Aravinda Pai, T.K.Mitra, T. Loganathan and Prabhat Kumar

Prototype Fast Breeder Reactor Project

Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI)

Department of Atomic Energy

Kalpakkam, India

DESIGN AND FABRICATION OF

SERPENTINE TUBE TYPE SODIUM

TO AIR HEAT EXCHANGERS FOR

PFBR SGDHR CIRCUITS

500MWe Prototype Fast Breeder Reactor (PFBR) flow chart

In PFBR, two diverse and independent Decay

Heat Removal (DHR) systems are provided

Operational Grade Decay Heat Removal (OGDHR) System

Safety Grade Decay Heat Removal (SGDHR) System

SAFETY GRADE DECAY HEAT REMOVAL SYSTEM (SGDHR)

There are 4 SGDHR loops

Each loop consists of 8MWt heat removal capacity

Diversity in design to avoid common cause failure

2 loops - Type-A design

2 loops - Type-B design

Sodium to Air Heat Exchanger (AHX) – Type A

Intermediate sodium inlet/outlet:

Temp. during poised state : 5400C/ 5400C

Nominal temp. during SGDHR : 4940C/3030C

Air inlet and outlet:

Temp. during poised state : 400C/ 5370C

Nominal temp. during SGDHR : 400C/2910C

Sodium to Air Heat Exchanger (AHX) is serpentine

tube design heat exchanger.

AHX transfers heat from the intermediate circuit

sodium to atmospheric air in Safety Grade Decay

Heat Removal (SGDHR) loop

One of the most critical component

The principal material of construction is modified

9Cr-1Mo steel.

CRITICAL AREAS IN FABRICATION

Hot forming of pullouts on headers

Header welding and fabrication

Tube to tube welding

Header pullout to tube welding

Post Weld Heat Treatment

Testing and surface treatment

There are 116 nos. of formed pullouts

exists on each header.

Each pullout has outside diameter of

38.1mm and wall thickness of 2.6 mm

with a height of 25mm.

Hot forming of pullouts in conventional

method is not possible due to

metallurgical and practical limitations.

Hot forming of pullout is carried out at

900-11000C by heating the local area of

the header shell segment using induction

heating process followed by die & punch

pressing.

The headers after pullout forming

undergoes normalizing at 1050-10900C

followed by tempering at 780±100C to

restore the material properties.

Hot forming of pullouts on headers

Conventional

nozzle concept Pullout concept

√ Χ

HEADER WELDING AND FABRICATION

After pullout forming, the

longitudinal & circumferential seam

welding of header shell segments

is carried out.

All modified 9Cr-1Mo joints are

preheated & interpass temperature

is maintained at 200-2500C.

After welding, post heating is

carried out at 2000C for 2 hours

and PWHT carried out at

760±10oC.

Even though Shielded Metal Arc

Welding (SMAW) process is

permitted as per PFBR

specification, 100% GTAW process

alone is executed to meet impact

properties.

The welding procedure is qualified with stringent destructive and non-destructive

examinations & testing before welding on the actual job.

During qualification, weld joints are subjected to

Thorough visual examination

Liquid Penetrant Examination (LPE)

Radiography Examination (RE)

Longitudinal tensile test at ambient temperature

Bend tests

Charpy V notch impact test

Hardness survey

Metallographic examination at 200X magnification for the complete transverse

section of the weld

Each production weld joint undergoes thorough visual examination, LPE and

Radiography Examination.

If RE is not possible, soundness of weld is evaluated by ultrasonic examination (UE)

After completion of PWHT, thorough visual examination, LPE and RE/UE is repeated

on the weld joints.

CHALLENGES IN HEADER

WELDING & FABRICATION

The welding and fabrication of

12mm thick slender header shell

having outer diameter of 457mm is

difficult and challenging task.

Due to small inside diameter, the

internal fixtures/spiders cannot be

used at many locations for distortion

control during welding.

Due to many formed pullouts on the

outer surface, fixtures/spiders

cannot be placed directly on the

headers during welding for

distortion control.

Even though distortion tendency is

less due to smaller diameter and

existence of formed pullouts, utmost

care is inevitable during welding to

avoid distortion and dimensional

deviations.

CHALLENGES IN HEADER

WELDING & FABRICATION

As no sufficient access from inside of

header shells, the welding has to be

carried out only from the outside.

Therefore single V type Weld Edge

Preparation (WEP) exists for welding

from outside.

In case of distortion, re-rolling after

welding for shape correction is not

permitted as per PFBR specification.

Many welding trials were conducted

to understand the behavior of shells.

Tremendous efforts were put to

achieve less than 1% ovality.

TUBE BUNDLE FABRICATION

There are 116 nos. of Modified

9Cr-1Mo tubes of OD 38.1mm and

2.6mm wall thickness.

Tube bundle fabrication mainly

consists of

Header pullout to tube welding

Manual GTAW process

Tube to tube welding

Automatic pulsed GTAW

process.

Tube bundle activities are carried out

in separate nuclear clean hall

conditions as per class-1 component

requirements of PFBR to ensure the

quality.

CHALLENGES IN HEADER PULLOUT TO TUBE WELDING

SERPENTINE ‘M’ TUBE WELDING AND FABRICATION

PREHEATING TUBE TO TUBE WELDING HELIUM LEAK TEST FOR M TUBE

POST WELD HEAT TREATMENT (PWHT)

AND CHALLENGES:

Due to complex constructional features, the heat

treatment is not straight forward.

The PWHT of individual tube to tube weld joint is

carried out by electrical resistance method using

metallic split cartridge.

Enormous nos. of trials were conducted to establish

the procedure for local PWHT of header pullout to tube

weld joints using metallic split cartridge.

The temperature control was extremely difficult due to

asymmetric shape and non-uniform mass of pullouts.

Hence, it was decided to carry out PWHT of header

pullout to tube weld joints along with PWHT of 12mm

thick header weld joints.

.

After fabrication of complete cylindrical

header, PWHT is carried out at 760±10oC

for 2 hours for 12mm thick longitudinal &

circumferential weld joints.

Then, welding of middle row header pullout

to tube joint is carried out and complete

header assembly is again heat treated at

760±10oC for 1 hour soaking time.

Subsequently, inner and outer row header

pullout to tube welding is carried out and

complete header is again subjected to heat

treatment for another 1 hour soaking time.

12mm thick weld joints undergoes heat

treatment for total 4 hours.

Middle row header pullout to tube weld

joint undergoes heat treatment for total 2

hours soaking time.

Outer & inner row header pullout to tube

weld joint undergoes heat treatment for 1

hours soaking time.

.

Final integration of M tubes

with header bend tubes

Testing & Surface Treatment

The tube side is subjected to pneumatic test at 8 bars gauge pressure.

Neither drop in pressure nor leakage/deformation is acceptable

After pneumatic test, surface treatment is carried out which involves degreasing, pickling, passivation by circulation of solutions.

Subsequently, tube side is subjected to Helium leak test

Global leak rate shall not be more than 6.66X10-9 Pa-m3/s

Local leak rate shall not be more than 2.66X10-9 Pa-m3/s

CONCLUSION:

The design, manufacture and construction of components

should employ proven techniques and it should be possible to

conduct analysis of the design as may be necessary for the

purpose of demonstrating adequate integrity at any specified

time throughout the plant life.

The important fabrication rules are use of high standard of

materials, use of high quality welding during all the stages of

manufacture supported by a quality assurance program which

ensures full approval of procedures and provides verification

of compliance with the procedures & practices.

Very high standard quality control and quality assurance

during design, material procurement, forming, welding,

fabrication, handling and testing has given confidence on

trouble free service from Sodium to Air Heat Exchangers for

the design service life of 40 years.

Thank you