Advanced Gas Reactors.pdf

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Nordisk Nordisk Pohjoismainen Nordick e r n - klrn- ydin- nuclearforskning forskning tutkimus researchsikkerheds- sllkerhets- turv alli suw safety

RAK-2

Description of the Advanced Gas CooledType of Reactor AGR)

Erik Nonbel

Ris0 National LaboratoryRoskilde, Denmark

November 1996


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AbstracThe present report comprises a technical description of the Advanced Gas cooledReactor AGR), reactor type which has only been built in G r a t Britain. 14 AGRreactors have been built, located at 6 different sites and each station is supplied withtwin-reactors.The Torness AGR plant on the Lothian coastline of Scotland, 60 km east of Edinburgh,has been chosen as the reference plant and is described in some detail. Data on the other6 stations, Dungeness B,Hinkley Point B, Hunterston B, Hartlepool,Heysham I andHeysham II, are given only in tables with a summary of design data.m e r e specific data for Torness AGR has not been available, corresponding data fiomother AGR plants has been used, primarily flom Heysham II, which belongs to the samegeneration of AGR reactors. The information presented is based on the open literature.The report is written as a part of the NKS/RAK-2 subproject 3: Reactors in NordicSurroundings, which comprises a description of nuclear power plants neighbouring theNordic countries.

NKS/RAK-2(96)TR-C2ISBN 87-550-2264-2Graphic Service, Riso, 1996The report can be obtained from:NKS SecretariatP.O.Box49DK-40o0 RoskildeDenmark

Phone: +45 46 77 40 45Fa: +45 46 35 92 73http://www.xisoe.dk/nkse-maii: [email protected]
http://www.xisoe.dk/nkshttp://www.xisoe.dk/nks


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Contents

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 S U M M A R Y OF DESIGN DATA ............................................................................ 103 SITE AND REGION ............................................................................................ 13

13.1 Selection of the site .................................................................................14SAFE TY CRITERIA ............................................................................................

5 TECHNICALESCRIPTION AND DESIGN EVALUATION ....................................... 155. 1 Plant arrangement .................................................................................... 15

16.2 Buildings and structures.. . . .5.3 Rea ctor core an d other reactor vessel internals.. . ..... ..... . 17

5 . 3 . 1 Mechanical de sign.. . . . . . . . 185 . 3 . 2Nuclear design . . . . . .205 . 3 . 3 Thermal and hydraulic design . . , , , . . . . . , , 2 8

5. 4 Rea ctivity Control system ....................... ................................................ 305.4. 1 Secondary shutdown system .................................................................... 3 132

5.5 .1 Rea ctor coolant piping .... . . . . . 3 25 . 5 . 2 Reactor coolant pumps ........................ ........................................ ......... . . 3 25.5.3 Steam gen erators.. . . 3

5 5 Reactor main coolant system . ....... .. . .............. .. ..

5.6 Residual heat removal systems ................................................................ 385.7 Emergency core cooling systems . . .. 4 05.8 C ontainment systems . . ............... . . . . . . ........ .... .. 42

5.8.1 Overall system information..5.8.3 Containment penetrations.. .5.8 .5 Pressure reducing sy

...............5 . 8 . 2 Containment structure . .5 .8 .4 Containment liner.. ...............

.......................................... 455.9 S team and power conversion systems . . . . . 4 646.9.1 Turbine-generator . . . . . . . . .5.9.2Main steam supply system . . . . . . . .47


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5.1 0 Fuel and com ponent handling and storage systems ................................ 485.11 R adioactive waste systems ..................................................................... 50

5.11.1Liquid waste system ............................................................................... 505.11 .2 Gaseous waste system ............................................................................ 505.11 . 3 Solid waste system ................................................................................. 51

5.12 Control and instrumentation systems ..................................................... 535.12.i Protection system ................................................................................... 535.12 .2 Regulating system .................................................................................. 54

5.13 Electrical power systems ....................................................................... 566 F I R E PR O TE C TI ON . W I G N E R E N E R G Y AN D GRAPHITEXIDATION .................... 587 PLAN T PERFORMANCE DURING NORMAL OPERATION ....................................... 608 PLAN NING AND ORGANISATION ....................................................................... 639 REFERENCES ................................................................................................... 6 510 APPENDICES................................................................................................ 66APPENDIX : DUNGENESSAGR STATION 67....................................................APPENDIX : HINKLEYOINT B AGR STATION ................................................ 71APPENDIX : HUNTERSTON AGR STATION ................................................... 75APPENDIX :HARTLEPOOLGR STATION ........................................................ 77APPENDIX : HEYSHAM GR STATION ............................................................. 83


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List of FiguresFigure 1.1. AGR stations in G reat Britain . . . . . . . . . . . . . .Figure 3.1. Site of Torness Nuclear Power Station. ..................Figure 5.1. Com ponents of a typically AGR nuclear power station. . . . . . . . . . . . .Figure 5.2. Layout of building structures.. .................................................

..................................... 9

Figure 5.3. Main co mponen ts of reactor.. ......................Figure 5.4 . Gas baffle with ga s flow paths. ........................................Figure 5.5. Intercon nection o f graphite bricks with keys.Figure 5.6. Core layout - nearly 114 core symmetry. .........................

Figure 5 .8AGR h e l element. ..................................................Figure 5.9Detailed view of AGR h e l element . . . . . . . . . . . . . . . .Figure 5.1O Refbelling machine ............................................. ....27Figure 5.1 1. Gas flow distribution in the core and vessel. ........Figure 5.12. AGR Control rod. . . . . . . . . . .Figure 5.1 3. AGR gas circulator. ...............

Figure 5.15 . AGR boiler unit. .Figure 5.16. Diverse boiler-feed systems. ............

Figure 5.18. The stressing gallery for the tendo ns at the top of the vessel. ................... 4 3Figure 5.1 9. Pressure vessel with penetrations.. . . . . . . . . . . .Figure 5.20. Location and size of &el assemb lyhe l elementFigure 5.21. Fuel route from loading of new h e l to unloading of irradiated h e l

Figure 6 .1 . Accumulation of stored energy in graphite, (Ref 6)

Figure 8. 2. Load factors for Torness AGR station.. ....Figure 1O 1. Location of Dun geness Nuclear Power Station.. .

Figure 10 .3. Location of Hinkley Point Nuclear Po wer P lant.. ...........................

........................... 21

Figure 5.7. Dimensions of an AGR h e l element ................................................................ 26

.................. 2830

............................................... 34.............................................

Figure 5 .14. The four boiler quadrants in the AG R. .......................................................................

.....Figure 5 .17. Decay heat and emergency boiler feed system .......................

............................ 45.......................... 48

49............................................ ' 5 6

.......................... 58.................................................................. 63

..................................... 64........................... 67

Figure 5.22. Electrical system .....................................

Figure 8.1 . AGR average load factors

Figure 10.2. Load factors for Dungeness B AGR stati

Figure 10 .4. Load factors for Hinkley Point B AGR station. .....................


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- 6 -Figure 1O 5 . Location of Hu nterston Nuclear Power Plant. ......................................... . 7 5

Figure 1O 7. Loca tion of Hartlepool Nuclear Power Station. ..Figure 10.6.Load factors for Hunterston B AGR station. . . . . . .

Figure 10.8.Dimensions of single- and multi-cavity preFigure 10.9.Pressure vessel layout for Hartlepool AGRFigure 10.10.Load factors for Hartlepool AGR stationFigure 1O 11. Location ofHeysham AGR station. .......Figure 1O 12.Load factors for Heysham I AGR station.. ..................Figure 1O 1 3 . Load factors for Heysham II AGR station. ..................................Figure 1O 14. Comparison o f layout of Heysham I and


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List of Tables

Table 1.1 . AGR stations in operation. ...........Table 2.1. Summary of design data for T orness AGR. nuclear pow er station..

Table 5 .2 . Main design data for the core. .........................

Table 5.4 . Heat balance for an AG R plant.Table 5 .5 . Design da ta for gas circulators.. ..............................Table 5.6 . Design data for boilers.. ..................................

............................................. 9.... 10

Table 5.1. Design data fo r pressure vessel. ....................

Table 5.3 . Main design data for k e l eleme............................................. 29

..................... 36Table 5.7. Design d ata for turbine plant. ......................................... 46Table 7.1. AGR Construction times and year for start of operation. . . . . . . . . . . . . . . 60Table 1O 1 . Summary of design data for Dungeness AGR nuclear power station . . . . . . . . . 68Table 10.2. Summary of design data for Hinkley Point AGR nuclear power statio n.. .. 7 2Table 10.3. Summary of design data for Hartlepool AGR nuclear power station.. . . . . . . . 8Table 10.4.Com parison of main data for all 7 sites of AGR's in UK ..................... 86


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1 IntroductionA new four-year nuclear research program w ithin the framework of NKS, NordicCom mittee for Nuclear Safety Research, was started in 1994 as a follow-on t o severalpreceding N ord ic prog ram me s. Joint research in this field is of interest for the fiveNo rdic co untries who have similar needs for maintaining their nuclear competence in thefield of reactor safety and waste m anagement, and wh o are exposed t o the same ou tsiderisks from reactors in neighbouring countries, from nuclear pow ered vessels, and fromrisks of contamination o f terrestrial and aq uatic areas.This report is written as a part of the N K S M - 2 subproject 3: Reactors in NordicSurroundings, which comprises a description of niiclear power plants neighbouring theNordic countries.The main objective of the project has been to investigate, collect, arrang e and evaluatedata o f reactors in the Nordic neighbourhood t o be used by the Nordic nuclearpreparedness and safety authorities.In the former NKS project, SIK-3, reactors within 150 km from the border of a Nordiccountry w ere treated, but it was decided to add a description of the British reactors,although the minimum distance to a No rdic border, the Nonveg ian, is about 500 km.Th e present report comprises a technical description of the Advanced Ga s cooledRe acto r (A GR ), a reactor type which has only been built in Great B ritain. 14 AGRreactors have been built, located at 6 different sites and seven stations, Figure 1.1 . andTable 1.1 . Each station is supplied with twin-reactors, the site of Heysham has twostations Heysham I and Heysham II.Dungen ess B was the first commercial AGR plant to be ordered and it representstogethe r with Hinkley Point B and Hunterston B the first generation of AGR s. All threestations ar e almost iden tically and ar e therefore called sister plants.Hartlepool and Heysham I represent a m odified version of first gene ration AG Rs with aso-called multi-cavity pressure vessel, while Torness and Heysham II constitute secondgeneration of AG R type of reactor with improved safety features.Th e Torness AG R plant on the Lothian coastline of Scotland, 60 km east of Edinburgh,has been ch osen as the reference plant and is described in som e detail. Da ta o n the other6 stations a re given only in tables with a sum mary of design data in appendix A-E .W here specific data for Torness AG R has been unavailable, corresponding data fromother A GR plants has been used, primarily from Heysham II, which belongs to the samegeneration of A GR reactor. Th e information presented is based on th e open literatureand the con tent of the report follows the format agreed on in the SIK-3 project (Re$ I ) .


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Generationof AGRWetation unitDungeness B 2 x 660 f i rs tHinkley Point B 2 x 660 firstHunterston B 2 x 660 first

Fipre I . I . AGR stations in Great Britain

Sister Start ofplant operation

a 1976a 1983-85a 1976-77

Heysham IHeysham IITorness

2 x 660 1983-842 x 660 second2 x 660 secondI Total number of AGR units : 14 at 6 sites and 7 stations I


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2 Summary of design dataIn Table 2.1a summary of the main design data for T orness AGR nuclear power stationis show n. All d ata app lies to a single unit o f the twin-rea ctor station.

Table 2.1 . Summary of design data fo r TornessAGR nuclear power station

Station designReactor typeElectrical output (gross)Thermal output (gross)E a c i e nc yHeat balancePower to turbinePow er loss to vessel liner cooling systemPower loss to circulator cooling systemPower loss to g as treatment plantTotal heat to gasPumping powerPower from reactorReactorModeratorCoolant gasNumber of h e l channelsLattice pitch (squa re)Active core diameterActive core heightNum ber of C ontrol rod channelsDiameter of Control rod channelsMean gas pressureMean inlet gas temperatureMean o utlet gas temperaturePeak channel outlet temperatureTotal gas flowPeak channel flowAverage channel flow

AGR Advanced Gas Cooled2 x 6 6 0 M W e2 x 1623 MWt40.7 Yo

1649 M W8.5 M W4.5 MW3.0 M W1665 MW42 MW1623 M W

GraphiteCO2332460 mm9.5 m8.3 m8912741 bar339 C639 C661 C4067 kg/s

14 kg/s12 kg/s


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Table2.1 continuedFuel elementsMaterialTypePellet diameterInner grap hite sleeve diameterChannel diameterCladding materialCladding thicknessElement lengthNumber of elements per channelEnrichmentPower densityMass of uranium per reactorAverage h e l ratingAverage h e l burn-upPressure vesselMaterialInner linerInternal diameterInternal heightExternal diameterTo p slap thicknessBo ttom slap thicknessDesign pressure

u 2

14.5 mm190 mm264 mm0.38 mm1036 mm8

3 kW/litre123 tonnes13.65 MWt/tU18,000 MWd/tU

36 pin cluster in graphite sleeve

Stainless steel

2.2 - 2.7 %

Pre-stressed concrete20.3 m21.9 m31.9 m5.4 m

7.5 m45.7 bar

Stainless steel

Gas circulatorsTypeRegulationNumber circulatorsPower consumption per reactor

BoilersNumber of boilersNumber of units per boilerFeedwater temperatureGas inlet temperature to reheaterGas outlet temperatureSuperhea ter outlet pressureSuperheater outlet temperatureSteam generationReheater o utlet pressureReheater outlet temperature

Centrifugalspeed8

Constan t variable inlet vanes

42 MWe

43158 C619 C293 C173 bar541 C525 kgis42 bar5 3 9 C


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3 Site and region3.1 Selection of the siteTorn ess is on the Lothian coastline, about 60 km east of Edinburgh, Figure 3 . 1 . The sitehas goo d ground conditions capable of supporting heavy loads, am ple supplies of seawater for cooling purposes, go od road and rail access and is supplied with fresh wate rfrom L othian Regional Council.The location of this base load plant in the east of the County provides a goo d ge o-graphical match betw een generating capacity and demand, and m inimises the need foradditional transmission.

Figure 3.I . Site of Torness Nucletzr Power Station.

Site work on To rness power station started in February 1977, with main construction followingin 1980. The first of the twin-station units was synchronised to the Scottish grid in M ay 1988 andthe second in February 1989, giving a total generating capacity of 1320 M We .


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4 Safety criteriaA primary concern for the UK nuclear programme has been to assure, that neither thepublic or the personnel at nuclear power stations are exposed to harmful radiation. Th epolicy for achieving these objectives embodies certain fundamental principles, of whichsome of the m ain features are:

As the result of normal operation of a power station, no person shallreceive doses o f radiation in excess cif the approp riate limits.The exposure o f individuals shall be ke pt as low as reasonable practicable.The collective dose-equivalent to opera tors and the general public as aresult o f the o peration of a nuclear installation shall be kept as low asreasonably practicable.All reasonably practical steps shall be taken t o prevent accidents.All reasonably practical step s shall be taken t o m inimize the radiologicalconsequences o f any accident.

Safety requirements are an important factor in the design, manufacture, construction andoperation o f the nuclear power plants and they shall assure defence in de pth.Th e first line of defence is high quality in design and manufacture o f com ponents andequipment so that th e plant will operate reliably, and operation of the plant by highlytrained staEThe second line of defence is a reactor design which assum es that if faults occur, theninstrum entation and Control systems will automatically bring the rea ctor t o a safeshutdown condition. Duplication o r triplication of safety equipment is provided whereappro priate, and it is designed to fail safe, so that if faults develop, the reac tor is shutdown automatically.The third line of defence is the examination of a whole range of extreme accidents orunlikely faults, which could point to th e need of additional safety features. The se safetyfeature s could be e xtra emerge ncy cooling systems, or additional electricity supplies.


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5 Technical description and design evaluation5.1 Plant arrangementIn an AGR nuclear power station, there are two reactor u nits and one turbine housecombined in a single complex with a central block for h e l handling, instrumentation andControl, a so-called twin-station. The reactors are selved by one refbelling machineoperating within a comm on charge hall. The principle comp onents of a typically AGRsystem ar e show n in Figure 5.1.

De aeraiw

Reserve eedwatet tankGasbaffle

Boleri e e d penetration

Gas carmlaorSecondaryshut-downplantmrn

Figure5.I . Componentsof a typically AGR nuclear power station.


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5.2 Buildings and structuresIn Figure 5.2 is shown th e site layout of buildings and structures o f the T orness PlantThe re actor building is a reinforced concrete co nstruction topped by the ch arge hallsupers tructure of steel portal frames to a height of '74 metres above groun d level.The re ar e four supplies buildings, housing diesel generators and sw itchgear. The sebuildings together with the reactor building are designed t o withstand seismic loads. T hefour buildings are located close to but separate from the reactor building to providemaximum segregation.The reactor building also houses the high level de-aerators, reserve feed w ater tanks andthe high pressure steam and feed pipework. The station Control room is also within thereac tor building, and turbine hall is a continuation c)f this building but w ith a lower roo f

Figure 5 2 Layout of budding structureslevel. It contains the turbine generators and associated auxiliary plant, and thecondensate and feedwater treatment plant. The circulating water pump house is areinforced co ncrete and steel structure located t o the north o f the reactor and turbine hallbuildings. It houses the four main circulating water pum ps and eight reactor coolingpumps. Between the pumphouse and the sea is an area containing four dm ms whichfilter the inlet sea w ater.


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Major features of the plant cons truction include the lifiing-in of the gas baffle enclosingthe cor e and liner roof assembly, installing the reactor and boiler com ponents and th etwo 700 MWe turbine generators, laying about 53 O00 cables for pow er, Control andinstrumentation, and constructing a 400 kV transmission substation.

5.3 Reactor core and other reactor vessel internalsFigure5.3 shows the main components of the reactor.

4 Circulator outlet gas duct5 Boiler6 Thermal insulation7 Reheat steaiii pcxietrations8 Main steam penetrations9 Boiler feed penetrations

1 Reactor core2 Supporting grid3 Gas baffle

Figure 5 .3 .Main components of reactor.


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5.3.1 Mechanical designIn a typical AGR system, the reactor core, boilers and gas circulators are housed in asingle cavity, pre-stressed concrete pressure vessel. The reactor m ode rator is a sixteen-sided stack of graphite bricks. It is designed to act a s a moderator and to p rovideindividual channels for h e l assemblies, Control devices and coolant flow (Figure 5.3andTable 5. ) .

Table 5 .1 . Design datafo r pressure vessel.MaterialInner linerInternal diameterInternal heightExternal diameterTo p slap thicknessBo ttom slap thicknessDesign pressure

Pre-stressed concreteStainless steel20.3 m21.9 m31.9 m5.4 m7.5 m

45.7 ar

The g raphite is covered by an upp er neutron shield of graphite and steel bricks andmounted on a lower neutron shield of graphite bricks, that rests on steel plates. Radialshielding is in th e form of steel rods located in two outer rings o f graphite bricks.The graphite stru cture is maintained in position by a steel restraint tank that surroundsthe graphite and is supported on a system o f steel plates.Th e shielding reduces radiation levels outside the core, so that when the reactor is shutdow n and depressurized, access to the boilers is possible.The re are tw o m ain effects of irradiation of the graphite moderator. One is dimensionalchange and the other is radiolytic oxidation by th e carb on dioxide c oolant. Th ere willalso be a significant change in the thermal conductivity, which decre ases with anincreasing temperature. Because of the relatively high temperatures in the core, there willbe little or no stored energy in the grap hite (Wigner energy).The dimensional change d ue to the anisotropic properties of graphite is reduced bymanufacturing the grap hite bricks by use of moulding rather than extrusion.W hen CO2 is radiolysed it breaks dow n giving CO and a very reactive chem ical specieswhich behaves like an oxygen atom

CO2+ CO + O (radiolytic)Most of these species recombine in the g as phase

O + CO + CO*


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How ever, som e of them will escape recombination in this way - the mean distance thatthe active species can trave1 before undergoing reaction is slightly greater than th e meanpore diameter of the graphite - and will reach the grap hite surface where they will react:O + c + C 0 ) (graphite surface reaction)

where C ( 0 ) is a s urfa ce oxide. This will subsequen tly break loose to give gaseou s CO.The consequences of the process is a weight loss of the graphite. Howev er, only that ga scontained in the po res of the graph ite takes part in the reaction, the bulk o f the gas in thereactor circuit is not involved.The criterion adop ted for the maximum permissible inean weight loss o f graphite hasbeen set to 5 % over a 30 years lifetime.Radiolytic oxidation is inhibited by adding methane to the coo lant. Howev er, methane inhigh concentrations can lead to carbon depositions, in particular on the h e l assemblysurfaces. Therefore, a compromise between protection of the m oderator and depositionon the fuel must be made by a careful choice of the concentration of the inhibitor.The co re and th e shield are completely enclosed by steel envelope called the g as baffle,the main function of which is to prod uce a downw ard flow of coolant gas (re-entrantflow) through paths in the graphite moderator to cool the graphite bricks and to separatethe hot from the cold gas (Figure 5.4).

Figure 5.4. Gas baffle with gasf l o w paths

Gasbaffle


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The gas baffle has three m ain sections - the dom e, the cylinder and the skirt. In the d oomthere ar e a number o f penetrations - one for each of the h e l channels in the graphitemod erator. Between the penetrations and the tops of the channels, system of gu ide tubesprovide the p aths for the h e l assemblies and the Control rods. T he skirt forms the low erpart of the gas baffle cylinder.The c ore and th e radiation shields are suppo rted on a structure called the diagrid, whichitself forms an integral part of the g as baffle. This diagrid is designed to carry the w eightof the reactor co re and to accommodate the thermal movements which arise fromcoolant temp erature variations during no rmal operating and in the case o f incidents.

5.3.2 Nuclear d esignReactor coreThe main design d ata for the reactor core is shown in Table 5.2 .

Table 5.2. Main design datafor the core.Moderator GraphiteNumber of h e l channels per reactor 332Number of Control rods 89Lattice pitch (square) 460 mmActive core diameter 9 . 5 mActive core height 8 . 3 mPower density 3 O kW/litre

3 3 9 CMean gas outlet temperature 639 CPeak channel outlet temperature 661 CTotal gas flow 4067 kg/sPeak channel flow 14 kg/s

Coolant ga s CO2

M ean inlet gas temperature

The reactor mo derato r is a sixteen-sided stack of graphite bricks. Th e bricks areinterconnected with graph ite keys to give the m oderator stability and t o m aintain thevertical channels on their correct pitch, despite dimensional changes due to irradiation,pressure loads and thermal stresses(Figure 5.5).


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- 22 -Furthe r more, a boron bead injection system is also provided in 32 of the 163 interstitialchannels designed t o give long-term -hold-down capabilities in the extreme ly unlikelysituation wh ere an insufficient number of Control rods ha ve been inserted into the cor eand depressurization of the core is required; in this case the pressure of the nitrogeninjection system cannot be maintained.

REFLECTORLEMENTS + CONIROLRODS 89BEAD NITROGENNJECTION 32

O NITROGENNJECTION 131UELCHANNELS 332

Figure 5.6. Core layout - nearly i / 4 core symmetry.


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Fuel assembliesThe main design data for h e l assemblies are shown in Table 5.3 .The h e l in AGR s consists of slightly enriched U02 xn the fo rm o f cylindrical pellets witha central hole. T hese a re contained w ithin stainless-steel cladding tubes, ea ch o f which isabout 900 mm long. A fuel element consists of 36 h e l pins surrounded by two con-centric graphite sleeves. The fuel pins are suppo rted by top and bottom grids which arefixed to th e ou ter graphite sleeve (Figure 5.8and Figure 5.9). The possible bowing of thepins is limited by sup port brace s. The support grids, braces, and inner sleeves are securedin position by a screwed graphite retaining ring at the top. The com plete unit forms a h e lelement (Figure 5.7).Eight o f these fuel elements are linked together with a tie bar toform a h e l stringer assembly. Each of the 332 h e l channels is provided with a h e lstringer assembly.

Table 5.3. Main design data o r uel elements.Material u 2TypePellet diameter 14 .5 mmInner graphite sleeve diameter 190 mmCladding material Stainless steelElement length 1036 mm8

Number of channels 332M ass of uranium per elementAverage h e l rat ing 13.65 MW t/tUAverage h e l burn-up 18,000 MWd/tU

36 pin cluster in graphite sleeve

Number of elements per channelEnrichment 2.2 - 2.7 ?4044 kg

The h nc tio n of the double sleeve in the h e l element is to provide a static gas gabbetween inner and outer sleeve to redu ce leakage of heat from the hot coo lant t o thegraphite moderator.The he l stringer assembly and its associated plug unit form a composite h e l assemblythat is handled by th e fuelling machine and loaded into th e reactor as on e unit. Since it isimportant that the cladding exhibit goo d heat transfer properties, the cladding is providedwith smal1 transverse ribs on the outer surface (Figure 5 .8 , and is compressed o nto thepellets during manufacture t o minimize the clearance gab betw een pellet and cladding.Th e remaining space is filled with helium, an inert gas with good heat-conductingproperties.The rea ctor has to single channel access, i.e. each channel is extended upw ards w ith itsown separate opening in the con crete vessel. This permits the g as temperature in eachchannel to be m easured and to adjust the flow of gas coolant remo tely.Finally it permits refuelling both when th e reacto r is on and off load and for any pressurefrom atmospheric to normal operating pressure, by ti fuelling mach ine that hand lescomplete h e l assemblies from the top of the vessel.


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iillI

Fipre 5 .7 . Dimensions of an AGR fuel element.


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Tie bar

Top brace.

inner graphi te sleeve

Outer graphit sleeve

Fuel p i n

- tainless steel can

M u 0 2 fuel pellet

Figure 5 8AGR fuel element.


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i ns u la to r pel let

Retaining p l a l e

Figure 5.9 Detailed view of AGR fuel elementThe h e l assembly plug consists of a closu re unit, a biological shield plug, a valve (gag )unit and actuato r and a neutron scatter plug. During normal operation, the h e l assemblyplug unit ac ts as the seal of the reactor pressure vessel at each h e l standpipe. They areprovided w ith closure/locking m echanisms, which are operated remotely.Th e biological shield plug is designed primarily to limit neutron and gam ma-radiationthroug h the standpipe. It consists of two mild-steel blocks joined tog ether by a m ild-steeltube. A steel ring loosely moun ted o n the low er block reduces radiation streamingthrough the annular gab between the plug and the standpipe liner.The valve unit is situated in the lower part of the h e l assembly plug. It contains a ductfor the hot coolant gas between the h e l stringer assembly and the outlet ports above thegas baffle dome. It includes a flow Control valve (gag) for adjustment o f coolant flow


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- 27 -throug h the individual channels. The valve is coupletl by a shaft passing through thebiological shield plug to a motor-driven valve actuator, the b as k function of which is toset the valve position. The ac tuator is opera ted by remo te Control from the centralControl room.Belo w the ga g unit a neutron scatter plug is located to prevent neutrons streamingup th e channel. The tie bar, attached to the to p of the gag unit carry both that unit andthe fuel stringer assembly during rehe lling operations.Fuel handlingAccess to the reactor for refuelling isprovided by standpipes located in thetop cap of the reactor pressurevessel, on e standpipe for eachreactor channel.One reh elling machine, designed tohandle both h e l and Controlassemblies, serves both reactors. Themachine runs on a travelling gantrythat spans the width of the chargehall and is suppo rted on rails whichrun along the ll length o f the hall,Figure 5.10. This gives the m achineaccess to both reactors and to thecentral service block. In this blockthere are facilities for storage o f newh e l elements and h e l s tringercomp onents and for their assemblyinto comp lete fuel assemblies. It alsoincludes facilities for tem porarystorage and dismantling of irradiatedh e l assemblies. The h e l s toragepound, used for the longer termstorage o f irradiated h e l elements, isalso located in the central block.The refuelling machine - essentially ahoist contained within a shieldedpressure vessel - is provided with atelescopic snout which can beextended to connect, seal and lockon to a short extension tube fitted tothe stan dpipe being serviced. Withinthe refuelling mach ine pressurevessel a three-compartment turretcan be rotated to align any of thecompartm ents with the m achinesnout. One turret tube is for Figure 5.10. Refuelling machine.


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- 28 -withdrawing of used h e l, one carry the new h e l and the last carry a spare plug unit. Thetop section of the pressure vessel above the turret contains the hoist drive shaft, whichpasses throug h seals in the vessel. At the end of the shaft is located the machine grab,suspended in roller chains. Th e grab is opera ted electrically by solenoids, and it can belowered through the turret tube aligned with the machine snout to pick up h e lassemblies.The m ovements of the machine and the connections to the standpipes are controlled froma platform at the bottom of the machine, Figure 5.10.All other o perations are controlledfrom a platform on the m achine located just above the gantry. The refuelling programm erequires about 5 h e l assemblies to be replaced per month.

5.3.3 Therm al and hydraulic designCarbon dioxide ga s is used to transfer the heat produced in the reactor to t he boilers.The ga s is pumped through the channels of the reactor a t high pressure by ga scirculators; its main flow paths are shown in Figure 5.1 1.

Gasbaff e

---

Figure 5.II Gasflow distribution iri the core and vessel.


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Th e gas circulator pumps the cooled gas from the bottom of the boilers and into thespace below the core. About half of this gas flows directly to the h e l channel inlets,while the remainder, known as the re-entrant flow, passes up through the annulussurrounding the co re along the inner surface of the gas baffle to th e top baffle. It returnsdow nwa rds through passages between the graphite moderator and the graphite sleeves ofthe h e l elements to rejoin the main coolant flow at the bottom of the h e l channels.(Probable som e kind of orifice is used at the bottom of the h e l channels to split the ga sflow into the re-entrant flow and the h e l channel flow).Th e re-entrant flow thus cools the graphite bricks, the co re restraint system and the ga sbaffle. The combined flow passes up the h e l channels and through the g uide tubes. Thenthe hot ga s flows into the space above the gas baffle and down through the boilers,whe re it is cooled, before re-entering the gas circulators below th e boilers.The main reason for the re-entrant flow from the top o f the co re to the bottom is to keepthe mo derator temperature below 450 C to avoid excessive thermal oxidation of thegraph ite bricks, and to limit temp erature gradients within a brick to ab out 50 C. his ismost econom ically achieved by a re-entrant coolant flow, in which part of the coolantwill pass dow nwards between the bricks before entering the bottom of the h e l channels.How ever, it doe s complicate the internal layout of the plant within the pre ssure vesselvault by necessitating a ga s baffle around the co re.Fuel element guide tubes, located at the top o f each channel, are used to duct the h ot g asthroug h th e space below th e gas baffle before it is discharged into th e space above.The core and the surrounding graphite reflector and shield are completely enclosed in thegas baffle which has a diameter of 13.7 and which is provided with a torispherical head.The baffle has to withstand th e ful1 core pressure differential of 1.9kg/cm2, and itstemperature is kept down to 325 C by insulation on the topside so that mild steel can beused.In Table 5.4are show n a heat balance schem e for a typical AGR plant.

Table 5 4 Heat balancefor m AGR planPower to turbine 1649 MWPow er loss to vessel liner cooling system 8.5 MW4.5MWPower loss to gas treatment plant 3. 0 MWTotal power t o gas 1665 MWPumping power 42 MWPow er from reactor 1623 MW

Pow er loss to circulator cooling system


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5.4 Reactivity Control systemTh e primary system for Control and shutdown of the reactor consists of 89 absorber rodsand drives housed in standpipes in the top cap of the reactor vessel. 44 of these ar e blackrods (Figure 5.12)of which 7 act as sensor rods for detecting any guide tubemisalignment that may o ccur between the graph ite moderator and the steel structure sabove it. Th e remaining 45 absorber rods a re grey regulating rods, of which 16 are usedas a safety group . This safety grou p can be moved o ut of the core when the rea ctor isshut down so that it can be moved into the c ore in case of an inadvertent criticality.Each black ro d consists of eight cylindrical sections linked togeth er by join ts.

Figure 5.12. AGR confrolrodEach of the low er six sections consists of a 9 % Cr, 1 % Mo steel sheath containing fourtubular inserts of stainless steel with a 4.4 % boron content to ensure blackness tothermal neutrons. Between the tubular inserts are two solid, cylindrical, graphite insertsto reduce n eutron streaming.The up per t w o sections, which form part o f the top reflector when fully inserted,con tain full-length, solid graph ite inserts only.The 45 grey regulating ro ds are o f a design similar to the black rods. How ever, the lowersix sections contain tubes of stainless steel without boron, but w ith graphite insertsarranged a s in the black rods.Th e Control assembly consists of a Control rod, a Control plug unit, a Control rod actu ato rand standpipe closur e unit and its housing. Th e cornplete Control assembly is designedfor removal by the refuelling machine both when the reactor is operating and when it isshut down.


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5.5 Reac tor main coolant system5.5.1 Reactor coolant pipingCarbon dioxide ga s is used to transfer heat from the reactor t o the boilers. Th e gas ispumped th rough th e channels of the reactor by gas circulators at a p ressure of about 40bar; its main flow paths have been described in section 5 .3.3.

5.5.2 Reactor coolant pumpsGas circulatorsEach reactor has 8 gas circulators driven by induction motors, Figure 5.13. Eachcirculato r, comple te with m otor and Control gear, is a totally closed unit loca ted in ahorizontal penetration at the b ottom of the reactor pressure vessel.In add ition t o its normal duties, the circulator unit, its mounting system and shaftlabyrinth act a s a secondary containm ent system, should the penetration closure fail. Th emounting system is pre-tensioned to provide nominally constant loading of the motorstator frame under all operating and fault conditions leading to depressurization.The moto r is provided w ith a variable frequency power supply to enable operation a tlower speeds especially at reactor trips. If a reactor trip occu rs, the blower speed dro psto 45 0 red min , but increases automatically to 3000 red m in in the case of accidental de-pressurization of the reactor.The normal regulation of the flow is via variable inlet guide vanes, w hich also Control thereverse flow when the pump motor has stopped.

Tab e 5.5.Design data or gas circulators.CentrifugalRegulation Constant speed hariable inlet vanesNumber per reactor

42 M W

The reason for the use of a totally enclosed ga s circulator design is partly to m ake sw ifiremoval and replacement of circulator units with a minimum loss of reactor outputpossible and partly to avo id the high pressure, rotating, oil-fed gas seal which has beenused so far on circulators for Magnox reactors.The complete gas circulator assembly with motor, impeller, and guide vanes (Figure5.13), can be withdrawn and sealed off from the reactor circuit while the reactor is atpressure. After th e internal seal is made operationai, the o uter pressure casing may beremoved and the g as circulator replaced.


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CO2 supplyA carbo n dioxide supply system is located on site. Ittj purpos e is to provide stora gecapacity for liquid carbon dioxide and supplies of gaseo us carb on dioxide for eachreacto r, th e refuelling machine and auxiliary plant facilities during normal o peration andfault condition s. The composition o f the gas coolant is maintained within definedopera tional limits by the reactor coolant processing system. A fraction of the reactorcoolan t flow is passed co ntinuously through th e processing plant and after treatm ent isreturned t o the main coolant circuit at a circulator inliet, the circulator providing th edriving fo rce.The plant is also provided with a reacto r coolant discharge system for the controlleddischarge of contaminated ga s from the reactor and associated equipment. It comp rises

The reactor vessel blowdown and purge system - one per reactorThe auxiliary blowdow n system - one per stationTh e reactor vessel safety relief-valve systerri - two per reactor

and will be described in section 5.8 of the report.


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5.5.3 Steam generatorsSteam sideThe boiler annulus situated between the gas baffle and the reactor pressure vessel, Figure5 .11 and Figure 5.14,is partitioned into fou r quadraiits, each containing one boiler andtwo ga s circulators. The boiler consists of three main boiler units, each com posed by aneconomiser, evapo rator and superheater section within a single casing, and supp ortedfrom below on beams anchored in the pressure vessel and the gas baffle skirt, Table 5 .6 .and Figure 5.15.On to p of each boiler a reheater unit is suspended from the pressure vessel roof withsliding joints b etween the boiler and the reheater casxng to accomm odate thermalmovements between th e two sections. Separate banks of tubes situated below theeconom iser sections in the boiler casing form the decay heat system (decay bo iler), whichis designed to provide cooling of the reactor during shut dow n.

Reactor core\Gas b affle

vessel

Each quadrant includes3 main boiler unitsand 1decay boiler

Figure 5.14. Xhe four boiler quadrunts in the AGR.The boilers are of the once-through type to minimize the number of pressure vesselpenetration required. Such boilers are characterized by the absence of steam-separatingdrums and fluid recirculation, and the working fluid is pumped continuously through theboiler. Heated feedwater is pumped via a manifold into the economiser at th e bottom ofthe boiler and flows upw ards through banks of tubes via the evaporator and superheaterzones to em erge through an outlet at the top of the boiler as superheated steam.


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Table 5.6. Design data- or boilers.Number of boilers 43Feedwater temperature 158 CGas inlet temperature to reheater 619 CGa s outlet temperature 293 CSuperheater ou tlet pressure 173 barSuperheater outlet temperature 541 CSteam generation 525 kg/sReheater outlet pressure 42 bar

Number of units per boiler

Reheater outlet temperature 539 C

Bo i le r annu ius

D e c a y h e a t boiler

Gas c i r cu l a t o r q u a d r a

- ..2 R e h e a t e ri superheater . -' r e s t r a i n t c - . - -.: .i ene t ra t ion ia i i u re-i y s i e m

i:.. * e i sm i c+....... .'..........

.. .:. a , .

1 -.:I . ..:..

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