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