirable. AECL specifications require the maintenance of2 to 10 mg/kg orthophosphate and a Zphos value of 2.2to 2.6. Variations outside this range can either lead tocorrosive conditions or a phenomenon known as ’phosphatehideout’.

CPT is maintained by phosphate additions based uponanalytical measurements of the orthophosphate content, andon determination of the Zphos The Zphos is determinedfrom the pH of the boiler-water "at 25C and the orthophos-phate concentration. A relationship exists among these threevariables (Figure 1).

Program for the developmentof an automated systemAutomatic analysis and control using continuous on-linesensors are potentially more rapid, precise and reliable thanmanual control using data provided by a laboratory. The two

Volume No. 5 October 1979 253


main requirements of such an automated system are (1)an ability to control the chemistry in AVT mode, and (2)the sensitive and reliable detection of a condenser leak and, ifthe leak is large enough, the initiation of automatic controlof the boiler using CPT. It is also desirable to provide anability to log data and diagnose system behaviour, with theadded advantage of giving the chemist a summary of paststeam-generator conditions at regular intervals or upondemand.

Figure 2 shows a simplified version of the design conceptsof the chemistry control system for a CANDU 600 steamgenerator. A central, dedicated minicomputer is the basiccomponent. Chemical analysis information is fed to it byon-line analyzers located at strategic points on the boilercircuit. In general commercially available industrial processanalysers were used. Extensive evaluation however wascarried out prior to final choice of instrumentation. Forrapid phosphate determination a system designed using theTechnicon Analyzer principles was used. Details of theanalyzer system have been presented elsewhere. Dependingon the information from these analyzers, the computeractuates valves and pumps which control the addition ofappropriate chemicals at the correct places. For example,operation of the phosphate system is such that the autom-atic addition of phosphate to the boiler will begin when acooling-water leak is confirmed by a signal from the sodiumanalyzers. The computer controls the phosphate concen-tration, and the Zphos set-points, by feedback, on the basisof information supplied by phosphate and pH analyzers onthe composite boiler blowdown line. In addition to initiatingCPT control, the computer can log and display data,interpret alarms, and execute preventive action to correctabnormal system conditions. It was initially perceived thatthe feed water hydrazine and pH control would be analog-

based and would be independent of the computer. In theactual event both were placed under direct digital controlby the computer.

Prior to installation of a prototype in a CANDU station,it was desirable to test several aspects of the design. Saskat-chewan Power Corporation expressed interest in suchdevelopment work. At their Boundary Dam station (Estavan,Canada) two older boilers, Units and 2, are operatedcontinuously in CPT chemistry and three others, Units 3,4 and 5, are operated continuously in AVT chemistry. Aco-operative program was undertaken to establish theconfidence necessary to proceed with a prototype nuclearstation installation.

For reasons of effort and economy, it was believedunnecessary to demonstrate a complete chemistry controlsystem at Boundary Dam. Key elements were thereforeselected for testing. These included the computer, controlalgorithm, sodium analyzers for leak detection, and on-line phosphate and pH instruments. In addition, commercialdissolved oxygen analyzers were compared, hydrazinemixing and reaction rates were studied and a hydrazineanalyzer was tested on-line with feedback control. The on-line calibration of analyzers, and data display and print-outwere also selected for demonstration. Concurrently a rapidon-line procedure for phosphate analysis was developed.A block diagram of the Boundary Dam Station test

arrangement is shown in Figure 3. Since Boundary Dam is alignite coal-fired station with dust and fly ash prevalent, thelocation and protection of the analyzers and computer werecarefully considered. The computer was housed in a clean,air-conditioned room serving other station instrumentation.It was connected by communication cables to the analyzerrooms of Units 4 and 5, and to a temporary shelter, housingthe phosphate and pH analyzer at Unit 1. The hydrazine

Analyser room unit#5 Analyser room unit#4 to boiler #1

7. tZaboiler #1,, ,,phosphate

II dosing pumps II

IIalibratio II

libration

"_" o. JL J ,--I sample fromboiler # 1

Relay room unit#5

r r-

PDP8E

computer

TTY;CRT

Temporary analyser roomunit#1

libratiounit

-1

Figure 3. Schematic of the Boundary Dam Station demonstration lay-out. TTY= teletype.


and dissolved oxygen analyzers were located in anotherclean instrument room. Data transmission lines of up toseveral hundred feet in length were used successfully.

The computer had three tasks: data-logging, CPTselection, initiation and control, and automatic analyzercalibration. The data4ogging mode included sampling datafrom all analyzers once per minute and checking for alarmconditions. The data collected at one minute intervalswere examined and discarded, except that at five-minuteintervals they were displayed on the screen, typed out onreports and stored in the computer files. In Figure 4, thecomputer values of sodium ion (#g/kg) dissolved oxygen(#g/kg), hydrazine (#g/g), phosphate (#g/g) and pH areshown for three time periods of 11 May, 1977. Alsoincluded in the listing is the Zphos value for the Unitboiler and the temperature of the sample delivered to thesodium ion monitor at these times. The number of sig-nificant figures is not an indication of precision and wouldbe ’rounded-off’ to three in a prototype installation.Similarly, the exponential notation would be converted to afloating point format. The stored data were typed threetimes daily, to provide a summary report.

NONTI- 5 55 5/I /77

T HI:: N 0 XY(I hi IIY i,’/

Figure 6. Portion of typical eight-hour report showingCongruent Phosphate control behaviour (Set Z refers toZphos setpoint and Set P refers to the orthophosphatesetpoint).’ZPHOS’ [Na]/CP04.


MONTP, 11:5:29 5/1 /77

MONTH 1b:33 )5/1 1177

MONTR lb:3b 5/11 /77

Figure 4. Typical five-minute report showing the concen-trations (pg/kg) of sodium ion in Unit 4, dissolved oxygenand hydrazine in Unit 5 feedwater and levels of ZPHOS,orthophosphate (pg/g) and pH in Unit 1 boiler. Thetemperature (F) refers to that of the sample flowing tothe sodium ion analyzer.’ZPHOS" refers to [Na] CPO4, the ratio oforthophosphate-derived sodium ion concentration to the sum ofphosphateion concentrations.

Figure 5. A typical CRT display showing the behaviourof hydrazine, dissolved oxygen and sodium ion over aten-hour period: ’ppb’ indicates #g/kg.



III

PO4 Setpoint

TestInjection

Zphos Setpoint

1800 1900

TIME,hours (March 16,1977)

2.35

2.25

Figure 7. Illustration of the behaviour of the CongruentPhosphate Treatment control system during an infectiontest. (Increasing Zphos, no blowdown).Zphos [Na]/CP04, the ratio of orthophosphate-derivedsodium ion concentration to the sum of phosphate ionconcentrations.

In the CANDU prototype, sodium analyzers wouldmonitor the condensate water and the computer wouldinitiate congruent phosphate treatment upon confirmationof a leak (4.5 L/h of sea water was taken as the maximumtolerable leak rate). For the Boundary Dam tests, it wasnecessary to simulate a leak by means of an electrical signal.(This meant that the sodium analyzer tests need not necess-arily be done on the boiler under phosphate control, and itwas convenient to install these analyzers on Unit 4 whichoperates in the AVT mode.) When this was done, thecomputer demonstrated its ability to confirm the indicationusing its leak detection logic, then bring the boiler intocongruent phosphate chemical control by initiating theoperation of monosodium phosphate and disodiumphosphate dosing pumps. Data from three of the analyzers(sodium ion, dissolved oxygen and hydrazine) could bedisplayed on a cathode ray tube, as illustrated in Figure 5.

=2.o

== 1.o

0

8 o.o

PO4 Setpoint

...__ZphsSetpoint

Test Injection

1430 1500 1530 1600

TIME, hours

2.35

2.30

2.25

2.20

2.15

Figure 8. Illustration of the behaviour of the CongruentPhosphate Treatment control system during an infectiontest. (Decreasing Zphos, no blowdown).

16.0

15.0

14.0

o

2.o

Test Injection

Z phos Setpoint

1530 1630 1730

TIME ,hours (March 16,1977)

2.45

2.40

2.35

2.30

2.25

Figure 9. Illustration of the behaviour of the CongruentPhosphate Treatment control system during an infectionand controlled climb test.



The deviations observable at 0030 and 0830 hours areartefacts of the automatic calibration process. In a reactorinstallation it is foreseen that all analyzer data could bedisplayed in this way upon demand. Figure 6 shows theformat of the automatic CPT control summary report. Thelast two columns indicate the Zphos set-point (SET Z) andthe phosphate set-point (SET P) during the time intervalreported.

Figures 7-11 show the results of several tests that wereconducted. In each figure, the upper plot is of the boilerwater orthophosphate concentration with its scale on theleft, and the lower plot is of the boiler water Zphos with itsscale on the right. The time-of-day scale is across thebottom. The set-point values are marked with horizontaldashed lines and the start of each test is indicated by avertical arrow. It should be recalled that since boilers#and 2 at Boundary Dam station operate continuouslyunder CPT conditions, our tests did not start at a zero PO4concentration but at some finite small value.

In the first test, shown in Figure 7, an initial rapidinjection into the boiler was performed. The conditionsin the boiler before the test began were a phosphate con-centration of 10.2 /.tg/g and a Zphos of 2.19. The setpoints

to be reached were 12.0 and 2.30/ag/g respectively. Withinabout 15 min, the setpoint levels were reached after an over-shoot of not more than 0.5 g/g in phosphate level and 0.01in the Zphos. For the CANDU prototype, the permissableerror between the actual boiler conditions and the setpointlevels is +2/.t/g for phosphate and +0.1 for Zphos.

The next test was also an initial injection (Figure 8). Inthis test a decrease in the Zphos level from its initial valueof 2.33 to a set-point at 2.25 was required. The phosphatelevel was to be increased to 17/.tg/g from 14/ag/g. Again theinjection was very rapid and the change was established towithin 0.5/.tg/g and 0.02 Zphos after about 20 minutes.

Figure 9 illustrates a situation in which the initial injectionfailed to change the Zphos value to the setpoint level and thecontroller took over. The starting conditions were 12.2/2g/gphosphate and Zphos 2.27. During the course of this test, itwas known that several disturbances were acting upon theboiler. As intended by its design, the controller detectedthese and managed within 2.5 hours to bring the phosphatelevel to its set point and come within 0.02 of the Zphossetpoint.

Figure 10 shows the results of a test in which there was noblowdown. Identifiable disturbances were imposed by causing

22

21

20

i--

o

17

PO4 Setpoint

Injection

1600 1700 1800 1900 2000 2100TIME, hours (Morch 17,1977)

Test Iection and controlledchmb under disturbed

Z PHOS Setpoint

2"45

24O

Figure 10. Illustration of the behaviour of the Congruent Phosphate Treatment control system during injection andcontrolled climb testing. (Controlled disturbances were introduced and boiler blowdown was inoperative during this test).



Elapsed Time, Hours

Figure 11. Illustration of the long-term ability of the CPT control system to maintain P04 and Zphos setpoints.

the controller to believe that the phosphate pump rates werelarger than they in fact were. The initial conditions were17/.tg/g phosphate and 2.2 Zphos. The setpoints were 20/g/gphosphate and 2.30 Zphos. The initial injection failed tocause the setpoints to be reached. However, the controllereventually adjusted to the ’disturbances’ and brought theZphos to within 0.01 of its setpoint. The controller allowedthe phosphate level to overshoot by 1/ag/g because of thelack of blowdown and as a consequence of adjusting theZphos.

Figure 11 illustrates the long-term ability of the CPTcontrol system to maintain the PO4 and Zphos setpoints.Under controlled climb conditions the boiler chemistry wasto go from initial conditions of 8g/g phosphate and

hOS 2.3 to setpoints of 10/ag/g phosphate and 2.4 Zphos.ile the boiler blowdown was in continuous operation, the

setpoints were reached after two hours. After 16 hours, theblowdown was inadvertently cut off. Up to that time, itwas counteracting the tendency of the boiler to increase inphosphate level while trying to adjust the Zphos precisely.After the shut-off of the blowdown, the tendency of thecontroller to adjust the Zphos precisely resulted in a 1.5/ag/govershoot on the phosphate level setpoint. However,throughout the test, the Zphos value was maintained for24 hours within 0.03 of the setpoint and the phosphate levelwas maintained close to the + 2/g/g specification.

All analyzers in the system for a CANDU prototypeinstallation would have temperature and flow monitor andalarms, and automatic calibration. In the Boundary Damtests, the alarms were demonstrated for the sodium analyzeronly, while three analyzers (hydrazine, sodium and phos-phate) were automatically calibrated. In response tocomputer signals, the calibration unit sequentially fed theappropriate analyzer a sample to each of two standardsolutions. The analyzer response to these standards was usedby the computer to calculate a calibration scale for conversionof subsequent sample signals to the correct correspondingchemical concentrations and units.

and that a technical foundation has been established whichpermits progression to a prototype installation.

The results of the program can be summarised in thefollowing sentences:

Hydrazine concentrations could be maintained autom-atically and dissolved oxygen levels controlled. The computeralgorithm was appropriate for control in either AVT or CPTmodes and for the selection of CPT upon detection of acondenser leak. The system could control the boiler’schemistry within set limits for an extended period of time.Automatic calibration of the on-line analyzers could be donesuccessfully. The data-logging, printout and display functionsof the computer were successful and an aid to good chemistrymanagement.

ACKNOWLEDGEMENTThis project was the work of many people of Saskatchewan PowerCorporation and Atomic Energy of Canada Limited. We wish partic-ularly to acknowledge the contributions of Messrs. B. Baker, A. Whyte,R. Portman, M. Myers, J. Westdal and G. Young.

Summary ofresults of the development workUpon completion of the experimental program at BoundaryDam, the chemistry control was successfully demonstrated inoperation, to the representatives of several Canadian electricalutilities. It was clear that .such a system is of interest tooperators of both nuclear- and fossil-fuelled power stations


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