Central Research Institute of Electric Power Industry

CRIEPI’s Studies on the SCC of the Canister for Spent Nuclear Fuel

2015

Int. Conf. on Management of Spent Fuel from Nuclear Power Reactors

IAEA, Vienna

June. 18, 2015

○M.Wataru, K.Shirai, J.Tani, H.Takeda, T.Fujii, S.Eto and T.Saegusa

1

SCC of the Canister

2

Deposition of sea

salt particles

Deliquesce of sea

salt

Pitting or crevice

corrosion

Crack occurs

around weld

Rust

Stainless

steel

Crack penetrates

through the plate

Deposition of sea

salt particles ①

②

③

④

⑤

⑥

Criteria I

Criteria II

⑦ Loss of confinement

2015

SCC Initiation Test with SUS304L

Minimum amounts of salt for SCC initiation should be set to 0.8g/m2 as Cl

No

rmal

ized

App

lied

Stre

ss

Chloride Density /g·m2 as Cl

O : Not occurred ＜100µm X : Occurred ＞100µm

O : Not occurred ＜50µm ∆ : Not occurred ＜100µm X : Occurred ＞100µm

Threshold value

No

rmal

ized

App

lied

Str

ess

Chloride Density /g·m2 as Cl

(Specimen A for 2000hr, Hardness 305Hv) (Specimen B for 5000hr, Hardness 247Hv)

3 2015

Salt Deposition on the vertical Surface

Salt Particle

Air Flow

Canister Surface

(1) The temperature of the canister surface

is hot.

(2) The surface of the deposition is vertical.

(3) The cooling air including the sea salt

particles goes upward in parallel with the

canister surface.

(4) The concrete cask is placed in a

building and the canister surface is not

exposed to wind and rain.

(5) Because the radiation dose is very high

near the canister surface and the gap

between the canister surface and the

concrete container is very narrow, it is

difficult to measure the amount of the

deposition and check the surface condition.

4 2015

Test Results of Salt deposition Test

Field Test (Choshi)

4km

Test Site in Choshi City

Sea

Laboratory Test

Salt Deposition Test on the Metal Surface

5

0.01

0.1

1

10

100

1000

10000

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Am

oun

t of

salt D

eposi

tion

（m

g/m

2as

Cl-）

Time（h）

Labo(1.4m/s, 30℃)

Field (Choshi）,30℃

Field (Yokosuka),Room Temp.

Filed (Yokosuka),100℃

Field (Yokosuka),200℃

（Salt in Air 10 mg/m3）

（Salt in Air 2μg/m3）

QLabo=2.68×t(1/2)

QField=0.0143×t(1/2)

（Salt in Air3.7 μg/m3）

QField=0.0504×t(1/2)

Field Test (Yokosuka)

2015 5

Prevention of the Initiation of SCC

• Estimation of Deposition rate at High Temperature MPC Salt Deposition Tests : Laboratory and Field Temperature Dependence of the Salt Deposition rate

• Temperature decay of the canister over time Performance Data with Full-Scale Demo Test

• Prevention of the Initiation of SCC more than 50years under an airborne salt concentration of 100μg/m3

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60

20

40

60

80

100

120

20 40100 250500

Sea Salt Concentration in Air μg/m3 as Cl

Storage Year

Canister T

emperature ( oC

)

Chl

orid

e D

ensi

ty /g

·m2

as C

l

Threshold Value for SUS304L Material

Canister Temperature

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60

20

40

60

80

100

120

20 40100 250500

Sea Salt Concentration in Air μg/m3 as Cl

Storage Year

Canister T

emperature ( oC

)

Chl

orid

e D

ensi

ty /g

·m2

as C

l

Threshold Value for SUS304L Material

Canister Temperature

6 2015

Evaluation of Crack Growth

7

Time (hour)

Cra

ck D

epth

(m

m)

Time (hour)

Cra

ck D

epth

(m

m)

4 Point Bending Test

Type 304 stainless steel, 80oC, 35%RH, 270MPa

About 10g/m2 as Cl of sea salt

RDCPD (Reverse Direct Current Potential Drop)

method

Potential drop data was converted to crack depth,

assuming half elliptical crack propagated

Crack Growth Rate as 2x10-11m/s

(S30403 stainless steel, 80oC, 35%RH, 270MPa, about 10g/m2 as Cl of sea salt)

Temperature

Chloride Solution

Pure Water

573K

572(F)

473K

(392F)

373K

(212F)

273K

(32F)

Crack Growth Rate in the literature (S30403)

2015

Crack Growth Depth (CGD) Control

• Humid Period in which the relative humidity exceeds 15% would be approximately 15,000 hours during a 60 year storage period crack growth test

• Consequently, the estimated crack propagation value during this period is only 1.1mm.

[Crack Growth Test Result]

19mm / 60years0.5mm / 15000h

CGR: 1x10-11m/s

19mm / 60years0.5mm / 15000h

CGR: 1x10-11m/s

38mm / 60years 1.1mm / 15000hr

CGR:2x10-11m/s

8 2015

Countermeasure for SCC

9

Method Example

Decrease residual stress Laser welding

Stress relaxation Annealing

Apply compression stress Shot peening, Ball-burnishing

Use high-Cr, high-Mo material

Duplex stainless steel, Super austenitic stainless steel

Use SCC free material Titanium, Ferrite Steel

Isolate from chloride Painting, Corrosion resistant plating

Decrease salt deposition Sea salt particle collection, Washing

Keep salt dry Keep surface temperature high

Design standard Threshold salt density, etc.

Operational standard Crack growth evaluation

2015

SCC Test using Small Scale Test Model with Surface Treatment

• Salt Concentration on the surface 10g/m2 as Cl over 10 times of threshold chloride density for SCC

initiation of S30403 stainless steel 80˚C with RH=35% over 1000 hours SCC initiation was not observed on the treated surface.

[Corrosion Test Results ]

Surface Condition after Cleaning

(Laser-Beam weld plates with LPB)

Circular Plate

Dia.100mm x 13 mm

After LPB As-Machined

Residual Stress

Measurement

Radial Direction

Residual Stress

Measurement

Thickness

Direction

Surface Condition after Cleaning

(Laser-Beam weld plates with LPB)

Circular Plate

Dia.100mm x 13 mm

After LPB As-Machined

Residual Stress

Measurement

Radial Direction

Residual Stress

Measurement

Thickness

Direction

Non-Treated surface (SCC occurrence)

Treated surface by LPB (only corrosion)

10 2015

SCC Test using Mock-up MPC Model

• Verify the effectiveness of the surface treatment technique MPC with full-scale diameter (1,836mm) and wall thickness (12.7mm) Half of the weld lines were treated by SP (Shot Peening) 4g/m3 as Cl , 80˚C with RH=35% over 2000 hours As the indication of the existence of the SCC was not visible due to the rust At the suspicious areas selected by PT, SCC initiation was detected only on

the as-weld surface (measured crack depth: 3mm) by SEM observation .

[Typical Indication detected in PT]

-1200

-1000

-800

-600

-400

-200

0

200

400

0.0 0.2 0.4 0.6 0.8 1.0

Depth from the surface (mm)

Resi

dual s

tress

(M

Pa)

PB

PCW

PLW

B

LW

[Measured Residual Radial Stress Distributions in the Mock-up MPC]

Weld line

11 2015

Monitoring salt concentration by LIBS

12

• Inspection of the occurrence of SCC Evaluation of the deposition of chloride attached on canister

during storage Development of the laser measurement system

Threshould Value for SUS304L

I Cl n

orm

aliz

ed b

y I O

Cl Concentration (g/m2)

Canister

Sensor head

Optical probe

[1] S. Eto et al., Spectrochim. Acta B. 87 (2013) 74-80. [2] S. Eto et al., Proc. EMSLIBS2013, Bari, Italy, Sep. 16–20, 2013, (2013) P_023.

2015

13

Monitoring Technology for Canister Confinement

Canister

TB

TT

Ref: H.Takeda, Development of the detecting method of helium gas leak from canister, Nuclear Engineering and Design 238 (2008)

2015

Summary

The items which the CRIEPI is continuing to study is as follows;

・Critical value of initiation of SCC

・Countermeasure technology for SCC

・Inspection technology for salt deposition by LIBS

・Monitoring technology for canister confinement

・Preparation of revision of JSME code

14 2015