Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2011, December 8-10, 2011

9760mm for supporting primary sodium pump (two with ID1900mm) and intermediate heat exchanger (four with ID2200mm). It has twenty other penetrations for supporting decayheat exchangers (four with ID 600mm), delayed neutrondetectors (eight with ID350 mm), cold pool level detector (onewith ID 600mm), hot pool level detector (one with ID 203mm),sodium purification lines (one with ID 480 mm), argon lines(two with ID 203mm), clad rupture detection argon lines (twowith ID 203mm), and an inclined (17o inclination with verticaland ID 600mm) penetration for in-out transfer of sub-assemblies. All the penetrations except the penetration forargon lines have machined support flanges with stringentdimensional and geometrical tolerances. It also have tencooling boxes, eighty-six spacer pads to avoid core disruptiveaccident, eighty-six concrete filling pipes for filling highdensity concrete, three hundred ten of thermocouples, hundredand forty-four tie rods holes, two hundred eighty-ninestiffeners, and thermal shield. It has twenty-one inlet and outletcoolant pipes, four horoscopes near IHX locations, fourhoroscopes near LRP location and twenty-eight support padsand gear mounting pads. Fig. 1 digital image during fabricationof roof slab.

Roof slab have large diameter to thickness ratio. Itsmanufacture needs many innovate methods for fabrication,

QUALITY ASSURANCE ASPECTS DURING MANUFACTURING OF ROOF SLAB FOR500 MWE PROTOTYPE FAST BREEDER REACTOR

Shripal, T.Loganthan , S. Ramesh, R.V.R. Govindarajulu, A. Ramu and Prabhat KumarPrototype Fast Breeder Reactor (PFBR) Project

Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI)Department of Atomic Energy,Kalpakkam-603102, Tamil Nadu, INDIA

ABSTRACT

The construction of 500 MWe Prototype fast Breeder Reactor is nearly in final stage of completion at BHAVINI,Kalppakkam, Tamil Nadu-603102. Roof slab, a critical component of PFBR, consists number of penetrations withspecified PCR, angular orientation, and flatness which need stringent dimensional requirements and the same arecontrolled by conventional and ECDS system. The plates for roof slab shell segments are produced by electric arcmelting with tight control on inclusion content to achieve sound weldments.The weldments of similar and dissimilarmetals are examined and evaluated by various NDE methods such as visual, LPE, radiographic examination, ultrasonicexamination, PSI (pre-service inspection) ultrasonic examination and finally tested for pneumatic and helium leaktesting. The highly radioactive concentration, radioactive nature of primary sodium and cover argon gas necessitatesboundaries with a high degree of reliability against failure for the roof slabs. Therefore, PFBR specifications requirementsare stringent compared to specifications of other industrial applications to enhance reliability. This paper details theoverview of the steps taken for implementing the quality assurance aspects during manufacture of roof slab for 500MWePFBR project.

Keywords: Ultrasonic, Radiographic, helium Leak testing, Electronic Coordinate Determination System.

INTRODUCTION

Top shield consist of roof slab (RS), rotatable plugs and controlplug. The main function of roof lab (RS) to provide thermaland biological shielding in the upper axial direction from thehot sodium pool to facilitate personnel access above and actsas part of primary containment boundary. It supports variouscomponents such as main vessel (MV), large rotatable plug(LRP),small rotatable plug (SRP), primary sodiumpumps(PSP),intermediate heat exchanges (IHX) ,decay heatexchanges (DHX), delayed neutron detectors (DND) , leveldetectors & other associated auxiliary equipments. Roof slabis a box type structure made mostly from special carbon steelplates confirming to AFNOR-A 48P2 (Mod.). It is selected asa principle material of construction to meets the requirementsof good through thickness ductility, high toughness, and goodresistance to lamellar tearing. The structural arrangementsconsist of a top and bottom plates inter-connected by innershell, outer shell, and radial stiffener plates.

DESCRIPTION OF ROOF SLAB AND NEED FORQUALITY ASSURANCE

Roof slab is massive structure in weight, ~230 tonne, and insize with the outer/inner diameters of 12900/6210 mm. It hassix major penetrations at pitch circle diameter (PCD) of

welding, and machining to control distortion, to achievestringent weld profile, and stringent geometrical (IS 2102, part1, fine class) and dimensional (IS 2102, part 2, H class)tolerances after onsite machining. There is a need for devisingmany QA procedures and methods for accurate and reliableinspection and development of non-destructive examinationtechniques to ensure these requirements. The welding ofspecial carbon steel, modified SS316LN, and modifiedSS304LN plates need special types of welding electrodes andfiller wires, which are not readily available and classified byAWS. These modified electrodes and filler wire developedafter very extensive tests and validations. The mainrequirements of special carbon steel welds to have low nilductility temperature and impact strength. The mainrequirements for austenitic stainless steel welds are to haveresistance to thermal shock and resistance to embrittlementdue to fast neutrons. PFBR specifications demands largeadditional numbers of tests/ inspections/examinations withmuch more stringent requirements than ASME, Section III,Division 1, subsection NB.

QUALITY ASSURANCE DURING PROCUREMENT OFRAW MATERIALS AND WELDING CONSUMABLES

The principal material of construction is special carbon steelconforming to AFNOR-A 48P2 (modified) except roof slabouter shell, step plate and thermal shield panels that are madeof modified stainless steel SS316LN and SS304 LN as perPFBER specifications. For welding of special carbon steelmaterial for roof slab, with SMAW process special lowhydrogen carbon steel electrode is used. Carbon steel plates,stainless plates, and welding consumables demands a verynarrow range chemical composition of plates and weldingconsumables as per PFBR specifications. For dissimilar weldbetween carbon steel and stainless steel, electrode and fillerwire conforming to AWS classification E309 and ER309 ofSFA 5.4 is used.

Carbon steel plates AFNOR A 48 P2: Carbon steel plateswith specific ductility requirements in short transversedirection used in the fabrication of roof slab. Plates made byelectric arc furnace melting process. Plates supplied in

Fig. 1 : Digital images during fabrication stages of roof slab

Fig. 2 : Digital images during machining and welding of roof slab

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normalised, killed and vacuum degreased condition forcontrolled inclusion content. Reference nil ductilitytemperature (RT NDT) of these plates determined as PFBRspecifications. Each plate tested for room temperature tensiletest- long transverse and short transverse test, RTNDT, impacttest, tensile after simulated stress relieving heat treatment, bendtest. Plates under goes 100% ultrasonic examination inaccordance with ASTM A -578.

Modified Austenitic stainless steel 316LN and 304LN plates:Stainless plates supplied in solution heat-treated, pickled, andpassivated condition and manufactured using electrical arcfurnaces melting process. Plates undergoes chemical analysis,room temperature tensile test, tensile test at elevatedtemperature, impact in solution annealed condition andembrittled condition, intergranular corrosion test, inclusiontest, delta ferrite (less than 1%), metallographic examination,grain size (finer than ASTM no. 2) determination as per PFBRspecifications. Plate are visually examined (in case doubt byliquid penetration examination) to ensure that are free fromscratches, blowholes, scales, cracks, hairline flaws. All platedundergoes 100% ultrasonic examination with 10% overlap bylongitudinal waves using pulsed-echo technique at a probefrequency of 2 MHz.

Austenitic stainless steel electrodes and wires: Austeniticstainless steel modified electrode E316-15 of SFA5.4 and 16-8-2 filler wires undergoes additional/modified examinationsand testing to ensure the service conditions of the components.These includes slag detachability, radiographic examination,all weld metal tension test, impact test, tensile test at ambienttemperature, tensile test at elevated temperature( 823K), inter-granular corrosion test in as welded state, fillet weld test, deltaferrite check, crack susceptibility test, and creep test.

Carbon steel electrodes and filler wires: carbon steel modifiedelectrodes of SFA 5.1 and modified filler wire of SFA 5.18,undergoes additional / modified examination and testing. Theseincludes all weld tensile test at room temperature, Charpyimpact test at -20oC, radiographic examination, nil ductilitytransition temperature determination, coating moisture test andfillet weld test.

QUALITY ASSURANCE DURING MANUFACTURINGOF ROOF SLAB (ROLLING, FABRICATION, ANDMACHINING)

LRP support flange (made in sectors and integrated) and allother flanges cut from a single plate are integrated to roofslab and has undergone heat-treatment at the holdingtemperature of 873K±10K. Holding time is 2 minutes per mmof thickness with minimum of 30 minutes and maximum of120 minutes. The temperature of loading in furnace is lessthan 200oC. Heat and cooling rate shall not exceed the greaterof two values (1) 220K/h divided by the maximum thicknessin multiple of 25 mm or (2) 55K/h. Other welds of thickness≤ 35 mm are under-gone to heat treatment. Stainless steelsurfaces under-goes passivation as per PFBR specifications.On site, machining (Fig. 2) was performed using specialpurpose machines in one setting without disturbing the roofslab. Dimensions of machined flanges ranges from 200mm to7000 mm in diameter with flatness requirement of 0.1 mm to0.3 mm. Bottom surface of support ring of support assemblyof diameter 13600mm also finish machined.

QUALITY ASSURANCE DURING WELDING OF ROOFSLAB

Recommended welding process is shielded manual arc welding(SMAW). For joints where back gouging is not possible, theroot pass made by GTAW process using argon gas purging onthe back side. The qualification of welding procedure andwelders for welding of carbon steel, stainless steel components,and dissimilar metal weld had done as per PFBR specificationsthat are much more stringent than code ASME Section III,Division 1, subsection NB and demands large number ofadditional tests, inspections, and examinations. Duringwelding procedure qualification for carbon steel, referencenil ductility temperature transition temperature (RTNDT) isdetermined as per ASME Section III, class I component withPFBR specification requirements. Specimens taken from basemetal, heat affected zone and weld metal with a requirementof RTNDT temperature -15oC or lower. After determining RTNDTtemperature as per ASTM E 208 by drop weight test, impact

Fig. 3 : Digital images during LPE and ultrasonic examination of weld of roof slab

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tests carried-out at the determined RTNDT temperature +33oCwith the requirement of 68J energy minimum and lateralexpansion of 0.9 minimum for all specimens. Fig. 2 digitalimage during welding of roof slab.

Welds are either ground flush or to required radius as indicatedin drawings. At other locations, reinforcement on the weldscontrolled as per PFBR specifications. The surface finishrequirements are better than 6.3 micron (CLA) on weldsground flush and ground surfaces. Sector integration weld aresuitably staggered with a minimum distance of 100 mmbetween them and sequenced to avoid build up of residualstresses and distortion. Written procedures made to controldistortion for each typical joint giving sequence of assembly,sequence of welding, and heat input to welds. The shrinkageand distortion of all weld joints measured and recorded. Duringwelding heat input, nearly 0.8 to 1.2 kJ/mm and 1.5 to2 kJ/mm reported for stainless steel welds and carbon steelwelds respectively. The number of repairs permitted for weldsis limited to maximum two times at any location. The mismatchlimit required at fit-up stage is as low as possible to achievethe tolerances specified on the geometrical shapes of thecomponents. The mismatch at fit-up stage is limited to (t/20)+1 mm with maximum of 3 mm, where t is the thickness ofthe plate. Production test coupons (PTCs) of about dimensionsof 150×1000mm welded to extension of plates for the job.PTCs welded simultaneously along with the job and samewelding parameters, heat treatment, destructive, and non-destructive testing followed as for the production welds. Totalthirteen PTCs prepared.

NON-DESTRUCTIVE EXAMINATION OF ROOF SLAB

The systematic and sequential methodology applied forensuring soundness of welds. All welds undergo 100% non-destructive examinations.

Visual examination: All welds undergoes visual examinationto ensure that welds are free from surface defects e.g. undercut, unfilled groove, slag, excessive penetration, lack of fusion,lack of penetration etc.

Liquid Penetration Examination of welds: All welds of roofslab subjected to solvent removal visible LP examination withwritten procedure for detecting surface discontinuities e.g.surface cracks, surface porosity, weld spatter etc. Weldssubjected to edge, root pass, final pass and one/two layer (forsome weld seams) LPE with evaluation and acceptance criteriaas per PFBER specifications. All liquid penetrant materialsused on austenitic stainless steel welds analysed batch wisefor sulphur (≤1%) and halogen content (≤25 ppm). All weldswith the temporary closures, fixtures etc., subjected to LPEand welded by qualified welder (Fig. 3).

Ultrasonic Examination: Many butt and fillet welds of roofslab undergo ultrasonic examination with written procedurewith evaluation and acceptance criteria as per PFBRspecifications. Weld surfaces merged smoothly in to surfaceof base metal. The surfaces adjacent to the weld shall be groundto eliminate any weld defect like arc strikes, weld spatter andto achieve surface not exceeding 6.3μm (CLA). Couplants (oil/grease) when used on austenitic stainless steel welds, analysedbatch wise for sulphur (≤1%) and halogen content (≤25 ppm).In principle, frequency of examination is 2MHz.The referenceblock for carbon steel welds is same as base material. Foraustenitic stainless steel simulated weld coupon used.Reference block with 2 mm side drilled hole employed forestablishing reference echoes height and DAC curves. Anyecho with amplitude equal to greater than 50% of the referenceecho recorded. Further, for joints with different thickness anddissimilar materials, suitable mock-ups have performed toestablish the ultrasonic examination sensitivity (Fig. 3).

Radiographic Examination: Many butt as well as T-Jointwelds of roof slab subjected to radiographic examination withwritten procedure with evaluation and acceptance criteria asper PFBR specifications. X-ray equipment used as radiationsource up to 20 mm thickness. For thickness higher than 20mm, X-ray or suitable γ-ray source used. In general, class-2 orclass-1 films along with lead intensifying screens used. Allweld surfaces merged smoothly into the base metal surface.For all films radiographic films density measurementsperformed. Geometrical un-sharpness requirements for X-rays

Fig. 4 : Digital images during ECDS measurement and helium leak testing of roof slab

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up to 400kV or Ir-192 or Tm-170 are less than 0.3 mm and forCo-60 is less than 0.6 mm. The finished surface of thereinforcement of any butt welds flushed with the plate. Themaximum reinforcement permitted is less than W/10+1 mm.Where W is the width of the weld seam in mm. Welds, radiographed with techniques, to achieve the sensitivity as per PFBR(ranging from 2-1T to 2-2T) specification. Sensitivity andacceptance criteria as per PFBR specifications are much morestringent than ASME, section III, division, subsection NB.

FINAL INSPECTION OF ROOF SLAB BY “ECDS”

Dimensional and alignment inspection of roof slab performedwith written dimensional and alignment inspection proceduresatisfies/generate data to meet all the requirements specifiedin the drawings. It includes both conventional and electronicco-ordination detection system (ECDS). ECDS measurementsperformed for verification of conventional measurements(Fig. 4).

PRE-IN-SERVICE INSPECTION (PSI) ULTRASONICEXAMINATION OF WELDS OF ROOF SLAB

Pre-in-service inspection (PSI) ultrasonic examinationperformed on welds of outer shell A1 and outer shell A2 fromexternal surface to generate baseline data for the purpose ofcomparison of these data during in-service inspection. Thesignals stored in retrievable digital form with appropriate weld/location identification.

LEAK TESTING OF ROOF SLAB

Welds of roof slab tested by pneumatic testing. An internalpressure of 0.06MPa applied for 10 minutes and welds checkedfor leaks. Then, pressure reduced to 0.5Mpa and roof slab ishold at this pressure for 60 minutes to ensure that there is nopressure drop and soap bubble (lissapol) solution applied todetect the leaks. The soap solution checked for ionic chloridecontent not more than 25ppm and sulphur content not morethan 1% by weight for application on stainless steel parts ofroof slab. Additionally, helium leak testing (HLT) underpressure of 0.06MPa performed on the welds of roof slab.During testing, it is ensured that global leaks shall not exceed1×10-6 Pa-m3/s per m3 of the internal volume and local leaksnot exceed 1×10-7 Pa-m3/s. Before testing, it is ensured thatconcentration of helium inside the roof slab is at least 60% involume. This had ensured by lab tests (Fig. 4).

CONCLUSIONS

The innovate methods for; manufacture, fabrication,integration of cooling boxes, alignments of various parts/sectors of roof slab, welding techniques, machining methods,non-destructive examinations, and leak testing all successfullyemployed to achieve required quality levels for roof slab -abox type hollow structure with very large outer diameter andlarge diameter to thickness ratio. This had achieved byimplementing the effective quality assurance methodologies.A well-planned inspection approach including ECDS methods,PSI-UT, and large number of mock-up etc. facilitated inachieving desired tolerances and stringent NDE requirementsfor roof slab and hence, meeting the challenges inmanufacturing the roof slab.