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)