Prof. K - Development of Al Added High-Cr ODS Steels for Fuel Cladding of Next Generation Nuclear...

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Development of Al added high-Cr ODS steels for fuel cladding of next

generation nuclear systems

A. Kimura a,, R. Kasada a, N. Iwata a, H. Kishimoto a, C.H. Zhang a, J. Isselin a, P. Dou a, J.H. Lee a,N. Muthukumar a, T. Okuda b, M. Inoue c, S. Ukai d, S. Ohnuki d, T. Fujisawa e, T.F. Abe f

a Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb Kobelco Research Institute, Inc., Takatsukadai 1-5-5, Nishi-ku, Kobe 651-2271, JapancAdvanced Nuclear System R&D Directorate, Japan Atomic Energy Agency, Narita 4002, Oarai, Higashi-ibaraki, Ibaraki 311-1393, Japand Graduate School of Engineering, Hokkaido University, N13, W8, Kita-ku, Sapporo 060-8628, Japane EcoTopia Science Institute, Nagoya University, Furo, Chikusa-ku, Nagoya 464-8603, Japan

fStructural Metals Center, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan

a r t i c l e i n f o

Article history:

Available online 1 January 2011

a b s t r a c t

A successful example of high-Cr oxide dispersion strengthened (ODS) steels development is introduced

with showing key technologies to overcome the issues to meet material requirements for next generation

nuclear systems as well as fusion blanket systems. Corrosion issue requires Cr concentration more than

14 wt.%, but aging embrittlement issue requires it less than 16 wt.%. An addition of 4 wt.%Al is effective to

improve corrosion resistance of 16 wt.%Cr-ODS steel in supercritical water (SCW) and leadbismuth

eutectics (LBE), while it is detrimental to high-temperature strength. An addition of small amount of

Zr or Hf results in a significant increase in creep strength at 973 K in Al-added ODS steels. Feasibility

of high-Cr ODSsteel without Al addition is assessed for fusion application in terms of corrosion resistance

in SCW.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Cladding materials development is a key issue to achieve high

burn-up operation of Generation IV nuclear energy systems such

as supercritical pressurized water reactor (SCPWR), sodium-cooled

fast reactor (SFR), and lead bismuth-cooled fast reactor (LFR) and

so on. The candidate cladding materials must have a high resis-

tance to neutron irradiation embrittlement and void swelling as

well as a good performance of mechanical properties at elevated

temperatures. In addition, a good corrosion resistance of the clad-

dings in the relevant environments has been certainly required for

practical long term operation of the advanced fission reactors [1,2].

Fusion blanket materials have been also required of high perfor-mances similar to the Generation IV nuclear systems, although

there are characteristic issues for the fusion application, such as

transmutation helium/hydrogen effects and material design

towards low activation [3]. Oxide dispersion strengthened (ODS)

ferritic/martensitic steels containing 912 wt.% chromium have

been developed as the fuel cladding material of SFR because of

their high creep strength at elevated temperatures and enough

resistance to neutron irradiation embrittlement [4]. However, the

(912)Cr-ODS ferritic/martensitic steels are not suitable for SCWR

owing to an insufficient corrosion resistance of the materials [57].

Corrosion resistance of iron based alloys is influenced by chro-

mium (Cr) and aluminum(Al). It is expected that an adequate com-

bination of the contents will be available for Cr and Al in the steels

for each blanket system. The Cr content can be balanced between a

merit of corrosion resistance and a demerit of aging embrittlement

with maintaining strength at elevated temperatures. The technol-

ogy development for strengthening of the Al-added steel has been

desired by controlling nano-scaled oxide dispersion morphology.

Furthermore, low activation material scenario limits the use of Al

as an alloy element of fusion materials. The feasibility check is also

demanded for Al free 16Cr-ODS steels as a candidate material for

fusion blanket structural material.In this work, alloy design methodology of high-Cr ODS steels is

shown to improve corrosion resistance with maintaining high

strength at elevated temperatures. The effects of Al addition on

the high-Cr ODS steels are discussed to assess the feasibility of

the Al free ODS steel to apply it to fusion blanket.

2. Alloy design

2.1. Chromium and aluminum

2.1.1. Corrosion properties

Ukai et al. developed 9Cr-ODS martensitic steel as a cladding

material for SFR. The application of 9Cr-ODS steel to the cladding

0022-3115/$ - see front matter 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jnucmat.2010.12.300

Corresponding author. Tel.: +81 774 38 3476; fax: +81 774 38 3479.

E-mail address: [email protected] (A. Kimura).

Journal of Nuclear Materials 417 (2011) 176179

Contents lists available at ScienceDirect

Journal of Nuclear Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j n u c m a t
http://dx.doi.org/10.1016/j.jnucmat.2010.12.300mailto:[email protected]://dx.doi.org/10.1016/j.jnucmat.2010.12.300http://www.sciencedirect.com/science/journal/00223115http://www.elsevier.com/locate/jnucmathttp://www.elsevier.com/locate/jnucmathttp://www.sciencedirect.com/science/journal/00223115http://dx.doi.org/10.1016/j.jnucmat.2010.12.300mailto:[email protected]://dx.doi.org/10.1016/j.jnucmat.2010.12.300


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of SCWR and LFR is limited because of insufficient corrosion resis-

tance of the steel [1,2]. Corrosion test results in supercritical pres-

surized water (SCW) and leadbismuth eutectics (LBE) were

referred to determine Cr and Al content. LBE has high solubility

of nickel, iron and chromium, which are most important alloy ele-

ments in austenitic stainless steels, which is the reason of the dif-

ficulty to use nickel super alloys and iron-based austenitic stainless

steels as the structural materials for LBE cooled systems, especially

at temperatures higher than 773 K [812]. Ferritic steels that con-

tain only a small amount of nickel have been considered to be more

adequate for LBE application. Above 873 K, however, the solubility

of iron and chromium in LBE also becomes significantly larger. In

order to prevent material from dissolving into LBE, alumina coating

was often conceived. Without alumina coating, however, it is also

considered that alumina film, which is spontaneously formed on

the surface of the steel containing Al in solution, may suppress dis-

solution. Fig. 1 shows the appearance of ODS steel specimens after

corrosion test in LBE with 106 wt.%O2 in solution for 10k h at

923 K. The Al free 19 wt.%Cr-ODS steel dissolved into LBE mark-

edly, while those of ODS specimens containing 4 wt.%Al almost

completely kept the specimen shape, indicating much higher resis-

tance to LBE corrosion. It is worth to noting that the corrosion

resistance in LBE is independent of Cr concentration from 13 to

19 wt.% in Al-added ODS steels. The addition of Al is considered

to be inevitable for structural materials to apply to LBE nuclear sys-

tems especially above 773 K.

As for the corrosion resistance in SCPW, the previous surveil-

lance test results showed that nickel super alloys were preferable

[13]. However, nickel alloys are rather susceptible to irradiation

embrittlement as well as helium embrittlement [14]. Fig. 2 shows

the weight gain after corrosion tests in SCPW(773 K, 25 MPa) up to

1800 h. The weight gain of 9Cr-ODS steel is much larger than

16Cr-ODS steel, indicating that 9Cr-ODS steel is not adequate for

application to SCWR. An EPMA and XRD analysis revealed that a

thick Fe3O4 film was formed on the surface of 9Cr-ODS steel but

not detected in the high-Cr ODS steels. It should be noted that the

weight gain is much largerin SUS430 (16Cr) than in 16Cr-ODS steel.This clearly indicates that the oxide particles dispersion plays an

effective role in the high corrosion resistance of ODS steels, since

the corrosion resistance in water environment is mainly controlled

by Cr concentration. The corrosion resistance of ODS steels in-

creases with chromiumconcentration. The effect of Al on corrosion

resistance depends on Cr concentration. In 19Cr-ODS steel, the

addition of 4 wt.%Al did not remarkably influence the corrosion

resistance [2]. However, in 16Cr-ODS steel, the addition of Al im-

proved corrosion resistance. The suppression of corrosion by Al

addition in 16Cr-ODS steel is due to formation of very thin alumina

film on the surface. In 19Cr-ODS steel, rather dense chromia film

was observed on the specimen surface [5]. It is considered that

16 wt.%Cr is not large enough to form homogeneous and stable

chromia on the whole specimen surface, while very thin aluminafilm covers the wholespecimen surface of the ODS steel added with

4 wt.%Al. Thus, an addition of Al is effective to improve corrosion

resistance of 16Cr-ODS steel but not in 19Cr-ODS steel.

2.1.2. A trade-off issue: thermal aging embrittlement

Although an increase in Cr concentration results in an improve-

ment of corrosion properties, it also often increases susceptibility

to aging embrittlement through the formation of Cr-rich secondary

phases. A trade-off between corrosion resistance and aging embrit-

tlement caused by increasing Cr content is one of the critical issues

for high-Cr ODS steels [15].

Aging effects were investigated for ODS steels with different Cr

content by measuring impact fracture energy at RT after aging at

773 K up to 10k h [16]. The Charpy V-notch specimen measures1.5 mmsquare rod with 20 mm length. As shown in Fig. 3, the frac-

ture energy at RT decreases with increasing Cr content before

aging. Aging causes the reduction of the fracture energy. The ODS

steels with Cr content higher than 18 wt.% showed a significant

reduction of fracture energy after the aging for 100 h. In contrast

to this, 16Cr4Al ODS steel showed very small reduction of the

fracture energy even after aging for 10k h. Microstructure observa-

tion by TEM revealed that fine secondary phases were formed in

high density after aging for 1k h at 773 K even in 16Cr ODS steels.

The secondary phases are considered to be Cr-rich phases. The ef-

fects of aging on tensile behavior are interesting. After the aging at

773 K for 1k h, Cr-rich secondary phases were formed in 16Cr-ODS

steels, and caused a small reduction of the fracture energy at RT as

shown in Fig. 3. However, the reduction of tensile elongation wasalmost negligible, but some hardening was observed at RT. This

Fig. 1. The appearance of ODS steel specimens after corrosion test in LBE solvedwith 106 wt.%O2 for 110

4 h at 923K.

Fig. 2. Weight gain after corrosion tests in SCPW (783 K, 25 MPa) up to 1800 h.

A. Kimura et al. / Journal of Nuclear Materials 417 (2011) 176179 177


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behavior was very similar to that of ODS steels after neutron irra-

diation that caused a hardening without a significant reduction oftotal elongation at RT.

Since the thickness of fuel cladding is around 0.5 mm, the duc-

tile-to-brittle transition temperature (DBTT) shift by aging is con-

sidered to be much smaller than those obtained for thicker

specimen (1.5 mm thickness) for evaluation of the DBTT of the

ODS steels in this work. Fracture toughness high enough to cold-

work piping is required for the fabrication of cladding. Finally, in

order to reduce the susceptibility to aging embrittlement, it is rec-

ommended that the Cr content is lower than 16 wt.%.

2.2. Tungsten and titanium

The addition of W or Ti influences on high-temperature

strength. According to the R&D of reduced activation ferritic steelsfor fusion blanket application, the addition of W to 9Cr martensitic

steel is effective to increase creep rupture time at 873 K [17].

Although W is ferrite former, martensite is still stable when W con-

tent is less than 2 wt.% [17].

Previous works on chemical compositions of oxide particles in

high Cr-ODS ferritic steel and 9Cr-ODS martensitic steel showed

that the addition of small amount of Ti resulted in decreasing oxide

particles size and increasing their number density, and conse-

quently increasing strength at elevated temperatures [4]. The

microstructural examinations of the oxide particles by TEM-EDS

and 3D-AP suggested that the fine oxide particles were pyrochlore,

Y2Ti2O5 [15]. Although Al addition may convert the crystal struc-

ture and chemical compositions of the oxide particles, Ti was nec-

essary to keep the size of the oxide particles being smaller.

2.3. Minor alloying element

2.3.1. Reduction of strength by Al addition

Most serious problem of Al addition is a loss of strength. Fig. 4

shows the effects of Al addition on the tensile strength of the 19Cr-

ODS steel, which clearly indicates that the Al addition significantly

reduces the tensile strength, although the tensile strength of 19Cr

4Al-ODS steel is still much higher than a reduced activation ferritic

steel, JLF-1.

In order to investigate the mechanism of this reduction of

strength by Al addition, TEM observations were performed, and it

was revealed that the structure and dispersion morphology of

the oxide particles were different between the steels with andwithout Al addition [18]. In the Al-added steel, the average

diameter of the oxide particles was about 7 nm but less than

3 nm in the Al free ODS steel. The number density of the oxide par-

ticles was reduced by almost one order of magnitude. The crystal

structure of the oxide particle was also changed by the Al addition.

As mentioned before, the fine oxide particles in the steel without Al

addition were mainly pyrochlore [19], while those in the steel with

Al addition were mostly rather larger yttrium aluminum perovsk-

ites (YAP) and yttrium aluminum hexagonal (YAH) [20].

2.3.2. Strengthening by oxide particles modification

Yttriumaluminum oxide particles are not adequate for

strengthening of ODS ferritic steels, while yttriumtitanium

pyrochlore is effective for strengthening. Although alumina and

yttria are well known as thermally stable oxides, the possible othersort of fine oxide particles are searched for their application to

strengthening of the ODS steels with Al and Y. There are many

stable oxide former elements, such as Y, Al, Ti, V, Ta, Nb, Hf, Zr,

and so on. Table 1 shows the oxide formation energies of each alloy

element at 1500 K. Among them yttria is familiar for ODS alloy

production. The oxide formation energy is ranging widely from

520 for Nbto995 kJ/mol O2 for Y at 1500 K. Since the formation

energy of yttria and alumina is 995 and 800 kJ/mol O2, respec-

tively, Hf and Zr may influence on the characteristics of oxide par-

ticles [21].

The effects of a small addition of Hf and Zr on the strengthening

were investigated for 16Cr4AlY2O3-ODS steel. Fig. 5 shows the

12 14 16 18 20 220

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

10,000h

4,300h

1,000h

100h

Absorbed

EnergyatRT,

J

Cr concentration, wt%

As-Received

Aged at 500C

Fig. 3. The dependence of the absorbed energy at RT on the Cr concentration of the

ODS steel after aging at 773 K for different period up to 10,000 h.

Fig. 4. The tensile strength of 19Cr-ODS steels with and without addition of Al as

well as JLF-1 steel.

Table 1

The oxide formation energies of each alloy element at 1500 K [18].

Family 3A 4A 5A 3B 4B

Elements Sc Ti V Al Si

Atomic# 21 22 23 13 14

Oxide Sc2O3 TiO2 V2O5 Al2O3 SiO2Formation energy 990 770 570 800

Y Zr Nb

Atomic# 39 40 41

Oxide Y 2O3 ZrO2 Nb2O

Formation energy 995 800 500

La Hf Ta

Atomic# 57 72 73

Oxide La2O3 HfO2 Ta2O5Formation energy 900 570

178 A. Kimura et al. / Journal of Nuclear Materials 417 (2011) 176179


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tensile strength at 973 K of each candidate ODS steel. 16Cr4Al is a

standard candidate ODS steel whose chemical compositions are

16Cr4Al2W0.15Ti0.35Y2O3. 16Cr is 16Cr-ODS steel without

Al addition, showing much higher strength than 16Cr4Al. The

extrusion temperature was 1423 K except for the steels without

notification in the figure. The addition of small amount of Hf andZr is very effective to increase the strength at 973 K.

TEM observation revealed that the addition of Hf andZr reduced

and increased the size and the number density of oxide particles,

respectively. Furthermore, the number density of grain boundary

precipitates, such as carbides and oxides were increased remark-

ably by addition of Hf or Zr. Grain boundary precipitates are well

known as obstacles for grain boundary sliding that is a typical

deformation mode at elevated temperatures [22]. This effect also

resulted in a significant increase in long term creep properties of

the ODS alloy at 973 K, as summarized in Table 2.

Thus, the positive effect of addition of Hf or Zr overcomes the

negative effect of Al addition on high-temperature strength. The

Zr addition is more recommended than Hf addition in terms of

thermal neutron cross section area or neutron absorption.

3. Al free 16Cr-ODS steel for fusion application

From a viewpoint of reduced activation, Al is not an adequate

element for application to fusion blanket. The advantage of Al addi-

tion is significant in corrosion resistance as mentioned above. The

corrosion rate of Al free 16Cr-ODS steel in SCPW at 873 K is about

3.5 times higher than that of Al added 16Cr-ODS steel. Based on the

corrosion test results, the corrosion depth of the Al free 16Cr-ODS

steel is estimated to be 35lm for 10 years inSCPW (25 MPa, 873 K,

C(O)= 8 ppm and flow rate = 16.7 ml/min) [23]. Although the effect

of flow rate of SCPWon the corrosion rate is necessary to assess the

feasibility for fusion application, it can be concluded that the Al

addition is not inevitable for 16Cr-ODS steel for the applicationto SCPW cooled blanket system for DEMO or beyond.

As for the corrosion resistance in LBE, it is considerably im-

proved by Al addition that causes the formation of thin alumina

on the specimen surface. In the case of Al free ODS steels, the dis-

solution of Fe and Cr increases considerably at elevated tempera-

tures higher than 773 K, which may limit the operation window

of the LBE cooled system [24].

4. Concluding remarks

(1) Corrosion issue requires Cr concentration more than

14 wt.%, but aging embrittlement issue requires it less than

16 wt.%. An addition of 4 wt.%Al is effective to improve cor-

rosion resistance of 16 wt.%Cr-ODS steel in SCW and LBE,

while it is detrimental to high-temperature strength.

(2) An addition of small amount of Zr or Hf results in a signifi-

cant increase in creep strength at 973 K in Al-added ODS

steels.

(3) The Al free 16Cr-ODS steel has an enough corrosion resis-

tance in SCPW (873 K, 25 MPa, C(O) = 8 ppm), although the

corrosion rate of the Al free 16Cr-ODS steel is about 3.5

times higher than that of the Al-added 16Cr-ODS steel.

Acknowledgement

Present study includes the result of Development of super ODS

steels with high-resistance to corrosion towards highly efficient

nuclear systems entrusted to Kyoto University by the Ministry

of Education, Culture, Sports, Science and Technology of Japan

(MEXT).

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Fig. 5. Effects of extrusion temperature and addition of minor elements on the

ultimate tensile stress (UTS) of 16Cr4Al ODS steel at 973 K.

Table 2

Creep test results at 973 K of each candidate ODS steel.

Materials Creep strength (973 K, 10 k h) (MPa)

16Cr-ODS 100

16Cr4Al-ODS 60

16Cr4Al-ODS-Zr 120

16Cr4Al-ODS-Hf 110

A. Kimura et al. / Journal of Nuclear Materials 417 (2011) 176179 179

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