Challenges in NDE of Welds in Nuclear Piping and Vessels

M .V. Kuppusamy, C.B. Rajeev, G.Ramesh, B.Anandapadmanaban and B.Venkatraman Quality Assurance Division, Indira Gandhi Centre for Atomic Research, India

Email: masikuppu@igcar.gov.in

Abstract

Austenitic Stainless Steel such as 304L has been widely used as work horse material for piping and components in nuclear fuel cycle facilities due to its inherent corrosion resistance in acidic environment and also ease of fabricability. Typical cell piping in a nuclear facility can have about 200 process vessels and allied piping works of length around 62 Km with over 70,000 welds. Inspection during and after construction in such a plant with high density piping is a real challenge. Also, once the plant goes into operation, many of the piping and components housed inside the concrete cells become nearly inaccessible for service (Zero Maintenance) due to high radiation fields. Right from the selection of materials, fabrication and inspection of various components of nuclear plants are important to ensure satisfactory operation during the design life since material fabrication related failures in similar kind of plants in the world resulted in prolonged shutdowns.

Various NDE methods such as Visual examination, Liquid Penetrant examination, Radiographic examination, Leak testing, Ultrasonic examination & Insitu-metallographic examination are employed to achieve the desired quality. The acceptance criteria for NDT are very stringent as compared to that of conventional construction codes, which in turn add to the complication for NDT qualification of various elements of the system. All volumetric NDT qualification of weld joints are coupled with advanced visual inspection aids such as videoscope, borescope, fiberscope to ascertain the degree of internal defects such as root concavity, root undercut, oxidation , pits etc. In-situ metallography has been used as a tool to check the effectiveness of solution annealing of dished ends of vessels. This paper dwells on all these aspects and the challenges faced.

Keywords: Austenitic Stainless Steel, High density, Advanced NDE Methods

1. Introduction: A nuclear fuel cycle facility under construction at Kalpakkam involves fabrication and installation of 200 process vessels and equipments weighing 147MT and allied piping works of length around 62 Km with varying sizes from 8 DN to 250 DN in about 5000 m3 of concrete cell volume. Austenitic Stainless Steel such as 304L has been widely used as work horse material for reprocessing plant cell piping and components due to its inherent corrosion resistance in acidic environment and also ease of fabricability. Fig. 1 is a typical photograph of cell piping to indicate the intricacy and high density of pipelines. FIG. 1 DENSITY OF PIPING IN A TYPICAL CELL

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In service inspection in fuel reprocessing plants is a challenge. In fact, once the plant goes into operation, many of the piping and components housed inside the concrete cells become nearly inaccessible for service due to high radiation fields. Hence right from the fabrication stage rigorous QA practices coupled with special NDE equipments and stringent acceptance criteria have been formulated and implemented so that in service we have minimum or zero maintenance of piping systems. Various NDE methods such as Visual examination, Liquid Penetrant examination, Radiographic examination (Conventional & Special X ray machines), Leak testing, Ultrasonic examination (Time of Flight Diffraction & Phased Array) & Audit radiography are employed to achieve the desired quality right from raw material inspection, in process inspection during fabrication and installation to final testing of piping systems. The main challenge in the implementation of NDE techniques is the high density of pipelines and limited accessibility. This paper mainly emphasizes on challenges faced in Non-destructive Evaluation in qualification of raw materials, machined components & qualification of weld joints. 2. Challenges in NDE of raw materials: As the raw materials irrespective of product form are going to be exposed to highly concentric boiling nitric acid with radioactive active species, freedom from defects especially stringers and inclusions shall be ascertained by appropriate NDE techniques which otherwise lead to end grain attack & intergranular corrosion by nitric acid during service. 2.1 Ultrasonic examination of raw materials: In order to detect defects like stringers and localized laminations, Ultrasonic examination of the plates are performed, as per the following table. This is carried out as per ASME Section V. 2.1.1Ultrasonic examination of plates All plates are inspected for lamination, inclusion etc. using both normal and angle beam even though the code of construction calls for only angle beam for less than 50 mm thickness and normal beam for above 50 mm thickness. The ultrasonic examination has to be performed in accordance with A578 and A577 for normal beam and angle beam examination, respectively. The scanning shall be done 100% on one major side with 10% overlap. The reference standard and acceptance criteria shall be followed as per the table given below for the class A service.

Sl No

Thickness of plate Reference Standard Acceptance Criteria

1. For plates of 6 mm to 10 mm thickness

1. Normal Beam using TR probe with 2mm-dia flat bottom hole/s (FBH).

2. Angle Beam: DAC Using 3% Notch

Any indication Above DAC Not Acceptable. Any indication with amplitude 50 to 100% DAC to be recorded.

FIG 2: ELECTROLYTIC DISSOLVER

2.1.2 Ultrasonic examination of pipes: The pipes are to be examined for any longitudinal and circumferential discontinuities as per by contact or immersion technique as per ASTM E213. The scanning shall be done 100% on outer diameter with 10% overlap. The reference standard and acceptance criteria shall be followed as per the table given below for respective classes.

Reference Acceptance Criteria Angle Beam: DAC Using 5 % Notch

Any indication above DAC Not Acceptable. Any indication with amplitude 50 to 100% DAC to be recorded.

3. Challenges in NDE of Machined components: An electrolytic dissolver as shown in Fig.2 involves challenges in machining of critical components such as dissolver chopper chute junction, right limb bottom block, annular tank bottom block and elctrolyzer(Fig.3a & 3b). As most of the components are machined from rod due to nonstandard size and commercial nonavailability of pipe form, ultrasonic testing is carried at rod stage as well as machined pipe stage.Machining of calibration blocks for small bore pipes such as 10 NB, 15 NB & 25 NB poses greater challenge in the ultrasonic testing of the components. The larger diameter components were grouped based on thicknesses from each heat no./lot no. Some component calls for extra material at ID for boring/finishing after welding with successive components. These components were inspected at intermediate stage as UT is possible at this stage only. As the technical specification demands for ultrasonic inspection of all product forms like plates, pipe & forgings, calibration block with required notches are mandatory requirement.

2. For plates greater than 10mm thickness and for plates (Thk. < 6 mm) examined in the mother plate stage. The mother plate stage examination shall be conducted at suitable higher thickness step (not more than 12 mm) of final required thickness.

1. Normal Beam: DAC using

a. 2mm-dia flat bottom holes (FBH) for 11 - 25 mm thk.

b. 3mm-dia flat bottom holes (FBH) for 26 - 75 mm thk.

2. Angle Beam: DAC Using 3% Notch

Any indication Above DAC Not Acceptable. Any indication with amplitude 50 to 100% DAC to be recorded.

FIG.3b RIGHT LIMB BOTTOM

BLOCK FIG. 3a ANNULAR LIMB

BOTTOM BLOCK

Preparation of required notch depth in calibration block and measurement of the same poses very big challenge especially for small bore / thickness pipes of range sizing from 8 NB to 100 NB pipes, where notch depth is 3 % of wall thickness ( e.g., 0.07 mm for 8 NB Sch 40 pipe, 0.11 mm for 10NB Sch

40 etc.).Wire cut Electrical discharge machining (EDM)is used for making the notches as shown in fig 4 and the same has been measured using techniques like replica method & profilometry etc.,

Critical Component with intricate shapes were inspected by ultrasonic examination for the possible maximum coverage and for remaining uncovered portion radiography was carried out. Advanced ultrasonic techniques such as Time of flight diffraction (TOFD) and Phased array are used for small bores nozzle in machined components as conventional Pulse echo technique is not possible due to inaccessibility and unfavorable OD/THK ratio. The nozzles machined are of varying sizes calls for the machining of calibration block of diversified sizes which is practically a costly and time consuming phenomenon. Hence, RT was proposed for all smaller sized nozzles instead of UT without sacrificing the quality of inspection. Thereby these machined components were inspected successfully. 4. Challenges in NDE of welded components:

4.1 Liquid Penetrant Examinations (LPE)

Root run as well as final run of all longitudinal & circumferential butt welds of Class-A components / equipments, fillet welds are subjected to LPE examination. Liquid penetrant examination shall conform to the requirements of ASME Section III NB-5000 and Article 6, T-600 to T-690 of ASME Section V. The below mentioned table gives the comparison of our nuclear piping specification to a conventional code of construction.

Defect type Class-A Section III

Rounded indication (RI) For vessels and pipes RI For vessel and Pipes

FIG. 4 UT CALIBRATION BLOCKS & ADVANCED UT

Defect type Class-A Section III

shall not be more than 1.5mm for all thicknesses

Max. size of any indication shall be less than 4.8mm.

4.2 Radiographic Examination (RT)

All longitudinal & circumferential butt welds of equipments and allied piping as per relevant product specification/drawing are subjected to 100% radiographic examination. Radiographic examination is performed using X-Ray source and in accordance with ASME Section III NB 5000 and Article 2 of Section V of ASME Boiler and Pressure Vessel code.

Sensitivity level of 2-1T has been specified for weld thickness upto 6.4 mm instead of 2-4T as required by ASME Section III Division 1 NB. The required sensitivity was achieved after number of trials by optimizing the radiography parameters. Acceptance criteria is also stringent than ASME codes in a way that even cluster and aligned porosities of any magnitude is not acceptable due to reduction in thickness locally.

The below mentioned tables give the additional requirement in radiography of nuclear facility components over ASME section III.

Radiographic Acceptance Standards Defect type Class-A Section III Div.1

Rounded indication (RI) Aligned Rounded indication

For vessels and pipes RI > 1/5t or 1.5mm whichever is less for thickness up to 25 mm is not permitted. Additionally for piping the cumulative length of isolated RI shall be less than 3 % of weld length of each pipe joints. Not permitted

For vessel and Pipes The maximum permissible size of any indication shall be ¼ t or 5/32 in. (4 mm), whichever is less, except that an isolated indication separated from an adjacent indication by 1 in. (25 mm) or more may be 1/3 t or ¼ in. (6 mm), whichever is less. Aligned rounded indications are acceptable when the summation of the diameters of the indications is less than t in a length of l2t

Elongated indication Not permitted ¼” for “t” upto 19mm 1/3 t for t from 19 to 57 mm

Group or aligned indication Not permitted. t” in a length of 12t aggregate length

Undercut 0.3 mm max. and shall not decrease the minimum required thickness

0.8mm max. shall not decrease the minimum required

Defect type Class-A Section III Div.1

thickness

Concavity Not permitted Acceptable subject to meeting the min. thickness requirements.

4.2.1 Audit Radiography: Audit radiography has been devised since more than 60000 joints with more than 120000 exposures involves in different cells. It is used as a QA tool to ensure that radiography has been carried out correctly on the joints with proper techniques and quality requirements. After radiography of every 100 pipe weld joints in each size, 3 % of the weld joints will be identified and audit radiography carried out. In general, on comparison if any discrepancy is found, radiography will be repeated on all the joints again.

4.2.2 Special Radiography System:

In addition to the regular radiography machine, a portable & compact ceramic X-ray tube head with very small focal spot & long cables is used due to high density of pipe lines and space constraint in compact cells. 4.3 Advanced UT of welded joints: The fig.5 shows sketch of a Jacketed Annular Tank to be used as process

vessel in our reprocessing plant. Qualifying the typical corner joint as shown in Fig.6 is not possible by conventional pulse echo techniques. Hence Time of Flight Diffraction (TOFD) & Phased Array techniques have been resorted to qualify these kind of critical joints with much better probability of detection.

5. Challenges faced in achieving close dimensional tolerances

The fuel right limb of electrolytic dissolver consists of four machined components namely, right limb bottom block, dissolver chopper chute junction and two segments of 100 NB Sch 40 pipe machined from round. Assembly of this fuel right limb by means of welding

FIG. 5 JACKETED ANNULAR TANK FIG. 6 TYPICAL CORNER JOINT

FIG.7 ALIGNMENT USING OPTICAL METHOD

poses a big challenge as close annular gap dimensional tolerance to be maintained between the fuel charging limb and basket so as to act as a gate valve for opening and closing of chopper chute linked to dissolver. Special fixtures are developed to carry out welding in vertical position (5G). A special optical alignment technique as shown in fig 7 with 4nos dial gauges at 900 intervals around the weld periphery with proper intermittent & block welding sequence has been used during welding to control the distortion in order to have tight control on concentricity and linearity 6. In-situ Metallography:

Insitu metallographic technique as shown in fig. 8 has been effectively used in ensuring the effectiveness of solution annealing of

formed dished ends in

different types of equipments used in reprocessing plant piping. Representative areas from Crown,

Knuckle and Straight face portions are selected from one dished end per heat treatment lot. Practice A test as per ASTM A 262 is carried out selected areas and viewed with 100x magnification using portable optical microscope for the presence of deleterious chromium carbide structure (Ditch).Freedom from ditch structure as shown in fig.9 and End grain attack –II ensures the complete absence of sensitization. Fig 10 shows a typical Step structure with Annealed Twins shows a effective solution annealing treatment. A cellulose acteate tape has been used to acquire the positive replica of the etched area and preserved for future assessment. 7. Visual Inspection of Welded Joints:

Root concavity, root undercut & oxidation of any magnitude is not acceptable as these flaws will act as potential crevice corrosion promoters.

FIG.8 IN-SITU METALLOGRAPHY

FIG.9 DITCH STRUCTURE

FIG.12 VIDEOSCOPE

FIG.11 FIBERSCOPE

FIG.10 STEP STRUCTURE

Sensitivity and probability of detection of visual inspection of weld joint of thin walled small bore pipes where either the accessibility is limited or eye doesn’t have sensitivity to locate the flaw are enhanced by using visual aids such as borescope, Fiberscope (Fig. 11) & Videoscope (Fig. 12) are use to carry out visual inspection of small bore pipes 8. Conclusion: A compact concrete cell with high density piping with associated equipments & vessels poses very big challenge in terms of qualifying base metal, machined components and the welded joints volumetrically by NDT methods to meet the technical specification. Stringent and meticulous NDE practices helped us to realize a many critical items in reprocessing plant such as electrolytic dissolver, Jacketed Annular tank etc with close dimensional tolerances. The well documented quality assurance & NDE practices for the fabrication of such a critical equipments and piping can be used successfully in the upcoming production plants.