CA

Future Developmentsin Nuclear Powerby G.J. Phillips

Future Developmentsin Nuclear Powerby G.J. Phillips

Dr. Phillips is an Associate Research Officer with the Applied MathematicsBranch, Chalk River Nuclear Laboratories. He has had over 20 years' experience

in nuclear physics and applied mathematics at Chalk River.

December 1978 AECL-6335

To date, the peaceful application of nuclear energyhas been largely restricted to the generation of elec-tricity. Even with such an application there is poten-tial for wider use of the nuclear energy generated inproviding heat for dwellings, control of climate for theproduction of vegetables and providing warm waterfor fish and lobster farming. It is possible to envisagespecific applications of nuclear power reactors to pro-cess industries requiring large blocks of energy.These and other future developments are reviewed inthis report by Dr. Phillips of the Chalk River NuclearLaboratories of AECL.

Jusqu'a present, les applications pacifiques deI'energie nucleaire ont touche principalement la pro-duction de I'electricite. Ce type d'application offrediverses possibilities comme, par exemple, assurer lechauffage des maisons, controler le climat en vue dela production de legumes et fournir de I'eau chaudepour I'elevage des poissons et de? homards. II estpossible d'envisager des applications specifiquespour les reacteurs nucleates de puissance dans lesgrandes industries necessitant beaucoup d'energie.C'est ce type de developpement futur qui est passeen revue dans ce rapport par le Dr. Phillips desLaboratoires Nucleates de Chalk River de I'EACL

INTRODUCTION

Ontario Hydro's CANDU* Nuclear Generating Station atPickering has produced more electricity than any othernuclear plant. During 1977, one of the Pickering reac-tors outperformed all 90 of the world's other power reac-tors that develop more than 500 megawatts of electricalenergy, MW(e), while the three remaining Pickeringunits placed third, fourth and fifth in the world perfor-mance table1". In doing so, Pickering has saved the On-tario consumers over one hundred million dollars during1977, and has improved Canada's cumulative tradebalance by nearly one billion dollars since the stationstarted operation.

Since exploiting success is a good principle, it isnatural to ask how nuclear energy can further substitutefor our rapidly dwindling fossil fuels. This question isconsidered under three general headings:

— evolutionary development of the currentCANDU system

— extension of the CANDU system to otherapplications

— other long-term nuclear developments.We will see in all areas that the choice of the

CANDU system provides an unequalled ability to ex-ploit future opportunities as they arise.

CURRENT AND FUTURE CANDUREACTOR SYSTEMS

Any discussion of the future of nuclear power inCanada must begin by considering future develop-ments of the CANDU reactor system. Although theCANDU reactor has achieved practical maturity andproven its commercial viability with the PickeringGenerating Station, the development potential of thesystem is nowhere near exhaustion. Current designsare capable of evolutionary development to minimizeenergy costs and conserve resources'2' (Fig. 1).

It is most important to recognize that the CANDUsystem can and will adjust to future requirements by aprocess of evolution, based on a proven, highly suc-cessful design. Most other advanced nations expectthat future energy requirements will be met by a changefrom current thermal reactor systems to fast breederreactors131. No doubt this can be accomplished, but onlyat the cost of developing new and highly advancedtechnology. This route is inevitably more costly and lesspredictable than our evolutionary one'4'.

Three major areas can be identified for the evolu-tionary development of the CANDU system. The first ofthese is reactor size, in terms of unit output.

The four Pickering reactors each develop 540MW(e). For the Bruce Generating Station, each of thefour units is rated at a nominal 750 MW(e), with thepossibility of additional thermal energy being suppliedas industrial process heat to a heavy water plant (Fig. 2).

Still larger units are being contemplated. A detaileddesign study of a 1250 MW(e) unit has been conductedto identify the areas where further development effortsare required. The conceptual design of a 2000 MW(e)unit has been carried far enough to indicate that thereare no fundamental technical limitations to stations ofthat size.

Although there are no immediate commitments forthe construction of these larger reactors, the studiesprovide information necessary to guide the develop-ment that will be required for their introduction, both inindustrial capability and in providing the evidence oftheir acceptability to satisfy the regulating authorities.Thus we will be in a position to take advantage of thereduced energy costs of these larger units when theutilities are able to accommodate them.

The second major area for the development of theCANDU system is in the fuel cycle. All present CANDUreactors operate on the once-through natural uraniumcycle, with retrievable storage of the spent fuel.Although this cycle is very efficient in its utilization ofour uranium resources, compared to other commercial-ly available reactor systems, it does consume fissileuranium, the supply of which is limited. Fortunately, the

Figure 1 An artist's Imprtstlon o' Picktrlng Gtntrating Stations 'A' and 'B'' CANada Deuterium Uranium

Figure 2 Bruce Generating Station

CANDU design is sufficiently flexible that with no majorengineering developments, it can be adapted to exploitthe considerable reserve of energy in spent fuel byrecycling plutonium, and may be further adapted to thetransformation of thorium into a fuel material, thus giv-ing access to vast new reserves of energy15'.

The research and development required to establishthese alternative fuel cycles for the CANDU system is amajor concern of AECL, and it is the subject of a com-panion report161.

Tfie third major area of possible development of theCANDU design is the choice of heat-transfer fluid, orcoolant, used to extract the heat energy from the reac-tor core. Most of the power reactors presently operatingin Canada are of the CANDU-PHW design, indicatingthat the coolant is Pressurized Heavy Water. Heavywater is an efficient but expensive heat-transfer fluid,and two other coolants have been examined for possi-ble savings in energy costs.

In the CANDU-PHW design, the coolant transfersheat from the reactor core to a heat exchanger, andthence to a secondary coolant circuit containing or-dinary light water. Steam generated in this secondarycircuit is then fed to the turbines. The first alternative isto replace the heavy water coolant with light water, anda full-scale design study has been conducted for theCANDU-BLW concept, in which the coolant is BoilingLight Water. The coolant is allowed to boil within thereactor core, and the steam is separated from thecoolant and fed directly to the turbines. This direct cy-

cle concept offers the possibility of cost reductions byeliminating the heavy water coolant and the heat ex-changers and thereby, incidentally, slightly increasingthe thermodynamic efficiency171 (Fig. 3).

Another potential coolant is a light heat-transfer oil,or "organic" fluid. This option has also been extensivelyexamined in a design study defining the CANDU-OCR,or Organic Cooled Reactor (Fig. 4). The organic coolanthas a lower vapour pressure than water, permitting asignificant increase in coolant temperature without pro-hibitive increases in pressure tube wall thickness.Although a heat exchanger and secondary light watercircuit for steam generation is required with the organiccoolant as with heavy water, the higher coolanttemperatures give an appreciable increase in ther-modynamic efficiency, leading to reductions in energycosts, and also a reduction in the amount of waste heatproduced'891.

The CANDU-BLW concept has been developed tothe point of constructing a 250 MW(e) demonstrationplant, the Gentilly-1 unit, built in co-operation withHydro-Quebec. This unit first went critical in November1970, and since then has yielded a wealth of informa-tion and experience for the further development of thistype of reactor"01.

At the Whiteshell Nuclear Research Establishment,in Manitoba (Fig. 5), the organic cooled WR-1 Reactorhas operated for over ten years, with coolant outlettemperatures up to 400°C. This is a research reactor,and not used for the generation of electricity, but its

Figure 3 Boding Light Water (BLW) Flow Diagram

'• Steam to turbine

Steam drum — M ,

Steam/water mixture-*/, >S

Heavy water _ J H H B Lmoderator " l i l i l i l i l ^ .l i l l l l l l l ReactorFuel •• UHlBlPressure T B B W Ptubes ^ y P"JP

JT •.•••-•—••••-.•h-wnr W a t e r f r o mcondenser

- Light water coolant

Figure 4 WR-1 Flow Diagram (OCR)

Cooling RMCtorWater Loop

i River Circuit

fluetorCoolingCircuit

CoolingWaterfrom River

Coolant tor !••!Exchanger fuel In loop

Reactor CoolantHelium Qas

HeatExchanger

River WaterModerator

Figure 5 Whltathell Nuclear Research Establishment

long record of dependable performance has establish-ed a significant degree of confidence in the organiccooling concept"11.

Both these approaches to future CANDU develop-ment have been examined in conjunction with the ad-vanced fuel cycles mentioned above. By combiningalternative coolants with advanced fuel cycles it seemscertain that additional reduction in capital cost may bepossible, since in such cases the physics of the designallows a reduction in the volume of the heavy watermoderator. Our analyses suggest that overall costreductions of 15 to 20 per cent may be achieved, prin-cipally in the capital cost component when the advanc-ed fuel cycles are available"21.

One further area of potential benefit with these ad-vanced reactor concepts concerns the amount of radia-tion emitted by the primary coolant circuits while thereactor is operating. While radiation exposures to theoperating staff do not limit the performance of reactorsof the current CANDU-PHW design, radiations from thecoolant circuits of the CANOU-BLW are of much lowerintensity, and in the CANDU-OCR, the primary circuit isalmost totally inactive. These options could thereforehave additional attractions if any future reassessmentof the biological effects of radiation should indicate areduction in the internationally recommended levels foroccupational exposure"31.

PROCESS HEAT

A very large fraction of Canada's energy demand is con-sumed either directly as heat, or is used for the genera-tion of electricity, much of which, in turn, is used forheating purposes.

The end uses of this heat energy cover a broadspectrum of domestic, commercial, and industrial ac-tivities, which may be categorized according to thetemperature range at which the heat is used. A recentsurvey of the temperature distribution of energy con-sumption in Canada shows that some 50 to 80 per centis used at temperatures below 100°C, 21 to 27 per centin the range of 100 to 140°C, 12 to 14 per cent in therange of 140 to 260°C, and less than 10 per cent above260°C. A small proportion, perhaps 5 per cent, is re-quired at temperatures of 1000°C or above for industrialprocesses such as steel manufacturing"41. Projectionsof future consumption up to the end of this centuryshow almost no change in these ratios"51 (Fig. 6).

Although power reactors are usually operated togenerate electricity, all or part of their output may be ex-tracted directly as heat, at temperatures almost up tothe outlet coolant temperature. For current CANDU-PHW reactors, this temperature is about 300°C andcould be up to about 400°C with the CANDU-OCR.Thus, even the presently operating reactors can supplyheat at temperatures covering more than 90 per cent of

1000

300

200

0 - 5

50 — 80%

0 10 20 30 40 50 60 70 80 90Per cent

Figure 6 Temperature Distribution ol Energy Consumptionin Canada

Canadian requirements, so it is appropriate to considerpossible applications of reactors as direct sources ofheat energy.

The large quantities of energy that are used forspace heating fall in the temperature range below100°C, generally referred to as "low grade heat". Verylarge amounts of low grade heat are produced as a by-product of electricity generation, but almost none of itis utilized. Most thermal electric generating stations,

whether nuclear powered or fossil-fuelled, operate in anefficiency range of approximately 25 to 40 per cent. Theremaining 60 to 75 per cent of the total energy producedappears as low grade heat, and is rejected to a body ofcooling water — river, lake or ocean, or to the at-mosphere by means of lagoons or cooling towers. Thisenergy could be utilized for space heating if somemeans can be found of matching supply and demandeconomically. However, in order to maximize their ther-modynamic efficiency, thermo-electric generators aredesigned to reject their waste heat at as low atemperature as possible. The Pickering Generating Sta-tion for example draws its cooling water from a depth ofabout eight metres in Lake Ontario, and raises itstemperature by only 11°C so that the dischargetemperature is only a few degrees higher than the lake'ssurface temperature. Higher discharge temperatureswould reduce the thermodynamic efficiency, and soreduce electrical production.

Large-scale space heating would require a sourcetemperature high enough to take care of losses in thedistribution system, and still deliver heat at a suitabletemperature to the consumer. Assuming such a sourceis provided, space heat and sanitary hot water can bedistributed for industrial and domestic use by districtheating systems. These are well established in someEuropean countries, particularly in Scandinavia, buthave been largely neglected in North America, where anabundance of cheap fossil fuel has established a tradi-tion of individually heated buildings. Replacement ofthe existing individual heating units in an establishedcommunity by a district heating system is probablyuneconomic, but may be considered where new in-dustrial and housing complexes are being established.

Tabte 1 Cost Comparison*

Reactor core, MW

Thermal powersupply, MW

Overall energy

utilization, %

Estimated systemload factor, % (a)

Nuclear stationsUEC', m#(kW-h)

Pipeline UEC,m$/(kW-h)

DHS" grid UECm$/(kW'h) (b)

TUEC"',m$/(kW-h)(c)

ThermalOnly

706

600.0

85.0

35.0

9.2

7.9

3-24

20.1-41.1

Thermal/Electrical

2200

600.0

36.6

- 80.0

4.4

7,9

3-24

15.7-36.7

Thermal /Heat Pump

1000

600.0

70.5

35.0

17.9

7.9

3-24

28.8-49.8

ElectricalResistance

Heating

-

600.0

27

67.9

-

_

-

30.1(d)

(a) This Is the load factor for the generating station. Theload factor for the pipeline and distribution system istaken as 35%.

(b) The value 3 is derived from the Toronto study for a ser-vice to large users. The value 24 is an estimate of thecost of a grid to serve individual homes. A system ser-ving mixed loads would cost somehwere In between.

(c) These are the rates that will have to be charged in 1979to recover the original investment (no markup for profitshas been Included).

(d) This Is the cost of electrical energy in 1979 assuming a1974 price of 17 m$/(kW-h)(18)escalated by 10% eachyear to 1979, taking into account the depression in theload factor on the anraga cost, due to resistanceheating.

•UEC = Unit Energy Cost"DHS = District Heating Scheme

••*TUEC = Total Unit Energy Cost

New district heating schemes would involve signifi-cant capital outlay for the distribution network, installa-tion, and possibly for back-up heat sources. In addition,there is the necessity for the load centre to be relativelyclose to the source, although separation of up to 100kilometres may be acceptable.

Our studies indicate that under present Canadiancircumstances, district heating schemes would havedifficulty in meeting the economic competition of elec-tric heating. Should these circumstances alter in thefuture, the heat energy could be obtained from nucleargenerating stations"61 (Table 1).

Although space heating is the chief application oflow grade heat in Canada, other methods of utilizingthis form of energy may be considered"71. Low gradeheat can be used to enhance food production in bothagriculture and aquaculture, and studies have beenconducted in both these areas.

Large complexes of climate-controlled green-houses could make use of the waste heat from powerstations for the year-round production of vegetablecrops. No extensions of existing technology are re-quired, but problems of financing, sponsorship, loca-tion, acceptance and marketing would have to beresolved"81. Similarly, a supply of warm water can be ap-plied to enhance the production from freshwater fishfarming. Fish farming on a commercial scale is alreadybeing practiced in several countries such as Japan, us-

Flgui* 7 Layout of OrttnhouM Facility

ing warm water from power stations. Again, the ques-tion of whether such ventures are feasible does not in-volve the supply of heat"91. It should also be noted thateven very large-scale schemes for food production canuse only a small fraction of the total heat output of anuclear power reactor (Fig. 7).

Low grade heat is an inescapable by-product of thethermal generation of electricity, but as indicatedabove, a reactor can also supply intermediate gradeheat (100 - 300°C) for industrial processes. A survey ofCanadian industry has been conducted to identifysituations where the power requirements could be mat-ched to the output of a nuclear reactor, operating eitheras a pure heat source, or in a dual role, supplying bothheat and electricity1201.

The smallest "standard" CANDU reactor producesabout 600 MW(e), equivalent to about 2000 MW of ther-mal energy. In most cases this far exceeds the require-ments of Canadian industrial establishments. A few ex-isting complexes, such as the aluminum smelter atKitimat and the petro-chemical industry at Sarnia havepower requirements of this order, but new de-velopments on this scale are not envisaged. Moreover,most process industries require highly reliable energysources, which implies either dual reactor installationsor large and expensive back-up systems. Similar con-clusions have been drawn for England, in spite of itshigher degree of industrialization*21'.

914 m Exclusion Limit

Greenhouse facility100 m

Figure 8 Bruce Nuclear Power Development

One possible situation where the full output of areactor could be applied to an industrial process is theextraction of oil from tar sands. This involves theseparation of the tar from the sand by heating the tar toreduce its viscosity, and the subsequent refining of thetar, which may involve the addition of hydrogen. Near-surface sands may be strip-mined, but deeper depositsmay be extracted by pumping steam into bore holes towarm the tar sufficiently that it can be pumped out. Thequantities of heat required for the large-scale applica-tion of these processes can be of the order of 1500 to2000 MW, with some electrical energy required in addi-tion. These requirements would appear reasonably mat-ched to the capacity of a reactor, and if tar were beingextracted by pumping steam into deep strata, it mightbe possible to operate with only modest back-up energysources, as the heat capacity of the system wouldpresumably be large enough to allow extraction to con-tinue during routine reactor shutdowns. This possibleapplication has not been studied in any detail1221.

Although there appear to be few opportunities inCanada for matching the full output of a reactor to asingle industrial operation, it is possible to extract aportion of the reactor output as process heat, while us-ing the remainder for electricity production. This ap-proach is now used in Canada for the production ofheavy water, which requires large quantities of heat and

significant amounts of electrical energy. For example, alarge plant, producing about 100 kg per hour of heavywater, requires about 600 MW of heat energy at temper-atures up to 130°C, and about 70 MW(e) of electricity1201.

At the Bruce Nuclear Power Development (Fig. 8)the Douglas Point reactor has for several years suppliedprocess heat for heavy water production, and the BruceHeavy Water Plant will use steam from the Bruce reac-tors. The La Prade Heavy Water Plant, under construc-tion near Trois Rivieres, Quebec will be supplied withsteam by the two reactors at Gentilly'23 *•• 25 26<.

ENERGY STORAGE

The energy supplied by an electrical utility may be con-sidered to consist of two components: a constant mini-mum requirement, or "base load", and a cyclic com-ponent which varies in response to daily, weekly andannual changes in demand. Utilities respond to thesevariations in demand by operating some units at highcapacity to supply the base load requirements, andother units at lower capacity factors to supply the peakloads. The choice of units for each class of service ismade on the basis of minimizing the overall cost ofpower. For each type of unit, the cost of generating

power is a function of its capital, operating, and fuellingcosts. Nuclear units, for example, have relatively highcapital costs and very low fuelling costs, making themideal for base load operation. Oil or gas burning unitswill have lower capital costs, but much higher fuellingcosts, and are better suited to peak load duty.

In a large system with a variety of units, it iseconomically sound to expand the system by addinglarge nuclear units to assume as much of the base loadas possible, while relegating the units that burn themore expensive fossil fuels to peak load duty.

As more nuclear units are added, the nuclearcapacity will eventually exceed the base load require-ments, and it may then be necessary to "load-follow"with some of the nuclear units. Nuclear reactors can bedesigned for this type of operation, but the reduction incapacity factor results in higher unit energy costs. Twoalternatives are available: one is to change the demandcurve, for example, by altering the rate-structure for thecost of energy127'. The other is to include some form ofenergy storage in the system. With suitable storagecapacity available, the units with the lowest fuel(operating) cost, can operate at full capacity, storingenergy during periods of low load, with much of it retur-ning to the system during peak load periods. Thestorage units may be distributed among the users, suchas the domestic storage heaters used in England, inconjunction with an adjusted rate structure, or may becentralized in a large storage facility, probably operatedby the utility.

Many schemes have been proposed for large-scaleenergy storage. The best established method is thepumped-storage hydro-electric system, which is ahydro-electric generating station that incorporatespumps to reverse the flow of water during trie off-peakperiods. Large pumped-storage installations haveoperated in America and Europe for a number of years.Geological and environmental considerations are ofmajor importance in establishing such systems. Insome cases, it may be possible to adapt existing hydro-electric sites to include pumped storage'28'.

Other systems of pumped-energy storage that havebeen considered are: the use of "steam-accumulators"to store high pressure hot water as a source of peakingsteam for turbines; storing of boiler feedwater in rockcaverns, and the storage of compressed air, also in rockcaverns, for use in gas turbines. The first of theseschemes has been demonstrated on a relatively smallscale in Europe; the latter two exist only asproposals129-30'.

In recent years there has been considerable interestin the concept of storing kinetic energy in largeflywheels. This interest stems from the development ofhigh-strength composite materials, reinforced withglass or carbon fibres. However, detailed examinationof the concept suggests that flywheels systems of suffi-ciently large capacity for load levelling in an electric

distribution system would be very expensive, and thetechnical feasibility of such systems remains to bedemonstrated'31 -32>.

The costs of any of these techniques of energystorage appear to be sufficiently high as to provide in-centive for investigating and possibly developing reac-tors with load-following capabilities. It may be difficultto justify a storage system on the basis that iteliminates the need for nuclear units to load-follow.However, as fossil fuel costs rise, it may becomecheaper to add storage facilities than to add new fossil-fired units for peak loading handling'29'.

With the current interest in the development ofrenewable energy sources, considerable attention is be-ing given to the direct use of solar energy, and a numberof experiments and demonstration projects are beingconducted. Due to its fundamentally intermittentnature, any substantial application of solar energy willrequire an energy storage facility as part of the system.Once the storage system is installed, solar heating mayfind itself competing with electric heating at off-peakrates, rather than the average or peak rates'33'34-35).

An alternative method of energy storage is the useof off-peak electricity-to generate hydrogen by elec-trolysis, thus transforming the electricity into a gaseousfuel. Hydrogen can be easily distributed by pipelines,and can be used in fuel cells, as a chemical feedstock,or as a replacement for natural gas. Used for the pro-duction of methanol it provides a source of portable li-quid fuel for transportation, which might eventuallycompete with liquid hydrocarbons derived from tarsands or coal liquefaction'36'.

There has been considerable discussion of pro-posals that developed countries should move to a"hydrogen economy" rather than to an "all-electriceconomy", but this appears to be an extreme sugges-tion, as hydrogen cannot replace electricity for manyapplications, and hydrogen produced by electrolysiscan never be cheaper than the off-peak nuclear energyused to produce ii137-38>.

AECL is particularly interested in the possibility oflarge-scale production of hydrogen as a source of heavywater through the application of the Combined Elec-trolysis and Catalytic Exchange Process (CECE). This isa method of deuterium exchange involving waterproofplatinum catalysts that is currently under developmentby AECL'39' (Fig. 9).

Currently TransCanada PipeLines, the Province ofManitoba, and AECL are taking part in a joint study ofthe feasibility of a 100 MW demonstration plant for elec-trolytic hydrogen-heavy water production.

A combined process such as this has the obviousvirtue of producing two saleable products simultane-ously. Possible drawbacks are the increased complex-ity of the system, compared to separate facilities, andthe necessity of maintaining a market for both productsif the system is to operate economically.

BasePower

• *

NuclearPowerPlant

Off-peakPower i

! r

°2Reservoir

H2

Reservoir

* | r— Feed H2O

AnodeSide

HJ/HJO DeuteriumStripper Column

CathodeSide

ConventionalElectrolysis Plant

Sewage treatment,Metallurgical processes

, etc.

*• steam turbines or fuel, cells

Gas pipeline for in-•• dustrial and domestic

use

Metallic hydrides or"*" liquid H2 for trans-

portation sytems

Deuterium enriched water to'inal concentration plant

Figure 9 Schematic diagram of complete system for producing, (a) hydrogen and oxygen by electrolysis, (b) heavy water,(c) peak power by gas turbine-generator or fuel cells and (d) other possible uses lor either or both H2 and O2

ELECTRO-NUCLEAR BREEDING

The physical basis for the operation of all nuclear reac-tors is the self-sustaining chain reaction. A fissile atomcaptures a neutron, and subsequently fissions, i.e.breaks up into two or more fragments, releasing energyand another neutron, or neutrons. The number ofneutrons released in fission is subject to statistical fluc-tuations, but on the average this number is greater thanone. Of the neutrons resulting from each fission, one isrequired to carry on the chain reaction, while the re-maining neutrons will either be captured by non-fissilematerial within the core, such as control elements orstructural components, or else "leak" out of the core,and be captured in the surrounding structure orshielding material (Fig. 10).

Uranium235

SlowNtutron

oFissiono

Ntutrons

Figure 10 The Fission Procatt

The natural uranium used to fuel the CANDU reac-tors is composed of two types of atoms. One type,which is fissile, is known as the uranium-235 isotope,and accounts for only 0.7 per cent of the uranium; theremaining 99.3 per cent consists of the non-fissileuranium-238 isotope. Since it constitutes such a largefraction of the fuel, a reactor fuelled with naturaluranium will have a significant fraction of its excessneutrons captured by the non-fissile component.

Fortunately, the uranium-238 isotope belongs to theclass of atoms known as "fertile". When a fertile atomcaptures a neutron, it undergoes a series of spon-taneous reactions that transform it into a fissile atom,which may then contribute to the chain reaction.Specifically, the uranium-238 is transformed intoplutonium-239, a fissile isotope that contributes asignificant fraction of the energy released in theCANDU reactors. Thus the excess neutrons from thefission process generate additional fuel for the reactor.

This process of converting fertile material to fissileis loosely referred to as "breeding". A true breeder reac-tor is one in which there is a sufficient excess ofneutrons for the reactor to produce more fuel than itconsumes. A CANDU reactor operating on the naturaluranium fuel cycle converts a useful proportion of thefertile uranium to plutonium, but does not generateenough excess neutrons for breeding. However, whenoperated on the thorium fuel cycle, the CANDU systemwill be a "near-breeder", producing as much fuel as itconsumes, and so capable of a self-sustainingequilibrium1'2*.

Although the total amount of energy available inthis equilibrium situation depends only on the supply offertile material, the ability to expand the system islimited by the supply of fissile material, because the ex-cess neutrons from the fissile atoms are required to in-itiate the process, although once started, the cycle isself-sustaining.

This possible limiting dependence on naturally oc-curring fissile material can be removed by providing analternative source of neutrons. Two possible means ofgenerating neutrons in substantial quantites are by fu-sion reactions and spallation reactions, both of whichmay be considered to be "electrical" means of neutrongeneration. Of these processes, fusion is perhaps thebetter known, but has yet to be demonstrated.

Spallation reactions occur when energetic protonsstrike heavy element targets*, and in effect knockneutrons out of the target nuclei. An electro-nuclearbreeding system might thus consist of a high energy,high current proton accelerator, producing spallationreactions in a heavy metal target, with a blanket of fer-

A A AFigure 11 Simplified diagram of a possible spallation reaction. A

total of 20 — 25 neutrons, each with a mean energy ofabout 3.S MeV, results from the cascade

Flgurt 12 Tharac 6 linear accalarator madical tharapy unit

' Proton energies in excess of 1 billion volts {1 GeV) are generallynecessary.

tile material surrounding the target to capture theneutrons. Fissile material would be generated in theblanket, and might have to be extracted chemical'ybefore fabrication into fuel for fission reactors. Altt"-natively, one might link the accelerator to a nudes,reactor in a hybrid system, and so directly increase thenumber of excess neutrons in the reactor core (Fig. 11).

Studies have shown the concept of electro-nuclearbreeding to be scientifically feasible, and that it wouldrequire relatively little development, mainly of the ac-celerators, to establish engineering feasibility. If thecost of uranium were to rise substantially, electro-nuclear breeding might be economically justified. Inany event, such systems will not be required until theadvanced fuel cycles using fertile materials are fullyestablished, so there is ample time for their orderlydevelopment1"01.

Meanwhile, AECL has a continuing program of ac-celerator development that has produced a team ofknowledgeable experts who maintain a currentawareness of progress in this general area of physicsand engineering, and who have also achieved a numberof advances in the development of medical ac-celerators, industrial accelerators, and physics ac-celerators for fundamental research1411 (Fig. 12).

FUSION

The fusion process is often described as the logical anddesirable successor to fission as a source of energy.The main arguments in favour of fusion are that the pro-blems of radioactive wastes would be much less severethan with the fission process, and that fusion wouldconsume only deuterium, of which there is an almostlimitless supply (Fig. 13).

In the fusion process, two light atomic nuclei arecombined to form a single larger nucleus, resulting inthe release of energy, and the emission of particles, in-cluding neutrons. All atomic nuclei are positively charg-ed and are mutually repulsive. To produce fusion, it isnecessary to overcome this repulsion by simultaneous-ly confining the nuclei and heating them to extremelyhigh temperatures. The existence of fusion reactions isapparent in the sun, and in the hydrogen bomb, butdespite a massive research effort in a number of coun-tries, self-sustained fusion reactions have not yet beenachieved in the laboratory142!.

Although a fusion process involving only twodeuterium nuclei, the so-called D-D reaction, is possi-ble, there is good reason to expect that the fusion ofdeuterium and tritium will be easier to achieve, and ef-forts to demonstrate controlled fusion are at presentconcentrated on this D-T reaction. Deuterium andtritium are both isotopes of hydrogen. Deuterium oc-curs naturally at concentrations of a few hundredths of

3 2 5 M e v

Deuterium Deuterium Helium 3 Neutron

Deuterium Tritium Helium 4 Neutron

Figure 13 Simplified diagra.ns ol two possible fusion reactions*

a per cent in all water. The oxide-of deuterium is heavywater, which Canada now produces in tonne quantities.

Tritium does not occur naturally, but can be produc-ed by the neutron bombardment of lithium. A fusionreactor using the D-T reaction would therefore consumelithium as well as deuterium. Although lithium is arelatively abundant element, on the basis of equivalentenergy, the reserves of lithium are roughly equivalent tothe total reserve.1. • fossil fuels'43'. The tritium could beproduced in the fusion reactor itself, by surrounding itwith a lithium blanket to capture the neutrons releasedin the fusion reactions. The system would have to incor-porate facilities for extracting the tritium from theblanket material.

Tritium is a radioactive material, and the high flux ofneutrons from fusion reactions will also induce intenseradioactivity in the reactor structure. Fusion reactorsare expected to be as intensely radioactive as fissionreactors, and will have similar requirements for contain-ment, shielding and precautions against exposure ofpersonnel.

Current attempts to demonstrate fusion reactionsfollow one of two approaches. The first is referred to as"magnetic confinement", and involves the localizationand compression of a hot plasma, i.e. an ionized gascontaining the deuterium and tritium nuclei, by meansof various configurations of magnetic fields1441. The se-cond is a more recent development called "inertial con-finement", and involves the compression of smallpellets containing the reacting nuclei by multiplebeams of charged particles, or by laser light. This com-pression should simultaneously produce the high den-sities and temperatures required for fusion to occur*'15'161.

While both techniques have their enthusiastic pro-ponents, a controlled fusion reaction has yet to bedemonstrated. After this technical feasibility has beenestablished, there are formidable engineering andmaterial problems in the design of a practical fusionpower reactor. Until solutions to : ' fse problems havebeen demonstrated, no detailed economic assessment

10 of fusion energy sources is possible, but it seems

' Scientilic American, 225(3). p. 67 (September 197))

unlikely that fusion energy will be appreciably cheaperthan fission energy. A major portion of the capital costof a fission power station is concerned with the "con-ventional" plant — the steam generators, turbines,alternators ar\d structures, and very s;:nilac units will berequired for a fusion-powered generating station.

It is clear from examining the history of existingenergy technologies that the time required todemonstrate, develop, introduce and establish a newsystem on the scale of a national energy program cantake many decades, and there seems little likelihood offusion power making a major contribution to our energysupply before the second half of the next century147 m.

Three advisory bodies: the Science Council ofCanada1491, the Project FCf Consortium1501, and the Ad-visory Committee on Project FC15" have recommendedthat the Canadian government should initiate andmanage a co-ordinated program in fusion energy. It isnot recommended that Canada attempt to develop a fu-sion reactor at this stage, but that we establish a co-ordinated national program of scientific researchrelated to controlled nuclear fusion, as a step towardsmeeting Canada's long-term needs for energy.

The program supports research in selected specificareas, where there is a good possibility of Canadian ef-forts making a significant contribution. At the sametime a general awareness of the complete field is beingmaintained, so that we are in a position to exploit anymajor breakthroughs when they occur.

The National Research Council has assumedresponsibility for co-ordinating the initial researchphase of the fusion program. AECL is participating withNRC, and maintains a modest in-house FusionAwareness Program, including theoretical studiesrelated to fusion research. It has been agreed with NRCthat AECL is the logical agency to assume responsibili-ty for the selection and execution of a Canadian fusionreactor project if, and when, the prospects for fusion im-prove sufficiently to warrant a major commitment.

CONCLUSIONS

The CANDU reactor system is established as a signfi-cant source of energy for Canada. A large and growingfraction of Ontario's energy requirements is now sup-plied by nuclear stations. Nuclear power systems arebeing introduced in Quebec and New Brunswick, andare being considered by other provinces. This trend willcontinue as the costs of fossil fuels rise relative tonuclear fuels, encouraging the substitution of the muchmore abundant nuclear fuels for the depleting suppliesof oil and natural gas'4S).

Although the CANOU design has now achieved acompetitive position as a source of electrical energy, itsdevelopment potential is very far from being exhausted.t FC = Fusion Canada

The inherent flexibility of the design gives it an excep-tional ability to cope with changing patterns of fuel sup-ply and energy demand. By adapting to advanced fuelcycles and possible alternative heat transfer sytems,we can confidently predict a continual, evolutionarydevelopment of the system that will maintain its com-petitive position, and ensure an economical secure sup-ply of energy well into the next century152- "•54>.

Our studies have indicated only very limited incen-tive for large-scale process heat or energy storagefacilities under present conditions, but nuclear fissioncan readily supply the energy for these applicationswhen changes in the economic and social patternsmake them viable.

Nuclear power will supply an increasing proportionof our energy requirements through a continuing pro-cess of scientific and engineering development. Thiscould include not only the further evolution of the fis-sion reactor and its fuel cycle, but also the expansion ofnuclear technology into the areas of electro-nuclearbreeding, and fusion power reactors. AECL maintains amodest level of effort related to these more advancedsystems so as to be in a position to assess and exploittheir long-term potential at the appropriate time155'.

11

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14

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