Weld Repair of Irradiated Materials · PDF filehelium bubble growth, ... –Auxiliary beam...

Weld Repair of Irradiated Materials

Keith J. Leonard, Zhili Feng, Wei Tang, Roger G. Miller, Brian T. Gibson,

Jian Chen, Scarlett R. Clark and Jeremy T. Busby

Oak Ridge National Laboratory

Greg Frederick, Jonathan Tatman, Artie Peterson and Benjamin Sutton

Electric Power Research Institute

4th International Conference on Nuclear Power Plant Life Management

Session 4-1 (presentation IAEA-CN-246-013)

October 23rd – 27th, 2017

Lyon, France

2 4th – International Conference on Nuclear Power Plant Life Management

Developing advanced weld technologies capable of addressing challenges associated with highly irradiated materials….

Helium generated in reactor internals

throughout the life of the plant, from boron

and nickel transmutations

Tensile stresses generated during the cooling

cycle of the weld exacerbate grain boundary

helium bubble growth, resulting in rupturing.

Diffusion and coalescence of helium occurs

at grain boundaries during welding (i.e.,

temperature in excess of 800°C)

Asano, K., et al., “Weldability of Neutron Irradiated Austenitic

Stainless Steels,” J. Nuclear Materials, Vol 264, 1999.


Technology Gap: Controlling Grain Boundary Helium Bubble Coalescence During Welding

• Key welding factors to control the helium bubble

migration and growth at the grain boundary

during welding:

1. Controlling welding heat input and weld

thermal cycle (i.e., reduce time above 800°C)

2. Controlling the tensile stress profile during

cooling (during maximum helium bubble

growth period)

• Conventional welding processes can not be

controlled to a level that reduces or eliminates

the He-bubble growth to prevent grain boundary

cracking

200

400

600

800

1000

1200

1400

1600

1800

0 2 4 6 8 10 12 14

Time (sec)T

em

pera

ture

(K

)

-100

-50

0

50

100

150

200

250

300

Str

ess (

MP

a)

He b

ub

ble

rad

ius (

nm

)

Temperature

Stress

He bubble radius

Point of

Interest

Source: Z. Feng ORNL Report


Advanced Welding Techniques May Provide Solutions to Repair and Mitigation Concerns

• Recent work performed on high helium

content stainless steel produced by power

metallurgy.

• Friction stir welding (FSW) suppressed voids

and cracks due to its solid state low welding

temperature. Huge voids and cracks with fusion welding

Friction stir welding and cross section


Advanced Welding Process Development

• Overall project objectives:

1. Obtain comprehensive understanding of the metallurgical effects of

welding on irradiated austenitic materials

2. Develop and validate advanced welding processes tailored for repair of

irradiated austenitic materials

3. Provide generic welding specifications and welding thresholds for

irradiated austenitic materials

• Welding processes under development

– Auxiliary beam stress improved (ABSI) laser beam welding

– Solid state friction stir welding/cladding


Auxiliary beam stress improved (ABSI) laser

welding


Proactive management of stresses during laser repair welding

• Integrated Computational Weld Engineering (ICWE) simulation to design and optimize ABSI conditions

• Significant reduction in tensile stresses during the on-cooling portion of the weld cycle when temperatures are in

excess of 850°C (1123 K) – reduces the amount of stress at elevated temperature for He bubble development

• Validated FEA analysis revealed significant reduction surface HAZ transverse tensile stresses with the incorporation

of the auxiliary beam ABSI technique

Bottom Surface

Weld nugget


Friction Stir Welding Process Development

• Initial parameter development performed using force control friction stir

welding, whereas the hot cell will rely on position control:

• Machine deflection was identified as a contributor to surface defect formation

during initial friction stir welding trials inside hot cell on unirradiated materials

• Software updated to incorporate z-axis position control (preprogrammed or

manual)


• Friction stir welding trials conducted with optimized process

parameters and new tool on unirradiated stainless steel

coupons

• Breakdown of the PCBN tooling during FSW of stainless

steels is a known issue

• Defect formation occurs in the form of a “worm hole” on the

advancing side of the rotating tool after 10 weld passes

• Process monitoring involved the examination of the spectral

content of weld forces (torque, traversing force, and side

force) and the utilization of an artificial neural network (ANN)

for identification of the conditions associated with significant

tool wear and the formation of volumetric defects

• With the proper combination of inputs, the ANN yielded a

95.2% identification rate of defined defect states in validation

Friction Stir Welding

Process Development

Rad

iogr

aph

ic N

DE

and

Des

tru

ctiv

e W

eld

Sec

tio

nin

g


Irradiated Materials Welding Facility at ORNL

• Located at the Radiochemical Engineering

Development Center (REDC) - hot cell

facility.

• The primary function of REDC is supporting

isotope production and transuranium element

product recovery, waste handling and

conversion. Therefore, significant adaptations

had to be made for the placement of the

cubicle.

• Weld cubicle and equipment installed in one

of the REDC hot cell bays.


Installation of the cubicle and testing of the systems

Installation of the cubicle QA testing of the various systems

Laser and FSW in cubicle Irradiated test coupon prep.


Facility Changes and Development of Procedures

REDC Facility Changes

• Associated infrastructure changes to accommodate weld cubicle

and support equipment

• Upgrades to cell shield doors and interlocks to function for the

associated radiological hazards of the project, which are new to

REDC – revision of facility radiological materials limits

• Refurbished manipulators and development of new tooling for

remote handling systems

• Installation of cubicle air handling system to maintain cubicle at a

slight negative pressure

• Enhanced laser safety through engineered controls to minimize

risks

• Revise technical safety requirements for REDC

Quality Assurance (QA) Subject Areas • Layered QA requirements established at the DOE, lab, program and

project levels, include:

– Establishing test materials pedigree and maintaining traceability

throughout irradiation, test preparation, conduct, post-weld analysis

and materials storage/archiving.

– Defining and documenting the design of welding equipment

– Fully defining material process, testing, and related work controlling

documents that support planned technical activities.

– Controlling the purchase of quality-impacting items and materials.

– Identifying and maintaining instrument calibration.

– Maintain records set to fully substantiate how work was performed.

– Software QA


Test Coupon Fabrication

• Custom made: 304L, 316L, and 182 alloys

• Targeted boron concentrations of 0, 5, 10, 20 and 30 wppm B.

• Low Co impurity levels.

• Processing:

– Vacuum arc re-melting (VAR) stock material

– Hot extrusion at 1100 ºC

– Homogenized at 1100°C for 5 hours in air

– Hot rolled to 19 mm thick, followed with cold rolling to 12 mm thick.

– Solution heat treatment (1000°C for 30 minutes for 304L and 1050°C for 30

minutes for 316L followed by water quenching)

– Machined to: 76 x 56 x 8.9 mm coupons

• PNNL and ORNL modeling to estimate helium concentrations based on

alloy composition and neutron spectra

• Thermal desorption spectrometry at ORNL to determine level of helium

content of the irradiated materials - current work


Test Coupon Irradiation

• High Flux Isotope Reactor (HFIR) Large-Vertical Experiment Facility

(VXF) positions (VXF-16, VXF-17, VXF-19 and VXF-21):

– 4.3x1014 n/cm2s thermal (E < 0.4 eV)

– 1.2x1013 n/cm2s fast (E > 0.183 MeV)

– 2 cycle irradiation (1 cycle ~ 24.5 days)

• 15 coupons per irradiation capsule, water cooled

• Flux monitors included during irradiation

• First irradiation campaign (304L and 316L) – Complete

• Second irradiation campaign (304L, 316L, and Alloy 182) - Complete

• Third irradiation campaign - Samples being prepared

Large VXF positions

coupons 1-3

4-6

7-9

10-12

13-15

spacers coupon extraction tool

irradiation capsule

coupons spacers


Irradiated Materials Characterization

• Characterization of pre-irradiated, post-irradiated and post-weld materials

• Examinations to take place at ONRL’s Low Activation Materials Development and

Analysis (LAMDA) Laboratory:

– Bulk chemistry

– Solute segregation

– Microstructure

– Mechanical behavior

• Current activities:

– Collaborative effort between EPRI, ORNL, PNNL, and University of Michigan

– Funded through the DOE – Nuclear Science User Facility (NSUF)

– Transmission electron microscopy of irradiated samples to observe He distribution

– Atom probe tomography of irradiated samples to observe radiation-induced segregation

– Thermal desorption spectroscopy (He concentration) of irradiated materials

Reconstructed APT datasets from the neutron irradiated 304L

(10 ppm B) sample showing distribution of Li, B and C along a

high angle grain boundary : courtesy of Emmanuelle Marquis

(U. of Michigan)

Li B C

Concentration profile across a high angle grain boundary : courtesy of

Emmanuelle Marquis (U. of Michigan)


Friction Stir Cladding (FSC)

• Using the friction stir weld subassembly to weld 304 stainless steel

sheet material onto the weld coupons.

• Examples of “cold runs” performed in the welding cubicle are shown

– Good FSC components were produce

– Cladded coupon shows no distortion in the sheet materials

(as shown in the side view)

• Sheet thickness is 0.9 mm for this experiment. 027C

028C

029C


Irradiated Materials Welding – to be started soon!

• “Cold Runs” using non-irradiated materials have been successfully

completed through the welding cubicle:

– Friction stir welding

– Friction stir cladding

– Laser welding with auxiliary beam stress improved technique

• Currently waiting for installation of radiation monitor / interlocks to be

replaced prior to start of testing phase using irradiated materials

• First set of irradiated welding tests to be performed using the laser welding

subsystem on irradiated 304L stainless steel of 5, 10 and 20 wppm B

doped concentrations – expected to start in the next 2 weeks

• Post-irradiated weld evaluations to follow

Example of multiple passes using the laser welding system during “cold” runs


Summary

• A welding cubicle has been constructed for use in the development of weld repair technologies for highly irradiated materials

• Laser welding system utilizes an auxiliary beam stress improvement (ABSI) configuration that has been optimized through

computational modeling and validated through experimental testing to reduce stresses near the weld zone

• An artificial neural network has been used to monitor process variables during friction stir welding to detect conditions

associated with tool wear that lead to the formation of weld defects

• Laser welding tests are about to begin on irradiated stainless steel of 5, 10 and 20 wppm B doped 304L

• Over the next few months - continued development and testing of laser and friction stir welding parameters for irradiated

304L, 316L and alloy 182 of varying He content

• Ongoing work includes the characterization of the unirradiated, irradiated and post-weld materials

• Future work – explore the effects of re-irradiation on weld repaired materials


For more information visit: lwrs.inl.gov

Contact:

Keith Leonard

Materials Aging and Degradation Pathway Lead

Email: [email protected]


Supplementary Slide -1

• Forecast of Helium Generation at 75 wppm Boron at 60 EFPY

(Typical PWR).

• Red Zone: >10 appm He (not weldable with current welding

processes)

• Yellow Zone: 0.1 to 10 appm He (weldable with heat input

control during welding repair)

• Green Zone: <0.1 appm He (No special process control is

needed in welding repair)

(EPRI BWRVIP-97A)

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