EPR – Areva brochure

■ EPR NUCLEAR ISLANDSteam generator cutawaySTEAM GENERATORSThe steam generators (SG) are the interface between theprimary water heated by the nuclear fuel and the secondarywater which provides steam to the turbine generator. Theprimary water flows inside the steam generator tube bundle andtransfers heat to the secondary water to produce steam.The EPR steam generator is a vertical, U-tube, natural circulationheat exchanger equipped with an axial economizer. It is anenhanced version of the N4 steam generator.It is composed of two subassemblies:• one ensuring vaporization of the secondary feedwater,• the other mechanically drying the steam-water mixture produced.In conjunction with an increased heat exchange area, the EPR axialeconomizer makes it possible to reach a saturation pressure of78 bar and a plant efficiency of 36 to 37% (depending on siteconditions). The tube bundle is made of a proven stress-corrosionresistant alloy: Inconel 690 with a specified mean value Co contentless than 0.015%. The steam generator bundle wrapper is made of18 MND 5 steel.To increase the heat transfer efficiency, the axial economizer directs100% of the cold feedwater to the cold leg of the tube bundle, andPressure shellBundle wrapper90% recirculated waterPressure shellADivider plateHot legSECTION ADouble wrapperDivider plate10% recirculated water100% feedwaterDouble wrapperABundle wrapperCold legabout 90% of the hot recirculated water to the hot leg. This is doneby adding a wrapper to guide the feedwater to the cold leg of thetube bundle and a partition plate to separate the cold leg from the hotleg. This design improvement increases the steam pressure by about3 bar compared to a conventional steam generator. There is aneasy access to the tube bundle for inspection and maintenance isprovided.Particular attention was given during the design of the EPR steamgenerator to cancel out secondary cross-flows to protect the tubebundle against vibration risks.The steam drum volume has been augmented. This feature, plus asafety injection pressure lower than the set pressure of thesecondary safety valves, would prevent the steam generators fromfilling up with water in case of steam generator tube rupture to avoidliquid releases.Compared to previous designs, the mass of water on the secondaryside has been increased to get a dry-out time, in the event of a totalloss of feedwater, of at least 30 minutes.The steam generator is fully shop-built, transported to the plant siteand installed in its reactor building cubicle in one piece.The axial economizer†Its principle primarily consists in directing the feedwater to the coldleg of the tube bundle and about 90% of the recirculated water tothe hot leg. In practice, this is done by adding to the standard naturalcirculation U-tube design a double wrapper in the cold leg of thedowncomer to guide the feedwater to the cold leg of the tube bundleand a secondary side partition plate to separate the cold leg andthe hot leg of the tube bundle. In conjunction with those two designfeatures, the internal feedwater distribution system of the steamgenerator covers only the 180° of the wrapper on the cold side.SecondarymanwayAuxiliaryfeedwaternozzleUpperlateralsupportbracketsTie rodBundlewrapperPartitionplate(secondaryside)TubesheetPartitionplate (primaryside)PrimarymanwayTransportation of a steam generator manufactured in China for Ling-Ao 2.Steam outletnozzleDryer frameSwirl vaneseparatorAuxiliaryfeedwater ringFeedwater ringFeedwater nozzleBundle doublewrapperAnti-vibrationbarTube supportplatesTube bundleFlow distributionbaffleChannel headPrimary coolantoutlet nozzleCHARACTERISTICSDATASteam generatorsNumber 4Heat transfer surface per steam generator 7,960 m 2Primary design pressure176 barPrimary design temperature 351 °CSecondary design pressure100 barSecondary design temperature 311 °CTube outer diameter/wall thickness 19.05 mm / 1.09 mmNumber of tubes 5,980Triangular pitch27.43 mmOverall height23 mMaterials• TubesAlloy 690 TT*• Shell 18 MND 5• Cladding tube sheetNi Cr Fe alloy• Tube support plates 13% Cr improved stainless steelMiscellaneousTotal mass500 tFeedwater temperature 230 °CMoisture carry – over 0.1%Main steam flow at nominal conditions2,554 kg/sMain steam temperature 293 °CSaturation pressure at nominal conditions78 barPressure at hot stand by90 bar* TT: Thermally treated† The steam generator is an enhancedversion of the axial economizer steamgenerator implemented on N4 plants.† The axial economizer allows increasing by3 bar the steam pressure output comparedto a conventional design, without impairingaccess to the tube bundle for inspectionand maintenance.† The very high steam saturation pressureat tube bundle outlet (78 bar) is a majorcontributor to the high efficiency of theEPR (37%).† The secondary water mass is consistentwith the 30 min. time period before steamgenerator dry-out in case of loss of allfeedwater systems.† The increase of the steam volume andthe set pressure of the secondary safetyvalves prevent any liquid release to theenvironment in case of steam generatortube rupture.26 II 27