Paper on critical issues in fabrication of stainless Steel equipment for NRP

1. Critical Issues in Fabrication of Stainless Steel Equipment for
Nuclear Recycle Plants
Santosh Takale, Ramakant and D. S. Rana
Nuclear Recycle Group, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085.
Key Words: Stainless steel, NAG, corrosion, IGC, filler wire, fabrication, quality control.
Abstract
Austenitic stainless steel (ASS) is the mainstay of nuclear recycle plants comprising spent
fuel-reprocessing plants and associated nuclear waste management facilities. Stainless steel
grade 304L is not only used in process equipment and piping, but also in other applications
such as cladding of concrete hot-cells, staging members, supports structures etc. However, as
the process equipment and piping handle highly radioactive and corrosive fluids, standard
ASTM grade austenitic stainless steel does not suit this application. ASS with modified
composition and metallurgical cleanliness was developed and has been successfully
deployed. Use of this specifically developed ASS viz. nitric acid grade SS 304L requires
especial attention during fabrication to achieve the requisite quality. The process equipment
once installed in the concrete hot-cells are not amenable for any kind of in-service repair or
maintenance and hence are required to be fabricated for life-time service. As these process
equipment encounter highly radioactive liquids, provisions have to be made for their in-situ
decontamination and decommissioning. All these requirements pose a great challenge to the
fabricator.
The paper brings out the critical issues in the manufacture of process equipment right from
material selection through fabrication methodology, inspection techniques and testing
procedures. Requirements over and above those stipulated in ASME codes have been
described. Process planning, manufacturing sequences, jigs & fixtures required in the
fabrication have been detailed out. Experience gained in the fabrication of over 350 Nos. of
process equipment is shared.
1. Introduction
Nuclear Recycle Group (NRG) of Bhabha Atomic Research Centre (BARC) is
responsible for carrying out all activities associated with the back-end of nuclear fuel cycle.
These include design, engineering, construction and operation of spent fuel reprocessing
plants and waste management plants. Besides, NRG is also responsible for research &
development activities for back-end of fuel cycle such as process & equipment development,
remote handling gadgets, waste matrix development, process simulation tools, development
of new systems for the management of radioactive wastes arising from the operation of
various nuclear facilities.
Chemical processing of spent nuclear fuel, besides separating useful fissile and
fissionable materials from fission products also generates large quantities of radioactive
wastes. The processing of spent fuel and radioactive waste processing are carried out behind
substantially thick shielding. RCC cells having wall & roof thickness ranging from 900 mm
to 1500 mm are normally constructed for housing process equipment and systems. The
radioactive wastes arising from spent fuel processing are categorised as high-level,
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2. intermediate-level, and low-level, depending upon the process streams from which these are
generated. The complete processing of spent fuel and subsequent waste management is
performed in nitric acid medium, containing a host of isotopes some of which are highly
corrosive. Further, presence of higher levels of radioactivity makes maintenance of the
equipment and vessels non-feasible. Hence, it is imperative that the equipment, vessels and
assemblies for process systems are designed and fabricated to their life time use.
The reprocessing of spent fuel and waste management employ special equipment viz.
dissolver assembly, extraction columns, thermo-syphon evaporators, conditioners,
denitrators, annular vessels, condensers, scrubbers, etc. Besides, large number of process
vessels is used for in-process hold-up of solutions and interim storage of liquid wastes. Each
of the above equipment and vessels is designed for different process conditions ranging from
normal temperature to boiling condition operations. As mentioned above, the processing is
carried out in nitric acid medium, concentration of which varies from 0.1 M to 12M. Thus the
process equipment and vessels are subjected to aggressive environment.
Austenitic stainless steel 304L (SS 304L) is the most suitable material for use in nitric
acid medium and same had been used in earlier generation of fuel reprocessing and waste
management plants. However, standard ASTM grade SS 304L contains large amount of
impurities and residual elements which render this material to corrosive attack during use.
Hence, need for SS 304L with controlled chemical composition and metallurgical cleanliness
was felt. Non-amenability of the process equipment and vessels to repair and maintenance
during use and to achieve life-time integrity made it is essential that the equipment are
fabricated beyond the call of code of fabrication with respect to quality so that no deficiency
is introduced during fabrication. Design features also provides for in-situ decontamination
and decommissioning of the equipment and vessels at the end of design life. Thus, stringent
specification for raw materials, material handling, fabrication, inspection and testing are not
only drawn but also rigorously implemented. Special techniques are developed for
manufacturing and quality assurance. Subsequent sections of this paper and attached
photographs bring out the experience gained by us during fabrication of about 350 Nos.
major and over thousand auxiliary equipment in the last couple of years.
2. Material Specification
2.1 Raw Materials:
SS 304L generally shows very good corrosion resistance in acidic oxidizing environment
and is widely used in process equipment and piping of chemical process industry. While the
process conditions in nuclear recycle plants are acid and oxidizing, presence of corrosion
promoters in process solutions makes ASTM grade SS 304L venerable to corrosive attack.
Hence, special composition for SS 304L was developed in consultation with corrosion
specialist from BARC to suit the process environment prevailing in our plants. A comparison
between standard SS 304L and modified SS 304L viz. SS 304L (NAG) is presented below.
The challenge was not only in developing the correct specification for SS 304L raw
materials such as plates, pipes, tubes, rounds etc. but also in producing these through
indigenous manufacturers. Due to prevailing technology control regime, it was not possible to
procure the bulk quantities of raw materials for our use from overseas producers, who could
readily meet the product specifications. Hence, considerable efforts were put-in first to
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3. convince the local manufacturers on the need for such materials and then during actual
production trials.
Typical composition of SS 304 L & SS 304L (NAG) materials with observed corrosion
rates as per ASTM-A 262 Practice` C’ (accelerated corrosion test) is given below:
Table 1 -.Composition
Grade C Mn Si P S Cr Ni N
min. - - - - - 18.0 8.0 -
304L
max. 0.030 2.0 0.75 0.045 0.030 20.0 12.0 0.10
min. - - - - - 18.0 10.0 -
304L
NAG
max. 0.02 1.8 0.35 0.025 0.005 20.0 12.0 0.05
Note: Other elements need to be controlled are O, Cu, Al, B, Ti, Mo, etc.
Table 2 - Corrosion rates (mpy)
Cycle – 1 Cycle – 2 Cycle – 3 Cycle – 4 Cycle – 5 Avg. for
Grade
48 Hrs 96 Hrs 144 Hrs 192 Hrs 240 Hrs 5 Cycles
304L 21.08 23.19 26.90 28.79 29.74 25. 94
304L
7.2 7.2 6.0 4.8 6.0 6.2
NAG
Another significant aspect to note that the lower corrosion rates in case of SS 304L
(NAG) material is achieved not only due to limiting conditions on chemical composition but
also due to its metallurgical cleanliness. In case of standard ASTM grade materials no such
specification exists. However, for SS 304L (NAG) this was controlled by specifying the
limits for inclusion rating and achieving in large scale actual production.
Table 3 – Inclusion Rating
A B C D
Grade
Thin Heavy Thin Heavy Thin Heavy Thin Heavy
304L
1 0.5 1.0 0.5 - - 1.5 0.5
NAG
Note: ASTM E – 45 overall limit of inclusion rating (A+B+C+D) < 4.0
2.2 Welding Filler Wires:
Developing specifications for raw materials were not the only the critical issue but
equally essential was developing suitable filler wires to be used in welding of the SS 304L
(NAG) materials. Special efforts were made to develop the welding filler for gas tungsten arc
welding (GTAW) which had controlled -ferrite content and could conform to low corrosion
rates, matching to parent materials, under inter-granular corrosion (IGC) tests as per Practice
‘A’ & ‘C’ of ASTM-A 262. Typical composition of the filler wire is presented below.
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4. Table 4 – Composition
Grade C Mn Si P S Cr Mo Ni Cu
min. - 1.0 0.3 - - 19.5 - 9.0 -
SFA 5.9 ER
308L
max. 0.030 2.5 0.65 0.03 0.03 22.0 0.75 11.0 0.75
min. - 1.0 - - - 18.0 - 10.0 -
SFA 5.9 ER
308L (spl.)
max. 0.025 2.0 0.4 0.02 0.01 20.0 0.20 12.0 0.1
3. Special feature of the Equipment
The process equipment for nuclear recycle application are clearly distinguishable from
the ones for other chemical process plant in terms of nozzle penetrations. Due to
inaccessibility of the equipment and to facilitate in-situ decontamination, provisions have to
be made for remote instrumentation, passive modes for agitating the contents of liquid and its
transfer, decontamination sprays, adequate spares for liquid inlet & outlet, remote liquid
sampling, cooling/ heating coils, etc. Thus the number of nozzles typically required per
equipment is in the range of 40-60 and makes internals of the equipment highly complex.
Conventionally used means for fluid transfer such as pumps are replaced by passive systems
such as steam jet ejectors & air-lifts. Similarly, instead of mechanical agitators, air is used as
the medium for agitating the liquid. An array of spray nozzles is arranged inside the
equipment in such as way that cleaning of internal of equipment is achieved by means of
externally fed solutions. Pressure, level, density, etc. are obtained by air purging method.
Temperatures are sensed by remotely inserted and replaceable thermocouples. All these
provisions inside the equipment make the fabrication even more challenging task.
4. Fabrication
The end-use specification and special design features demands that equipment are
fabricated to nil defect. This requires numerous steps; only a few of which are detailed below:
4.1 Manpower Orientation:
This is the first and most important step. Our equipment being non-standard with custom-
built requirements, it was essential that technical & supervisory staff to be engaged in the
fabrication was fully briefed on the Do’s & Don’ts. This was the real challenging task as
most of shops under jobs with varied specifications at the same time. Orienting them was
necessary so as to understand our requirements on aspects such as material storage &
handling, cleanliness, contamination, process planning, manufacturing sequence, welding, in-
process inspections, testing, quality assurance etc. It is advisable these are written down and
prominently displayed such that the staff is regularly reminded of the special requirement of
the job. Clear, crisp, correct & effective communication among the managerial, supervisory
& technical staff is vital. Necessity of retaining the staff once trained for a particular
fabrication work is essential.
4.2 Shop Floor Practices:
Implementing the so called `Good Shop Practices’ both in letter and spirit was the next
difficult step. Clean and dust free shop floor was the first pre-requisite before receipt of the
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5. raw materials for fabrication. Well segregated bay, which ensures protection against rain and
windy conditions, exclusively for fabrication of stainless steel equipment was a necessity.
Cross contamination through tools, material handling accessories, plant & machinery etc. is
to be avoided. Walking over the raw materials such as plates, pipes etc. is prohibited so is the
direct contact between the carbon steel and stainless steel surfaces. All material handling
gadgets, rolls for plate bending, forming tools, jigs & fixtures etc. were cladded with stainless
sheets. Care was taken that welding fixtures, clamps or manipulators do not have any surface
made from lead, zinc or copper/copper alloy that can cause contamination of the stainless
steel work-piece.
4.3 Mock-ups Trials:
SS 304L (NAG) is not only very costly material but is also difficult to work with, unlike
carbon & commercial grade SS steels. Hence, before the start of fabrication, besides
numerous written down procedures and approvals, it was felt necessary to conduct mock-up
trials on every important and critical parameter & process. This was to establish and achieve
the written down procedures on mock-up pieces rather than on actual job and if necessary to
modify the procedure & process.
4.4 Fit-ups:
Achieving good fit-up for any weld joint is very vital. A good fit-up not yields defect-free
weld joint but also reduces residual stress to a large extent. Hence, wherever possible
machined fit-ups were provided. For flush nozzles, inside profile were pre-matched by
grinding to suit inner profile of the vessel. For through nozzles/penetrating nozzle at curved
surfaces, the profile of the opening was prepared to provide uniform gaps. Different types of
fixtures were developed to suit specific need to avoid distortion, deflection, misalignment etc.
during welding such as spider for cir-seam joint of shell sections, three-leg support
arrangement for cooling coil subsection joints, sandwiching mechanism with clamps for
multiple nozzle to flat end closure joint etc. The all the mating clamps, jigs, fixtures were of
stainless steel only.
4.5 Welding:
Owing to high thermal expansion and low thermal conductivity controlling heat in-put
during welding of stainless steel is essential. Therefore, manual welding GTAW over
automatic welding processes were preferred to have better control primarily because of the
presence of internals in the equipment. Purity of argon gas for welding found to have great
impact on the weld joint quality. Therefore, only high purity argon gas (>99.995%) for
shielding as well as purging was used in all welding operations. Care was taken to envelope
all surface expose to weld heat during and after welding to avoid any heat tint or oxidation.
HF units were used for arc initiation. Only qualified welders were permitted to do welding
including temporary and attachment welds. Weld procedures were qualified not only to meet
ASME Section IX criteria but also to meet the requirements of corrosion, ferrite etc. To the
extent possible all the welds were full penetration joints, back chipped & re-welded. Inside
weld joints were finished smooth with uniform contour. This way it was ensured that no
crevices were left during fabrication, which otherwise is highly detrimental during the use of
the equipment. Reinforcement on welds was not allowed to exceed 10% of the material
thickness.
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6. Reduction in residual stresses introduced during forming, welding etc. was one of the
critical requirements of fabrication, as this may result in stress related corrosion during use.
Solution heat treatment was resorted to wherever it was found necessary. Skip-welding, back-
step welding, specially designed fixtures etc. were used to avoid distortion and control
residual stress.
4.6 Cleanliness & Contamination Checks:
Earlier experience from the failure analysis of some of the equipment reveled that the
contamination getting introduced at any point during fabrication may have affected its
performance under use, even though no apparent defects were noted during its fabrication.
Hence, it was a great challenge to convince the fabricators of not only providing
contamination free environment but also to maintain it all stages of fabrication.
Cleaning of raw materials before use of any paint, oil, grease, dust or any other
contamination was strictly adhered to. Cleaning of prepared edges for welding and completed
weldments was done on immediate basis. Wire brushes used were of stainless steel and
reserved specifically for this job to avoid contamination of the weld surfaces and these
brushes by carbon steel material. All scales, dents, burrs, weld spatter, oxide, oil and other
foreign materials were completely removed from inside and outside of the vessel. Chloride &
Iron contamination checks were carried out on a continuous basis.
5. Quality Assurance
Finally, the result of any fabrication work is revealed through quality surveillance and
control regime followed during fabrication. Towards this, a detailed quality assurance plan
was drawn well before the start of fabrication activity. Requisite inspection & tests were pre-
qualified on mock-up trial pieces so as to achieve satisfactory results on production job.
Visual inspection is the most important tool and much emphasis was given to it during
fabrication. Acceptance criteria specified were more stringent than the codal requirements.
Example of acceptance for liquid penetrant test (LPT) is given below:
Codal requirement NRG’s specification
(a) Rounded indications above 1.6 mm relevant (a) Rounded indications above 0.8 mm
& above 4.8 mm unacceptable unacceptable
(b) Cluster indications could be acceptable (b) Cluster indications unacceptable
as per chart
(c) No definite criteria on indications inside (c) No indication on equipment inside
the equipment wall acceptable
6. Conclusion
It has been possible to address only a few critical issues in this presentation. Nevertheless,
in conclusion it can be said that considering the nature of end use of the equipment for
nuclear recycle plants, material specification including welding filler wire is the first critical
issue. Orientation of fabricator towards specific fabrication requirements and its
implementation throughout is the next issue to be handled. Equally critical is extensive mock-
up trials for establishing fabrication as well as inspection & test procedures. Contamination
control remains the central issue to be addressed throughout the fabrication work.
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7. Typical Layout of Equipment in side a Process Cell
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8. Back side of the penetrating nozzle weld
Hollow bar nozzle to avoid burn through of nozzle
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9. Full vessel inside purging with high purity argon during closure joint
Annular vessel (at final inspection stage)
Regards.
Santosh Takale,
Scientific Officer, BARC
Ph - 0-9967584554.
santoshatbarc@gmail.com
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