metal release behavior of alloy 690 coated with cr-oxide

© 2015 NIPPON STEEL & SUMITOMO METAL CORPORATION All Rights Reserved.

Metal Release Behavior of Alloy 690 Coated with Cr-oxide in Simulated PWR

Primary Water at High Flow Velocity

August 9-13, 2015 Yumi Momozono, Yasuyoshi Hidaka, Manabu Kanzaki

Yasuhiro Masaki, Akihiro Uehira, Osamu Miyahara Nippon Steel & Sumitomo Metal Corporation (NSSMC)

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17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Ottawa, Ontario, Canada


Outline

1) Introduction Ni release in PWR and Pre-filming for SG tubing 2) Experimental Recirculation type metal release test at high flow velocity 3) Results 3.1) Ni release rates 3.2) Physical analysis of pre-film 4) Discussion 4.1) Inhibitory effect of pre-film on Ni release 4.2) Ni release behavior on pre-film 5) Conclusions 2


1) Introduction

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Alloy 690 is SG tubing material, which contains 60% Ni. Ni converts into 58Co by neutron irradiation.

reaction①：235U+n→fission product+n reaction②：58Ni+n→58Co（β decay，half life 71days）

Ni is transmuted to radioactive Co

neutron

Core

Primary water

Ni reaction① reaction②

58Co

Ni is released from inner surface of SG tubing

Ni Primary Water

Steam Generator(SG)

(TT690：60Ni-30Cr-9Fe)

SG tubes release

58Ni

58Ni

58Co

Deposit on the surface of primary tubing

fuel

Transport to the reactor core

Figure 1 Ni release from SG tubing in PWR.

Background

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Figure 2 Image of reduction of Ni release by pre-filming.

Pre-filming on the inner surface of SG tubing is expected to reduce Ni release in the early operation period.

Operation Cycle

Reduction of Ni release by pre-filming on SG tubing surface

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Reduction of Ni release by pre-filming

Ni R

elea

se

SG tube

Pre-filmed barrier

Prevent Ni release.

58Ni

Primary water


Alloy 690 contains 30% Cr. Cr-oxide can be formed on the surface of Alloy 690 by control of temperature and potential of oxidation.

Figure 3 Relationship between temperature and potential of oxidation.

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-300

-250

-200

-150

-100

-50

0

0 500 1000 1500 2000

温度(℃）

酸化

ポテ

ンシ

ャル

ΔG

0=RTlnPO2(kcal)

Temperature (°C)

Pote

ntia

l of o

xida

tion

(kca

l)

Cr is oxidized selectively under this condition.

What is pre-filming?

Forming Cr-oxide film on Alloy 690


Objective

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• The amount of Ni released from Alloy 690 is small in PWR primary water environment.

→ It is not easy to evaluate the amount of released Ni. So there is no standardized way to evaluate Ni release.

• To simulate an actual PWR primary water environment,

the recirculation type metal release test system, which mainly focused on high flow velocity, was introduced.

To clarify the effect of pre-film on Ni release and the Ni release behavior on pre-film.


2) Experimental

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Specimens

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C Si Mn S Ni Cr Fe Cu 0.02 0.3 0.3 <0.001 59.3 29.5 10.0 <0.1

• The specimens are Alloy 690 tubing having an outer diameter of 19 mm and an inner diameter of 17 mm.

• Pre-filming: Annealed at 1100°C in hydrogen with 2000 ppm water

vapor in order to form Cr-oxide film on the surface. • Non pre-filming (as a reference): Annealed at 1100°C in dry hydrogen. • Both test tubing were thermally treated at 725°C in

vacuum for 10 h.

Table 1 Chemical composition (mass%)


[Specification] Test solutions are contacted to pure Ti or resin. Max. temp. is 330 ° C. Max. pressure is 17.5 MPa Flow rate 5 L/min (calculated to 1.7m/s(Re=22000))

Heater

High pressure pump

Circulation Pump

Water chemistry control (Con., DO, DH, pH)

Ni, Cr and Fe are free by using pure Ti and resin.

Storage tank

Ni is sampling from outer side of the specimen

Ni is sampling from Inner side of the specimen

High flow velocity (using inner bar)

Specimen (1mL)

Ni release is measured directly

Cooler

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Figure 4 Recirculation type metal release test equipment.

Recirculation type metal release test


( )( )

( )( )

S

WinVinn

outVoutn

×

−

Figure 5 Outline of sampling method.

n(out): Ni amount from SG tubing(n1+n2) (g) V(out): Volume of test solution from outer samplingV1 (L) n(in): Ni amount in test solution (n3+n4) (g) V(in): Volume of test solution from inner samplingV2 (L) W : Flow rate (L/h) S : Inner surface of specimen (m2)

Ni release rate (g/m2/h)

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Ni release rate is calculated by the amount of Ni in the outlet of test tube subtracted from that in the inlet of the tube. Ni amount is sum of the amount in test solution and in cleaned solution.

Dilute nitric acid solution for cleaning sampling line

Sampling line from outlet of SG tube

Sampling line from inlet of SG tube

Cleaned solution

n1 n2

n3 n4

V1

V2

SG tube

Simulated primary water

Test solution

Test solution

Cleaned solution

Recirculation type metal release test


Test condition of recirculation type metal release test

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Test solution 1000 ppm B + 2 ppm Li Temperature 325°C

Pressure 15.5 MPa Dissolved oxygen <10 ppb

Dissolved hydrogen 2.6 ppm Flow rate 5 L/min

Flow velocity 1.7 m/s (calculated) Test time Pre-filmed: 620 h, Non Pre-filmed: 1129 h

Table 2 Test condition of recirculation type metal release test

The flow velocity in the actual plant is estimated* at approximately 5.5 m/s. * “Handbook of Water Chemistry of Nuclear Reactor System”, Atomic Energy Society of Japan, (2000), p.122


Physical analysis of pre-film

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• X-ray diffraction (XRD; RIGAKU, RINT-2500H) X-ray source: Co Kα (30 kV, 100 mA) Angle of incidence: 0.3° Scanning zone of 2θ: from 10° to 105° • Glow discharge spectroscopy (GDS; HORIBA, GD-Profiler 2) Analyzing area: 4 mm in diameter Power capacity: 35 W Pressure of Ar gas: 600 Pa • Transmission electron microscope (TEM; RIGAKU, JEM-200CX) Acceleration voltage: 200 kV


3) Results

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Ni release rates

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0

0.0002

0.0004

0.0006

0.0008

0.001

0 500 1000 1500

Ni r

elea

se ra

te (

g/m

2 /h)

Test time (h)

Pre-filmed Alloy 690

Non Pre-filmed Alloy 690

• The Ni release rate of non pre-filmed Alloy 690 tubing decreased with test time like conventional studies.

• The Ni release rate of pre-filmed Alloy 690 tubing also decreased with test time promptly.

• The Ni release rate of pre-filmed Alloy 690 tubing is much lower than that of non pre-filmed Alloy 690 tubing.

Drastically decreased by pre-filming

Figure 6 Ni release rates in simulated PWR primary water at 1.7 m/s.


XRD analysis before metal release test

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0

50

100

150

200

250

300

350

10 20 30 40 50 60 70 80 90 100

Inte

nsity

(CP

S)

2θ (degree)

△

△

△

△△ △

○

○

○

○

○○

○

○○○

○ ○○

□

□ □

○: Cr2O3

△: MnCr2O4□: Austenite

• Cr2O3 and MnCr2O4 were detected from the pre-film. • According to Ellingham phase diagrams, it was

considered that Cr was mainly oxidized in the main elements (i.e., Ni, Cr, and Fe) of Alloy 690.

Figure 7 XRD analysis of pre-filmed Alloy 690 tubing before the metal release test.


GDS depth profile

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0

10

20

30

40

50

60

70

80

90

100

0 0.2 0.4 0.6 0.8 1

Ele

men

t con

cent

ratio

n (m

ass%

)

Depth (μm)

Dashed line:Before metal release testSolid line:After metal release test

Cr

NiFeO

Mn

0123456789

10

0 0.02 0.04 0.06 0.08 0.1

Elem

ent c

once

ntra

tion

(mas

s%)

Depth (μm)

NiFe

O

Mn

• The oxide film was highly Cr enriched before and after the metal release test.

• Concentrated Ni on the surface before the test considerably decreased with the decrease of Mn, in contrast to the increase of O in the pre-film.

Figure 8 GDS depth profile of pre-filmed Alloy 690 tubing.

Decrease Decrease

Metal

Cr2O3

(Mn, Ni)Cr2O4Pre-film

Before metal release test

2 layers


Cross sectional TEM observation

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• The pre-film on Alloy 690 was stable even at a high flow velocity of 1.7 m/s.

Before metal release test After metal release test

Figure 9 Cross sectional TEM observation of pre-filmed Alloy 690 tubing.


4) Discussion

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4.1) Inhibitory effect of pre-film on Ni release 4.2) Ni release behavior on pre-film


Inhibitory effect of pre-film on Ni release

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• The reaction resistance of metal release was measured by an electrochemical method of anodic polarization resistance.

• Specimens were immersed in autoclaves filled with simulated PWR primary water for 100 h before the electrochemical measurement.

Test solution 0.5 M Na2SO4 Temperature 35°C

Gas Bubbled with Ar gas

Potential From Open circuit potential (OCP) to OCP + 10mV

Potential sweeping rate 2mV/s

Table 3 Test condition of electrochemical measurement

Personal computer

Aqueous solution

C. E.

R. E.

ArInOutPotentiostat

Specimen


Inhibitory effect of pre-film on Ni release

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0

0.002

0.004

0.006

0.008

0.01

0 0.1 0.2 0.3 0.4

⊿E

(V)

I(μA/cm2)

Pre-f ilmed Alloy 690

Non Pre-f ilmed Alloy 690

• The anodic polarization resistance of pre-filmed Alloy 690 was approximately 10 times higher than that of non pre-filmed Alloy 690.

• This result indicates that pre-filming is protective toward metal release.

Pre-filmed: 8.1 × 10 -1 MΩ∙cm2

Non pre-filmed: 3.3 × 10 -2 MΩ∙cm2

Figure 10 Anodic polarization resistance after immersion test in simulated PWR primary water.

I: Current density

E: Voltage R: Resistance

10 times higher E=IR

(Ohm’s law)


Metal

Cr2O3

(Mn, Ni)Cr2O4Pre-film

Metal

Cr2O3 Pre-film

Ni Mn Cr(Mn, Ni)Cr2O4

Before metal release test After metal release test

Ni release behavior on pre-film

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• A small amount of Ni was released from pre-filmed Alloy 690 at the beginning of the metal release test.

Figure 11 Schematic diagram of Ni release behavior on pre-film.

• The source of Ni released from pre-filmed Alloy 690 could be the surface layer of the pre-film.

• As shown in GDS depth profile, Ni and Mn decreased after the metal release test. It is assumed that Ni in MnCr2O4 is released.


Conclusions • It was clarified that the Ni release rate of pre-filmed

Alloy 690 at 1.7 m/s was much lower than that of non pre-filmed Alloy 690 because the pre-film mainly contains Cr2O3 which is protective to Ni release.

• However, a small amount of Ni was released from pre-filmed Alloy 690 at the beginning of the metal release test. The source of the small amount of Ni released from pre-filmed Alloy 690 could be the surface layer of the pre-film.

• It was also clarified that the pre-film on Alloy 690 was stable even at a high flow velocity of 1.7 m/s.

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Thank you for your attention.

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metal release behavior of alloy 690 coated with cr-oxide

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