Atomic Energy of Canada Limited

HEAVY WATER MODERATED

NUCLEAR POWER REACTORS

by

J.L. GRAY and C.L. MOON

This paper was specially prepared for theInternational Meeting on Modern Electric, Power Stations, heldat Liege, Belgium, from the 11th to the 15th May, 1970, underthe auspices of the Association of Electrical Engineers trained at the

Montefiore Electrotechnical Institute.

Egalement disponible en français sous le N° AECL - 3660F

Ottawa, Ontario

June 1970

AECL3660

HEAVY WATER MODERATED NUCLEAR POWER REACTORS

bV

J.L. GRAY,

President Atomic Energ)' of Canada Limited

and

C.L. MOON,

SuperintendentStandards and Publications Branch Atomic Energy of Canada Limited

SUM MARY

The heavy water moderated reactor is receiving increased attention and consideration as a prac-tical and economic source of power. After many years in which its potential seemed to he of more interestto research scientists than to the electrical utilities, it has now emerged as a fully engineered and developedsystem. !ts capabilities and its economics are being confirmed by the operation of full-scale powerstations. It is in the process of taking its full place alongside other reactor types for the economic genera-tion of power. Among heavy water moderated reactors, many variations exist. A number of these varia-tions are discussed and their further commercial development is considered.

Special attention is given to tlv CANDV series of heavy water moderated, natural uraniumUielled reactors.

June, 1970

AKCL - 3660

LES RÉACTEURS ÉLECTRONUCLEAIRES MODERES PAR EAU LOURDE

par

J.L. GRAY

Président de l'Energie Atomique du Canada, Limitée (EACL)

et

CL. MOON

Directeur du Bureau des normes et publicationsdu Groupe d'études électronucléaires de l'EACL

RESUME

On considère de plus en plus le réacteur modéré par eau lourde comme une source électriauepratique et économique. Après de nombreuses années au cours desquelles il intéressait davantage leshommes de sciences que les producteurs d'électricité, ce type de réacteur est maintenant parfaitementau point. Ses avantages techniques et économiques sont d'ores et déjà confirmés dans de grandes centralesélectronucléaires. Le réacteur modéré par eau lourde va bientôt occuper une place de choix parmi lestypes de réactew capables de produire de l'électricité à bon compte. Les principales variantes du réacteurmodéré par eau lourde sont passées en revue, surtout en ce qui concerne leur développement commer-cial. La filière CAN DU (réacteurs dont le combustible est de l'uranium naturel et le modérateur del'eau lourde) fait l'objet d'une attention particulière.

1. INTRODUCTION.

The heavy water moderated power reactor hasbeen the subject of continuous and vigorous researchand development for many years. It has evolved ina number of variations which have been the subjectof scientific and economic evaluations in many lear-ned papers. It has now emerged from the area ofthe research laboratory and, in the hands of theengineering designer, has become a practical andeffective producer of electric power. In this phasetoo, it exists in several forms, each with its merits,each with its limitations.

A detailed review and evaluation of thj variousheavy water reactor concepts which have emergedis beyond the scope of this paper. An outstandingreview in this area was presented to this Society atthe 1966 meeting, in the excellent paper by A.Bahbout and J.C. Leny [1]. It is not proposed tocover the same ground in this paper, but to moveahead to other considerations.

The proliferation of reactor types, all using heavywater moderator, is at the same time a tribute tothe flexibility inherent in the concept, and a sourceof weakness which could affect its commercial exploi-tation. If too many types exist, the effort requiredto develop fully any one of them may not be availa-ble. In this respect I would like to quote directlyfrom a paper produced by a large utility which iscommitted to an extensive heavy water reactor pro-gram.

« Each nuclear concept which promises low costnuclear power must be developed and adequatelydemonstrated before a utility or nation can commit alarge program based upon that concept. The totalunit energy cost will be a function of the inherentpotential of the concept, the development status, thecapability of project personnel, the program magni-tude, the economic ground rules and the industrialcapability of the sources of goods » [2].

In the above statement the emphasis on the prac-tical aspects of power production is obvious. Inconsidering the status of heavy water power reactors

STATION

I — C O , —

— HjO-

COOLANT-

TUBEI

MODERATOR.COOL

LOW PRESSURE

VESSELI

MODERATOR-HIGH PRESSURE

TUBEI

MODERATOR-COOL

LOW PRESSURE

D2O

TUBEI

MODERATOR-COOL

LOW PRESSURE

•VESSEL

MODERATOR-HOT

HIGH PRESSURE

-INDIRECT KKNEl i

-INDIRECT BOHUNICE

NOMINALOUTPUT

MWe

10070

no

— BOILING- -DIRECT -GÊNTULYSGHWR

25093

PRESSURIZED- • I N O I R E C T -

• B O I L I N G - -DIRECT-

• PRESSURIZED INDIRECT-

-DOUGIAS POINTPICKERINGBHUCERAPPMAPPKANUPP

-MARVIKP.N

-MZF»ATUCHA

2004 » 5004 x 7502 » 2002 x 200

125

132

SO319

Fig. 1. — I" "\\y Water Moderated Power Reactor Systems.

- 2 -

BOILING H?O COOLEDDIRECT

SGHWRGENTIIAY

HjO FROM CONDENSE»

PRESSURE TUBE REACTORS

3 - ^ H2O STEAMTO TURBINE

MODERATORDUMP PORT

PRESSURIZED D2O COOLEDINDIRECT

DOUGLAS POINTPICKERINGBRUCEETC.

TO BOILER FROM BOILERTO BOILER FROM BOILER

TO DUMP OR STORAGE

BOILING D2O COOLEDDIRECT

MARVIKEN

PRESSURE VESSEL REACTORS

OjO FROM CONDENSE»

DjO STEAM TO TURBINE

PRESSURIZED D2O COOLEDINDIRECT

MZFRATUCHA

FROM BOILER TO BOILER

Fig. 2. — Schematic Arrangements of Reactors.

TABLE 1.HEAVY WATER MODERATED NUCLEAR POWER REACTORS.

Unit

1. ATUCHA

2. DOUGLASPOINT

PICKERING

GENTILLY

BRUCE

3. BOHUNICE

4. EL-4

5. MZFR

KKN

6. RAPP

MAPP

7. KANUPP

8. MARVIKEN

9. SGHWR

NetOutputMWe

(Nominal)

319

200

4 X 500

250

4 X 750

110

73

52

100

2 X 200

2* X 200

125

132

93

Country

Argentina

Canada

Canada

Canada

Canada

Czechoslovakia

France

Germany

Germany

India

India

Pakistan

Sweden

U.K.

Heat Transport

Fluid

D,O

DL,O

D,O

H,O

DUO

CO,

CO,

D,O

CO,

D,O

D,O

D,O

D,O

H2O

Cycle

Ind.

Ind.

Ind.

Direct

Ir.d.

Ind.

Ind.

Ind.

Ind.

Ind.

Ind.

Ind.

Direct

Direct

System

State

Press.

Press.

Press.

Boiling16 %Steam

Press.,some

Boiling

Press.

Press.

Press.

Press.

Press.

Press.

Press.

Boiling

Boiling

PressureEnvelope

P. Vessel

P. Tube

P. Tube

P. Tube

P. Tube

P. Vessel

P. Tube

P. Vessel

P. Vessel

P. lube

P. Tube

P. Tube

P. Vessel

P. Tube

Fuel

Nat. UO,

Nat. UO,

Nat. UO,

Nat. UO2

Nat. UO,

Nat. UO :

Enr. UO,

Nat. UO2

Enr. UO,

Nat. UO,

Nat, UO,

Nat. UO2

Enr. UO;

Enr. UO,

NuclearDesigner

Siemens AG

AECL

AECL

AECL

AECL

—

Indatom

Siemens AG

Siemens AG

AECL

DAE India

Can. Gen.Elec.

ASEA

UKAEA

Operator

CNEA

Ont. Hydro

Ont. Hydro

Hydro Quebec

Ont. Hydro

—

Edf

KKW-B

KKN

DAE India

DAE India

PAEC

Swedish StatePower Board

UKAEA

Status

ConstructionCrit. 1972

Operational 1967

ConstructionCrit. 1971, 71, 72 & 73

ConstructionCrit. 1971

Constru tionCrit. 1975, 7-J.77& 78

ConstructionCrit. 1970

Operational 1966

Operational 1965

ConstructionCrit. 1970

ConstructionCrit. 1971,74**

ConstructionCrit. 1974**

ConstructionCrit. 1970

ConstructionCrit. 1970

Operational 1967

* One unit authorised •* Estimated

_ 4 _

in this, the year 1970, it quickly becomes apparentthat we have progressed beyond the stage wherewe are mainly interested in the relative merits ofcompeting system concepts : we now must considerthe status of functioning hardware, in terms of com-pleted and committed stations. It is therefore thepurpose of this paper to explore this area and tohighlight some of the trends which may indicate thedirection in which we are moving. In this way wecan better estimate our future position and the abilityof the heavy water moderated reactor to competesuccessfully in the world markets.

2. SURVEY.

ences are responsible for the proliferation of designswith which we are faced.

It is fortunate that in addition to the stations ofseveral types which are operating we have a numberof larger stations at an advanced stage of construc-tion. Information from these larger stations shouldsoon be available. In the meantime there is emer-ging, both from those vhich are operating and areunder construction, some indications on which validcomment can be based.

Refering to Figures I and 2, may we now considerthe relative effects of several of the main parameterson the viability of the Jesign. These can be groupedas in Table 2.

A survey of the present situation might be agood place to start. As our avowed aim is to explorethe commercial possibilities of heavy water moderatedpower reactors, we propose to omit from the surveythose reactors which were built to demonstrate thefeasibility of a concept, and which are thereforemainly experimental in nature. With a few excep-tions, this limits us to reactors which are designedeither to approach or to achieve the production ofpower on a competitive basis with other sources,i.e. power reactors which are sufficiently large todemonstrate the operating features and economicsof the design, through regular operation in a powersystem. !n Table No. I the heavy water reactorswhich appear to meet these criteria are listed. Thewide range of types which have evolved is obvious.These types are illustrated graphically in Figure 1,where reactors are grouped according to their mainparameters. The physical arrangement of each type,except for the CO.. cooled reactors, is shown in sim-plified schematic form in Figure 2.

3. ANALYSIS,

Having highlighted the fact that we have thiswide range of reactor types we must recognize thatfor each type we will have a corresponding techno-logy, with its inherent problems to be detected andsolved. Looking at the entire range of heavy waterreactors we suggest that it would be a brave niunindeed who would undertake to point to one or otherof them as being the more advanced or more suitablefor further commercial development. Nonetheless,some kind of comparative analysis would seem to bein order so we can sec more clearly where we aregoing. If this analysis is limited to s series sf quali-tative observations, we believe it will become apparentthat the various designs exist as the result of a combi-nation of political and economic rather than technicalinfluences, and that to some extent these same influ-

TABLE 2.

COMMON FEATURES ANDCOMPETING VARIABLES

Common Feature

1. D,O Moderator

2. Water cooled,pressure tubereactor

3. D...0 cooled primarysystem

4. D-..O cooled pressurevessel reactor

Competing Variable

Water cooleu versusCO, cooled

D...O cooled versusH.O cooled

Pressure vessel versuspressure tube

Direct versus indirectcycle

DISCUSSION OF TABLE 2.

I) Water Cooled versus COa Cooled Reactors.

The demarcation between D..O moderated reactorscooled by water, either H.O or D..O, and thosewhich are cooled by COL. is perhaps one of the mostpronounced. The advantages, in terms of thermalcycle efficiency, which result from high temperaturegas cooling arc well known. These advantages must bepurchased at a price, however. High temperaturesimpose added restrictions on the designer in hisselection of materials for use within the cure of thereactor and on the level of stress at which the mate-rials can be used. It appears safe to assume that theeffect of these restrictions is to increase the parasiticabsorption of neutrons by the structural materialsin the reactor. Another acknowledged problem areawith the CO. cooled reactor is that of obtaining anadequate power density. Operation at a higher fueltemperature improves this factor, but may accentuatethe materials problems indicated above.

- 5 -

With no attempt at making a value judgement, itmay be noted that of the three reactors which arecooled by CO2, both KKN and EL-4 will use enri-ched fuel, thus giving up one of the advantagesoffered by the heavy water moderated reactor.

Concerning their future, we are not aware at mistime of definite plans by either the French or Ger-man authorities to build additional and larger CO2cooled units for commercial use. Evidence availableconcerning the pressure vessel Bohunice reactor inCzechoslovakia indicates that this system will notbecome a major factor in the CSSR power reactorprogram. In summary, it seems difficult to avoid theconclusion that for the immediate future the CO:,cooled, heavy water moderated reactor will notbecome an active competitor in the commercialfield.

2) D,O versus H.O Cooled Pressure Tithe Reactors.

As members of a company involved with the designand construction of pressure tube reactors cooledwith pressurized D.O and of one cooled with boilingH...O, we feel that the two types have much in com-mon. The design, manufacturing and constructionproblems are of a similar order. The choice betweenthem would appear to depend on an economic eva-luation of the differences arising from the selectionof coolant.

With the pressurized D._,O cooled reactor we havecertain known problems. These include the capitalcost of a higher D,.O inventory, the control of leakagefrom the hot, high pressure coolant system to theatmosphere or to light water circuits, th? separationof light water areas from heavy water areas to faci-litate recovery of escaped D..O, upgrading facilitiesfor D.O, a tritiated atmosphere, and others lessobvious.

A boiling H..O cooled pressure tube react<T offersa number of comparable problems also, as well assome advantages. In using a direct cycle, the costof heat exchangers is eliminated and some gain ismade in the sleam conditions at the turbine. Also,the DjO inventory and the containment and recoverycosts arc reduced. However, this same system may beexpected to yield a higher activity in the turbine asthe whole heal transport system becomes a reactorprimary circuit. The use of light water in a boilingmode also introduces a range of thermodynamic andhydraulic problems related to two phase flow in paral-lel systems.

Additionally, if natural uranium fuel is used theproblem of controlling excess reactivity, in the eventof accidenta! voiding of the coolant tubes, is moresevere. This problem is overcome in SGHWR by« under moderation » involving the use of enrichedfuel and relatively close fuel spacing.

Solutions are available for all the problems, relatedto both coolants, mentioned above. But they costmoney. The relative merits of each system must inthe final analysis be measured in economic terms.At the present time neither coolant has demonstrateda clear cut advantage in the overall cost of powerproduction. They must both be considered as viabledesigns at this time.

3) D...O Cooled Pressure Tube versus Pressure VesselReactors.

In outward appearance and in written descriptionspressure tube reactors and pressure vessel reactorsgive the impression of being radically different. Acloser look reveals a surprising similarity, the essen-tial difference being whether the heavy water modera-tor is contained inside or outside the high pressureboundary. Both types have tubes in which the fuelresides and through which the coolant flows. In thepressure vessel reactors, however, these tubes tonot have to withstand a pressure differential. Accord-ingly they may be made from thinner material, thusreducing the amount of structure in the core. As acompensating feature the entire moderator at fullprimary system pressure and at an elevated tempe-rature must be contained by (he pressure vessel.Thus, on the structural side we have the relativeproblems of (a) designing suitable pressure tubeswhich will have an acceptably long service life andwill not be unduly absorbent of neutrons, accom-panied by a relatively simple low-pressure vessel tocontain the moderator, and (b) using comparativelylight tubes to guide the coolant over the fuel, whilebeing required to design a much heavier vessel tocontain the moderator.

When considering these however, a size limitationaffecting pressure vessels must be recognized, inthat it appears to be economically impractical, using1970 technology, to construct a natural uranium pres-sure vessel reactor in the higher power range, i.e.500 MW and over. The use of enriched fuel reducesor removes this limitation. The knowledge and thetechnology to produce both pressure vessel and pres-sure tube designs arc available and have been demon-strated. Therefore any choice between these conceptsmust be based on their total economics, both ofconstruction and of operation, and as further affectedby any limitation on size.

In considering the economics of the constructionof a reactor, the cost in absolute terms is only onefactor. Of perhaps equal importance, and sometimesmore important, is the cost in terms of the hostcountry. For a reactor which is both designed andconstructed within one country this does not presenta pioblem because one of the precepts of a gooddesign is that it will be suitable for manufacturingby techniques known to be available. But a designwhich is economically correct, in the country in

which it originates may not be the best economicchoice in another country. This is particularly truewhere a reactor is soid to a foreign buyer who wishesto do a considerable portion of the manufacturing inhis own country. In such an instance the demandswhich a design makes on the size or competence ofthe manufacturing industry in the host country canaffect its economic suitability for that buyer. In thisrespect, we fcei that an analysis will show that themanufacture of a steel pressure vessel for a naturaluranium reactor will require a more sophisticatedheavy industry than that required to manufacture acorresponding vessel for a pressure tube reactor. Theintroduction in future of prestresscd concrete pressurevessels suitable for heavy water reactors may mode-rate this factor but there is not yet evidence that itwill reverse it. The fact that pressure tube reactorsin any size within the range of power envisaged todaycan be manufactured using the same techniques andthe same levci of industrial skills provides the desi-gner and the buyer with a freedom that must beacknowledged. It is our belief that consideration ofsuch matters should enter into any discussion of therelative economic merits of pressure tube and pres-sure vessel reactors.

One undoubted advantage which is possible withpressure vessel reactors, as a class, is the eliminationof the numerous individual coolant feeder pipes whichare necessary wilh pressure tube reactors. The asso-ciated phy:';:al complication and possibility of leakswhich they entiil are therefore reduced correspond-ingly. It should be noted, however, that the Marvikendesign takes i..ily partial advantage of this possibility,as it contains a considerable number of individualD,O feeder lines. Another point for comparisonbetween pressure vessel and pressure tube reactorsis the matter of direct openings into the primarycoolant circuit. It should be noted that three out ofthe four pressure vessel reactors listed in Figure 1utilize external openable pressure connections toeach coolant channel, for purposes of loading andunloading fuel, in an arrangement which corres-ponds tc that used for pressure tube reactors. Thereis bound to exist, therefore, a similar possibility ofleakage. This possibility will be affected by the num-ber of such openable channels and by the soundnessof the design of the mechanisms involved.

From the two observations noted above it seemsfair to conclude that the use of a pressure vesseldesign may reduce but will not automatically elimi-nate all potential sources of leakage at or adjacentto the reactor.

We have had several years during which we havebeen able to observe the operating history of pres-sure vessel reactor MZFR in Germany and the smal-ler Agesta pressure vessel reactor in Sweden. Whileneither of these is large enough to be competitive in

the full sense, they have indicated that the conceptis sound and that the design does not contain inhe-rent flaws. The problems experienced in operationhave been of a type which reasonably might beexpected in a new design, and all have been capableof remedy.

Correspondingly, we have had experience withpressure tube reactors on which to base comment.In length of time, the major part of this experiencehas been with the Canadian 25 megawatt NuclearPower Demonstration reactor, NPD, which has beenin operation since 1962. The experience, which iswell documented in the technical literature, has beenvery satisfactory. The 100 megawatt SGHWR andthe 200 megawatt Douglas Point, both much larger,have been in operation over a shorter period of time,but nonetheless have provided significant indications.Their teething troubles have been well documented ;but these troubles hnse not been due to the useof pressure tubes as the high pressure containmentenvelope. They luive been due to malfunctioning ofmechanisms common to both types of reactor. Exam-ples of these are leaks at the point of attachment ofthe fuelling machine to a fuel channel, malfunctioningof components within the fuelling machine, and pro-blems with conventional equipment such as pumpbearings and seals, leaking valve stems, and poweroperated valves. Other problems have involved waterchemistry, atmosphere drying systems, control ofprocess systems, and others.

When the operating experience acquired to datewith pressure tube and pressure vessel D..O reactorsis reviewed it becomes apparent that there is verylittle which would place one in a preferred positionto the other. From the practical evidence of seeingnew stations of both types currently being construc-ted, it is obvious that we will have both types withus for the foreseeable future. Within the limitationsof size discussed above they well may complementeach other in the world markets, each fitting into itsparticular economic niche.

4) Pressure Vessel Reactors. - D..O Direct Cycleversus Indirect Cycle Cooling.

In making a comparison between DL.O cooledpressure vessel reactors having a direct cooling cycleand those having an indirect cooling cycle we are, ineffect, comparing the Marviken reactor unit withMZFR and Atucha. It might be felt that such a com-parison is not valid, because Marviken uses enrichedfuel and consequently has a smaller core than wouldotherwise be required. Enrichment does not affectthe characteristics of the cooling cycle however, andthe comparison is felt to be quite valid.

The direct cooling cycle in the form used byMarviken possesses some very attractive features. Theabsence of multiple external connections directly into

- 7 -

the high pressure primary coolant system for refuel-ling removes the possibility of leakage which is alwayspresent with such an arrangement. At the same timeit appears that this advantage is not inherent in theconcept but occurs due to the fact that the designersextended the pressure shell to include the refuellingmachine. On the surface it appears that the directcycle concept would be equally valid if a refuellingarrangement similar to that adopted for MZFR orAtucha had been used.

Aside from the considerations of two phase flowas discussed previously, the main technical points ofinterest in the direct cycle reacior are the range andseverity of problems which will accompany the exten-sion of the DO primary coolant circuit through theturbine-condenser system. Problems associated withmechanical joints and with shaft sei.ls have existedfor a long time, and perfect solutions are rare. Adirect cooling system which would be satisfactorywith H O coolant may prove to be troublesome whenthe coolant becomes DO, We have been impressedby the information which has been published concer-ning the sealing of the turbine shaft for Marviken,but believe that we must await long term trials beforepassing judgement. A similar view must he expressedconcerning other sealed joints in the system. Thepossibility of leakage at other points in the extendedsystem, for example in the condenser area or in thecondensate return system, must also be borne in mindand the economic effects must be included in theoverall assessment. The economic considerationsmust also include the possibility of a radioactiveturbine-condenser ..ystcm, a tritium bearing atmo-sphere in the turbine area and safety precautions:.gainst ihe effects of massive leakage even thoughihis is considered to he a remote possibility.

In comparison, the indirect coolant system used inMZFR and Atucha must bear the costs, '••oth incapital and thermal terms, of the heal exchangers,or boilers. The flow conditions in the reactor may bemore predictable and the conventional part of thestation can be less tightly grouped since it docs notcontain expensive heavy water.

When viewed objectively, the relatively few pointsdiscussed above appear to cover the major differencesbetween direct and indirect pressure vessel reactorsystems. As in the other comparisons, the relativemerits will be determined only after the stations haveoperated under commercial conditions. The accepta-bility of a D,O primary circuit which includes theturbine and condenser systems must be proven tech-nically before it can be evaluated in term.- of itseconomics. We must wait until we have the insults.These should be available before too long.

4. PROGRESS AND TRENDS.

In considering the merits of the various reactorsystems discussed in the foregoing, there is one pointwhich should not be overlooked. It is ;hat the eco-nomic viability of any reactor system depends notonly on the current state of its development, but onits capacity for being improved. If a design standsstill it is economically vulnerable ; in time it will beovertaken by a rival design which through technolo-gical advancement has improved its competitive eco-nomic position. It seems prudent therefore to look ata number of key technical features in heavy waterreactor stations to see what progress has been madeand to determine what trends exist.

It would be advantageous to be able to study eachtype of reactor and to report on all the stages throughwhich it has evolved. To do so would be a formi-dable task. It would also involve a great deal of repe-tition as advancing technology has undoubtedly affec-ted different designs in similar ways. It is hopedtherefore that we may be permitted to limit our studyto one reactor family. The one selected is the CANDÛseries of pressure tube reactors. It has the advantagethat it is now in the third design generation overand beyond the original 25 megawatt demonstrationreactor NPD. Considering the whole family it has adesign history extending over fifteen years, a cons-truction history of twelve years and a combined netpower capacity of 6200 megawatts electrical.

COMPARISON OF DOUGLAS POINT,PICKERING AND BRUCE REACTORS

Three reactors are selected for comparative review:Douglas Point, Pickering and Bruce. They are allrepresented by the horizontal pressure tube schematicarrangement shown in Figure 2, All are designed bythe same design team, and arc operated by the sameutility. The successive generation aspect is apparentfrom their respective dates of criticality.

Selected parameters for the three stations are listedin Table 3. Other features and considerations, moredifficult to tabulate, will be discussed on an indivi-dual basis later. First, a review of Table 3.

The first and most striking feature is the progres-sive growth in net power, both of the individualreactor and of the complete station. There is anobvious economy with increasing size that docs notneed elaboration. The capital and units costs discus-sed later in this paper are, in part, a reflection ofthis fact.

- 8 -

TABLE 3.SELECTED PARAMETERS.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

Net Output (nom)

Diameter ofCalandria 1 Dia

Length of Core

No. of Pressure Tubes

Diameter of Tubes(and fuel)

Tube Material

Tube Design Stress

Station Efficiency

Reactor Outlet Temp.

Specific Fission Power :MaximumAverage

Max. Channel Power

Burnup, average

Total Weight U/reactor

Date Critical

MWe

cm

cm

cm

psi

%

"C

W/gm,,W/gm,,

MWth

MWd/te

Tonnes

DouglasPoint

203

599

500.4

306

8.25

Zr-2

16,000

29.1

293

3116.82

2.75

8400

41.5

1966

Pickering1,2,3,4

4 X 508

804

594

390

10.34

1 & 2 Zr-23 & 4 Zr-Nb(cold worked)

1 &2 16,0003 & 4 21,000

29.1

293

32.218.9

5.125

8000

92.3

1971,71,72&73

Bruce1,2,3,4

4 X 750

846

594

480

10.34

Zr-Nb(cold worked)

21,000

29.8

299

35.821.7

5.82

9600

114

1975,76,77&78

Items 2, 3, 4 and 5 of Table 3 illustrate that agreatly increased power output does not requireradical changes in the arrangement and dimensionsof the reactor vessel and its component parts. Theincrease in diameter of the pressure tubes from8.25 cm for Douglas Point to 10.34 cm for Pickeringand Bruce stations, and an increase in the heat ratingof the fuel, as shown in item 10, contribute to thisfact.

Items 6 and 7 reflect advancing metallurgicalknowledge and a willingness on the part of the desi-gners to take advantage of higher strength materialsas they become available. The higher strength mate-rials involve more difficult forming techniques inmaking joints, but we have learned how to live with

them. Since creep, as well as ultimate strength, is adesign criterion for the pressure tubes, the adoptionof cold worked Zr : Nb yields a double benefit. Itshigh strength permits the walls to be thinner withconsequently less absorption of neutrons, and its lowcreep rate extends the service life of the tube sothat it is equal to the projected life of the reactor.

Items 8 and 9, Station Efficiency and ReactorOutlet Temperature, are interrelated and show only aslight increase in the third generation. These condi-tions are related to the overall optimization of thestation and the values were selected on a basis oftechnical feasibility and to yield the lowest costpower rather than the highest thermal efficiency.

9 -

The Specific Fission Power, item 10, is a directmeasure of the intensity or rate of heat productionin the fuel. The increasing values in succeeding gene-rations reflect an increasing confidence based ondirect operating experience, analysis of defects andimprovements in fuel design.

The Maximum Channel Power, item 11, is affectedprimarily by the increased channel dimensions inPickering and Bruce, by the increased specific powerproduction in the fuel, and by changes in the primaryheat transport system.

The values for average fuel hurnup at dischargeshown as item 12, reflect an uneven progress. ThePickering reactor carries a cobalt load in the form ofadjuster rods, thereby producing irradiated cobalt atthe expense of burnup. The cobalt adjuster rods arcwithdrawn as a control manœuvre when a boost inreactivity is needed to overcome the effects of xenonpoisoning in the fuel. The %00 MWd te value forBruce is achieved in part due to the economics ofsize and the use of enriched boosters rather thanremovable adjuster rods (absorbers) for xenon poisoncompensation.

The total inventory of uranium in the reactor inrelation to the net power becomes less in succeedinggenerations, reflecting the increase in the average spe-cific fission power in the fuel.

In addition to the parameters listed in Table 3,examination of other major areas where cor-responding progress has been made in the samethree stations is revealing.

Reactor Arrangement ani Shielding.

The general arrangement of the reactors and theprincipal shielding structures are shown schemati-cally in Figure .?. For Douglas Point two reactorvessels arc involved, the calandria and a moderatordump tank located directly below it. They are housedin a calandria vault which provides shutdown shiel-ding. The vault is a massive reinforced * heavy * con-crete structure, for which ilmenite ore was used asaggregate. To remove the heat which is generatedby the radiation within the concrete, double rows ofcooiing pipes are embedded in ihe concrete. As thesewould not be adequate in themselves to prevent sur-lace damage to the concrete, a four-layered systemof liners and steel plates was installed inside thevault to provide « thermal » shielding. The thermalshields are heavy and costly, and they extended theconstruction period of the vault considerably. Alsothey required a separate cooling air system to limitthe temperature rise.

End shields, in the form of heavy steel slabs andcooling water passages arc independently suspendedat each end of the reactor to permit approach to theface of the reactor during shutdown periods.

The Douglas Point reactor arrangement permitsvault air to circulate through the annular spacebetween the pressure tubes and the calandria tubes.This results in some production of argon 41, with itsattendant problems. Experience at Douglas Pointhas shown that the separate cooling water circuitsrequired for the end shields and for cooling the vaultconcrete, and the air cooling system for the thermal

DOUGLAS POINT PICKERING BRUCE

IHEtMAl•SHIELDS. '•

1 1t.' '• :•' î * .

: ! T T : ; f1 1 • : { . ' •

Fig. 3. — Arrangement of Reactors and Shielding.

- 10-

DOUGIAS POINT PICKERING

O

BRUCE

Fig. 4. — Containment Systems.

shields resulted in complexity and uneconomical useof space.

For Pickering, the reactor arrangement is basicallysimilar to that at Douglas Point hut economies havebeen realised by placing the steel thermal shieldsinside the calandria, immersed in the moderator, andby making the end shields integral with the calandriathus eliminating one set of tube sheets. The calandriavault arrangement was thus greatly simplified. Theargon 41 problem is eliminated at Pickering by sca-ling the annuli between the pressure tubes and calan-dria tubes and connecting them to a closed nitrogen-filled circulation and drying system. This reducesthe chances of corrosion and provides a means ofdetecting small leaks into the annuli.

Simplification has been carried a long step for-ward in the Bruce design. By modifying the reactorshutdown system the dump tank has been eliminated.This has made it possible to eliminate the entirecalandria vault structure and to replace it with asimple water-filled shielding tank. The integral endshields of the ealandria are supported by and formpart of the tank. They are filled with small steelballs, easily installed on site, rather than the heavysteel slabs used for Pickering. Cooling of the endshields is by the shield tank circulation system. Thecalandria internal thermal shields required for Picke-ring are not necessary for the Bruce reactor.

The various changes described above remove seve-rul major items from the critical construction path.They reduce the complexity both of system andstructures and therefore reduce costs.

In Douglas Point the system is self-containedwithin the reactor building, and a maximum failurecould result in an internal pressure of 1.4 atmo-spheres absolute, which would persist for a period oftime.

For Pickering the discharged steam is not retainedwithin the reactor building. A two-stage system isprovided in which a 'urge « vacuum building »,which contains water spatys and is maintained at anabsolute pressure of approximately 50 millimeters ofmercury, is connected to the four reactor buildings.A rise in pressure following a major failure activatesa multiple valve system which permits the steam andair mixture to discharge into the vacuum building.The reactor building is thus subjected to a transientpressure of 1.4 atmospheres absolute. Because theincreased pressure is of short duration the reactorbuilding has less stringent requirements to be leak-tight than does the Douglas Point building. The useof one vacuum building and equipment to serve fourreactor units results in an overall economy.

The basic arrangement for Bruce is the same asfor Pickering. Due to the more compact arrange-ments of the Bruce reactor, a more compact buildingstructure has resulted. A smaller volume is exposedto the discharge from a failure. The escaped steamtherefore carries less air with it into the vacuumbuilding, thereby requiring a smaller vacuum buil-ding than that for Pickering, despite the fifty percentincrease in station power. The ratio of volumes ofthe vacuum buildings is 2/2.9.

Containment System.

For each of the three reactors a containmentsystem is provided which will contain the dischargefrom a major failure in the primary coolant system.Water sprays arc used in each instance to condensedischarged steam, thereby limiting the rise in pres-sure inside the containment structure. The contain-ment systems are shown schematically in Figure 4.

Reactivity and Shutdown Control

The devices used for control in the three reactorsare listed in Table 4. Where several methods areshown under a single control function they operatesequentially, in the order listed.

An explanation of the various control devices,which may not be apparent from their titles, follows :

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TABLE 4.CONTROL DEVICES

Control Function

Power level

Flux tilt

Rapid shutdown

Xenon override

Douglas Point

a. 4 absorberunits

b. Moderatorlevel

c. Boron inmoderator

Item a. above,used differentially

Moderator dump

Insertion of8 booster units

Pickering

a. 14 zonecontrolcompartments

b. Boron inmoderator

Item a. aboveused differentially

a. 11 shut-offrods

b. Moderatordump (slow)

Removal of 18adjuster rods

Bruce

a. 14 zonecontrolcompartments

b. Boron inmoderator

Item a. aboveused differentially

a. 28 shut-offrods

b. Poisoninjection

Insertion of27 booster rods

— Douglas Point absorber units are stainless steelelements which are inserted from the top andbottoin of the reactor, together or differentially.

— Pickering and Bruce zone control compartmentsare tubular compartments located spatially in thecore. They may be filled to any level with ordi-nary water, thus acting as variable absorbers.

— Pickering adjuster rods arc cobalt rods, whichnormally are fully inserted in the core. Whenthey are removed the reactivity of the system isincreased.

Douglas Point Control Methods.

The Douglas Point design inherited from its pre-decessor, the 25 megawatt station NPD, the conceptof controlling reactivity by changing the level of themoderator in the calandria. The design therefore in-cludes a relatively complex system to control thelevel of the moderator. It was realised that a reducedmoderator level would distort the flux, and that therequired control over short term changes in reacti-vity could be obtained by inserting four absorber ele-ments, one into each quadrant of the core. This useof absorbers offered simplicity and economic gains,and was selected as the primary method. The othermethods shown are for intermediate and long termsuppression of reactivity.

Flux tilt occurs when a local perturbation causesthe flux pattern and hence the power distribution inthe core to become distorted. To correct this condi-

tion the absorber units are inserted differentially. Ina reactor the size of Douglas Point it has been foundthat satisfactory control of this imbalance can beobtained using four absorbers.

The booster units, containing enriched uraniumfuel, are inserted to delay the onset of xenon poison-out after a shutdown or to enable an earlier start-upfollowing a xenon poison-out.

Pickering Control Methods.

Because of its effect on the flux pattern, moderatorlevel was abandoned as a device to control the powerlevel of the Pickering reactor. Fourteen zone controlcompartments are used instead. By progressivelyfilling the zone control compartments with ordinarywater, the power level can be reduced as required.A boron injection system which puts boric acid intothe moderator provides a « back-up » method ofcontrolling reactivity.

Flux tilt is controlled by filling zone controlcompartments differentially.

Rapid shutdown of the Pickering rea tor is pro-vided by 11 shut-off rods which drop into the core.Slow moderator dump is provided as a « back-up >method of shutdown. The resulting moderator sys-tem is less complex than that in the Douglas Pointreactor.

The use of cobalt filled adjuster rods, as a remo-vable absorbing load to produce an increase in the

1 2 -

reactivity of the system, is not essential to the design.The designer or reactor operator has a choice of usingan absorber load or enriched booster rods. The Finalselection is b-ised on an economic evaluation whichassumes certain values for the cobalt-60 produced.

Bruce Control Methods.

The zone control compartment method developedfor Pickering has been carried forward to the Brucereactor. Moderator dump has been eliminated enti-rely, and the calandria is normally maintained full.For rapid shutdown, the shutoff rods are backed upby a system for injecting poison into the moderator.

A major advance has been the use of booster rodsin two groups, one at each end of the reactor. Thisarrangement permits local regions of the core to bebrought to criticality following a shutdown evenwhen the maximum amount of xenon is present inthe fuel. The excess xenon poison is then burnedout progressively across the reactor. This providesfreedom to start up the reactor at any time followinga shutdown. Also the reactor can operate at varyingpower levels between 50 % and 100 % of fullpower for extended periods of time. In other words,it provides « load-following » capability.

Primary Heal Transport System.

The successive designs of the primary heat trans-port systems for the three reactors have evolved ona basis of acquired experience and are affected by theincreasing size of commercially available equipmentand by improvements in its design. Major objectives

have included improving the steam conditions, sim-plifying circuits, reducing costs and reducing thenumber of components, thus reducing the numberof potential leakage sources. Table 5 lists some of thechanges which have evolved.

The main coolant pumps for Douglas Point wereselected on a basis of using multiple units of mode-rate size so that extra pumps would be available forstandby duty if required. Experience has shown,however, that multiple units can add to operatingproblems and increase the potential points of leakagesufficiently to outweigh the benefits of extra flexi-bility and redundancy. This experience was loo lateto affect the decision for Pickering, but for theBruce reactor the largest practical size of pump wasselected, and the number was reduced to four.

A major problem area with pumps has been thatof shaft seals. At the time that the Douglas Pointpumps were selected, failures of mechanical face sealswere numerous. Controlled leakage seals, in the formof throttle bushings, were therefore specified. Al-though they passed extensive shop and laboratorytests, they developed a high flow rate in service.This included a high inflow into the primary system,which has created pressure control problems. Themotor bearings also caused trouble. New mechanicalseals for the pumps have been developed and otherassociated problems have been successfully solved andthe resulting improvements incorporated into thePickering and Bruce pumps. Experience indicatesthat an intensive development program by the nucleardesigner i>; necessary to overcome problems of thistype. The pump shaft sealing arrangements areshown schematically in Figure 5.

DOUGIAS POINT PICKERING

Fig. 5 Primary Pump Sealing Arrangements.

- 1 3 -

TABLE S.EVOLUTION OF HEAT TRANSPORT SYSTEM DESIGN.

Net reactor power - MWe

Main pumps per reactor - No. x kW

Boilers per reactor - No. x Mg/h(evaporation rate)

Percent of channels with flow measured

Approximate no. of non-welded jointsper reactor, excluding fuel channelclosure seals

Approximate no. of valves per reactor— packed stem— bellows sealed

Douglas Point

203

10 X 930

8 X 145

100%

3,000

2,000

Pickering

508

16 X 1400

12 X 245

5 %

1,000

170570

Bruce

750

4 X 8200

8 X 525

5 %

250

75500

The sizes of the boilers have increased witheach design generation, and follow a pattern similarto that of the main pumps.

The last three items listed in Table 5 clearly reflecta continued move toward simplicity and a reductionin the number of joints which could contribute toleakage of heavy water. It is confidentally anticipatedthat these moves will produce the desired results.

Heavy Water Management.

It would be most gratifying if we could assumethat there would be no leakage of heavy water fromthe various systems which contain it. But such is notalways the case, and provisions must be mac! • toprevent its loss on the assumption that some leakagewill occur. Two basic principles to be observed arei) limit to the maximum practical extent the escapeof DjO liquid or vapour, and ii) do not let any esca-ped D-jO become mixed with ordinary water.

Considering the question of DX» leakage to atmo-sphere, methods to retain, segregate and recover theescaped D.O have evolved steadily. Douglas Pointwas originally designed for relatively normal treat-ment of the boiler room atmosphere. It was assumedthat few potential sources of leakage of D.O existed,in the boiler room. Experience proved this to beotherwise. To reduce the loss of D,O, the DouglasPoint boiler room has been converted recently to a« dry atmosphere » room, with both recirculationand exhaust drying systems. Pickering contains theseprovisions in its original design : the arrangements of

the building floors and partitions contributes to thisend. In the Bruce station hot high pressure heavywater systems are fully segregated from regions wherehigh pressure light water systems arc located. Thisarrangement should substantially eliminate the down-grading of heavy water with light water vapour andshould reduce D.O loss at Bruce to a very lowlevel indeed.

Cosis.

The improvements in design with each generationof reactor have had a pronounced effect on costs.Table 6 illustrates the reduction in capital costs whichtiave been realised in a number of areas of thestations.

The total capital costs in terms of installed capa-city and the unit energy costs are shown in Table 7.It may be of interest to note that the comparable unitenergy costs from a new coal-fired plant built by thesame utility under identical conditions is 4 mills perkilowatt hour.

From the foregoing comparison of three stationsit is evident that continued improvements in the engi-neering, construction and manufacturing areas canresult in great progress within a given reactor concept.Standardization at many levels can be realised, simpli-fications can be effected and costs reduced. This evo-lutionary approach, we suggest, is the quickest, themost economical and the most certain way in whichheavy water reactors can be brought to their fullpotential.

- 1 4 -

TABLE 6.CAPITAL COSTS IN SELECTED AREAS.

Reactor and its immediate shielding (2)

Fuel Handling (3)

Primary Heat Transport System (4)

Safety Features (5)

Remainder of Nuclear Plant (6)

Installed Cost in U.S. Dollars/kWe (1)

Douglas Point

30

18

25

4

55

Pickering

20

6

16

3.5

29

Bruce

15

3

14

2.5

21

NOTES :

(1) 1968 U.S. dollars/kWc: direct costs for materialsand installation.

(2) Includes calandria, fuel channels, calandria vault,end shields, shield tank, dump tank, reactivitymechanisms.

'3) Includes fuel transfer, fuelling machines, shield-ings doors and all associated instrumentation.

(4) Includes main pumps, boilers, etc. and all asso-ciated instrumentation.

(5) Includes dousing system, vacuum building andducts, etc.

(6) Includes control centre equipment for conven-tional as well as nuclear plant. Includes costof civil works which house the nuclear plant.

TABLE 7.UNITS COSTS

Capital cost (iota!)

Unit energy cost, mills/kWh

— capital charges

— operation and maintenance

— heavy water upkeep

— fuel

Total, unit energy cost

Douglas Point(completed

1966)

$ 407/kWe

4.5

0.8

0.3

0.8

6.4

Pickering(1968

dollars)

$ 246/kWe

2.6

0.3

0.1

0.6

3.6

Bruce(1968

dollars)

$ 227/kWe

2.3

0.2

0.1

0.6

3.2

NOTES :

1. Capital costs include heavy water and half initialfuel charge.

2. Capital costs exclude escalation after 1968.

3. Capital charges 7.07 % on heavy water (40-vearlife) and 7.66 % on remainder of plan (30-yearlife). Interest rate 6.5 %.

4. Capacity factor assumed 80 %.5. Costs are in U.S. dollars.

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5. LOOKING AHEAD.

No review of the statuts of heavy water reactorswould be acceptable without a look into the future.Many statements in this area have already beenmade. Traditionally they have concentrated onimproved system concepts, and have involved newtechnology and extensive development programs.We confidently expect that in time these new con-cepts will evolve and will become the commercialreactors of the future. They will combine the eco-nomy inherent in a heavy water moderated systemwith high thermal efficiency and the ability to accepta variety of fuels and fuelling cycles. But they arenot available now. They must be considered aslong term projects which can be built only when therequired technology has been developed and the eco-nomic conditions warrant their construction. In themeantime there is much real progress to be made,some of it in the technical field and some in the areasof economics and organization.

In the technical field, improvement in the effi-ciency of the thermal cycle offers a logical path toprogress. To do so requires higher steam tempe-ratures than those at present realised. A numberof methods of achieving this are available. The useof fuel having a density higher than that of UO;, isanother inviting area for improvement. There areothers, the most obvious of which is reduction incapital costs. Work is underway to exploit all pos-sibilities which show promise. To the extent thatchanges are evolutionary, they will form part of thepattern of reducing costs of power discussed earlierin this paper. The economic evaluation of majortechnical innovations, which always involve somerisk, must take account of this moving target.

Let us examine briefly the effect of national poli-cies and economics on the choice of reactor i pe.One of the attractions of the heavy water moderatedreactor is the relative degree of political and eco-nomic freedom it offers because of its ability to usenatural uranium fuel. A country that has suppliesof uranium can be totally self-sufficient in respectto fuel ; alternatively, uranium can be bought atcompetitive prices on the world market. The heavywater reactor therefore may be expected to appealto governmental authorities in countries wherefreedom to manoeuvre is held to be important. Thesame governmental authorities may be expected tofavour a type of reactor which can be produced withthe desired degree of participation by their industryand labour.

At the same ti.ne it must be noted that the heavywater power reactor has been a late-comer on theworld scene and as such must fight for recognition.It has had its share of teething troubles and thesehave been widely publicized and frequently misinter-

preted. As a result, electric utilities, whose primaryfunction is to produce reliable power at the lowestpossible cost, may understandably take a detachedview. They are interested in proven performanceover a reasonable length of time and on a commer-cial scale. In Europe, where a number of compet-ing types of heavy water reactor have been developedconcurrently, no such demonstration has occurred.

A matter of practical concern to any country orutility planning to build a heavy water nuclear powerstation is the availability of a guaranteed supply ofheavy water. (It is estimated that one tonne ofheavy water per megawatt of electrical capacity willmeet initial requirements and piovide annual make-up for several years). There is a general, althoughshort term, scarcity of heavy wnier today which mustbe recognized. However, it should be recognizedequally clearly that there will be a plentiful supplysome five or six years hence, when it will be neededin quantity. In Canada alone heavy water plantshaving a total capacity of 1500 metric tonnes peryear are under construction at three locations.Completion dates range from 1970 to 1973. Weexpect that other countries will also he in the heavywater production business, and contribute to a supplysufficient to meet all requirements. Alternatively,any authority committing itself to a heavy watermoderated reactor program could choose to establishits own heavy water production piant.

It is assumed in the foregoing discussion that thetotal economics of power production are a matter ofnational concern. If this view is accepted, it followsthat the best choices will be made when there isfull co-ordination of a nation's political-economicaims and the technical objectives of or means pur-sued by its industrial complex. May we offer, asan example, the path we have followed in Canada.

When in the 1950s a need for nuclear power wasforeseen, Canada's assets included a high level ofknowledge of the technology of heavy water researchreactors and a large supply of uranium ore. Itwould have been quite possible to have followed theenriched uranium light water practice adopted in theUnited States if we bought the enriching services.The decision to follow an independent course wastaken on grounds that seemed of national importanceto Canada ; and so the CANDU series evolved.When is came time to build Douglas Point, the Cana-dian government, through Atomic Energy of CanadaLimited, supplied the funds, designed the station,managed the project and assumed all financial risk.The utility, Ontario Hydro, supplied the site and theoperating staff. For Pickering, the Canadian go-vernment participated to the extent of financing33 percent of the cost of the first two (of four) units,on a repayable basis. For the third and fourthPickering units and for Bruce, the entire financingand cost are the responsibility of the utility.

- 1 6 -

This form of technical-economic co-operation hasbeen for Canada the logical way in which to dealwith projects of such magnitude and national impor-tance. While it would be most unlikely that iden-tical conditions would apply elsewhere, we feel thatthe principles are sound and worthy of consideration.By following such a course the element of risk tou single utility is reduced and a nation can developa specific power policy. By concentrating all effort

on a chosen nuclear power concept, success is reaso-nably assured. The technology to achieve this isavailable now : we need but use it.

This then is the status of the heavy water mode-rated nuclear power reactor : in the technologicalsense, an accomplished fact ; in the commercialsense, a competitor which will move increasingly intothe forefront in the field of power production.

REFERENCES :

1. A. BAHBOUT AND J. C. LKNY : Les réacteursà eau lourde : Bilan et perspectives.

2. L. G. McCONNELL : Operating Performanceand Economies - Heavy Water Moderated NuclearStations. Ontario Hydro NOD-2.

3. D. L. S. BATE : The Evolution of CANDU-PHWPower Reactors in Canada. AECL documentPP-1, or Technical Meeting No. 3/1 of the Inter-national Nuclear Industries Fair (Nuclcx 69),October 1969; Basel, Switzerland.

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