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CRIEPI 1/37
Basis and Safety Case of Spent Fuel Storage
T. Saegusa, K. Shirai, M. Wataru, H. Takeda, K. Namba
CRIEPI, Japan
May 2014
IAEA Work Shop on DPC Safety Case May 19-21, 2014
CRIEPI 2/37
Safety Case of Spent Fuel Storage
Concept of safety case in
GSR Part 5;
The safety case is a collection
of arguments and evidence in
support of the safety of a
facility or activity. The safety
case will normally include the
findings of a safety
assessment, and will typically
include information
(including supporting
evidence and reasoning) on
the robustness and reliability
of the safety assessment and
the assumptions made therein.
CRIEPI
ERC
CRIEPI 3/37
Contents 1. Spent Fuel
Characteristics and Need for Storage
2. Safety Regulations, Code and Standard
3. Metal Cask Storage
4. Concrete Cask Storage
5. Vault Storage
6. Spent Fuel Integrity
7. Others
2.1 New Regulatory Requirements
3.1 Design Concepts and Economy
3.2 Heat Removal
3.3 Containment
3.4 Sub-Criticality
3.5 Structural Integrity
3.6 Seismic Performance
3.7 Severe Accident Performance
3.8 Interaction between Transport and Storage
CRIEPI 4/37
2.5 Safety Regulations, Code and Standard -Four Safety Functions
1. Confinement of the radioactive material
2. Shielding (control of external radiation level)
3. Criticality Prevention
4. Heat Removal (prevention of damage caused by heat)
CRIEPI 5/37
2.1 New Regulatory Requirements after Fukushima
Air in
CRIEPI 6/37
2.1(1) Other Safety Measures Consideration of Natural Phenomena
Safety shall not be lost by earthquake, tsunami, etc.
If the storage building were collapsed by the natural phenomena, the basic safety functions shall not be affected.
With appropriate measures and period, the shielding and heat removal functions shall be recovered.
Multiple initiating events occurring simultaneously shall be considered.
CRIEPI 7/37
2.1(2) Other Safety Measures Consideration of External Event
Safety shall not be lost by accidental external man-made disaster.
Spent fuel storage facilities shall be designed to protect airplane crash with a probability more than 10-7 /year.
Spent fuel storage facilities shall be designed by appropriate measures to protect illegal access of outsiders.
CRIEPI 8/37
2.1(3)Comparison with International Regulations
The new regulations includes requirements of IAEA GSR Part 5 “ Predisposal Management of Radioactive Waste“ and SSG-15 “ Storage of Spent Nuclear Fuel“ .
The new regulations added a heat removal requirement referring German “Safety Guidelines for dry Interim Storage of Irradiated Fuel Assemblies in Storage Casks”.
CRIEPI 9/37
3.2(1) Heat Removal -Cask building with chimney-
Video of an experiment on natural cooling (Thermal Hydraulic Phenomena).
排気スタ
ック開閉
部
模
擬
キ
6000
7700 3300
Inlet
Outlet
Cask model
Unit : mm
Open or Closed
Fan
Scale: 1/5
CRIEPI 10/37
3.2(1) Results of Heat Removal Test 1) The ceiling height hardly influences the heat removal
characteristics.
2) The ceiling temp. is seriously affected by ceiling height. Therefore, the ceiling height should be determined considering the temp. restriction of concrete and electrical parts.
3) There are two kinds of flow in the storage area, e.g., an upward flow induced by buoyant force on the cask surface and a horizontal flow induced by chimney effect. These two flows contribute to cool the casks.
4) The chimney (stack) height directly influences the heat removal characteristic.
CRIEPI 11/37
Type I model
Type II
model
0 2 4 6 8 10 12 14 16 18 20
漏洩
率(Pa・m3 /s)
経過時間 (年)
基準漏洩率
Ⅰ型モデル
平 均 6.50×10-10
標準偏差 3.40×10-10
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 2 4 6 8 10 12 14 16 18 20
漏洩
率(Pa・m3 /s)
経過時間 (年)
基準漏洩率
Ⅱ型モデル
平 均 5.70×10-11
標準偏差 2.25×10-11
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
Elapsed year
Elapsed year
Type I model
Type II model
Lea
k R
ate
(P
a-m
3/s
) L
eak
Rate
(P
a-m
3/s
)
Standard Leak Rate (Pa-m3/s)
Standard Leak Rate (Pa-m3/s)
Ave.
σs
Ave.
σs
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100 110
残留
反発
力(
N/m
m)
時間(year)
試験の温度条件(139℃一定)での解
析結果使用済燃料の発熱低下を考慮した条
件での解析結果(初期温度139℃)
密封喪失限界残留反発力(12N/mm)
Storage period (year)
Critical repulsion force to loose containment (12
N/mm) Resi
du
al
rep
uls
ion
fo
rce
(N/m
m)
Temperature is 139 °C constant.
Initial temp. was 139 °C and
decreases with time.
Analytical result of containment due to stress relaxation of metal gasket (Type I)
3.3 Containment -(1) Long-term containment of metal gasket-
CRIEPI 12/37
The trends of the gasket temperature
depends on the cask design.
3.3(1) Evaluation of the Long-term Sealability
7942
7781
6500
7000
7500
8000
8500
0 10 20 30 40 50 60 70 80 90 100
Year
L.M
.P.
Temp. of 2nd Lid (Type Ⅰ)Initial Temp. 140℃Initial Temp. 135℃Initial Temp. 130℃Initial Temp. 120℃Temp. of 2nd Lid (Type Ⅱ)
40
60
80
100
120
140
160
0 20 40 60 80 100Year
Temperature of 2nd Lid(℃
)
Initial Tem p. 150℃Initial Tem p. 140℃Initial Tem p. 130℃Initial Tem p. 120℃M easured Tem p.(TypeⅠ)M easured Tem p.(TypeⅡ)
0
10
20
30
40
50
60
70
80
90
100
120 125 130 135 140 145 150 155 160
Initial Tem p. of 2nd Lid(℃)
Evaluation Time (yea
r)
TypeⅠ
TypeⅡ125℃ 134℃
CRIEPI 13/37
3.3 (1) Containment- Summary
Two kinds of cask lid structure models are being tested for more than 19 years at constant temperature.
The very reliable containment performance has been demonstrated.
By applying the Larson-Miller parameter, the results indicate a longer period of sealing performance taking account of the decay heat of the spent nuclear fuel.
After finishing the test, all of the lids were opened. The degradation data of the gaskets were obtained.
CRIEPI 14/37
3.5 Structural Integrity -(1)Drop test of ductile cast iron cask-
CRIEPI 15/37
3.5(2) Heavy Weight Drop Tests onto Cask by Building Collapse
CRIEPI 16/37
Background Conventionally, leakage tests are performed before and after drop tests of the package. On the other hand, it has been known that packages may leak momentarily at the moment of the mechanical impact. However, such momentary leakage has not been measured quantitatively. Purpose To quantitatively measure momentary leak from a full-scale metal cask without impact limiters in a full scale drop test.
3.5 Structural Integrity -(3)Instantaneous leak in cask drop test-
CRIEPI 17/37
Horizontal drop test Rotational impact test
The cask was dropped
horizontally from 1 m high. The cask was rotated around
an axis of a lower trunnion.
Height 1m 1m
The front trunnion attacked the
concrete floor, directly.
Both the cask corner and the
front trunnion attacked the
concrete floor, directly.
3.5(3) Test Conditions
Concrete floor
1m
12.6°
Concrete floor
1m
12.6°
gasket
Concrete floor
1m
trunnion
gasket
Concrete floor
1m
trunnion
CRIEPI 18/37
3.5(3) Concrete floor after Horizontal Drop Test
A
A
0
Lid
-14
-12
-10
-8
-6
-4
-2
0
-4 -3 -2 -1 0 1 2 3 4 5 6
Length(cm)
Depth
(cm
)
A-A Section
The Center of
Trunnion
Floor Level
Lid Direction
The depth of penetration to the concrete floor of the trunnion was about 10 cm.
CRIEPI 19/37
0 10 20 30 40 50 60
Time(min)
Leak rate(
Pa・m
3/s)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
3.5(3) Leak Rate from the Primary & the Secondary Lids at Horizontal Drop Test
0 10 20 30 40 50 60
Time(min)Leak rate(
Pa・m
3/s)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
The total amount of helium gas leakage from the primary and secondary lids was 2.0×10-6Pa・m3.
This value is about 10-
8% of the initially filled helium gas.
The amount of leakage was insignificant.
Cask Body
Secondary Lid
Primary LidHelium leakDetector 2
Helium leakDetector 1
He:4atm
CRIEPI 20/37
3.5(2) Concrete floor after Rotational Drop Test
Lid Cask
corner
Trunni
on
-1
0
1
2
3
4
5
6
-40 -20 0 20 40 60 80 100
Length(cm)
Dept
h(c
m)
Trunnion
Cask corner
Floor Level
The depth of penetration to
the concrete floor of the
trunnion was about 5 cm.
CRIEPI 21/37
0 10 20 30 40 50 60
Time(min)
Leak rate(
Pa・m
3/s)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 10 20 30 40 50 60
Time(min)
Leak rate(
Pa・m
3/s)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
The total amount of leakage from lids was 1.7×10-5Pa・m3.
This value is about 10-7% of the initially filled helium gas.
This value was larger than that of the horizontal drop test.
Nevertheless, the amount of leakage was also insignificant.
Cask Body
Secondary Lid
Primary LidHelium leakDetector 2
Helium leakDetector 1
He:4atm
3.5(2) Leak Rate from the Primary and the Secondary lid at Rotational Drop Test
CRIEPI 22/37
3.5(3) Summary of Momentary Leak from Metal Cask w/o Impact Limiters
Momentary leak rates were quantitatively measured at the drop tests of a full scale metal cask simulating drop accidents in a storage facility.
Negligible helium leak was observed in both
cases. At the rotational impact test, the amount of leakage was larger than that of the horizontal drop test. However, the amount of leakage was insignificant.
CRIEPI 23/37
3.6 Seismic Performance -Cask tipping over-
CRIEPI 24/37
Stainless/ Pb/Stainless Cask
3.7 Severe Accident Performance -(1)Cask burial in concrete debris-
CRIEPI 25/37
3.7(1) Test cases in building collapse
I : Debris covered lower part of the vertical cask.
II : Debris covered upper part of the vertical cask.
III : Debris covered upper part of the horizontal cask.
IV : Debris fully covered the horizontal cask.
I II II
I IV
CRIEPI 26/37
3.7(1) Results of thermal performance tests & analyses at various building collapses
Temp. (℃) at various cases of coverage with debris
Cask
component
Design
criteria on
max.
allowable
temp.
Spent fuel 335 349 320 423 500
Lead in
cask body
208 226 201 239 327
Gasket in
primary lid
171 207 156 242-1 month
248-2months
Gasket in
2nd lid
147 190 136 NA
250-1
month
II III IV I
CRIEPI 27/37
3.7 Severe Accident Performance -(2)Airplane crash on cask-
Objective : To evaluate integrity of a metal cask under a hypothetical airplane crash accident.
Key Issue : Cask Lid Sliding & Opening “Leak tightness of the metallic gasket is very sensitive to lid
movements”
Animation Video
CRIEPI 28/37
3.7(2) Cases for Airplane Crash Tests
Horizontal Crash Test of 2/5
Reduced Cask Model
Crashed by a Simulated
Engine
Vertical Crash Test of Full
Scale Model of Cask Lid
Crashed by a Simulated
Engine
Reduced Model Cask (2/5)
Simulated Engine
Full Scale Model of Cask Lid
Simulated Engine
Case 1
Case 2
CRIEPI 29/37
3.7(2) Crash Test Result
(measurement)
widen
narrow
The measured leak rate was within the permissible value for transport casks.
CRIEPI 30/37
• Transport casks receive mechanical vibration in transport. The containment performance of metal gaskets is influenced by large external load or displacement .
• Quantitative influence of such vibration in transport on the containment performance of the metal gasket has not been known, but is crucial information particularly if the cask is stored as it is after the transport.
3.8 Interaction between Transport & Storage -(1)Vibration in transport impacts containment in storage-
Background and Objective
CRIEPI 31/37
3.8 (1) Measurements of Leak Rate and Radial Displacement with Time under Cyclic Loading
Leak
rate
(P
a・
m3/s
)
Time (s)
Leak rate
Rad. dispmt
Rad
ial D
irecti
on
Dis
pla
cem
en
t (m
m)
amplitu
de
230
cycles
If the amplitude exceeded 0.02mm, the leak rate did not recover.
CRIEPI 32/37
3.8 (1) Vibration in Transport Impacts Containment in Storage- Summary
1.Mechanical vibration in transport would
influence the containment performance
of the metal gasket for storage if the
amount of sliding exceeded a threshold
value.
2.The threshold values in the model were:
0.1~3 mm of static displacement, or
±0.02 mm of cyclic displacement.
To evaluate influence of vibration force during transportation on sealing performance of the aged gasket
Calculated opening disp. of the metal gaskets perpendicular to the flange surface
With acceleration measured during actual sea transportation
vertical
horizontal
axial -30
-25
-20
-15
-10
-5
0
5
10
15
20
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
acce
lera
tio
n (
m/s
2 )
time (s)
axial
vertical
horizontal
Analytical model Time history of acceleration
3.8 Interaction between Transport & Storage -(2)Ageing in storage impacts containment in transport-
Dynamic Analysis for Cask Lid (1/3)
CRIEPI 34/37
Opening disp. evaluated by the dynamic analysis Primary lid : maximum value was smaller than 0.001 mm. Secondary lid : maximum value was smaller than 0.003 mm.
-0.010
-0.005
0.000
0.005
0.010
0.000 0.020 0.040 0.060 0.080 0.100
Open
ing D
isp.
at
1st
Lid
(m
m)
Time (sec)
0deg
45deg
90deg
135deg
180deg
225deg
270deg
315deg
270° 90°
0°
180°
widen
narrow
-0.010
-0.005
0.000
0.005
0.010
0.000 0.020 0.040 0.060 0.080 0.100
Open
ing D
isp.
at 2n
dL
id (
mm
)
Time (sec)
0deg
45deg
90deg
135deg
180deg
225deg
270deg
315deg
270° 90°
0°
180°
widen
narrow
Time history of opening displacement
Each opening disp. << ru (spring back distance) of the gasket used for 60 years. The sealing performance will be maintained in good condition.
3.8 -(2)Ageing in storage impacts containment in transport- Dynamic Analysis for Cask Lid 2/3
(Primary lid) (Secondary lid)
3.8-(2)Ageing in storage impacts containment Summary 3/3
Numerical methodology of sealing performance of metal gasket after long term usage was proposed.
Spring back distance ru of the gasket used for 60 years :
0.09 mm
Opening disp. by dynamic analysis with the accelerations measured during actual sea transportation was evaluated.
Opening disp. of the primary lid and the secondary lid : smaller than 0.003 mm.
Opening disp. << spring back distance ru
The sealing performance will not be lost by lid opening during the sea transportation within the acceleration measured.
CRIEPI 36/37
Conclusion
CRIEPI published a book of safety case for spent fuel storage.
The book includes experiments and analyses supporting evidence and reasoning on the robustness and reliability of the DPC .
These information will become a basis for further development by advanced techniques on experiments, analyses, lessons learned, etc., in the future.
CRIEPI 37/37
Acknowledgement
Parts of the researches in this book were carried out by contracts from the Japanese governments, i.e.,
Agency for Natural Resources and Energy of Ministry of Economy, Trade and Industry (METI).
Nuclear and Industrial Safety Agency of METI (now, Nuclear Regulatory Authority)
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