No. 11

PAPER 58

DESIGN AND DEVELOPMENT OF STEAM GENERATORS

FOR THE AGR POWER STATIONS AT HEYSHAM II/TORNESS

XA0055820

A N Charcharos, A G Jones, National Nuclear Corporation Ltd.

SYNOPSIS

The current AGR steam generator design is a development of the successful once-through units supplied for theOldbury Magnox and Hinkley/Hunterston AGR power stations. These units have demonstrated proven control andreliability in service. In this paper the factors which have dictated the design and layout of the latest AGRsteam generators are described and reference made to the latest high temperature design techniques that havebeen employed. Details of development work to support the design and establish the performancecharacteristics over the range of plant operating conditions are also given.

To comply with current UK safety standards, the AGR steam generators and associated plant are designed toaccommodate seismic loadings. In addition, provision is made for an independent heat removal system for postreactor trip operations.

1 DESCRIPTION

1.1 General

There are four boilers in each reactor and eachboiler comprises three once—through HP units andthree single stage reheater units which in normaloperation generate steam to drive the main turbine.A further bank of tubing is provided beneath each HPunit and operates in conjunction with the HP unit toremove decay heat when the reactor is shut down.This bank also assists in the maintenance of thereactor gas inlet temperature within the requiredrange during reactor start-up and shutdownoperations. The general layout of the steamgenerators within the reactor is shown in Fig 1 anda cross-section of an HP unit with an associatedreheater and decay boiler is shown in Fig 2.

The boiler units, rectangular in section, arelocated in the annulus between the reactor gasbaffle and vessel wall. The tube arrangement forthe heating surfaces, consisting of plain and finnedtubing, is formed from horizontal parallel straighttubes and associated return bends. Tubes axes arearranged in the circumferential direction to takeadvantage of the maximum length of straight tube.

1.2 Plant layout and steam'and feedconnections

The gas is constrained to flow through the reheaterand HP boiler units by means of permanent casings,gas seals and annular plates. The boiler units aresupported from below by two carbon steel beams whichare suspended from supports on the gas baffle andfrom the vessel wall. The reheater banks aresuspended from the vessel roof and connected to theHP unit with a flexible seal.

Each quadrant has its own feed water systemincorporating the usual complement of main andstart-up feed regulating valves, see Fig 3.Pipework distributes the feed water to sixpenetrations through which tailpipes pass to thethree boiler units. A control valve in the pipeworkto each penetration is provided to ensuredistribution stability and steam temperaturecontrol. High pressure steam is conveyed from eachboiler through nine penetrations in the pressurevessel wall and is collected into steam headers.The reheat steam is fed into and out of the pressurevessel through a total of six combined headers andpenetrations per quadrant. External reheaterpipework incorporates a bypass to limit the outletsteam temperature by means of steam attemperators.

1.3 Tube elements for the HP and reheater units

The HP elements are continuous (2 flow/platen) fromthe feed inlet penetration to the superheater outletat which point the elements are bifurcated intotailpipes which are routed through the steampenetrations. The material of the HP tube elementsis graded from austenitic stainless steel type 316 Hat the top through 9% Cr 1Z Mo to 1% Cr \% Mo at thebottom. The tubes for the primary economiser are25.4 m o.d. made of \X Cr i% Mo with carbon steelfins on a staggered arrangement; the 9% Cr banksare made of 28 mm o.d. plain tube on an in-linepitch; and the stainless steel tube bank consistsof 36 mm o.d. plain tube on a staggered pitcharrangement. Inconel 600 and 5% Cr transitionjoints are used at the upper and lower materialchange locations respectively.

The elements are supported by means of a weldedspacer system of the same material as the tubing.The elements are connected at various intervals downthe unit by links to transverse support beams whichtransfer tube bank loads to the main casingstructure. Support beams and associated links areof austenitic stainless steel.

The reheater elements comprise 38 mm o.d. plaintube, each element having four flow paths. Thetubes are bifurcated to reduce the number ofbranches on the large bore headers. The elementsare supported by means of 316 H stainless steelspacers and hangers In a similar manner to that ofthe HP boiler. The tailpipes between the tube banksand the headers are attached in the shops and theassembly is transported to site for unit erection.

1.4 Decay heat boiler

The tube banks for the decay heat boiler are locateddirectly beneath the main boiler economiser sections(Fig 2 ) . One inlet feed penetration and one outletsteam/water penetration are provided for each boilerquadrant. External valve Isolations allow theoperation of the decay heat system when fewer thanfour quadrants are required.

Each decay heat bank consists of twelve rows offinned tubing of the same geometry and material asthe main boiler primary economiser. Ferriticmaterial is used throughout the platens so thatoperation with lower quality water than thatspecified for the main boilers can be tolerated.The boilers are brought into service for reactorstart-up duty and are also initiated automaticallyafter reactor trip as part of the reactor shutdownsequence. Facilities for control of feed flow andpressure are provided.

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1.5 Tailpipes

1% Cr i% Mo tailpipes pass from the tube plate inthe feed inlet penetration to the bottom of theboiler units with sufficient flexibility toaccommodate Che relative thermal movements. Super-heater internal tailpipes are routed from the top-ofthe boiler units through a tube plate in each outletpenetration to external headers. Tailpipes connectthe platens of the reheaters to large bore internalheaders.

The boiler casings are designed to support the tubebanks in a horizontal position during assembly atworks, transportation and up-ending prior to liftinginto the reactor vessel. In addition to supportingthe tube banks, the casing is designed to withstandgas pressure and temperature differentials duringnormal operation of the plant. Provision is made atthe bottom of the HP unit casings for supportpedestals to the main support beams. Reheatercasings and main unit casings are fabricated fromaustenitic stainless steel. Vertical baffles areprovided over the full height and breadth of theceheater and HP units to improve the acousticbehaviour of the casing cavity. These baffles forman integral part of the casing structure. Flightsare included to minimise gas bypassing between tubebanks and casing walls.

1.7 Boiler supporting structure

An annular ring common to all four boilers isattached to the lower end of the HP units to controlradial expansion of the units and provide convenientattachment points for the main gas seal. Each mainunit is supported at its base by two fabricatedradial beams through pin joint connection betweenthe beams and the base of the unit casing'. Theradial beams are supported by flexible slingsconnected to tubular supports attached to the gasbaffle and to dummy penetration in the liner. Theslings have hinged connections to permit transversemovement and are preset so that in normal operationthe flexural stresses are low, see Fig 4. Connect-ions are provided at the top of the HP unit betweensuperheat penetrations and casing to control thermalmovements in a radial direction, and to accommodatevertical and horizontal expansion of the units.

Each reheater is supported by four slings suspendedfrom the concrete pressure vessel roof, see Fig 1and 2. The reheaters are also connected to the HPboiler unit3 by means of a rolling seal which allowsvertical and radial differential expansion betweenthe reheater and the HP units. A sliding bracketmaintains radial and circumferential alignmentthereby preventing undue loads on the seal. Seismicloads on the boiler structure are reacted throughrestraints connected between the casings and dummysupports on the vessel liner wall and differentialloadings on the reheater slings and the steampenetrations.

1.8 Ga3 seals and division plates

The main gas seals are continuous around the Innerand outer walls of the annulus at the bottom of themain unit casing. Each seal comprises two tubesspaced apart and sandwiched between layers of shimswhich are backed and protected by plates. One tubeof the seal assembly is connected via a membraneplate to the boiler units, the other tube via amembrane plate to ledger sections on the liner andgas baffle skirt plates. Joints In the system arecovered by lap sealing plates. The seals allowrelative thermal movement in both the horizontal andvertical directions, see Fig 5. Low temperaturecoolant tapped off the gas baffle is piped aroundthe bottom of the units to maintain the surroundingvoids cool and to offset leakage through the seal.The gas leakage through the seals will be

approximately 1% of the boiler gas flow. Thecoolant flow, in association with baffles around theunit at various levels serves to prevent hot CO2bypassing the boiler units and flowing down to thelower part of the annulus. Division plates areprovided between each quadrant below the main gasseals to isolate adjacent boiler/circulator systemsin the event of one quadrant being shut down. Thedivision plates are made of mild steel platessuitably stiffened, the structure being adequate towithstand the full differential pressure across theboilers. Differential loads on the division plateare reacted via attachments to the liner and gasbaffle. A door is provided in the division plate atfloor level for inter-quadrant access.

1.9 Penetrations

Two economiser penetrations are provided for eachboiler unit, see Fig 6. The penetration has a tubeplate at the outer end to which the economiser tubesare welded. Flow stabilising orifices are providedat each tube location. The tube plate extension iswelded to the penetration liner tube on site.Attached to the tube plate is a large diameterflanged header comprising feed inlet branch anddrain branch. The flange can be removed forplugging tubes in the event of a boiler elementleak.

The three superheater outlet penetrations associatedwith each boiler unit are fabricated into unitscomprising steam tubes and insulated sheath tube andcomplete with a forged end plate at the inboard end,see Fig 7.

The annulus formed by the sheath and the insulatedpenetration liner tube is provided so that thepenetration sheath tube acting as a cantilever cantake up the vertical expansion of the boiler units.

Cold pull is applied to limit the stresses in thesheath tube during normal working conditions. Theindividual steam tubes pass through the forged endplate and connect into 3team headers mounted on theoutside of the reactor vessel. Access is providedto the steam tubes for blanking in the event of aboiler element leak.

Plate baffles and insulation assemblies are providedon both feed and steam penetrations to control gascirculation and heat transfer.

The reheater inlet and outlet steam penetrationscomprise large bore stainless steel tubes externallyinsulated and contained in a sheath tube. The outerend of the steam tubes is flanged to provide accessto the header for inspection and remote tubeplugging in the event of a reheater element leak. Abranch is provided for connection to the reheatersteam pipework.

Shielding at the outer end of the feed and steampenetrations limits external radiation to acceptablelevels. All boiler feed and steam penetrations areprovided with external secondary retention featuresto limit gas side discharge to acceptable levels inthe unlikely event of a penetration weld failure.

1.10 Boiler annexe components

Each boiler has an independent feed supply withassociated control and trip valves which dividesinto six branches which connect to the outer end ofthe feed penetrations. Separate branches areprovided for connection of emergency feed systems,see Fig 3.

The HP steam pipework is routed from the outletbranch of the external superheate headers; eachheader serves three boiler units. The pipework isanchored back to the concrete vessel. In additionto the steam mains to the turbine, connections fromthe HP steam pipework permit discharge of steam or

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water to the start-up vessels, and to the LP ventsystem, via appropriate control valve stations.Provision is also made for pipework to bypass the HPboiler for circuit clean-up operations.

Reheater bypass pipework is. provided between thereheat inlet pipework and the branches on thereheater outlet penetrations, complete with anattemperator device to obtain the required finalreheat steam temperature.

2 FACTORS DICTATING LAY-OUT AMD DESIGN

2.1 Materials

Apart from the requirement to use materials withadequate strength at the specified temperature andpressure, there are four factors governing thechoice of materials in the various sections of thesteam generator. These are:

(i) Gas side oxidation(ii) General water/steam side corrosion(iii) Stress corrosion(iv) Erosion corrosion.

Gas side oxidation

Temperature limits are necessary to avoid excessivegas side oxidation of components for an economicboiler design. It is also necessary to use alloysteels that are compatible with other operationalfactors. The following metal temperature limits areemployed with associated metal loss and oxide growthfor 30 years operation exposed to AGR gas.

Material

Carbon steel(0.1% Si min)

Carbon steel(0.2% Si min)

1% Cr i% Mo(0.2% Si min)

9% Cr 1% Mo(0.62! Si min)

316 SS

Temperature

350"

370°

370°

550"

700°

C

C

C

C

C

Metal

0

0

0

0

0

.17

.29

.29

.51

.33

loss

mm

mm

mm

mm

mm

Oxide growth

0.29

0.38

0.38

0.81

0.28

mm

mm

mm

mm

mm

For structural components interface joints attemperatures above 250°C are fully seal welded wherepossible. Where gas tightness cannot be guaranteed,prescribed gaps are adopted beween faces of weldconnections to accommodate oxide growth. Boltedarrangements are designed to cater for oxide jackingstrains and metal loss at thread forms to preventdisengagement.

Water/steam side corrosion

Allowances for general water/steam side corrosionare included in the tube wall thickness assessmentfor the respective materials of construction. Inaddition allowances to cater for periodic chemicalcleaning of the boiler units due to accumulatedoxides result in the following totals:

Carbon and 1% Cr i% Mo steels

9% Cr 1% Mo steel

316 H SS

Stress corrosion

0.61 mm

0.89 mm

0.18 mm

Occasional wetting of austenitic tube material Isacceptable in the short term provided water qualityIs controlled within defined limits. Potential longterm problems are avoided by using high purity feedwater during power operation and by controllingsteam temperature at entry to the austenitic sectionof the secondary superheater to a nominal minimum of70°C superheat at which droplets are absent. Allweldments are heat treated to condition thematerials.

Erosion corrosion

In regions of high water velocities such as down-stream of control orifices and at tube bends,erosion corrosion can occur resulting In severemetal wastage. The process is a function of anumber of factors including the material compositionof the attacked surface. Tests have shown thatalloy steels are resistant to attack and so thesesteels are used for the water sections of the boilerunits.

Taking account of all the above factors 1% Cr i% Mosteel Is used for the feed Inlet, primary ecotiotnlserand decay heat tube banks. The secondaryeconomiser, evaporator and primary superheater aremade from 9% Cr 1% Mo steel while the upper tubebanks are made of 316 H austenitic stainless steel.

The transition from one tube material to another isprovided by welded joints at interbank locations. A5% Cr \% Mo tube Insert is used between the 1% Crand 9% Cr tubes and an Inconel 600 insert is usedbetween the 9% Cr tubes and the 316 H tubes. Alltube welds are subject to heat treatment.

TABLE 1

BASIC DIMENSIONS AND MATERIALS

Because of metal temperature limits due to gas sideoxidation, it Is necessary to use aus-tenitlc stain-less steel tube for the second stage superheater andreheater tube banks. This material is subject tostress corrosion in aggressive chemical environ-ments .

Component

Reheater tubes

Reheater tailpipes

Reheater platens

Reheater slingsReheater casing

Reheater headers

Secondary super-heater tubesSecondary super-heater tailpipeSecondary super-heater platensTransition tubesPrimary super-heater tubesPrimary super-heater platensEvaporator tubes

Evaporator platens

Secondary economisertubesSecondary economiserplatensTransition tubes

Primary economisertubesPrimary economiserplatensDecay heat boilertubes

Dimensions

38 o.d. x4 thick51 o.d. x4 thick36/unit(staggered)4/unitStiffenedplate 1.5 mlong368 o.d. x3036 o.d. x4 thick38 o.d. x

44/unit(staggered)36 o.d. x 428 o.d. x3.5 thick44/unit(In-line)28 o.d. x3.544/unit(in-line)28 o.d. x0.3544/unit(In-line)28 o.d. xT cJ. J

25.4 o.d. x4 (finned)44/unit(staggered)25.4 o.d. x4 (finned)

Material

316 H SS

316 H SS

Nimonic 80A316 H SS

316 H SS

316 H SS

316 H SS

316 H SS

Inconel 6009% Cr 1% Mo N4T

9% Cr 1% Mo N4T

9% Cr 1% Mo N4T

9% Cr 1% Mo N4T

5% Cr 1% Mo

1% Cr i% Mo

(MS fins)

1% Cr iZ Mo

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TABLE I ( c o n t ' d )

Component

Decay heat boilerplatensBoiler unit casing

Dimensions

44/unlt

Stiffenedplate 12 mlong

Economiser tailpipes 25.4 o.d. x4 x 44 off

Feedtube restrictors 22 o.d. x 3x 44 off

Decay heat inlettailsDecay heat outlet

Decay heatmanifoldsBoiler supportbeamsBoiler supportslings

38 o.d. x 4x 12 off51 o.d. x4.5 x 12 off76 o.d. x 8x 4 off2/unit

4 sets/beam

2.2 Inspection/repairs

Material

(MS fins)

316 H SS

1% Cr \% Mo

1% Cr \% Mo-

1% Cr \% Mo

U Cr \% Mo

1% Cr \% Mo

Carbon steel

2i% Cr U Mo

To assist inspection/repair of the steam generatorsunder man access conditions permanent ladders andplatforms are provided in the reactor annulus and tothe boiler and reheater casings. Doors are fittedin the main boiler unit casing at two levels topermit access to superheater tailpipes and to the 9%Cr/316 H stainless steel transition joints andassociated thermocouples. Viewing panels areavailable also for inspection of the 9% Cr/1% Crtransition joints and other components. Accessdoors are fitted in the angular ring cheese plates,platforms and quadrant division plates for passagefrom one quadrant area to another.

For remote inspection additional standpipes arelocated at pilecap level to enable TV cameras to beintroduced into the reactor vessel annulus.Identifiers are provided on the boiler units toestablish locations and for record purposes. TVcamera access to the annulus below the main gas seallevel is arranged via standpipe penetrations in theannulus floor. Access route provisions incorpora-ting guide tubes and tundlshes allows inspection ofthe following items:

(i) reheater headers, tailpipes, casings andsling supports

(ii) main unit casing(ill) 9% Cr/316 transition joints(iv) seismic restraints(v) main gas seal(vi) internal cooling pipework(vii) main unit support system(viii) economiser and decay heat tailpipes(ix) quadrant division plates.

Camera routes are also used to permit access tocorrosion specimen containers attached to the mainboiler casings. These containers are installed aspart of the component oxidation monitoring system.

It is a requirement that isolation of a single mainboiler tube element Is possible by external tubeblanking. This is made possible by single feed tailtube connections to each flow circuit of which thereare two per element, and by a single steam tubeconnection at exit to the element. The feed tailtubes are welded to tube plates located in externalwater headers that are fitted with bolted flangedplates giving hand access to tube ends when removed.The steam tailpipes however, are welded to internalnozzles of a tube plate located at the inboard endof a penetration sheath tube. The 3team circuitsare continued through the tubeplate to externaltail-pipes which pass through the sheath tube toexternally mounted superheater headers.

Should a boiler unit tube leak occur (detected bywater ingress to reactor gas) the associatedquadrant would be shutdown, isolated and dried out.When operationally convenient the reactor would beshut down and depressurised and the flanged endplate connections at Eeed headers removed to gainaccess to the orifice holders. The affected tubecircuit would be identified using CO2 analysersand then isolated by fitting blanks in place of thecontrol orifices. At the superheater outletpenetration the affected tube circuit would beisolated by cutting the associated externalsuperheater tailpipe and welding on domed ends.Access to blank off tubes In the reheater is madepossible by end flange plates on each penetration.A purpose made, remote-ly operated tube pluggingmachine can be inserted into the penetration to thetailpipes headers to plug weld the nozzle servingthe affected platen.

2.3 Fabrication

The tube platens are supported by means of a weldedspacer system of the same material as the tubing.In preference to manual welding an automated weldingprocess is employed for the majority of the boilerand reheater spacer welds using robot welding heads.As a consequence the spacers are arranged longitud-inally with single spacers at tube centreline whereaccess for the welding heads is permitted by thetube pitch and tangential spacers at close pitchconfigurations.

2.4 Vibration

Tube plates are supported at two locations by hangerbars pinned to welded spacer attchment. To preventexcessive gas flow induced vibration, tube spacers

are arranged so that naturaltrequencies are in excess of generated frequencies.The fundamental frequency of acoustic resonancewithin the boiler casings is determined by the unitdimensions. In order to Increase the acousticfrequency of the cavity and provide separation ofthe acoustic frequencies and the flow inducedfrequencies and hence avoid coupling, a full lengthbaffle is incorporated in the tube banks at .midposition, thus splitting the boiler into two halfunits.

2.5 Special requirements

2.5.1 Poat-trip cooling

The boiler system performs a major role in thesafety of the reactor by virtue of its heat removalfunctions. This role is enhanced by the provisionof a separate and independent decay heat removalboiler that is accommodated by an additional tubebank of finned tube platens located below theprimary economiser and contained within the mainboiler unit casing. In each quadrant the threedecay heat tube banks are Integrally connected atinlet and outlet by tailpipes that are formed intotube bundles and pass through two penetrations toconnect to external pipework. .The associated feedsystem is independent of the main supply and drawsfeed water from reserve tanks. The discharge fromthe decay heat tube banks passes through a pressurecontrol valve (35 bar setting) to a flash vessel andhence to the dump condenser.

The prime safety function of the system is toprovide decay heat removal for pressurised reactorfaults and is capable of continued long termoperation. It is designed to be aseismic and tocool a pressurised shutdown reactor to approximately100°C gas temperature. As an added function thesystem is used to maintain reactor gas inlettemperature within limits during reactor start-up orcontrolled shutdown, or following a pressurisedfault. It is also employed during commissioningtests to control reactor gas temperatures.

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2.5.2 Seismic

The stations are required to withstand the effectsof an earthquake having a prescribed maximum freefield ground motion considered applicable to UKsites, with a peak acceleration of 0.25 g. Dynamicanalysis of the complete nuclear island has enabledaseismic design conditions to be derived forindividual boiler components. These conditions arein the form of floor response spectra and staticcoefficients.

Initial design has been based on loadings defined bystatic coefficients derived from a design baseearthquake having a peak ground acceleration of0.125 g. These seismic loadings are combined withoperational loads and the resulting stresses arerequired to be within the elastic limits of thematerials of construction. This approach isstrictly applicable only to components having alowest natural frequency of 33 Hz or greater. Asmajor components in the steam generators have lowernatural frequencies a dynamic response of a completerepresentation of the twelve boiler and reheaterunits together with annular ring, supports andrestraining structures has been performed with safeshutdown earthquake inputs. The resultant loads anddisplacements are applied to the individualcomponents in combination with operationalconditions.

Restraining structures include seismic restraintbrackets located at two levels of the boiler casing.A /5th scale model of a boiler unit has beendynamically tested to substantiate the theoreticalmodelling. For external pipework and associatedequipment conventional dynamic analysis methods havebeen used to establish positions of seismicallyqualified restraints and snubbers necessary toaccommodate the safe shutdown earthquake.

Shaker tests have been carried out on valves andactuator assemblies in operating mode to demonstrateseismic capability. Control and instrument cubicleshave been similarly tested.

2.5.3 Safety

It is a requirement for reactor safety that in theunlikely event of a failure of a gas pressure con-tainment component, the rate of depressurlsation ofthe reactor is limited to an acceptable value.Consequently, all boiler steam and feed penetrationsare fitted with gas flow restrictors designed tolimit the free flow area of Q.006 m 2 for anysingle weld failure. The restraints for the majorpenetrations take the form of long seamless pipesextending internally from the outboard end of thepenetrations to low stress regions of the linershutter tubes, (see Fig 7 ) .

For the superheater tailpipe bundles and the minorpenetrations, restrictor plates are used asindicated. Externally, secondary retention flangesare provided at the end forgings and are tied backto the reactor vessel to an anchor plate.

In order to limit external radiation to anacceptable level (a maximum of 20 ntrem/hour duringrefuelling), radiological shielding is provided atthe outer end of the reheater and superheaterpenetrations and on the inner end of the reheaterheaders. The feed inlet and decay heat boilerspenetrations are similarly shielded. Internalshields are fitted to the instrument penetrations.The layout of the external shielding is arranged insections to permit examination of penetration gaspressure retaining welds during station life.

3 PLANT OPERATING CONDITIONS

3.1 General

From a study of the operational envelope for the

reactor and boiler plant a series of Plant OperatingConditions has been identified, together withassociated frequencies over reactor life, andsubsequently assessed for the design substantiationof boiler components. Table 2 gives a simplifiedsummary of POCs and frequencies.

The POCs are classified as Normal, Frequent, Infrequent orLimiting events; test POCs are also identified.

Normal POCs occur during the course of plannedoperation of the reactor and include start-up, poweroperation over the load range, boiler quadrant shut-down and start-up, refuelling activities andcontrolled reactor shutdown operations.

Frequent POCs include reactor trips with variants ofpost-trip cooling, turbine trips, quadrant trips andother plant faults expected to occur several timesduring reactor life.

Infrequent POCs are expected to occur once or lessduring reactor life and component design must assumea once per lifetime basis. Included are majorboiler leak, steam and feed pipework failure, minorreactor depressurisation faults and quadrant tripprotection failures.

Limiting POCs are events which are not expected tooccur but are included in the design otherwise theconsequences could include release of significantradioactivity. It is accepted that these POCs mayrequire extensive remedial action or even write-offof plant. Included in these POCs are major reactordepressurisation, hypothetical failure of a boilerhalf unit feed tube plate, reactor trip from fullpower with minimum post-trip cooling by one mainboiler supplied with emergency boiler feed or by twodecay heat boilers in service.

Test POCs include boiler testing during the combinedengineering tests and power phase commissioning ofthe station.

TABLE 2

SUMMARY OF NORMAL AND FREQUENT POCs

Plant Operating Conditions Frequency

Reactor temperature raising to standby 35

Reactor start-ups with three and four 300boilersCold turbine start-ups to 100% load 100Hot turbine start-ups to 100% load 200Power cycling 100%-80%-100% load 5400Power cycling 100%-60%-100% load 2400Power cycling 100%-40%-100% load 650Refuelling cycles at loads from 30% 200to 100% loadSingle boiler planned shutdowns 150Single boiler trips 150Controlled shutdown from three or four 150quadrant operation to hot reactor standbyReactor trip from operating power level 60Turbine trip - reactor trip 60Loss of feed trip - turbine trip - 30reactor trip

3.2 Operating procedures

Operation of the reactor at power requires the useof three or four boiler quadrants in serviceproducing steam conditions scheduled as appropriatefor the turbine load. The station auto controlsystem ensures that the boilers are operated overthe control range within the defined constraints ofthe boiler plant, in particular tube metaltemperatures and steam superheat at the 9 Cr 1 Mo/TP 316 transition jonts.

For the start-up of the boilers a 'dry start'technique is employed with feed admitted toinitially empty boilers which are uniformly at

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approximately 300°C. Low load steaming of theboilers allows the generation of steam at 360°C/90 bar a, which is controlled by means of steampressure reducing/desuperheating valves at entry tothe boiler start-vip vessels, see Fig 3. Thesesteam conditions are suitable for starting a coldturbine. For a hot shutdown turbine the boilersteam conditions are raised to 460°C/140 bar abefore admission to the turbine takes place.

Over the scheduled load range the high pressuresteam conditions from the boiler rise from the hotstart conditions to 538°C/160 bar a. Power cyclingfrom the 100% MCR load to reduced outputs takesplace under control of the station auto controlsystem to match reactor/boiler operation tovariations in turbine load demand.

The station auto control system is based on a gasforcing/steam pressure governing strategy to matchturbine load demand. As part of the overall controlsystem the boiler half unit feed valves areregulated to adjust feed flow and control the degreeof steam superheat at the 9 Cr 1 Mo/316 transitionjoints, or to control the decay heat boiler outletgas temperature at loads below 50%. In addition theaverage differential pressure across the half unitfeed valves is maintained at a constant value overthe load range by regulation of the boiler feed pumpspeed.

In the event of a reactor trip or turbine trip thehigh pressure boiler steam is diverted from theturbine, to a low pressure vent system which allowsdischarge of steam to atmosphere via a set of steampressure reducing valves and flash vessels providedwith silencers. These control valves also serve toreduce boiler pressure at a predetermined race downto 80 bar a. This approach allows the boilers tocontinue to remove heat from the reactor gas circuitafter the trip, whilst ensuring that the boilerplaten stresses remain acceptable and also permit-ting any subsequent use of low pressure emergencyfeed pumps if this is necessary. In parallel withpost-trip operation of the main boilers the decayheat boilers are brought into service automaticallyafter a reactor trip generating steam at 35 bar aand providing the longer term heat removal system.

The above start-up and shutdown procedures describedin outline are all based on the successful exper-ience gained with the boilers at Hinkley Point B andHunterston B Power Stations.

3.3 Analysis of POCs

The POCs have been studied extensively utilisingcomputer models for the complete station as well asspecific boiler models to establish thermalperformance data during steady-state and transientoperations. Particular development of the boilermodels has allowed the most critical tube temper-ature profiles to be identified during rapidtransients such as following reactor trip.

Validation of the theoretical boiler models has beendemonstrated by a programme of laboratory testing onrepresentative platen configurations covering bothsteady-state and transient thermal performance.

Results from the POC calculations provide thedefinitive thermal input data for the structuralanalysis models used in the design assessment.

Steady-state temperature profiles throughout theboiler are shown In Fig 8, whilst typical temper-ature variations with time following a reactor tripare presented In Fig 9.

3.4 Design assessment

From a study of the POCs and associated frequencies,loading histograms have been compiled for thevarious components in the boiler systems. The

histogram is established to envelope the duty cycleson the plant including loadings from:

(I) pressure differentials between reactor gas andboiler fluid

(ii) structural loadings arising from constraint ofdifferential thermal expansion of tubingwithin the platen

(iii) local thermal stresses in the welded spacer totube connections

(iv) steady and fluctuating aerodynamic loadings(v) self weight and seismic loadings(vi) system loadings from connected tailpipes or

structural members accommodating gross thermalexpansions of the boiler system and supportswithin the reactor vessel.

In undertaking the component design analyses dueattention is given to the load combinations andappropriate conditions of design. The loadcombinations comprise the parameters associated withthe plant operating condition either alone, or,where relevant, in combination with an Internal orexternal hazard.

The boiler system performs a major role in thesafety of the reactor by virtue of its heat removalfunctions and therefore safety class 1 designationis appropriate to certain components. Thesecomponents are the main boiler reheater and decayheat boiler pressure parts and their respectivesupport provisions, penetration assemblies andsecondary containment devices. The loadingcombinations on these items are analysed to providedesign substantiation against the following modes offailure where applicable:

(i) ductile or creep rupture(ii) instantaneous or creep Instability(ill) excessive deformation(iv) incremental collapse or ratcheting(v) creep and fatigue Including creep/fatigue

interaction.

No single design code is available which meets allrequirements for substantiation against the abovemodes of failure. It has therefore been necessaryto develop a substantiation route based on theprinciples of code BS 5500 and ASME Code Case N47-21, together with concepts of limit load andreference stress techniques as applicable using ISOmaterial data and corresponding UK data for 9 Cr1 Mo and 1 Cr 0.5 Mo steels. The above approachallows for primary stress assessment, shakedownassessment and creep-fatigue damage assessment.Fatigue strength reduction factors for specificgeometrical configurations e.g. spacer tube welds,have been derived experimentally in order to relatemaximum stresses from analysis to peak stresses forfatigue assessment.

Cumulative damage evaluation of componentsrecognises the contribution of low cycle fatigue dueto plant cycling from the POCs, high cycle fatiguedue to aerodynamically induced vibration whereappropriate, and creep damage to componentsoperating at the higher temperature levels. Theresult of this substantiation work demonstrates thatthe components can accommodate the duties imposed bythe plant operating conditions over the reactorlife.

4 PERFORMANCE ASPECTS

4.1 General

Optimisation of the operating parameters has beenbased on total plant model studies covering reactor,boiler and turbine performance. Layout of thereactor circuit and boiler arrangements within thepre—stressed concrete pressure vessel provides for adownwards gas flow through the boiler units, withcarbon dioxide gas conditions at reheater inlet ofapproximately 615°C/41 bar a. These provisions

R737-PAPER58( 11)HB R737-PAPER58( 12)HB

allow the design of once through main boilers forgeneration of high pressure steam at 538°C/160 bar a, together with reheat steam at 538°C/39 bar a, to suit 660 MW(e) turbo-alternatormachines as developed in the UK for conventionalfossil fuelled power stations. Operating parametersare summarised in Table 3.

TABLE 3

OPERATING PARAMETERS AT 100% CMR LOAD

Gas flow/reactorGas pressureGas temperatures:

Inlet to reheaterInlet to HP boilerOutlet from HP boiler

HP steam flow/reactorHP steam outletHP feed temperatureHot RH steam afterattemperatorCold RH steam

4203 kg/s41 bar a

615°C573°C290°C500 kg/s540°C/166 bar a156°C538°C/41 bar a

342°C/43 bar a

4.2 Tube bank heat transfer and gas mixingcharacteristics

4.2.1 From the definition of overall thermal dutyand interface data to suit turbine andreactor operation, the gas 3ide and watersteam side boundary parameters are estab-lished for the steam generators. Sizingcalculations for the installed heat transferareas are based on heat transfer correlationsestablished from laboratory tests on theselected tube pitching configurations andpublished data. In addition development workhas been undertaken to confirm gas side heattransfer correlations for tube part rows andtailpipe geometries.

Performance tests at Hinkley Point B andHunterston B Power Stations have also beenanalysed and shown to give overall confirm-ation of the correlations used in the currentsteam generator design.

4.2.2 Laboratory tests on the tube bank geometrieshave been carried out to establish the gasmixing characteristics. Particularly goodmixing has been demonstrated for the 9 Cr1 Mo in-line tube pitch arrangement. Themixing data is important for the assessmentof effects of blanked flow paths. Appro-priate boiler models are used to determineany required changes in tube orifice sizes,whilst ensuring that platen temperatureconstraint values are still satisfied.

4.3 Boiler water flow stability

Dynamic models have been developed to predict thethreshold parameter values associated with the onsetof instability, and to define the necessarystabilising orifice pressure drops at inlet to eachfeed tube. This analysis work has also been valid-ated by laboratory tests on full scale electricallyheated rigs.

Stability beween the parallel flow paths is assuredby the provision of orifice assemblies at the tubeplate of each economiser penetration, which are alsoaccessible in service when the plant is shut down.In the event of a tube leak, access is thereforepossible to permit blanking at the tube plate and tore—orifice neighbouring flow paths as required.

For stability between half boiler units, in additionto the restrictor tube and orifices, the half unitfeed control valves also serve to ensure stabledistribution. To minimise post-trip instabilitybetween half units the post-trip sequence equipmentwill initiate closure of the half unit controlvalves to their minimum opening.

Development work in manufacturing and service per-formance aspects of restrictor assemblies has beencarried out and also erosion corrosion studies. Inthe case of the decay heat boilers, stabilisingrequirements are satisfied by the inclusion oforifice holes in the distribution manifolds at thebottom of the boiler units.

4.4 Tube bank vibration

Boiler equipment in the reactor vessel is designedto resist damage from gas flow or noise inducedvibrations. Component design is suitable foroperation at gas flows up to 375 kg/s per unit whichassumes utilisation of circulator output marginbeyond the best estimate operating point. For theshort term transient associated with transfer fromfour quadrant to three quadrant operation gas flowsup to 425 kg/s per unit have been considered.

Tube platens are supported at two locations by meansof a system of welded hanger/spacer attachments.Spacers are also provided at the mid-span of theplaten which, together with the supports, providessufficient restraint to prevent excessive vibration.In addition, welded spacers are provided on astaggered arrangement between the spans formed bythe support and centre restraint system, and alsolocal to the bends at the outer ends of the platento give further restraint against vibration.

The fundamental frequency of acoustic resonancewithin the boiler casings is determined from theunit dimensions. A full height baffle is incorpor-ated in the tube bands at the mid-position, in orderto increase the acoustic frequency of the cavity andso prevent coupling between acoustic and flow-induced frequencies.

The welded spacer support system for the tube banksensures natural frequency values in excess of thefrequencies which could be generated by flow-inducedphenomena. Furthermore, the design of the supportsystem includes dynamic effects due to fluctuatingaerodynamic loadings which are treated asdistributed loads on the platens both in-plane andout-of-plane.

The general arrangement of boiler tubing and supportsystems is similar to the Hinkley Point B/HunterstonB design, which has been demonstrated to have satis-factory vibration characteristics from laboratoryand site tests together with service operation.Hence there is confidence that no fundamentally newvibration problems will occur.

In order to demonstrate that platen support and tubespacer design is adequate to prevent damage, vibra-tion tests at gas flows in excess of lOOX CMR arebeing carried out in the laboratory on represent-ative platen and tailpipe configurations.

On completion of construction and during the commis-sioning phase, vibration testing will be carried outon the first reactors at Heysham II and Torness asfollows:

(i) determination of natural frequency, mode andamplitudes of vibration of components in thereactor

(ii) determination of response of the gas withinthe boiler geometries

(iii) combined system tests for which strain gauges,accelerometers, and pressure transducers willbe monitored during the tests and signalsrecorded and analysed. The tests will bedesigned to simulate as far as possible, orallow assessment of the relevant operatingconditions applicable to the boilercomponents, including maximum flowconditions.

High temperature vibration monitoring equipment isprovided to monitor vibration responses duringoperation at high power.

R737-PAPER58( 13)HB R737-PAPER58( 14)HB

2415 DEVELOPMENT

Extensive development work has been undertaken insupport of the design of the steam generators and aschedule of topics covered is given below. Some ofthese will be seen to have application not only togas cooled nuclear steam generators but also toboiler designs in general. On-going work requiringlong term data includes mechanical testing andoxidation/corrosion studies.

5.1 Fabrication development

Spacer-tube geometries. Transition joints. Tubeplates. Casings. Penetrations. Non-destructiveexamination techniques. Heat treatment gascomposition.

5.2 Performance

Temperature measurement at transition joints. Heattransfer at spacer-tube connections. Gas flowdistributions and mixing. Seal leakage. Tube inletflow measurement.

5.3 Tribology

Fretting and wear tests on selected components.

5.4 Mechanical testing

Tensile tests, creep tests, ambient bursting testson tube butt welded and spacer attachment specimens.Transition joint creep tests.

5.5 Noise vibration, fatigue

High cycle fatigue testing spacer tube assemblies.Natural frequencies and damping studies. Acoustictests in pressurised facility. Flow inducedvibration tests in CO2 pressurised rig on platensand tailpipe configurations.

5.6 Oxidation and corrosion

Gas side oxidation. Erosion-corrosion studies.Stress corrosion, corrosion fatigue of transitionjoints.

R737-PAPER58( 15)HB

GENERAL LAYOUT OF STEAM GENERATORSWITHIN REACTOR

FIG 1

113

SECTION THROUGH ANWLU5 SECTION THROUGH REHEATER^ MAIN UNIT

CROSS-SECTION OF STEAM GENERATOR UNIT FIG 2

I

•afo

REHEATER

MAIN BOILER

DECAY HEATBOILER

OOCIRCULATORS

ATTEMPERATOR

: REHEATBYPASS

OTHERREHEATERS

,-&-&STEAMo

— t S } — 1 * | — dWATER T

L.P. VENTFLASH VESSEL

TURBINESTOP VALVE

«>TEAM

WATER

-iXj 1*»—•-SPRAY WATER OTHER

BOILERS HP L.R

TURBINES

PAIREDBOILER

START UP VESSEL DEAERATOR

HALF -UNIT —VALVES —

EMER6tNCY

FEED

POST TRIP FEED

START UPFEED

OTHER J ^BOILERS

-©-

•A {

MAIN FEED

DECAY HEATFLASH VESSEL

CONDENSER

JSTART UP/STANDBY

PUMPS

M A I N FEED PUMP

RESERVEFEED WATERTANK

• & •

1CONDENSER

DECAY HEATFEED PUMPS

OOAIRCOOLER

CHECK PLATES

DUMMY PENETRATION

OlACRID SUPPORT

SOILER M A I N SUPPORT BEAM

ARRANGEMENT OF MAIN BOILER SUPPORT BEAMAND SLINGS

FIG 4

GAS SEAL TU9ES

MAIN BOILER UNITS ANNULAR RING AND GAS SEAL FIG 5

RETENTION FI_*.MSL.

- END CAP RESTRICTOH.

—:THERMOCOUPLE. PENETRATION

t OUTCD PQCSTOEMIN6

TIE BOLT

ANCMOD FLANGE.

FEED THERMOCOUPLE AND INSTRUMENT PENETRATIONS FIG 6

REHEATER PENETRATION

SUPERHEATER PENETRATION

STEAM PENETRATIONS FIG 7

XI?

STEADY-STATE TEMPERATURE PROFILES FIG 8

© R.H. INLET &flSTEMPERfffURE

© H.P. INLET (&flS TEMPERATURE

(3) H. P. OUTLET STEW-1 TEriP.

© M.P. OUTLET

© STEflM

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REACTOR TRIP TEMPERATURES FIG 9