Ceramic Coatings for Nuclear Fuel Cladding to

Increase Accident Tolerance

E. Alat, J. Hu (Argonne) , D. Wolfe,

and and A.T. Motta (Penn State)

19th International Symposium on Zirconium in the Nuclear Industry

Manchester, UK, May 2019

Background

• Motivation: the large amount of hydrogen generated during the Fukushima-

Daichii accident during the 2011 Japan earthquake and tsunami motivated a

look at Accident Tolerant Fuels, that is, fuels that are more forgiving in the

case of loss of coolant accident and give an improved time of response

before hydrogen generation and fuel failures start to occur.

2

Background

• Among the several concepts (SiC, FeCrAl), coatings for Zr

alloys have been proposed. In collaboration with

Westinghouse, Penn State created a concept based on physical

vapor deposition of TiN/TiAlN multilayer coatings on ZIRLO.

3

Outline

• Approach: Physical Vapor Deposition of TiN/TiAlN multilayer

coatings on ZIRLO

• Potential show-stoppers; what can derail the concept?

• Neutronics; what is the penalty

• Optimization of the Coating

– Deposition Parameters

– Architecture

• Corrosion Testing and Characterization

• Conclusions/Perspectives

4

Large Collaboration

• M.Fratoni, I. Younker (UC Berkeley) => neutronics

• J. Partezana and R. Comstock (Westinghouse) => corrosion testing

• Manchester: M.Preuss group => interfacial shear strength

• K. Sickafus (UTK) => project director

Some publications• E. Alat, A.T. Motta, R.J. Comstock, J.M. Partezana, and D.E. Wolfe, "Ceramic Coating for Corrosion (C3) Resistance of Nuclear

Fuel Cladding," Surface & Coatings Technology, 281, (2015) 133-143.

• E. Alat, A.T. Motta, R.J. Comstock, J.M. Partezana, and D.E. Wolfe "Multilayer (TiN, TiAlN) ceramic coatings for nuclear fuel cladding," Journal of Nuclear Materials, 478, (2016) 236-244.

• Michael J. Brova, Ece Alat, Mark A. Pauley, Rachel Sherbondy, Arthur T. Motta, Douglas E. Wolfe "Undoped and ytterbium-doped titanium aluminum nitride coatings for improved oxidation behavior of nuclear fuel cladding", Surface & Coatings Technology 331 (2017) 163-171.

• Y. Liu, I. Bhamji, P.J. Withers, D.E. Wolfe, A.T. Motta, and M. Preuss, "Evaluation of the interfacial shear strength and residual stress of TiAlN coating on ZIRLOTM fuel cladding using a modified shear-lag model approach," Journal of Nuclear Materials (2015), http://dx.doi.org/10.1016/j.jnucmat.2015.06.003

• In situ ion irradiation and annealing experiments under TEM on multilayer (TiN, TiAlN) accident tolerant fuel (ATF) coatings on Zr cladding alloys, Jing Hu1, Jie Wang1, Meimei Li1, Mark Kirk1, Ece Alat2, Douglas Wolfe2,3, Arthur Motta, in preparation

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Goals of the project

Develop a fuel concept based on an advanced ceramic coating for Zr-alloy cladding. The coated cladding must exhibit demonstrably improved performance compared to conventional Zr-alloy clad. Potential impact:

1. During normal service, the ceramic coating should decrease cladding oxidation and hydrogen pickup to reduce cladding embrittlement.

2. During a reactor transient (e.g., a loss of coolant accident), the ceramic coating must minimize or at least significantly delay oxidation of the Zr-alloy cladding, thus reducing the amount of hydrogen generated and the oxygen ingress into the cladding.

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Shannon Bragg-Sitton, Nuclear News •

March 2014

Penn State Multilayer PVD Coatings

ZIRLO

TiAlNTiN

TiAlN

TiAlN

TiAlN

TiN

TiN

TiN

Ti bond

coating

~10 mm

• Physical vapor deposition parameters of

multilayer coatings studied to create the optimal

architecture for corrosion resistance

• Corrosion tested in autoclave at Westinghouse at

360 C, very little weight gain, spallation or

delamination for optimum architecture

• Tested for thermal stability and scratch test

• Limited tests at high temperature

• Corrosion resistance

• Low neutronic penalty

• Easy to fabricate

• Mechanical properties

• Adhesion to substrate

• Scalability to production size

8

Coating Deposition Method

Cathodic Arc Physical Vapor

Deposition

Advantages• Target/cathode geometry:

Flexible• Quantity of ionized target

atoms: 50-100%• Higher kinetic energy• Indistinct

coating/substrate interface

• Tailorability of the interfacial products

• Scalability to production size.

Cathodic Arc Physical Vapor Deposition

Process• A voltage is applied to the cathode

while a grounded trigger contacts

the cathode to initiate the arc.

• Magnets are used to constrain the

arc movement to the target face.

• Cathode spots accumulation creates

a dense plasma which is ionized

with energetic ions that promote the

adhesion and dense coating

formation on the substrate.

• Plasma generation is proportional to

the supplied current by the power

supply.

• TiAl and Ti targets in N atmosphere

9

Ref: http://www.ing.unitn.it/~colombo/RIVESTIMENTI_DA_FASE_VAPORE

/Le%20applicazioni%20delle%20tecnologie%20PVD.htm

Cathodic Arc Physical Vapor Deposition

Process

• Target TiAl alloy ratio: 33 at% Ti to 67

at% Al.

• Nominal substrate temperature: 325oC

• Cathode vaporization stage:

– 1.2 x 10-2 mTorr Ar atmosphere

– -1000 V bias

• Deposition stage:

– 12 m Torr N2 or N2 and Ar atmosphere

– -50 V substrate bias

– 65 A evaporator current for Ti cathode

– 60 A evaporator current for TiAl

cathode

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Possible Show Stoppers for the Concept

A. Adhesion and Wear.

B. Permeation/Corrosion.

C. Localized Attack.

D. Dimensional Stability and Strain Effects.

E. Mechanical Resistance (fretting, etc.)

F. Accident Tolerance.

G. Radiation Damage

In addition there are significant development and production

issues to turn the process into large scale cladding fabrication.

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✓

✓

✓

✓

✓

0. Neutronics

Determine the impact of the coatings on reactor core operation

▪ Parameters

▪ Coating thickness

▪ Coating composition

▪ Metrics

▪ Reactivity penalty

▪ Cycle length

▪ Fuel enrichment

▪ Case Study

▪ AP-1000

AP-1000 assembly model used for

assessing the impact of coatings

Neutronic Analysis, Max Fratoni and Ian Younker

Neutronic evaluation of coating and cladding materials for accident, tolerant fuels, I. Younker and M. Fratoni, Progress in Nuclear Energy 88 (2016) 10-18

A small increase in the enrichment compensates for

the reactivity loss

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Ti3AlC2 Ti2AlC Nb2AlC TiAlN ZrC Zircaloy-4

Enri

chm

ent

Incr

emen

t (a

t.%

)

10 μm

50 μm

100 μm

Comparison of enrichment increment (at.%) necessaryfor PWR to match reference cycle length as a function

of coating composition and thickness

Concept: Use Ternary Nitrides• Ternary nitrides composed of combinations of two binary nitrides can produce coatings with

properties which exceed that of the individual binary coatings ( i.e. solid solution hardening)

• These two phases coexist in Ti1-xAlxN for 0.7>x>0.6 nano composite

• Selected TiAlN due to improved oxidation properties

Ti(Al)N: NaCl structure

Basic cubic structure

Al(Ti)N: Wurtzite Structure

Similar to hexagonal diamond

Yang, 2004; Hedenqvist, 1997

• Possibilities for Ternary Systems

– Multiphase nano-composite coatings; TiN+CrN or TiN+AlN

– Single phase coatings such as (TixCr1-xN) and TixAl1-xN

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Ti (Al)N

Al (Ti)

NGreen: Alternating Ti and Al atoms depending on composition

Blue: Nitrogen atoms

Autoclave Testing at Westinghouse

• Gen 1 coating

– 3 days in 360ºC, static purewater

• Gen 2 coating

– 3 days in 360ºC , static purewater

• Gen 3 coating

– 3 days in 360ºC , static purewater

• Gen 4 coating

– up to 67.8 days in 360ºC , static pure water

• Gen 5 coating

– Up to 88.8 days in 360ºC , static pure water

• Gen 6 coating

– 30 min in 800oC

PF61

Before coating

After TiAlN coating

17After autoclave testing (test T871 680⁰F)

PF65 PF65 PF71 PF75

Front Back Front Back Front Back Front Front

Digital Images of Generation 2 before and after Autoclave Testing at 360 C for 3 days

Applied Research Laboratory

MATERIALS PROCESSING DIVISION

Coating Deposition Parameters and Coating Architecture

layer

BC

Ra

TiN

TiN

TiAlN

TiAlN

Ti

ZIRLO substrate

Parameter Effect

Substrate bias Film microstructure, coating

composition, residual stress,

density

Nitrogen partial

pressure

Coating composition,

crystallography, hardness,

toughness, wear performance,

adhesion degree

Substrate surface

toughness, Ra

Coating adhesion

Surface preparation Coating adhesion

Bond coating

thickness, δBC

Coating adhesion

Layer chemical

composition

Crystallography, hardness,

toughness, adhesion, corrosion

resistance

Layer thickness,

δlayer

Residual stresses, corrosion

resistance

Number of layers Corrosion resistance

Multilayer Samples• Nitrogen Pressure

• Substrate Bias

• Ion pre-clean

• Heat treatment

• Flux to ion ratio

• Unalloyed TiAlN (TiN)

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Very even deposition and little evidence of

oxygen permeation and oxide formation

Corrosion testing

shows very little

weight gain

compared to

uncoated ZIRLO

(red), on the order of

1/6 of corrosion.

Samples which

showed weight loss

were not kept

Applied Research Laboratory

MATERIALS PROCESSING DIVISION

EDS cross section line spectrum after autoclave test at 360°C and saturation pressure of 18.7 MPa for 33 days, (a) oxygen profile through coating

starting from the outer layer towards the substrate and (b) oxygen, titanium, aluminum and nitrogen profile through cross section of E25-(TiN/TiAlN 8-

layer)/Ti/ZIRLO® sample.

• No significant oxidation

• No aluminum diffusion/depletion

• TiN layer enhanced coating performance by reducing/eliminating Al diffusion

through the coating.

• Oxygen penetrated only to a distance of ~1.5 μm, corresponding only to the

outermost TiN layer.

• A very thin (~1.5 μm) outer TiN layer should be sufficient to protect the

underlying TiAlN coatings.

21

Optimal Coating Design Approach

22

Deposition parameters

Titanium bond coating thickness

Substrate surface roughness

Multilayer design

ZIRLO®

Ti-BC= 0.6 μm

TiAlN

TiN

Roughness: ~0.25 µm Ra

~10 µm

Substrate bias: -50VNitrogen partial pressure: 1.6 Pa

Alat et al., J. Nucl. Materials 478 (2016) 236-244.

3

Intermediate Voltage Electron Microscope (IVEM)

Ion Acceleator(on 2nd floor)

Specimen holder for in situ ion irradiation

Electro

nb

eam

In situ studies of nuclear materials and fuels under irradiation• Use Ion beam to create irradiation damage in the sample• Use electron beam to observe damage in real-time

IVEM – Tandem Facility

24

Irradiation test • IVEM (Intermediate Voltage Electron Microscopy) facility at Argonne National

Laboratory – Hitachi 9000 microscope operating at 200 keV

• TEM thin foils were irradiated with 1 MeV Kr ions at a dose rate of 6x1011

ions/cm2/sec at 300°C.

• The fluence was converted to dpa using a Kinchin Pease calculation on the SRIM-2013code with 100 nm foil thickness, 1MeV Kr++ ions and displacement energy of 30 eVfor Ti, 25 eV for Al and 28 eV for N.

• The final dpa level on TiAlN/TiN layers was around 20 dpa and ~45 dpa on thesubstrate.

• Essentially all the Kr ions penetrated through the thin foil. After irradiation, thesamples were annealed in situ to 800°C for 30 min.

• Detailed TEM bright field (BF), dark field (DF) imaging and diffraction analysis wassystematically carried during the irradiation.

Applied Research Laboratory

MATERIALS PROCESSING DIVISION

25

(a-1) (a-2)

20 dpa 300 ⁰ C 20 dpa 300 ⁰C + anneal 800 ⁰C

(a-4)Pt

Ti20Al80N

TiN

Ti20Al80N

Ti

Ti20Al80N

TiN

Ti20Al80N

TiN

TiN

Zr

Pt

Ti33Al67N

TiN

Ti

TiN

TiN

TiN

Zr

Ti33Al67N

Ti33Al67N

Ti33Al67N

Ti20Al80N

TiN

Ti20Al80N

TiN

Ti33Al67N

TiN

TiN

Ti33Al67N

(a-3)

(b-1) (b-2) (b-4)(b-3)

Before irradiation

microcracks

TEM-Bright field (BF) images of sample A (Ti20Al80N + TiN) and

sample B (Ti33Al67N + TiN) : (a-1)(b-1) overview of coatings

before irradiation, (a-2) (b-2) higher mag images showing TiN

and TiAlN layers before irradiation, (a-3)(b-3) after 20 dpa/300°C ion irradiation, (a-4)(b-4) after 20 dpa/300 oC ion

irradiation + anneal at 800 oC.

Post-Irradiation

• Good irradiation resistance of the

coatings

• The coatings retained their integrity

after irradiation to a dose of 20 dpa

at 300°C.

• No irradiation defects above 1 nm

and no obvious changes in the

existing columnar grain structure and

size.

• In particular, micro cracks present in

the Ti20Al80N layer in sample A before

irradiation (arrowed in the figure) did

not grow under irradiation.

• No evidence of layer intermixing was

observed after ion irradiation.

Post-Anneal caused no change

Irradiation TestsIrradiated with 1 MeV Kr ions to 20 dpa at 300 C in

cross section

Applied Research Laboratory

MATERIALS PROCESSING DIVISION

27

TEM-BF images of coating/Zr matrix interface microstructure of sample A

(Ti20Al80N + TiN) and sample B (Ti33Al67N + TiN): (a-1)(b-1) before irradiation

overview, (a-2) (b-2) after 20 dpa ion irradiation and 800C anneal. The thickness of

the Ti layer is approximately 350 nm, the columnar grain size is around 30-50 nm

wide and 300 nm long.

In the Ti20Al80N layer sample, a 20 nm thick void layer developed at the interface of

the Ti bond coat layer and the Ti20Al80N layer near the metal, which could lead to a

potential failure by coating delamination. No change in the sample B (Ti33Al67N +

TiN):

The metal matrix is damaged by irradiation

TEM BF images of loop growth vs ion irradiation dose in the ZIRLO®

(Zr-1.0Nb-1.0Sn-0.1Fe) substrate (1 MeV Kr++ 300 °C); <a> loop, g=

(01-10). (a) 0 dpa; (b) 0.1 dpa; (c) 0.2 dpa; (c) 1 dpa; (d) 2 dpa.

• Fabricated 10 micron thick TiN/TiAlN coatings on ZIRLO for accident tolerance using physical vapor deposition. Very small neutronic effect

• Coating architecture was optimized in detail (bond coating thickness, surface roughness, number of multilayers, composition, phases, deposition parameters).

• Corrosion tested at 360 C and higher temperatures showing little weight gain and little degradation of the coating (also thermal testing, scratch testing).

• Coated samples were corrosion tested in autoclave at 360°C to 89-128 days and showed essentially no oxygen ingress and no spallation or delamination. The Ti33Al67N showed better corrosion performance in autoclave than Ti20Al80N .

29

Summary and Conclusions

• The coating performance depended strongly on deposition parameters. Previously identified optimized parameters for coating adhesion and corrosion performance were:

– Ti33Al66

– substrate surface roughness (0.25 μm Ra),

– titanium bond coating thickness (0.6 μm),

– substrate bias (-50 V),

– nitrogen partial pressure (1.6 Pa) and

– multilayer design (8-layer) with an overall layer thickness of about 10 microns.

• In situ ion irradiation of both samples on the coatings showed good irradiation resistance of the coatings. After irradiation to 20 dpa followed by anneal at 800°C, no irradiation defects above 1 nm during irradiation were observed and no obvious change in existing coating columnar grain structure and size were detected in the 33-67 sample. 30

Summary and Conclusions

• Fretting

• Corrosion test in Nuclear reactor chemistry

• Temperature gradient

• Growth and creep

31

Recommendations for Future Work

END

33

33 at.%Ti-67 at.%Al provides better adhesion

Scratch direction0 N ~50 N

Alat et al., J. Nucl. Materials 518 (2019) 419-430.

Scratch Tests

4

TEM Specimen Holder Vacuum Storage Station

• LaB6 filament provides superior electron brightness,ideal for imaging irradiation defects using diffractioncontrast

• Image defects near the atomic resolution (pointresolution of 0.25 nm at 300 keV)

• Accelerating voltage of 100 – 300 keV allowsobservation of thicker samples

• Minimum degradation of image resolution from theinsertion of the ion beamline into the microscope

• Large pole piece gap allows accommodation of a widerange of specimen holders

‒ Heating stage up t o 1000C‒ Helium cooling stage to 20K‒ High and low temperature straining stage‒ Triple axis stage for tomography

• Digital image recording and video recording with 15frames/sec

• Vacuum of 10-8 Torr (free of contamination andoxidation at high temperature)

Hitachi H-9000NAR Microscope

5

• NEC implanter and 911Danfysik ion source

• Available ions include H, inertgases, and many elements fromAl to Au

• Energies 50 – 500 keV singly charged and up to 1 MeVdoubly charged.

Pete Baldo tunes the electrostatic deflector to direct the ion beam down into the electron microscope on the 1st

floor.

• Real time dosimetry by a Faraday cup installed in the column at 4 cm from the sample

• Beam size of 1.5 mm dia. on the sample center with a uniformbeam

• Ion flux: 1010 – 1012 ions/cm2/s(corresponding to 10-5 – 10-3

dpa/s)• Can achieve a dose of 100 dpa in a

day.

Ion accelerator

36

University of Tennessee(PI Kurt Sickafus)

University of Sheffield

Pennsylvania State University University of Oxford

University of Colorado University of Manchester

University of Michigan University of Huddersfield

Los Alamos National Laboratory Australian Nuclear Science and Technology Organization

Westinghouse Electrical Company

Participating Institutions

Penalty on burnup and cycle length increases

linearly with thickness

37

Coating 10 μm 50 μm 100 μm

Ti3AlC2 -3.7 -19.0 -39.8

Ti2AlC -3.7 -17.4 -35.0

Nb2AlC -2.8 -14.5 -28.0

TiAlN -3.4 -16.9 -35.3

ZrC -0.7 -4.0 -8.0

Zircaloy-4 -1.2 -3.9 -8.8

Comparison of cycle length penalty (EFPD) for PWR

as a function of coating composition and thickness

Flat sample mounting

• Samples are mounted into the sample holders after cleaning

• The sample holders clamp along the bottom edge and have straight vertical

bars that are inline with the sample edges with an approximate 1/8” gap

• It should be noted that no coating is deposited where the samples are held

38

Sample

holderUncoated

Participants (US side)

• University of Tennessee: Kurt Sickafus, Maulik Patel, Brian Wirth

• Penn State: Doug Wolfe, Ece Alat

• Westinghouse: Bob Comstock

• University of Michigan: Gary Was

• University of Colorado: Rishi Raj

• University of California Berkeley: Max Fratoni

• LANL: Kendal Hollis

39