Performance of Metal and 

Polymeric O-Ring Seals in 

Beyond-Design-Basis 

Temperature Excursions

Office of Nuclear Regulatory Research

NUREG/CR-7115, Rev. 1
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Performance of Metal and 

Polymeric O-Ring Seals in 

Beyond-Design-Basis 

Temperature Excursions

Manuscript Completed: September 2015

Date Published: November 2015

Prepared by

Jiann C. Yang

Edward J. Hnetkovsky

Doris Rinehart 

Marco Fernandez

National Institute of Standards and Technology

Engineering Laboratory

Gaithersburg, Maryland 20899

Felix Gonzalez, NRC Project Manager

Prepared for: 

Division of Risk Analysis 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

NRC Job Code N6550

Office of Nuclear Regulatory Research

NUREG/CR-7115, Rev. 1

iii

ABSTRACT

This report documents the beyond-design-basis thermal exposure test results of the 

performance of metallic and polymeric seals typically used in radioactive material transportation 

packages. The overall objective of the project is to provide test data and insights regarding the 

performance of these O-ring seals (metallic or polymeric) when exposed to beyond-design-basis 

temperature conditions that could result due to a severe fire. Tests were conducted using a 

small-scale test package (pressure vessel) made of stainless steel SS 304 filled with helium 

initially pressurized at either 5 bar or 2 bar at room temperature. The test package was then 

exposed in an electric furnace to temperatures equal to, or above the specified maximum 

operating temperatures of these seals (up to 900 °C) for a pre-determined period (typically 8 h 

to 9 h). These O-ring temperatures and test periods are more severe than the 800°C for 30 

minutes hypothetical accident conditions described in 10 CFR Part 71. In addition, the 800°C 

and 900°C seal temperatures are much higher than would be expected in real world conditions 

because transportation packages generally are designed with the seals protected by being 

located within the package and, therefore, not directly exposed to the exterior conditions of the 

environment. The pressure drop technique was used to determine if leakage occurred during 

thermal exposure. There are three test phases in this study. Revision 1 adds test results from 

Phases II and III.

In Phase I, one metallic and two polymeric (ethylene-propylene compound (EPDM) and Teflon

(PTFE)) seals were used. A total of fifteen tests were performed: twelve using metallic seals, 

two using EPDM, and one using Teflon. Leakage was observed in a number of the thermal 

exposure tests; the time when leakage occurred varied.

In Phase II, additional test data on EPDM, and PTFE O-rings were obtained. For exploratory 

purposes, three additional polymeric O-rings (butyl, Viton, and silicone) were also tested in this 

phase for comparisons. For the EPDM, butyl, PTFE, no leak was detected. However, when 

tested at 900 °C, EPDM and butyl seals leaked before the test fixture reached 900 °C. One test 

using silicone O-ring did show leakage. For all the tests that used Viton O-rings, the vessel 

pressure peculiarly increased during the thermal exposure. The reason for the pressure 

increase remains unclear.

In Phase III, the test series involved the use of a similar test package, but with a double O-ring 

configuration tested at 800 °C or 900 °C for 9 h. The outer-inner O-ring combinations were 

metallic-metallic, EPDM-metallic, blank-metallic, EPDM-EPDM, and butyl-butyl. When the same 

torque for the single metallic O-ring was used to tighten the bolts in the double metallic-metallic 

or blank-metallic tests, leaks were observed in the metallic-metallic, but not in the blank-metallic, 

tests. Even when the appropriate torque value was applied for the double O-ring configuration, 

the metallic-metallic tests leaked, but only after being at a 900°C environment between two and 

eight hours; there was no leakage prior to that period. For the EPDM-EPDM and butyl-butyl 

tests, leaks were observed in all cases even before the vessel reached the test temperature. 

No leaks were observed with the EPDM-metallic double O-ring configuration.

v

CONTENTS

ABSTRACT................................................................................................................................iii

LIST OF FIGURES ................................................................................................................... vii

LIST OF TABLES..................................................................................................................... xiii

EXECUTIVE SUMMARY ..........................................................................................................xv

ACKNOWLEDGMENTS.......................................................................................................... xvii

ABBREVIATIONS.................................................................................................................... xix

1 BACKGROUND ................................................................................................................ 1-1

1.1 Introduction............................................................................................................ 1-1

2 TECHNICAL APPROACH................................................................................................. 2-1

2.1 Experimental Apparatus......................................................................................... 2-1

2.1.1 Phase I ..................................................................................................... 2-1

2.1.2 Phase II .................................................................................................... 2-4

2.1.3 Phase III ................................................................................................... 2-5

2.2 Experimental Procedure ........................................................................................ 2-7

2.2.1 Phase I ..................................................................................................... 2-7

2.2.2 Phase II .................................................................................................... 2-8

2.2.3 Phase III ................................................................................................... 2-8

2.3 Test Matrix............................................................................................................. 2-9

2.3.1 Phase I ..................................................................................................... 2-9

2.3.2 Phase II .................................................................................................. 2-11

2.3.3 Phase III ................................................................................................. 2-12

3 RESULTS AND DISCUSSION.......................................................................................... 3-1

3.1 Phase I .................................................................................................................. 3-1

3.1.1 Metallic O-Rings ....................................................................................... 3-1

3.1.2 EPDM O-Rings ....................................................................................... 3-30

3.1.3 PTFE O-Rings ........................................................................................ 3-36

3.2 Phase II ............................................................................................................... 3-40

3.2.1 EPDM O-Rings ....................................................................................... 3-40

3.2.2 Silicone O-Rings..................................................................................... 3-47

3.2.3 Butyl O-Rings ......................................................................................... 3-53

3.2.4 Viton O-Rings ......................................................................................... 3-57

3.2.5 PTFE O-Rings ........................................................................................ 3-62
vi

3.3 Phase III .............................................................................................................. 3-67

3.3.1 EPDM-Metallic O-Rings.......................................................................... 3-67

3.3.2 Blank-Metallic O-Ring ............................................................................. 3-71

3.3.3 EPDM-EPDM O-Rings............................................................................ 3-73

3.3.4 Butyl-Butyl O-Rings ................................................................................ 3-75

3.3.5 Metallic-Metallic O-Rings ........................................................................ 3-77

4 SUMMARY........................................................................................................................ 4-1

5 REFERENCES.................................................................................................................. 5-1

APPENDIX A DESIGN DRAWINGS OF TEST VESSEL.....................................................A-1

APPENDIX B VESSEL INTERNAL VOLUMES ...................................................................B-1

APPENDIX C UNCERTAINTY ESTIMATE OF MEASUREMENTS .....................................C-1

C.1 Uncertainty Estimate in Thermocouple Measurements ..........................................C-1

C.2 Uncertainty Estimate in Pressure Transducer Measurements................................C-2

APPENDIX D VESSEL VOLUME EXPANSION AND LEAK RATE ESTIMATION...............D-1

D.1 Vessel Volume Estimation due to Thermal Expansion ...........................................D-1

D.2 Isothermal Leakage Rate Estimation .....................................................................D-4

APPENDIX E O-RING MASS LOSS MEASUREMENTS.....................................................E-1

APPENDIX F LEAK RATE MEASUREMENTS WITHOUT O-RING .................................... F-1
vii

LIST OF FIGURES

Figure 2-1. A schematic illustration of the experimental apparatus..................................... 2-3

Figure 2-2. Photographs of the experimental apparatus..................................................... 2-4

Figure 3-1. Post-test photographs showing silver from the metallic O-ring deposited on 

the O-ring groove and the O-ring bonded to the test vessel (Vessel #1) 

body................................................................................................................. 3-2

Figure 3-2. Temporal variations of vessel pressure at 24 °C without metallic O-ring. ......... 3-3

Figure 3-3. Temporal variations of vessel pressure and temperature in Test # Metallic-2 

(Test #2). ......................................................................................................... 3-4

Figure 3-4. Temporal variations of vessel pressure and temperature in Test # Metallic-2 

(Test #2) with time scale extended, including the cool-down phase. ................ 3-5

Figure 3-5. Temporal variations of vessel pressure and temperature in Test # Metallic-3

(Test #3). ......................................................................................................... 3-6

Figure 3-6. Temporal variations of vessel pressure and temperature in Test # Metallic-3 

(Test #3) with time scale extended, including the cool-down phase. ................ 3-7

Figure 3-7. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-3 (Test #3). .............................................................................. 3-8

Figure 3-8. Temporal variations of vessel pressure and temperature in Test # Metallic-4 

(Test #4). ......................................................................................................... 3-9

Figure 3-9. Temporal variations of vessel pressure and temperature in Test # Metallic-4 

(Test #4) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-10

Figure 3-10. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-4 (Test #4). ............................................................................ 3-11

Figure 3-11. Temporal variations of vessel pressure and temperature in Test # Metallic-5 

(Test #5). ....................................................................................................... 3-12

Figure 3-12. Temporal variations of vessel pressure and temperature in Test # Metallic-5 

(Test #5) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-13

Figure 3-13. Post-test photograph of Vessel #5 (vessel body with O-ring removed) after 

exposure at 427 °C (800 °F) for 9 h. .............................................................. 3-13

Figure 3-14. Temporal variations of vessel pressure and temperature in Test # Metallic-6 

(Test #6). ....................................................................................................... 3-14

Figure 3-15. Temporal variations of vessel pressure and temperature in Test # Metallic-6 

(Test #6) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-15

Figure 3-16. Temporal variations of vessel pressure and temperature in Test # Metallic-7 

(Test #7). ....................................................................................................... 3-16

Figure 3-17. Temporal variations of vessel pressure and temperature in Test # Metallic-7 

(Test #7) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-17

Figure 3-18. Temporal variations of vessel pressure and temperature in Test # Metallic-8 

(Test #8). ....................................................................................................... 3-18

Figure 3-19. Temporal variations of vessel pressure and temperature in Test # Metallic-8 

(Test #8) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-19

Figure 3-20. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-8 (Test #8). ............................................................................ 3-20

Figure 3-21. A photograph showing the pressure gauge used to check the pressure 

transducer performance................................................................................. 3-21
viii

Figure 3-22. Temporal variations of vessel pressure and temperature in Test # Metallic-9 

(Test #9). ....................................................................................................... 3-22

Figure 3-23. Temporal variations of vessel pressure and temperature in Test # Metallic-9 

(Test #9) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-23

Figure 3-24. Comparison of pressure measurements from pressure transducer and 

pressure gauge.............................................................................................. 3-24

Figure 3-25. Temporal variations of vessel pressure and temperature in Test # Metallic-10 

(Test #10). ..................................................................................................... 3-25

Figure 3-26. Temporal variations of vessel pressure and temperature in Test # Metallic-10 

(Test #10) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-26

Figure 3-27. Temporal variations of vessel pressure and temperature in Test # Metallic-11 

(Test #13). ..................................................................................................... 3-27

Figure 3-28. Temporal variations of vessel pressure and temperature in Test # Metallic-11 

(Test #13) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-28

Figure 3-29. Temporal variations of vessel pressure and temperature in Test # Metallic-12 

(Test #14). ..................................................................................................... 3-29

Figure 3-30. Temporal variations of vessel pressure and temperature in Test # Metallic-12 

(Test #14) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-30

Figure 3-31. Temporal variations of vessel pressure and temperature in Test # EDPM-a 

(Test #11). ..................................................................................................... 3-31

Figure 3-32. Temporal variations of vessel pressure and temperature in Test # EPDM-a 

(Test #11) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-32

Figure 3-33. Temporal variations of vessel pressure and temperature in Test # EPDM-b 

(Test #15). ..................................................................................................... 3-33

Figure 3-34. Temporal variations of vessel pressure and temperature in Test # EPDM-b 

(Test #15) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-34

Figure 3-35. Isothermal reference helium leakage rate during the 25 h heating in 

Test # EPDM-b (Test #15). ............................................................................ 3-35

Figure 3-36. Photograph of the EPDM O-ring after thermal exposure. ............................... 3-36

Figure 3-37. Photograph showing the disintegration of the tested EPDM O-ring when an 

attempt was made to remove the O-ring from the groove............................... 3-36

Figure 3-38. Temporal variations of vessel pressure and temperature in Test # PTFE-a 

(Test #12). ..................................................................................................... 3-37

Figure 3-39. Temporal variations of vessel pressure and temperature in Test # PTFE-a 

(Test #12) with time scale extended, including the complete cool-down 

phase............................................................................................................. 3-38

Figure 3-40. Isothermal reference helium leakage rate during the 22 h heating in 

Test # PTFE-a (Test #12). ............................................................................. 3-39

Figure 3-41. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-1. ............................................................................................. 3-41

Figure 3-42. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-2. ............................................................................................. 3-42

Figure 3-43. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-3. ............................................................................................. 3-43

Figure 3-44. Temporal variations of vessel pressure and temperatures in 
ix

Test # EPDM-4. ............................................................................................. 3-44

Figure 3-45. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-5. ............................................................................................. 3-45

Figure 3-46. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-6. ............................................................................................. 3-46

Figure 3-47. Photographs showing the thermally degraded EPDM O-ring after 8-h thermal 

exposure at 316 °C (600 °F) from the four EPDM O-ring tests 

(Tests # EPDM-1, EPDM-2, EPDM-3, and EPDM-4). Tests # EPDM-5 and 

# EPDM-6 were conducted at 900 °C for 9 h.................................................. 3-47

Figure 3-48. Photographs showing the vessel interiors after 9 h thermal exposure at 

900 °C from Tests # EPDM-5 and EPDM-6.................................................... 3-47

Figure 3-49. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-1. ........................................................................................... 3-48

Figure 3-50. Calculated isothermal reference helium leakage rate in Test # Silicone-1. ..... 3-49

Figure 3-51. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-2. ........................................................................................... 3-50

Figure 3-52. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-3. ........................................................................................... 3-51

Figure 3-53. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-4. ........................................................................................... 3-52

Figure 3-54. Photographs showing the thermally degraded silicone O-ring after 8-h 

thermal exposure at 316 °C (600 °F) for the four silicone O-ring tests............ 3-53

Figure 3-55. Temporal variations of vessel pressure and temperatures in Test # Butyl-1... 3-54

Figure 3-56. Temporal variations of vessel pressure and temperatures in Test # Butyl-2... 3-55

Figure 3-57. Temporal variations of vessel pressure and temperatures in Test # Butyl-3... 3-56

Figure 3-58. Photographs showing the thermally degraded butyl O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the three butyl O-ring tests........................... 3-57

Figure 3-59. Temporal variations of vessel pressure and temperatures in Test # Viton-1... 3-58

Figure 3-60. Temporal variations of vessel pressure and temperatures in Test # Viton-2... 3-59

Figure 3-61. Temporal variations of vessel pressure and temperatures in Test # Viton-3... 3-60

Figure 3-62. Temporal variations of vessel pressure and temperatures in Test # Viton-4... 3-61

Figure 3-63. Photographs showing the thermally degraded Viton O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the four Viton O-ring tests. ........................... 3-62

Figure 3-64. Temporal variations of vessel pressure and temperatures in Test # PTFE-1.. 3-63

Figure 3-65. Temporal variations of vessel pressure and temperatures in Test # PTFE-2.. 3-64

Figure 3-66. Temporal variations of vessel pressure and temperatures in Test # PTFE-3.. 3-65

Figure 3-67. Photographs showing the thermally degraded PTFE O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the three PTFE O-ring tests. ........................ 3-66

Figure 3-68. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-1 (Vessel #10). ........................................................... 3-68

Figure 3-69. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-2 (Vessel #11). ........................................................... 3-69

Figure 3-70. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-3 using the cap of Vessel #12 and the body (with boss 

re-surfaced) of Vessel #10. ............................................................................ 3-70

Figure 3-71. Photographs showing the thermally degraded EPDM-Metallic O-rings after 

9-h thermal exposure at 800 °C (1472 °F) for the two EPDM-Metallic O-ring 

tests............................................................................................................... 3-71

Figure 3-72. Temporal variations of vessel pressure and temperatures in 

Test # Blank-Metallic-1 using the cap of Vessel #13 and the body (with boss 
x

re-surfaced) of Vessel #11. ............................................................................ 3-72

Figure 3-73. Photographs showing the thermally degraded Metallic O-ring after 9-h 

thermal exposure at 900 °C (1652 °F) for the Blank-Metallic O-ring test. ....... 3-72

Figure 3-74. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-EPDM-1 using Vessel #16. ...................................................... 3-73

Figure 3-75. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-EPDM-2 using Vessel #17. ...................................................... 3-74

Figure 3-76. Photographs showing the thermally degraded EPDM O-rings after 9-h 

thermal exposure at 900 °C (1652 °F) for the EPDM-EPDM O-ring test......... 3-75

Figure 3-77. Temporal variations of vessel pressure and temperatures in 

Test # Butyl-Butyl-1 using Vessel #16............................................................ 3-76

Figure 3-78. Photograph showing the thermally degraded butyl O-rings after 9-h thermal 

exposure at 900 °C (1652 °F) for the butyl-butyl O-ring test........................... 3-76

Figure 3-79. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-1 using the cap of Vessel #14 and the body of 

Vessel #12..................................................................................................... 3-78

Figure 3-80. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-1 using the cap of Vessel #14 and the body of 

Vessel #12..................................................................................................... 3-79

Figure 3-81. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-2 using the cap of Vessel #15 and the body of 

Vessel #13..................................................................................................... 3-80

Figure 3-82. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-2 using the cap of Vessel #15 and the body of 

Vessel #13..................................................................................................... 3-81

Figure 3-83. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-3 using Vessel #18.................................................... 3-82

Figure 3-84. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-3 using the cap of Vessel #18. .................................. 3-83

Figure 3-85. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-4 using Vessel #19.................................................... 3-84

Figure 3-86. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-4 using Vessel #19.................................................... 3-85

Figure 3-87. Photographs showing the thermally degraded Metallic-Metallic O-rings after 

9-h thermal exposure at 900 °C (1652 °F) for the two Metallic-Metallic O-ring 

tests............................................................................................................... 3-85

Figure A-1. Design drawing of single O-ring test vessel body..............................................A-1

Figure A-2. Design drawing of removable flange for single metallic seal (vessel cap). ........A-2

Figure A-3. Design drawing of removable flange for single polymeric seal (vessel cap). .....A-3

Figure A-4. Design drawing of double O-ring test vessel body. ...........................................A-4

Figure A-5. Design drawing of removable flange for double O-ring (outer EPDM-inner 

metallic) seal (vessel cap)................................................................................A-5

Figure A-6. Design drawing of removable flange for double O-ring (outer polymeric-inner 

polymeric) seal (vessel cap).............................................................................A-6

Figure A-7. Design drawing of removable flange for double O-ring (outer metallic-inner 

metallic) seal (vessel cap)................................................................................A-7

Figure D-1. Schematics showing the systems used in the thermodynamic analysis............D-2

Figure E-1. Photographs showing EPDM and butyl O-rings after thermal exposure at 

316 °C (600 °F) for 8 h.....................................................................................E-2

Figure E-2. Photograph showing an EPDM O-ring after thermal exposure at 
xi

450 °C (842 °F) for 8 h.....................................................................................E-2

Figure F-1. Temporal variation of normalized vessel pressure at 24 °C without O-ring at 

different torques used to tighten the vessel flanges.......................................... F-1

xiii

LIST OF TABLES

Table 2-1. O-ring seal material information .......................................................................... 2-5

Table 2-2. Outer O-ring seal material information................................................................. 2-6

Table 2-3. Nominal test conditions and parameters (Phase I in chronological order).......... 2-10

Table 2-4. Nominal test conditions and parameters (Phase II in chronological order)......... 2-11

Table 2-5. Nominal test conditions and parameters (Phase III in chronological order)........ 2-12

Table 4-1. Summary of tests where constant vessel pressure was not observed during 

thermal exposure................................................................................................ 4-3

Table 4-2. Summary of all the test results ............................................................................ 4-4

Table B-1. Measured internal volumes of pressure vessels at room temperature.................B-1

Table C-1. Summary of standard uncertainty components in thermocouple 

measurements....................................................................................................C-1

Table C-2. Manufacturer’s specifications of the pressure transducer ...................................C-2

Table C-3. Summary of standard uncertainty components in pressure measurements ........C-2

Table D-1. Calculations of vessel volume expansion at high temperatures ..........................D-3

Table E-1. Test matrix for O-ring mass loss measurements.................................................E-1

Table E-2. O-ring mass loss measurements at 316 °C (600 °F) for 8 h ................................E-3

Table E-3. O-ring examinations after thermal exposure at 316 °C (600 °F) for 8 h...............E-4

Table E-4. O-ring mass loss measurements at 450 °C (842 °F) for 8 h ................................E-4

xv

EXECUTIVE SUMMARY

The objective of this work is to provide experimental seal performance data for metallic and 

polymeric seals typically used in radioactive material transportation packages for beyonddesign-basis temperature excursions. There are three phases in this project. Phases I and II 

used a single O-ring configuration, whereas Phase III used a double O-ring configuration.

Tests were conducted using a test package (a pressure vessel) made of stainless steel SS 304 

filled with helium initially at either 5 bar or 2 bar at room temperature. Since it is difficult to 

control a real fire environment for testing, a programmable electric furnace was used to provide 

various controlled thermal environments to heat the vessel to a specified temperature for a predetermined duration, which varied from several hours to one or two days. The vessel pressure 

was monitored to determine if a leak occurred at the exposure temperature.

In Phase I, 15 tests including two shakedowns were performed with some at beyond-designbasis thermal exposure conditions. Of the tests conducted, 12 tests used metallic seals, two 

used ethylene-propylene compound (EPDM) seals, and one used a PTFE (Teflon) seal.

Of the five repeat metallic-seal tests (Tests # Metallic-2, # Metallic-3, # Metallic-4, # Metallic-8, 

and # Metallic-12), leakage (decreasing vessel pressure) was observed in three of the tests 

(Tests # Metallic-3, # Metallic-4, and # Metallic-8) during the 9 h 800 °C (1472 °F) exposure. 

The times when the leakage occurred (the vessel pressure started to decrease) varied in the 

three tests performed. In Test # Metallic-3, measurable leakage occurred approximately 6.9 h 

after the test temperature 800 °C (1472 °F) had been reached. In Test # Metallic-4, measurable 

leakage occurred about 2.8 h into the 800 °C (1472 °F) exposure. In Test # Metallic-8, leakage 

was observed roughly 3 h into the test. The two shakedown tests (Tests # Metallic-1 and # 

Metallic-9) were conducted using a 30 min and 4 h exposure to 800 °C (1472 °F), respectively, 

and the seal appeared to hold vessel pressure. No leakage (unchanged vessel pressure within 

the pressure measurement uncertainty) was also observed in the two metallic seal tests that 

used 100 °C (212 °F) incremental heating from 427 °C (800 °F) to 627 °C (1161 °F) and from 

427 °C (800 °F) to 727 °C (1341 °F), respectively, with at least 9 h exposure at each 

temperature increment. Three repeat metallic seal tests were also conducted at the seal 

maximum operating temperature of 427 °C (800 °F) for 9 h. The seal maintained vessel 

pressure within the measurement uncertainty of the pressure transducer in all three tests.

No leakage was observed in one EPDM seal tested at 300 °C (572 °F) for more than 20 h (Test 

# EPDM-a); however, leakage was observed immediately after the vessel had attained the 

nominal target temperature of 450 °C (842 °F) in another EPDM seal test (Test # EPDM-b). 

Leakage was also observed in the test (Test # PTFE-a) that used a PTFE seal after it had been 

subjected to 300 °C (572 °F) exposure for 22 h during the cooling phase.

In Phase II, additional experimental seal performance data for the two polymeric (EPDM and 

PTFE) O-ring seals were measured. For exploratory purposes, three additional O-ring seals 

made of other polymeric materials (butyl, Viton, and silicone) were also studied. Leakage rates 

were measured using the same stainless SS 304 steel pressure vessel sealed with a polymeric 

O-ring and following the same test procedure in Phase I. The vessel was filled with helium 

initially at 2 bar at room temperature and then placed and heated in a programmable electric 

furnace to a nominal temperature of 316 °C (600 °F) for 8 h. The vessel pressure was 

monitored to determine if a leak occurred as a result of O-ring failure during the thermal 

exposure.
xvi

A total of 18 tests were performed: four tests used EPDM O-rings, four used Viton, four used 

silicone, three used butyl, and three used PTFE. For the EPDM, butyl, PTFE, and silicone Orings, the results, except one silicone O-ring test, showed that within the measurement 

uncertainty of the pressure gauge, there were no discernible decreases in vessel pressure 

during the thermal exposure and that the initial pressure was restored after cool-down to room 

temperature, which implied that any leak was undetectable. For all the tests that used Viton Orings, the vessel pressure peculiarly increased during the thermal exposure, and the reason for 

the pressure increase remains unresolved. The post-test examinations of the O-rings revealed 

that all the O-rings (except PTFE) were irreversibly deformed and severely damaged after the 

extreme thermal exposure, and the seal material was found to be squeezed out of the O-ring 

groove of the pressure vessel.

In Phase III, leak tests were performed using the same test pressure vessel with a double Oring configuration. The outer-inner O-ring combinations were metallic-metallic, EPDM-metallic,

blank-metallic, EPDM-EPDM, and butyl-butyl. The vessel was filled with helium initially at 5 bar 

at room temperature and was then thermally exposed at 800 °C (1472 °F) or 900 °C (1652 °F) 

for 9 h in the electric furnace to determine if a leak occurred. These O-ring temperatures and 

test periods are more severe than the 800°C for 30 min hypothetical accident conditions 

described in 10 CFR 71. A total of eleven tests were carried out using a double O-ring 

configuration: four tests used metallic-metallic, three used EPDM-metallic, one used blank (no 

O-ring)-metallic, two used EPDM-EPDM, and one used butyl-butyl. For the four metallicmetallic O-ring tests, leaks were observed in the two tests when the same torque for a single 

metallic O-ring was used. Even when the appropriate torque value was applied for the double 

O-ring configuration, the other two metallic-metallic tests leaked, but only after being at a 900°C 

environment between two and eight hours; there was no leakage prior to that period. No leak 

was observed in the blank-metallic test. Leaks were detected in all the EPDM-EPDM and butylbutyl tests, even before the test temperature was reached. No leaks were observed with the 

EPDM-metallic double O-ring configuration.
xvii

ACKNOWLEDGMENTS

This work was sponsored by the U.S. Nuclear Regulatory Commission (NRC). The authors 

would like to thank Felix Gonzalez (Project Manager), Chris Bajwa, Earl Easton, Robert 

Einziger, Joseph Borowsky, Mark Adams and Shivani Mehta of NRC, Anita Aikins-Afful from the 

NRC for technical editing the report, Harold Adkins of Pacific Northwest National Laboratory 

(PNNL), William Luecke and Michael Moldover of NIST for many helpful discussions, and Gary 

L. Stevens of NRC for checking the pressure vessel design calculations.

DISCLAIMER

Certain commercial entities, equipment, or materials may be identified in this document in order 

to describe an experimental procedure or concept adequately. Such identification is not 

intended to imply recommendation or endorsement by the National Institute of Standards and 

Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily 

the best available for the purpose.

xix

ABBREVIATIONS

ANSI American National Standards Institute

ASME American Society of Mechanical Engineers

CFR Code of Federal Regulations

CRUD Chalk River Unidentified Deposits

DAQ Data Acquisition

EPDM Ethylene-Propylene Compound (Diene Monomer)

FDS Fire Dynamics Simulator

HAC Hypothetical Accident Conditions

ISO International Organization for Standardization

NIST National Institute of Standards and Technology

NRC Nuclear Regulatory Commission

PTFE Polytetrafluoroethylene (Teflon)

RES NRC Office of Nuclear Regulatory Research

1-1

1 BACKGROUND

1.1 Introduction

The U.S. Nuclear Regulatory Commission (US NRC) is collecting data to better characterize the 

performance envelope of seals used on radioactive material transportation packages, during fire 

exposures that exceed the hypothetical accident conditions (HAC) fire (800 °C for 30 min) 

described in 10 CFR Part 71.73. Examples of an accident that could potentially produce an 

exposure beyond the HAC fire were the Caldecott Tunnel fire in 1982 (Adkins et al., 2007a), the 

Baltimore Tunnel fire that occurred in 2001 (Adkins et al., 2007b), and the MacArthur Maze fire 

in 2007 (Dunn et al., 2009). The performance of package seals is important for determining the 

potential for release of radioactive material from a package during a beyond-design-basis 

accident because the seals, in general, have lower temperature limits than other package 

components. It should be noted that the regular approach for evaluating seal performance in 

any HAC or beyond-design-basis fire is to assume that seals fail completely if their normal 

operating temperature limits are exceeded at any point in the transient thermal evaluation of a 

given package. Such conservative and bounding approach yields the maximum possible 

estimates of potential for release of radioactive material from transportation packages in fire 

exposures.

NUREG/CR-6886, “Spent Fuel Transportation Package Response to the Baltimore Tunnel Fire 

Scenario,” describes in detail an evaluation of the potential release of radioactive materials from 

three different transportation packages (Adkins et al., 2007b). This evaluation used estimates of 

temperatures from a simulation of the Baltimore Tunnel fire using the NIST Fire Dynamics 

Simulator (FDS) (McGrattan and Hamins, 2003) as boundary conditions for numerical models to 

determine the temperature of various components of the packages, including the seals. For two 

of the packages evaluated, the model-estimated temperatures of the seals exceeded their 

continuous-use rated service temperature, meaning a potential release of radioactive material 

(i.e., CRUD) if one assumed zero effectiveness (complete failure) of the seal. However, for both 

of those packages, the analysis determined, by a bounding calculation, that the maximum 

expected release was well below the regulatory limits for the release allowed during the HAC 

series of events in 10 CFR Part 71.

Previous work reported in the literature has mainly focused on elastomeric seals and 

temperatures well below 800 °C (1472 °F). The test fixtures in previous work typically consisted 

of two flanges or two plates with two concentric O-ring grooves, one for the test seal and one for 

the secondary external seal, and a small cavity for helium tracer gas (e.g., Bronowski, 2000, 

Marlier, 2010). Testing of package seals to determine their performance in beyond-designbasis fire scenarios can provide physical data needed to understand the likelihood of a release 

of radioactive materials.

The objective of this work is to provide experimental seal performance data for metallic and 

polymeric seals for thermal exposures beyond their rated temperatures. Both materials are 

used in the design of seals, although metallic seals tend to have higher allowable temperatures 

because of their design and material properties. The data is obtained using a test fixture 

consisting of a vessel body and flange cap; the scope of the testing does not evaluate the size 

of the test fixture as a test parameter. The scope of the testing does not evaluate the 

performance of aged seals under beyond-design-thermal exposure. Since it is difficult to control 

and replicate a real fire environment for testing, an electric furnace was used to provide various 

controlled and repeatable thermal environments.
1-2

The pressure drop method was used in this study to examine the seal performance at elevated 

temperatures. The implementation of the pressure drop technique in a harsh thermal 

environment proved to be less challenging than other more sensitive test methods, as described 

in ANSI N14.5-1997. The use of any temporal decrease in vessel pressure as a means to 

detect potential leakage is best applied to isothermal conditions (ANSI N14.5-1997) where for 

an ideal gas the molar leakage rate is simply proportional to the rate of decrease in pressure. 

However, under transient heating conditions, even in the absence of a leak, the rate of increase 

in vessel volume due to thermal expansion could also contribute to the temporal decrease in 

pressure, which may be construed as a leak. Therefore, all the leak tests were conducted by 

maintaining the pressure vessel at a constant elevated temperature in the furnace. Although 

the monitoring of pressure drop is not the most sensitive way to detect leaks, the sensitivity to 

detect a small pressure drop could be greatly enhanced if the vessel pressure is monitored over 

a very long duration. In addition, the sensitivity of the method can further be improved by using 

a smaller test volume since the sensitivity of a pressure drop is inversely proportional to the test 

volume (ANSI N14.5-1997). This work make use of these two attributes, long measurement 

duration and small vessel volume.

The testing was divided into several phases. In Phases I and II, a single O-ring configuration 

was used. Phase I focused on the thermal performances of a metallic and two polymeric 

(EPDM and PTFE) O-rings. In Phase II, the focus was solely on polymeric O-rings, and the test 

series was expanded to include butyl, silicone, and Viton O-rings. In Phase III, a double O-ring 

configuration was used. The outer-inner O-ring combinations were metallic-metallic, EPDMmetallic, blank-metallic, EPDM-EPDM, and butyl-butyl.
2-1

2 TECHNICAL APPROACH

2.1 Experimental Apparatus

2.1.1 Phase I

The test fixture consists of a seamless vessel body with a flange machined from a stainless 

steel (SS 304) cylindrical stock and a removable SS 304 flange (vessel cap) with seal groove 

machined to O-ring manufacturer specifications. The flange dimensions were made in 

conformity with the ASME Standard B16.5-2009 [6], Flange Class 2500 with a design pressure 

rating up to 29.2 bar (424 psi) at 800 °C (1472 °F) (Table 2-2.1, ASME Standard B16.5-2009). 

The vessel cavity had a nominal internal volume1 of 100 mL. The design drawings of the test 

vessel body and the removable flange (vessel cap) are provided in Appendix A. Nine test 

vessels with the same dimensional tolerances were constructed and used for this test series.

The metallic seal was a Garlock Helicoflex2 metal O-ring (GHx P/N H-311291) made of Inconel 

718 and silver with an outer diameter of 6.35 cm (2.5 in.) and a cross section of 0.32 cm (0.125 

in.). The metallic O-ring groove size and flange surface roughness (minimum 0.80 μm and 

maximum 1.6 μm) were based on the specifications listed in the Garlock Helicoflex technical 

literature. The polymeric seals were ethylene-propylene compound (EPDM) O-ring (DBR 

Industries, 2-228 E0740-75, Lot # 2Q060080040952) and PTFE O-ring (DBR Industries, 228 

PTFE; Lot # 04/07 12730-1).

The vessel body and the cap were joined together using four bolts (SS 304 1-1/8 in. 7 TPI). 

Each bolt was tightened using a micrometer torque wrench (KD Tools 2953 3⁄4 in. drive 100-600 

lb ft, 153-830 N·m, with a resolution of 3.4 N·m). A torque of 416 N·m (307 lb ft) ± 2 N·m 

(expanded uncertainty with a coverage factor of 2) was used for the metallic seals and 271 N·m 

(200 lb ft) for the EPDM and PTFE O-rings.

3

 In addition, a silicone-base O-ring lubricant 

(Parker Super O-Lube® from Parker Seal, Lexington, Kentucky) was applied to the EPDM O-ring

before the two flanges were bolted together. A 24 cm long stainless steel tubing with an inside 

diameter of 0.48 cm (0.189 in.) and an outside diameter of 0.953 cm (0.375 in.) was inserted 

into the bottom of the vessel body flush through a straight-hole with a bevel-groove and was allaround fillet welded to the vessel.

4

 The exposed end of the tubing was connected to one arm of 

a union cross (Swagelok SS-400-4) via a reducing union (Swagelok SS-600-6-4) and a port 

connector (Swagelok SS-401-PC). Two needle valves (Swagelok SS-1RS4) or bellow valves 

(Swagelok SS-4BK)5 for filling and evacuating the test vessel were mounted on the other two 

respective arms of the union cross using two port connectors (Swagelok SS-401-PC). A union 

tee (Swagelok SS-400-3) was connected to the fourth arm of the union cross via a port 

 1 Actual internal volume (see Appendix B) was measured using an internal micrometer with a resolution of 

0.005 mm).

2 The seal material and manufacturer were selected and specified by the U.S. NRC.

3 The torque requirement for the polymeric O-ring (used as a face seal) was recommended by the 

manufacturer to be metal-to-metal fit.

4 Three vessels (Vessels #1, #6, and #7) were tested for surface flaws in the welds using the ASME liquid 

penetration test (ASME, BVPC-VIII-1-2007) by a local testing laboratory. Vessels #6 and #7 were tested 

brand new while Vessel #1 was tested after two thermal exposures, one at 800 °C (1472 °F) for 0.5 h and 

one at 427 °C (800 °F) for 9 h. The test results showed no surface flaws in the welds in all three vessels. 

Note that performing x-ray of this type weld is difficult at best and with the tube passing through it makes 

obtaining a good x-ray of the entire weld impossible.

5 For the first three tests, needle valves were used.
2-2

connector (Swagelok SS-401-PC). A pressure transducer (Omegadyne, PX01C1-500A5T)6 

was attached to one arm of the union tee via a tube adapter (Swagelok SS-4-TA-7-4) to 

measure the vessel pressure. A grounded thermocouple (Omega K-type CAIN-18G-24)6 to 

monitor the vessel internal temperature was inserted into the vessel interior through a reducing 

union (Swagelok SS-400-6-2) which was connected to the other arm of the union tee via a port 

connector (Swagelok SS-401-PC), the union cross, and the stainless steel tubing.

Due to the large mass and thermal inertia of the test vessel, it was anticipated that the vessel 

temperature might not be completely uniform when the vessel was heated. Two additional 

thermocouples (Omega K-type CAIN-18G-24)7 were placed near the O-ring groove to monitor 

the temperatures experienced by the seal. In order to securely attach the two thermocouples to 

their desired locations, two divots 1.25 mm deep and 3 mm in diameter were drilled 90 ° apart in 

the side of the flange that contained the O-ring groove. The thermocouple tips were placed in 

these divots, and the thermocouples were clamped onto the sides of the vessel.

Figure 2-1 is a schematic illustration of the experimental apparatus, two photographs of which 

are shown in Figure 2-2. The upper photograph of Figure 2-2 shows the test vessel placed 

inside the furnace before testing, and the lower photograph shows the furnace opening was 

covered with a thermal insulation board during testing. Note that all the connection fittings, the 

two valves, and the pressure transducer were located outside the furnace.

The exposure of the seal to a high temperature environment was achieved using a 

programmable temperature-controlled electric furnace (Carbolite CWF 1200) with an internal 

capacity of 25.4 cm × 25.4 cm × 40.64 cm (10 in. × 10 in. × 16 in.). The electric furnace has a 

maximum operating temperature of 1200 °C (2192 °F). Although the furnace was equipped with 

a digital temperature display for internal furnace temperature, a grounded thermocouple 

(Omega K-type CAIN-18G-24) was placed inside the furnace interior to monitor the furnace 

temperature for crosscheck. The differences between these two readings were less than 10 °C.

 6 See Appendix B for measurement uncertainty discussion.

7 Omega WTK-10-60 (Inconel cladding) thermocouples were used subsequently to facilitate their mounting 

onto the two flanges.
2-3

Figure 2-1. A schematic illustration of the experimental apparatus.

Electric

furnace

Pressure 

vessel

Pressure

transducer

Vacuum

Helium

Thermocouple

Valve

Thermocouple

Thermocouple

Thermocouple
2-4

Figure 2-2. Photographs of the experimental apparatus.

2.1.2 Phase II

The experimental apparatus used in this Phase was identical to the one used in Phase I, except 

a torque of 271 N·m (200 lb ft) was used8 to tighten the vessel body and cap for all the 

polymeric O-rings tested in this Phase. The five polymeric O-rings used were EPDM, silicone, 

butyl, Viton, and PTFE. Table 2-1. O-ring seal material information lists the O-ring product 

descriptions and their maximum operating temperatures. All the O-rings were obtained from 

DBR Industries.

 8 The original torque wrench used in Phase I was broken, and the wrench manufacturer went out of 

business; the new replacement wrench can only be set at 6.8 N m (5 lb ft) increments. The applied 

torque was 414 N m (305 lb ft), not 416 N m (307 lb ft) in Phase I.
2-5

Table 2-1. O-ring seal material information

O-ring material Product information

EPDM Ethylene-propylene compound (Item ID 2-228 E0740-75, Lot # 2Q06 

0080040952); maximum operating temperature: 149 °C (300 °F)

Silicone Silicone (Item ID 228 S570, Lot # 1Q00 20003195501); maximum 

operating temperature: 232 °C (450 °F)

Butyl Butyl 70 (Item ID 228 Butyl 70, Lot # 4Q12 107110/60); maximum 

operating temperature: 121 °C (250 °F)

Viton Viton Durometer 75 (Item #83248/1-228, Lot # 3Q09 9206002); 

maximum operating temperature: 204 °C (400 °F)

PTFE PTFE (Item ID 228 PTFE, Lot # 04/07 12730-1); maximum operating 

temperature: 260 °C (500 °F)

2.1.3 Phase III

Five double O-ring configurations were used in this test series: (1) an outer metallic O-ring and 

an inner metallic O-ring, (2) an outer EPDM O-ring and an inner metallic O-ring (Garlock 

Helicoflex/Technetics) and (3) an outer blank (no) O-ring and an inner metallic O-ring), (4) an 

outer EPDM O-ring and an inner EPDM O-ring, and (5) an outer butyl O-ring and an inner butyl 

O-ring. The outer O-ring material information is provided in Table 2-2. The inner metallic O-ring 

had the same dimensions as the ones used in the single O-ring configuration test series in 

Phase I. The inner EPDM and butyl O-rings were the same ones used in the single O-ring 

configuration test series in Phase II. Other than having a wider boss to accommodate the 

double O-ring grooves on the vessel cap (see Appendix A), the vessel geometry was identical to 

that used in Phases I and II. For the metallic-metallic O-ring configuration, two tests used the 

same torque value of 416 N m (307 lb ft) as was used in the single metallic O-ring tests, and two 

tests used a torque value of 976 N m (720 lb ft)9

. For EPDM-metallic O-ring and blank-metallic 

O-ring configurations, a torque value of 416 N m was used. For EPDM-EPDM and Butyl-Butyl 

O-ring configurations, a torque value of 637 N m (470 lb ft)9 was used. 

 9 The torque value was calculated based on the proportionality between torque and O-ring diameter. For 

double O-rings, the sum of the two O-ring diameters was used as the equivalent effective O-ring diameter 

for torque estimation (per manufacturer’s technical information).
2-6

Table 2-2. Outer O-ring seal material information

O-ring material Product information

Metallic Technetics Group Columbia P/N: H-312522

EPDM

Ethylene-propylene compound (DBR Industries, Item ID 2-235 E0740-

75, Lot # 1Q140080218069); maximum operating temperature: 149 °C 

(300 °F)

Butyl Butyl 70 (Item ID 2-235-B0612-70, Lot # 3Q140060012619/M0553228); 

maximum operating temperature: 121 °C (250 °F)
2-7

2.2 Experimental Procedure

2.2.1 Phase I

This test series involved the thermal exposure of a test vessel with a metallic or polymeric seal 

in an electric furnace to determine the seal performance at elevated temperatures. Two 

nominal temperatures, 800 °C (1472 °F) and 427 °C (800 °F), were selected for thermal 

exposure testing of the metallic cask seals; 10 CFR Part 71 uses a fire environment thermal 

exposure temperature of 800 °C (1472 °F) and the published maximum operating temperature 

of the metallic seal used in this study is 427 °C (800 °F)10. For most of the metallic seal tests, 

the thermal exposure time was 9 h after the flange with the O-ring groove had attained the test 

temperature. Other thermal exposure protocols were used for some tests (see section 2.3

below). For the polymeric seals, the test seal was exposed to a flange temperature of 150 °C 

(302 °F) for 1 h, 200 °C (392 °F) for 1 h, 250 °C (482 °F) for 1 h, and 300 °C (572 °F) for 22 h. 

The maximum operating temperatures of the EPDM and PTFE O-rings are 149 °C (300 °F) and 

260 °C (500 °F), respectively.

The assembled test vessel with the test seal installed was placed in the electric furnace, and the 

tubing system was connected to the vessel. A vacuum line was attached to one valve of the 

tubing system, and a helium supply line from a 1A cylinder of compressed helium (analytical 

grade) was connected to the other valve. With the two valves opened and the pressure 

regulator of the cylinder closed, the entire test system was then evacuated for 60 s using an oilless rocking piston vacuum pump (GAST Manufacturing Corporation Model 71R645-V110-

UAP). The valve that was connected to the vacuum line was closed, separated from the 

vacuum line, and capped using a plug (Swagelok SS-400-P). The pressure regulator of the 

compressed helium cylinder was then slowly opened to fill the test vessel with helium at room 

temperature to a nominal pressure of 5 bar. The helium supply line was disconnected from the 

fill valve, and the valve was capped using a plug (Swagelok SS-400-P). The tubing connections 

of the test apparatus were immediately tested for leaks using soapy water. The vessel pressure 

was then monitored for more than 48 h. If the vessel pressure variations over this period were 

within the expanded uncertainty of the pressure transducer (0.11 bar)6

, it could be confidently 

assumed that the vessel pressure was unchanged, and the invariance of vessel pressure was 

used as an additional “no leak” indicator.

With no leaks detected, the electric furnace was turned on to heat the test vessel from room 

temperature to the target temperature. Depending on the specific target temperature, the 

heating process could take more than 4 h. Once the vessel flange temperature reached the 

target value, the vessel was heated according to the pre-determined exposure duration (see 

Table 2-3 below). After the thermal exposure, the furnace was turned off, and the vessel was 

allowed to cool to room temperature inside the furnace. The internal temperature and pressure 

of the vessel, the furnace temperature, and the flange temperatures were recorded during the 

entire heat-up and cool-down phases using a LabVIEW-based 16-bit DAQ (Data Acquisition) 

system (National Instruments NI USB-6251) with an input/output connector block (National 

Instruments NI SCC-68). The DAQ system sampled at a rate of 100 Hz; however, the data 

were logged at 1 min intervals.

 10 Generally, in HAC analyses using computer codes, Transportation package design seals do not reach 

the HAC temperature of 800 °C (1472 °F).
2-8

2.2.2 Phase II

The pressure vessel preparation and experimental procedures were identical to those used in 

Phase I although the test conditions and parameters were different (see Table 2-4). Since the 

exposed temperature was not high and the tested polymeric O-ring could be easily removed 

from the O-ring groove, the vessel was re-used in this test series, except that brand new bolts 

were used. Before each test, the O-ring groove and the vessel interior were properly and 

thoroughly cleaned using acetone and/or vinegar. The vessel used was Vessel #3 from 

Phase I.

2.2.3 Phase III

Aside from the test conditions and parameters and the double O-ring configuration (see Table 

2-5), the test preparation and experimental procedures followed those used in Phases I and II.
2-9

2.3 Test Matrix

2.3.1 Phase I

Table 2-3 summarizes the test conditions and parameters used in Phase I. The first column in 

the table lists the revised test number designations used in this revised NUREC/CR-7115 with 

their corresponding test numbers in the original NUREC/CR-7115 listed in the second column. 

The revised test designations provide more clarity, better reflect the test O-ring material, and 

conform to those used in Phase II and III.
2-10

Table 2-3. Nominal test conditions and parameters (Phase I in chronological order)

Test # Test # Vessel # Nominal initial 

vessel conditions Exposure duration

Metallic-1 1* 1 24 °C at 5 bar Heat-up + 30 min at 800 °C + cool-down

Metallic-2 2 2 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

Metallic-3 3 3 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

Metallic-4 4 4 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

Metallic-5 5 5 24 °C at 5 bar Heat-up + 9 h at 427 °C + cool-down

Metallic-6 6 2** 24 °C at 5 bar Heat-up + 9 h at 427 °C + cool-down

Metallic-7 7 1** 24 °C at 5 bar Heat-up + 9 h at 427 °C + cool-down

Metallic-8 8 6 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

Metallic-9 9 1*** 24 °C at 5 bar Heat-up to 427 °C and then to 800 °C for 

about 4 h + cool-down

Metallic-10 10 7 24 °C at 5 bar

Incremental heating from 427 °C to 

627 oC with 100 oC increment§ + cooldown

EPDM-a 11 3** 24 °C at 2 bar

Incremental heating from 150 oC to 

300 °C with 50 oC increment§§ + cooldown

PTFE-a 12 3** 24 °C at 2 bar

Incremental heating from 150 oC to 

300 °C with 50 oC increment§§ + cooldown

Metallic-11 13 8 24 °C at 5 bar

Incremental heating from 427 °C to 

727 oC with 100 °C increment§ + cooldown

Metallic-12 14 9 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

EPDM-b 15 3** 24 °C at 2 bar Heat-up + more than 24 h at 450 oC

*Shakedown test; during this test DAQ malfunctioned, and no temporal data was collected.

**Flange and groove surfaces refurbished.

***Flange and groove surfaces refurbished again.

§Vessel was heated at each set temperature for 9 h or more; exact exposure duration for each test is described in 

Section 3.

§§Vessel was heated at 150 °C (302 °F) for 1 h, 200 °C (392 °F) for 1 h, 250 °C (482 °F) for 1 h, and 300 °C 

(572 °F) for more than 20 h.
2-11

2.3.2 Phase II

Table 2-4 summarizes the test conditions and parameters used in Phase II.

Table 2-4. Nominal test conditions and parameters (Phase II in chronological order)

Test # Vessel # Nominal initial vessel 

conditions Thermal exposure duration

EPDM-1 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Silicone-1 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Silicone-2 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Silicone-3 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Silicone-4 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Butyl-1 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Butyl-2 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Butyl-3 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Viton-1 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Viton-2 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Viton-3 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

Viton-4 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

EPDM-2 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

EPDM-3 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

EPDM-4 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

PTFE-1 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

PTFE-2 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

PTFE-3 3 24 °C at 2 bar Heat-up + 8 h at 316 °C + cool-down

EPDM-5 3 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

EPDM-6 3 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down
2-12

2.3.3 Phase III

Table 2-5 summarizes the test conditions and parameters used in Phase III.

Table 2-5. Nominal test conditions and parameters (Phase III in chronological order)

Test # Vessel #

Nominal initial 

vessel 

conditions

Thermal exposure duration

EPDM-Metallic-111 10 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

EPDM-Metallic-2 11 24 °C at 5 bar Heat-up + 9 h at 800 °C + cool-down

Metallic-Metallic-1 See Note 1 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Metallic-Metallic-2 See Note 2 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

EPDM-Metallic-3 See Note 3 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Blank-Metallic-1 See Note 4 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

EPDM-EPDM-1 16 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

EPDM-EPDM-2 17 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Butyl-Butyl-1 16* 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Metallic-Metallic-3 18 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Metallic-Metallic-4 19 24 °C at 5 bar Heat-up + 9 h at 900 °C + cool-down

Note 1: Body of Vessel #12 and cap of Vessel #14; body of Vessel #14 was never fabricated.

Note 2: Body of Vessel #13 and cap of Vessel #15; body of Vessel #15 was never fabricated.

Note 3: Body of Vessel #10 (with boss re-furbished) and cap of Vessel #12

Note 4: Body of Vessel #11 (with boss re-furbished) and cap of Vessel #13

*Cleaned and re-used

 11 In the double O-ring Test # designation, the following convention is adopted: the outer O-ring is 

identified first, and the inner O-ring is identified second. For example, “EPDM-Metallic” represents the 

outer O-ring as EPDM and the inner O-ring as Metallic.
3-1

3 RESULTS AND DISCUSSION

In this section, test results are presented and discussed. It should be noted that only after the 

vessel had reached the specified test temperature was the pressure drop method applied to 

determine if a leak had occurred under this isothermal thermal exposure condition. During the 

transient heat-up phase of the vessel, the temporal variation of vessel pressure could not readily 

be used to determine if there was a potential leak unless a catastrophic seal failure occurred,

causing a significant drop in pressure. As the vessel was heated, the vessel pressure and 

temperature increased. The temporal volumetric thermal expansion of the pressure vessel also 

introduced an additional parameter. The interplay among these parameters, vessel 

temperature, pressure, and volume, precluded the use of the transient pressure data to 

conclusively determine whether a leak had occurred during the heat-up phase. Another 

indicator for leak was to determine whether the vessel pressure could be restored to the initial 

pressure after the thermal exposure and cool-down. The restoration of the initial pre-test 

charged vessel pressure after cool-down indicated no leak had occurred during a test.

In the following sub-sections, the figures, which show the measured temporal variations of 

vessel pressure and temperatures in each test, contain five curves, one for vessel pressure and 

four for temperatures measured at the vessel interior, two locations on the flange where the test 

O-ring was housed, and the furnace interior. These figures share a common feature; all exhibit 

three stages in the temperature measurements. The initial rise in temperature is due to the 

heating of the test vessel from room temperature to the targeted test temperature in the furnace 

(the heat-up period). The plateau portion of the temperature curve represents the thermal 

exposure duration at the test temperature (the test period), and the subsequent decrease in 

temperature indicates the cooling of the test vessel after turning off the electric furnace at the 

conclusion of the thermal exposure at the test temperature (the cool-down period). In some of 

the figures, overshooting of the targeted temperature is apparent during the heat-up phase as a 

result of an initial furnace set-point higher than the targeted temperature. This was in attempt to 

rapidly ramp up the furnace temperature to shorten the heat-up time.

3.1 Phase I12

3.1.1 Metallic O-Rings

3.1.1.1 Test # Metallic-1 (Test #1; Vessel #1)

The thermal exposure test using Vessel #1 was intended to be a shakedown of the 

experimental apparatus. Four and a-half hours (4.5 h) was required for the flange temperature 

to reach the equilibrium furnace temperature of 800 °C (1472 °F). The vessel was then 

maintained at this temperature for an additional 30 min before turning off the electric furnace.

During the heat-up phase, the DAQ readings from the pressure transducer and the 

thermocouples became erratic due to unknown reasons; the problem was later diagnosed to be 

improper grounding and connection to the DAQ system. The readings from the pressure 

transducer and thermocouples were recorded manually using a voltmeter and a thermocouple 

reader, respectively. At 800 oC (1472 °F), the vessel pressure reached 14.6 bar and was 

holding at 14.6 bar for the additional 30 min of heating. After the vessel was cooled to room 

temperature, the vessel pressure recovered its initial pressure of 5.0 bar at room temperature, 

 12 For clarity, the results are presented and grouped based on the O-ring material rather than on the 

chronological order in the test series for Phases I, II, and III.
3-2

which indicated the no leakage.

A post-test inspection of the test vessel revealed that the metallic seal was soldered to the 

flange of the vessel body and a silver-color coating was imprinted on the surface of the O-ring 

groove, as shown in Figure 3-1. The high-temperature exposure also discolored the test fixture.

Figure 3-1. Post-test photographs showing silver from the metallic O-ring deposited on 

the O-ring groove and the O-ring bonded to the test vessel (Vessel #1) body.

3.1.1.2 Test # Metallic-2 (Test #2; Vessel #2)

Before Test # Metallic-2 (Test #2) was conducted, an exploratory test was performed to 

examine the severity of the leak without using a metallic O-ring. The vessel was charged with 

helium to 5.62 bar at room temperature, and the vessel pressure was monitored at room 
3-3

temperature. Figure 3-2 is the test result, which indicates that the vessel pressure began to 

drop and leakage occurred immediately at the start of the test and continued slowly over the 

entire experimental run-time (26.43 h). The reference helium leakage rates LR,He (ref·cm3

/s) 

were estimated based on the method described in Appendix D. The time-averaged leakage 

rate is estimated to be 2.4 × 10-3 ref·cm3

/s.

Figure 3-2. Temporal variations of vessel pressure at 24 °C without metallic O-ring.

Figure 3-3 shows the temporal variations of the vessel internal pressure and temperature, two 

flange temperatures and furnace temperature during the heat-up, the constant-temperature 

heating (isothermal), and a portion of the cool-down phases for Test # Metallic-2 (Test #2). The 

rapid decrease in the furnace temperature identifies when the furnace was turned off and the 

beginning of the cool-down phase. Figure 3-4 shows a re-plot of Figure 3-3 with the time scale 

extended to include the entire cool-down phase, which normally takes more than several days 

to naturally cool the test vessel inside the powered-down furnace from 800 °C (1472 °F) to room 

temperature.

The scale of the ordinate for pressure in Figure 3-3 is magnified to show that the pressure starts 

to decrease shortly after the vessel temperature has reached 800 °C (1472 °F) and continues to 

decrease very slowly during the remainder of the 9 h constant-temperature heating phase. 

Although the continuous downward trend does seem to imply the occurrence of a very tiny leak, 

the decrease in pressure is within the expanded measurement uncertainty of the pressure 
3-4

transducer (0.11 bar) and is not considered significant in this test. However, the complete 

thermal exposure history, shown in Figure 3-4, indicates that the vessel pressure is restored to 

its original charge pressure (≈ 5 bar) after the thermal exposure, indicating no leakage. For this 

test, the occurrence of a leak is inconclusive based only on the small pressure drop.

Figure 3-3. Temporal variations of vessel pressure and temperature in Test # Metallic-2 

(Test #2).
3-5

Figure 3-4. Temporal variations of vessel pressure and temperature in Test # Metallic-2 

(Test #2) with time scale extended, including the cool-down phase.

3.1.1.3 Test # Metallic-3 (Test #3; Vessel #3)

Test # Metallic-3 (Tests #3) is a repeat of Test # Metallic-2 (Test #2). Figure 3-5 presents the 

test results which show the occurrence of a leak during the 9 h constant-temperature heating 

phase at 800 °C (1472 °F). In Test # Metallic-3, the vessel pressure decreases slowly initially 

and then significantly after about 25,000 s (6.9 h) at 800 °C (1472 °F). Since the leakage rate is 

directly proportional to the time rate of change of pressure, this two-stage decrease in pressure 

indicates a slower leak rate (small time derivatives of vessel pressure [dP/dt]) at first and then a 

faster leak rate (large dP/dt) at the end. Another leak indicator is revealed in Figure 3-6, which 

shows the entire time history of the test. After cooling to room temperature, the vessel does not 

recover its initial charge pressure of approximately 5 bar because of the leak. Isothermal 

reference helium leakage rates (LR,He) during the 9 h heating in this test are shown in Figure 3-7.
3-6

Figure 3-5. Temporal variations of vessel pressure and temperature in Test # Metallic-3 

(Test #3).
3-7

Figure 3-6. Temporal variations of vessel pressure and temperature in Test # Metallic-3 

(Test #3) with time scale extended, including the cool-down phase.
3-8

Figure 3-7. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-3 (Test #3).

3.1.1.4 Test # Metallic-4 (Test #4; Vessel #4)

Test # Metallic-4 (Test #4) is another repeat of the thermal exposure condition used in Test # 

Metallic-2 (Test #2). Figure 3-8 shows the test results from Test # Metallic-4. In this test, the 

pressure remained relatively constant for about 10,000 s (2.8 h) initially during the 9 h constanttemperature heating phase, begins to drop significantly for about 15,000 s (4.2 h) with large 

dP/dt, and decreases slowly with small dP/dt for the remaining duration. Figure 3-9 shows the 

complete time-pressure and time-temperature histories of the test. The calculated reference 

helium leakage rates are shown in Figure 3-10.

It is interesting to note that despite the occurrence of a leak in these two tests (Tests # Metallic3, and # Metallic-4), the vessel pressure did not equilibrate to atmospheric pressure at the end 

of the cool-down-phase, which took several days. This observation indicated that re-sealing 

could potentially occur as the vessel was cooled down.
3-9

Figure 3-8. Temporal variations of vessel pressure and temperature in Test # Metallic-4 

(Test #4).
3-10

Figure 3-9. Temporal variations of vessel pressure and temperature in Test # Metallic-4 

(Test #4) with time scale extended, including the complete cool-down phase.
3-11

Figure 3-10. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-4 (Test #4).

3.1.1.5 Test # Metallic-5 (Test #5; Vessel #5)

The results for Test # Metallic-5 (Test #5) are shown in Figure 3-11. The vessel pressure 

remained constant during the entire 9 h constant-temperature heating period at 427 °C (800 °F). 

The seal held vessel pressure. This was confirmed as the initial pressure at room temperature 

was recovered after the cool-down phase. In addition, the pressure remained unchanged for 

over 140 h after the cool-down. Figure 3-12 shows the complete thermal exposure cycle of the 

test.

A photograph of the exposed vessel is shown in Figure 3-13. The extent of discoloration of the 

vessel depends on the test temperature (Figure 3-1 vs. Figure 3-13). A post-test inspection 

revealed that the metallic O-ring seal was not soldered to either flange surfaces, and no silver 

coating from the seal was transferred to the O-ring groove.
3-12

Figure 3-11. Temporal variations of vessel pressure and temperature in Test # Metallic-5 

(Test #5).
3-13

Figure 3-12. Temporal variations of vessel pressure and temperature in Test # Metallic-5 

(Test #5) with time scale extended, including the complete cool-down phase.

Figure 3-13. Post-test photograph of Vessel #5 (vessel body with O-ring removed) after 

exposure at 427 °C (800 °F) for 9 h.
3-14

3.1.1.6 Test # Metallic-6 (Test #6; Vessel #2 Refurbished)

The results for Test # Metallic-6 (Test #6) using the refurbished Vessel #2 are shown in Figure 

3-14. The refurbishment only involved the re-facing of the surfaces of the flanges and O-ring 

groove to the specified tolerances. For this test, the vessel pressure started to decrease very 

slowly during the constant-temperature heating period. The scale of the ordinate is magnified in 

Figure 3-14 to elucidate the continuous decrease in pressure. However, the small pressure 

drop is within the pressure measurement uncertainty of 0.11 bar. Although the continuous 

decrease in pressure does seem to imply that a very small leak might have occurred during the 

test, the initial charge pressure was restored when the vessel was cooled to room temperature 

after the thermal exposure (as shown in Figure 3-15), an indication of no leakage.

Figure 3-14. Temporal variations of vessel pressure and temperature in Test # Metallic-6 

(Test #6).
3-15

Figure 3-15. Temporal variations of vessel pressure and temperature in Test # Metallic-6 

(Test #6) with time scale extended, including the complete cool-down phase.

3.1.1.7 Test # Metallic-7 (Test #7; Vessel #1 Refurbished)

Figure 3-16 shows the results for Test # Metallic-7 (Test #7) using the refurbished Vessel #1. 

The same refurbishing process used in Vessel #2 was applied to Vessel #1. As indicated in the 

figure, the vessel pressure remained unchanged during the 9-h constant-temperature heating 

period. No leakage was observed during Test # Metallic-7. Figure 3-17 is the complete thermal 

exposure history of Test # Metallic-7.
3-16

Figure 3-16. Temporal variations of vessel pressure and temperature in Test # Metallic-7 

(Test #7).
3-17

Figure 3-17. Temporal variations of vessel pressure and temperature in Test # Metallic-7 

(Test #7) with time scale extended, including the complete cool-down phase.

3.1.1.8 Test # Metallic-8 (Test #8; Vessel #6)

The test conditions were similar to Tests # Metallic-2, Metallic-3, and Metallic-4. Figure 3-18

shows the thermal exposure results for Vessel #6. The pressure trace indicates that leakage 

occurs after about 3-h exposure to 800 °C (1472 °F). For this test, the pressure initially 

decreases very rapidly, followed by a gradually decrease in vessel pressure. However, the 

pressure never dropped below 2 bar, indicating that the seal had some residual sealing 

capacity. Figure 3-19 is the complete time-pressure and time-temperature histories of Test # 

Metallic-8 (Test #8). Figure 3-20 shows the calculated isothermal reference helium leakage 

rates during the 9-h heating.
3-18

Figure 3-18. Temporal variations of vessel pressure and temperature in Test # Metallic-8 

(Test #8).
3-19

Figure 3-19. Temporal variations of vessel pressure and temperature in Test # Metallic-8 

(Test #8) with time scale extended, including the complete cool-down phase.
3-20

Figure 3-20. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-8 (Test #8).

3.1.1.9 Test # Metallic-9 (Test #9; Vessel #1 Re-refurbished)

Test # Metallic-9 (Test #9) was performed using Vessel #1, which had been used once at 800 

°C (1472 °F) for 30 min and once at 427 °C (800 °F) for 9 h and refurbished twice. The sole 

intent of this test was to check the performance of the pressure transducer against a pressure 

gauge13 to ensure that the pressure transducer functioned properly. Figure 3-21 shows the setup used for this comparative test. The only difference between this arrangement and the 

original was the addition of a pressure gauge.

The test protocol involved the heating of the vessel to 427 °C (800 °F). Once the vessel 

reached 427 °C (800 °F), the furnace temperature was raised to 800 °C (1472 °F), and the 

vessel was heated at 800 °C (1472 °F) for a little more than 3 h. Figure 3-22 and Figure 3-23

show the pressure and temperature data using different time scales respectively. No leak is 

indicated in the figures since the attainable pressure at 800 °C (1472 °F) remained constant. 

However, the pressure at 800 °C (1472 °F) was lower than those recorded in previous tests 

under similar conditions (≈ 12 bar vs. ≈ 14.5 bar). This is due to the increase of the 

measurement volume with the addition of the pressure gauge which used a C-shape Bourdon 

tube with a surprisingly large volume. The gauge was dissembled and the internal volume of 

the Bourdon tube, which had a nearly elliptical cross section, was estimated to be more than 

 13 WIKA model with a range of 0 bar (0 psig) to 13.79 bar (200 psig) and a resolution of 0.14 bar (2 psig).
3-21

12 mL. Figure 3-24 correlates the pressure readings from the pressure transducer with those 

from the pressure gauge at different times during the heating phase. The pressure transducer 

clearly functioned properly, and the transducer readings correspond, within the measurement 

uncertainties, to those obtained from the pressure gauge.

Figure 3-21. A photograph showing the pressure gauge used to check the pressure 

transducer performance.
3-22

Figure 3-22. Temporal variations of vessel pressure and temperature in Test # Metallic-9 

(Test #9).
3-23

Figure 3-23. Temporal variations of vessel pressure and temperature in Test # Metallic-9 

(Test #9) with time scale extended, including the complete cool-down phase.
3-24

Figure 3-24. Comparison of pressure measurements from pressure transducer and 

pressure gauge.

3.1.1.10 Test # Metallic-10 (Test #10; Vessel #7)

In Test # Metallic-10 (Test #10), an incremental heating protocol was used to assess the 

metallic seal performance. The vessel was initially heated to 427 °C (800 °F) and held for 9 h at 

this temperature. Then, the furnace temperature was raised to 527 °C (981 °F), and the vessel 

was heated at 527 °C (981 °F) overnight (more than 9 h). The vessel was then heated to 627 

°C (1161 °F) for 9 h. Figure 3-25 and Figure 3-26 show the test results. No leak was observed 

based on the pressure traces at the test temperatures, 427 °C (800 °F), 527 °C (981 °F), and 

627 °C (1161 °F), and the recovery of the initial pressure at room temperature. Post-test 

removal of the O-ring from the groove was very easy. A post-test inspection of the vessel 

indicated that the metallic seal did not bond onto the O-ring groove.
3-25

Figure 3-25. Temporal variations of vessel pressure and temperature in 

Test # Metallic-10 (Test #10).
3-26

Figure 3-26. Temporal variations of vessel pressure and temperature in 

Test # Metallic-10 (Test #10) with time scale extended, including the 

complete cool-down phase.

3.1.1.11 Test # Metallic-11 (Test #13; Vessel #8)

For Test # Metallic-11 (Test #13), a metallic seal and a new vessel were used. The vessel was 

evacuated (for 1 min) and filled with helium (to 5 bar) and then evacuated and filled again twice. 

Contrary to the previous vessel filling process used in Tests # Metallic-1 (Test #1) to Test # 

PTFE-a (Test #12), two additional evacuating and filling cycles were applied in order to 

determine if the evacuation process had any effect on the results.14 In this test, an incremental 

heating protocol was used. The test vessel was first heated at 427 °C for more than 10 h, then 

at 527 °C for 9 h, then at 627 °C for more than 10 h, and finally at 627 °C for 9 h. Figure 3-27

and Figure 3-28 show the results. No leakage was observed during the entire incremental 

heating processes. However, after the vessel was cooled down to room temperature, the 

vessel pressure did not recover to its original (starting) value (≈ 5 bar) but remained at a slightly 

higher constant value of ≈ 5.2 bar over a duration of more than 50 h, albeit within the uncertainty 

of the transducer. This was not due to the drift of the pressure transducer because when the 

vessel content was vented to the atmosphere, the pressure transducer showed an atmospheric 

 14 Whether the test vessel was evacuated once or three times, no discernible difference in the final 

attainable vessel pressure was observed under the same test conditions.
3-27

pressure reading.

Figure 3-27. Temporal variations of vessel pressure and temperature in 

Test # Metallic-11 (Test #13).
3-28

Figure 3-28. Temporal variations of vessel pressure and temperature in 

Test # Metallic-11 (Test #13) with time scale extended, including the 

complete cool-down phase.

3.1.1.12 Test # Metallic-12 (Test #14; Vessel #9)

In Test # Metallic-12 (Test #14), a new vessel with a metallic seal was used. The vessel was 

evacuated and charged with helium three times to prepare for the test. This test was a repeat of 

Tests # Metallic-2, Metallic-3, and Metallic-4. Figure 3-29 shows the first 50,000 s (≈ 14 h) of 

the pressure-time and temperature-time histories to illustrate the absence of a leak (constant 

pressure at constant temperature). The pressure-time and temperature-histories during the 

complete heating and cooling cycle is shown in Figure 3-30. However, similar to the 

observation in Test # Metallic-11, after the test vessel was cooled down to room temperature, 

the vessel pressure did not recover to its starting value (≈ 5 bar) but remained at a slightly 

higher constant value of ≈ 5.2 bar over a duration of more than 50 h. Again, this was not due to 

the drift of the pressure transducer because when the vessel content was released to the 

atmosphere, the pressure transducer gave an atmospheric pressure reading.
3-29

Figure 3-29. Temporal variations of vessel pressure and temperature in 

Test # Metallic-12 (Test #14).
3-30

Figure 3-30. Temporal variations of vessel pressure and temperature in 

Test # Metallic-12 (Test #14) with time scale extended, including the 

complete cool-down phase.

3.1.2 EPDM O-Rings

3.1.2.1 Test # EDPM-a (Test #11; Vessel #3 Refurbished)

An EPDM O-ring (DBR Industries 2-228 E740-75, Lot # 2Q060080040952) with an outer 

diameter of 6.35 cm (2.5 in.) and a cross section of 0.32 cm (0.125 in.) and refurbished 

Vessel #3 were used in this test. Vessel #3 was refurbished by re-facing the flange to remove 

the O-ring groove originally machined for the metallic seal and re-machining a new O-ring 

groove for the EPDM O-ring which has different groove dimensions from its metallic counterpart.

Figure 3-31 and Figure 3-32 are the test results plotted using different time scales. The spikes 

(overshoots) in the furnace temperature measurements reflect the way the furnace temperature 

was intentionally ramped and manipulated to accelerate the incremental heating of the vessel to 

the set temperatures. During the first 1 h heating at 150 °C (302 °F), the vessel pressure 

remained constant. The subsequent 1 h heating at 200 °C (392 °F) and the following 1 h 

heating at 250 °C (482 °F) also indicated the maintenance of vessel pressure. The final heating 

at 300 °C (572 °F) for more than 20 h shows a slow decrease in pressure, albeit within the 

pressure measurement uncertainty. However, the vessel pressure was restored to its initial 

value after the thermal exposure test, as shown in Figure 3-32. Post-test inspection of the 

exposed vessel revealed that the O-ring was still pliable and could be easily removed from the 
3-31

O-ring groove.

Figure 3-31. Temporal variations of vessel pressure and temperature in Test # EDPM-a 

(Test #11).
3-32

Figure 3-32. Temporal variations of vessel pressure and temperature in Test # EPDM-a 

(Test #11) with time scale extended, including the complete cool-down 

phase.

3.1.2.2 Test # EPDM-b (Test #15; Vessel #3 Refurbished)

In Test # EDPM-b (Test #15), an EPDM seal together with the refurbished Vessel #3 (used in 

Test # EDPM-a) was employed. The refurbishment simply involved cleaning of the O-ring 

groove and the cavity of the used vessel with ethanol. The vessel was evacuated and charged 

with helium three times to prepare for the test. The difference between this test and 

Test # EPDM-a was the temperature used for the thermal exposure test. Instead of 300 °C (572 

°F) as used in Test # EPDM-a, a higher temperature, 450 °C (842 °F) was used.

Figure 3-33 shows the first 39 h of the pressure-time and temperature-time histories obtained 

from this test. The pressure and temperature histories for the complete heating and cooling 

cycle are given in Figure 3-34. The pressure trace in Figure 3-33 clearly indicates a leak 

occurred soon after the vessel had attained the nominal target temperature of 450 °C (842 °F). 

Figure 3-35 shows the calculated reference helium leakage rates over the 25 h isothermal 

heating period at 450 °C (842 °F) with a time-averaged reference leakage rate of 9.2 × 10-4

ref·cm3

/s.
3-33

Figure 3-33. Temporal variations of vessel pressure and temperature in Test # EPDM-b 

(Test #15).
3-34

Figure 3-34. Temporal variations of vessel pressure and temperature in Test # EPDM-b 

(Test #15) with time scale extended, including the complete cool-down 

phase.
3-35

Figure 3-35. Isothermal reference helium leakage rate during the 25 h heating in 

Test # EPDM-b (Test #15).

From Figure 3-33 and Figure 3-34, a peculiarity is noted. The measured attainable pressure 

(≈ 8.5 bar) at 450 °C (842 °F) before the leak occurred is noticeably higher than the pressure 

calculated using the ideal gas law (≈ 4.9 bar), even assuming no thermal expansion of the 

vessel volume. The addition of vapor mass from the potential thermal decomposition and offgas of the O-ring and the lubricant used at 450 °C (842 °F) is conjectured to be the cause for 

this unusually high pressure. Another abnormality is that after the vessel was cooled down to 

room temperature, the pressure never recovered to its original value (≈ 2 bar) but remained 

relatively constant at ≈ 2.16 bar over a duration of more than 100 h, implying the O-ring still 

possessed some residual sealing capability. A post-test inspection of the tested vessel 

revealed that the interior vessel wall and the thermocouple inserted into the vessel interior were 

coated with a thin layer of black substance. Although the O-ring was still properly seated in the 

groove (see Figure 3-36), it had turned into a packed layer of powdery charred material and 

could not be removed from the groove without destroying its structural integrity (see Figure 

3-37). Additional tests would be needed to further examine this finding.
3-36

Figure 3-36. Photograph of the EPDM O-ring after thermal exposure.

Figure 3-37. Photograph showing the disintegration of the tested EPDM O-ring when an 

attempt was made to remove the O-ring from the groove.

3.1.3 PTFE O-Rings

3.1.3.1 Test # PTFE-a (Test #12; Vessel #3 Refurbished)

A PTFE O-ring (DBR Industries 228 PTFE, Lot #04/07 12730-1) with an outer diameter of 6.35 

cm (2.5 in.) and a cross section of 0.32 cm (0.125 in.) and refurbished Vessel #3 were used for 
3-37

this test. The refurbishment of the vessel simply consisted of cleaning of the O-ring groove 

previously used for the EPDM seal test (Test # EPDM-a). Figure 3-38 and Figure 3-39 show 

the results during the heating phase and the complete heating-cooling history respectively. 

During the 22 h heating at 300 °C (572 °F), there was a slight drop in vessel pressure at the end 

of the isothermal heating phase; however, the pressure decrease was within the measurement 

uncertainty of the pressure transducer. Figure 3-40, which shows the calculated reference 

helium leakage rates during the 22 h heating period, indicates a time-averaged reference 

leakage rate of 4.5 × 10-5 ref·cm3

/s. The fluctuation in the leakage rate data was due to the 

taking of numerical time derivatives of the unsmoothed pressure-time data. As indicated in the 

pressure trace, leakage was observed in this test during the cooling phase. The original 

pressure (≈ 2 bar) did not recover, and the vessel pressure dropped below the original pressure

at room temperature. Post-test inspection of the exposed vessel revealed that the compressed 

O-ring was still intact and could be easily removed from the O-ring groove.

Figure 3-38. Temporal variations of vessel pressure and temperature in Test # PTFE-a 

(Test #12).
3-38

Figure 3-39. Temporal variations of vessel pressure and temperature in Test # PTFE-a 

(Test #12) with time scale extended, including the complete cool-down 

phase.
3-39

Figure 3-40. Isothermal reference helium leakage rate during the 22 h heating in 

Test # PTFE-a (Test #12).
3-40

3.2 Phase II

3.2.1 EPDM O-Rings

Figure 3-41, Figure 3-42, Figure 3-43, Figure 3-44, Figure 3-45, and Figure 3-46 show the 

vessel pressure and temperature measurements obtained in Tests # EPDM-1, EPDM-2, EPDM3, EPDM-4, EPDM-5, and EPDM-6 respectively. Tests # EPDM-1, EPDM-2, EPDM-3, and 

EPDM-4 were conducted at 316 °C for 9 h, whereas Tests # EPDM-5 and EPDM-6 were 

performed at 900 °C for 9 h.

In Test # EPDM-1 (Figure 3-41), the vessel pressure decreased by about 0.1 bar over the 8-h 

test period; however, the corresponding decrease of the vessel internal temperature by 10 °C 

over the same period was commensurate to a drop in pressure close to 0.1 bar and could easily 

contribute to this pressure decrease. The temperature fluctuation in this test, first in the leakrate measurement test series, was the result of constantly adjusting the furnace set-point in an 

attempt to shorten the vessel heat-up time to reach the test temperature. Therefore, the slight 

decrease in pressure in this particular test should not be construed as a leak but rather as a 

result from temperature variation. In Test # EPDM-2 (Figure 3-42), Test # EPDM-3 (Figure 

3-43), and Test # EPDM-4 (Figure 3-44), the vessel pressure remained constant during the 8-h 

test period, and the initial charged pressure was recovered after cool-down, indicating no leak 

had occurred. Leaks were observed in Tests # EPDM-5 and EPDM-6 before the test 

temperature was reached in both tests (see Figure 3-45, and Figure 3-46). Since leak occurred 

during the heat-up period, the isothermal leakage rates could not be estimated.

Post-test inspections of the disassembled test vessels revealed that the EPDM O-rings were 

thermally degraded due to prolong thermal exposure in all six tests (see Figure 3-47). In Tests 

# EPDM-1, EPDM-2, EPDM-3, and EPDM-4, when the tested vessel was removed from the 

furnace, what appeared to be thermally degraded O-ring material was found seeping out from a 

location of the two-flange interface. The location was where the vessel made contact with and 

rested on the base of the furnace and was always aligned with the direction of the gravity 

vector. The location was also marked by a stain on the flange from the O-ring material, which is 

clearly shown in Figure 3-47. Note that the locations where the seeping of the O-ring material 

occurred appeared to be different in all four photographs in Figure 3-47 because the 

disassembled flange was not oriented and placed in the original test position in the furnace 

when the photographs were taken. The flange with an opening and without an O-ring groove 

shown only in Test # EPDM-3 in Figure 3-47 was not the vessel cap flange (as shown in the 

other three photographs) but rather the flange of the vessel body. The O-ring was removed 

from its groove in the vessel cap flange and adhered to the boss of this flange during the 

disassembling of the vessel. In Tests # EPDM-5 and EPDM-6, a thin layer of material was 

found adhered to the vessel interior wall (see Figure 3-48). Similar observations were made in 

double EPDM-EPDM O-ring tests in Phase III (see Section 3.3.3) although the extent of the 

layer thickness/area differed in both cases.
3-41

Figure 3-41. Temporal variations of vessel pressure and temperatures in Test # EPDM-1.
3-42

Figure 3-42. Temporal variations of vessel pressure and temperatures in Test # EPDM-2.
3-43

Figure 3-43. Temporal variations of vessel pressure and temperatures in Test # EPDM-3.
3-44

Figure 3-44. Temporal variations of vessel pressure and temperatures in Test # EPDM-4.
3-45

Figure 3-45. Temporal variations of vessel pressure and temperatures in Test # EPDM-5.
3-46

Figure 3-46. Temporal variations of vessel pressure and temperatures in Test # EPDM-6.
3-47

Figure 3-47. Photographs showing the thermally degraded EPDM O-ring after 8-h thermal 

exposure at 316 °C (600 °F) from the four EPDM O-ring tests

(Tests # EPDM-1, EPDM-2, EPDM-3, and EPDM-4). Tests # EPDM-5 and 

# EPDM-6 were conducted at 900 °C for 9 h.

Figure 3-48. Photographs showing the vessel interiors after 9 h thermal exposure at 

900 °C from Tests # EPDM-5 and EPDM-6.

3.2.2 Silicone O-Rings

Figure 3-49, Figure 3-51, Figure 3-52, and Figure 3-53 show the temporal variations of the 

vessel internal pressure and temperature, two flange temperatures and furnace temperature for 

the four silicone O-ring tests, Test # Silicone-1, Test # Silicone-2, Test # Silicone-3, and Test # 

Test # EPDM-5

Layer of material

Test # EPDM-6

Layer of material
3-48

Silicone-4, respectively.

In Test # Silicone-1 (Figure 3-49), the vessel internal pressure started to drop after about 2 h 

into the test. At the end of the test period (8 h), the pressure had decreased by approximately 

0.5 bar. The drop in vessel pressure during the test period and the subsequent further 

reduction of the vessel pressure to ambient pressure (~ 1 bar) and the inability to regain the 

initial charged pressure during the cool-down phase were indicative of a leak. Figure 3-50

shows the calculated reference helium leakage rate, and a time-averaged reference leakage 

rate of 2.4 × 10-3 ref cm3

/s was obtained over the time interval from 25,000 s to 37,000 s.

Figure 3-49. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-1.
3-49

Figure 3-50. Calculated isothermal reference helium leakage rate in Test # Silicone-1.

In Test # Silicone-215 (Figure 3-51) and Test # Silicone-3 (Figure 3-52), the vessel pressure 

decreased by less than 0.1 bar, which was within the measurement uncertainty of the pressure 

transducer, over the 8-h test period. However, the initial charged pressure was recovered after 

the cool-down phase. The occurrence of a leak was proved to be inconclusive in these two 

tests.

In Test # Silicone-4 (Figure 3-53), the vessel pressure remained unchanged during the 8-h test 

period, and the initial charged pressure was also regained after the cool-down phase. Based on 

these observations, the absence of a leak was concluded.

Post-test inspections of the disassembled test vessel revealed that the silicone O-rings were 

thermally degraded due to prolong thermal exposure in all four tests (see Figure 3-54). When 

the tested vessel was removed from the furnace, a globule of O-ring material was observed to 

ooze out from a location of the two-flange interface. Similar to the EPDM O-ring tests, the 

location was where the vessel made contact with and rested on the base of the furnace and was 

always aligned with the direction of the gravity vector. As explained in Section 3.2.1, the 

locations where the seeping of the O-ring material occurred were different in all four 

photographs in Figure 3-54 as the result of how the disassembled flange was placed when the 

 15 The DAQ was inadvertently turned off during the first hour of the heat-up period, and the data was not 

logged. The time zero in the figure was the time when the DAQ was switched back on. The data collected 

over the 8-h test period were never affected.
3-50

photographs were taken. The flange with an opening and without an O-ring groove shown only 

in Test # Silicone-1 in Figure 3-54 was the result of the O-ring adhering to the boss of the 

opposite flange of the vessel body during disassembly of the vessel. In addition, the degree of 

thermal damage to the O-rings, as shown in the figure, might not completely represent the 

actual in situ damage condition because the already thermally degraded O-ring could potentially 

be further damaged when the vessel cap and body were pulled apart during disassembly of the 

vessel for post-test inspection.

Figure 3-51. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-2.
3-51

Figure 3-52. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-3.
3-52

Figure 3-53. Temporal variations of vessel pressure and temperatures in 

Test # Silicone-4.
3-53

Figure 3-54. Photographs showing the thermally degraded silicone O-ring after 8-h 

thermal exposure at 316 °C (600 °F) for the four silicone O-ring tests.

3.2.3 Butyl O-Rings

Figure 3-55, Figure 3-56, and Figure 3-57 show the temporal variations of the vessel internal 

pressure and temperature, two flange temperatures and furnace temperature for the three Butyl 

O-ring tests, Test # Butyl-1, Test # Butyl-2, and Test # Butyl-3, respectively. The variation in 

pressure within the measurement uncertainty and no loss in pre-test vessel pressure after cooldown indicate the absence of a leak in these three tests.

Figure 3-58 shows the photographs taken during post-test inspections of the thermally damaged 

butyl O-rings in the three tests. The explanations of the features in the photographs are similar 

to those described in the EPDM and silicone O-ring tests in Sections 3.2.1 and 3.2.2.

Test # Silicone-1 Test # Silicone-2

Test # Silicone-3 Test # Silicone-4
3-54

Figure 3-55. Temporal variations of vessel pressure and temperatures in Test # Butyl-1.
3-55

Figure 3-56. Temporal variations of vessel pressure and temperatures in Test # Butyl-2.
3-56

Figure 3-57. Temporal variations of vessel pressure and temperatures in Test # Butyl-3.
3-57

Figure 3-58. Photographs showing the thermally degraded butyl O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the three butyl O-ring tests.

3.2.4 Viton O-Rings

Figure 3-59, Figure 3-60, Figure 3-61, and Figure 3-62 show the temporal variations of the 

vessel internal pressure and temperature, two flange temperatures and furnace temperature for 

the four Viton O-ring tests, Test # Viton-1, Test # Viton-2, Test # Viton-3, and Test # Viton-4, 

respectively.

In all four tests, the pressure was not constant but varied during the 8-h test period. In Test # 

Viton-1 (Figure 3-59), Test # Viton-3 (Figure 3-61), and Test # Viton-4 (Figure 3-62), the vessel 

pressure increased in the early part of the 8-h test period but remained relative constant at the 

later part. In Test # Viton-2 (Figure 3-60), the pressure increased continuously over the entire 

8-h test period. The rate of pressure increase was irregular and not reproducible in all four 

tests. The increase in pressure at the isothermal test condition and the subsequent attainment 

of vessel pressure greater than the pre-test initial charge pressure after cool-down suggested 

the addition of material to the vessel interior. The origin of the material remains unknown. This 

peculiarity in vessel pressure behavior was also observed in Test #15 in Phase I using an 

EPDM O-ring exposed at 450 °C (842 °F) for more than 25 h.

Figure 3-63 shows the photographs taken during post-test inspections of the thermally damaged 

Viton O-rings in the four tests. The explanations of the features in the photographs are similar 

to those described in the EPDM, silicone, and butyl O-ring tests (see Sections 3.2.1, 3.2.2, and 

3.2.3).

Test # Butyl-1 Test # Butyl-2

Test # Butyl-3
3-58

Figure 3-59. Temporal variations of vessel pressure and temperatures in Test # Viton-1.
3-59

Figure 3-60. Temporal variations of vessel pressure and temperatures in Test # Viton-2.
3-60

Figure 3-61. Temporal variations of vessel pressure and temperatures in Test # Viton-3.
3-61

Figure 3-62. Temporal variations of vessel pressure and temperatures in Test # Viton-4.
3-62

Figure 3-63. Photographs showing the thermally degraded Viton O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the four Viton O-ring tests.

3.2.5 PTFE O-Rings

Figure 3-64, Figure 3-65, and Figure 3-66 show the temporal variations of the vessel internal 

pressure and temperature, two flange temperatures and furnace temperature for the three PTFE 

O-ring tests, Test # PTFE-1, Test # PTFE-2, and Test # PTFE-3, respectively. The lack of 

detectable pressure drop during the 8-h test period and the recovery of initial charged pressure 

after the cool-down phase are indicative of the absence of a leak.

Figure 3-67 shows three photographs of the thermally exposed PTFE O-rings taken during posttest inspections. The tested O-rings remained intact but flattened due to the clamping forces of 

the bolts and thermal exposure, and no PTFE material had oozed out at the flange interface. 

However, a minute amount of PTFE powder was observed when the two vessel flanges were 

separated and the O-ring groove was exposed.

Test # Viton-1 Test # Viton-2

Test # Viton-3 Test # Viton-4
3-63

Figure 3-64. Temporal variations of vessel pressure and temperatures in Test # PTFE-1.
3-64

Figure 3-65. Temporal variations of vessel pressure and temperatures in Test # PTFE-2.
3-65

Figure 3-66. Temporal variations of vessel pressure and temperatures in Test # PTFE-3.
3-66

Figure 3-67. Photographs showing the thermally degraded PTFE O-ring after 8-h thermal 

exposure at 316 °C (600 °F) for the three PTFE O-ring tests.

Test # PTFE-1 Test # PTFE-2

Test # PTFE-3
3-67

3.3 Phase III

3.3.1 EPDM-Metallic O-Rings

Figure 3-68, Figure 3-69, and Figure 3-70 show the temporal variations of the vessel internal 

pressure and temperature, two flange temperatures and furnace temperature for the Test # 

EPDM-Metallic-116, Test # EPDM-Metallic-2, and Test # EPDM-Metallic-3, respectively. New 

vessels (body and cap) were used in the first two tests17. For Test # EPDM-Metallic-3, a new 

vessel cap and a used vessel body with re-surfaced boss were used, and the test was 

conducted at 900 °C (1652 °F), instead of 800 °C (1472 °F).

In all three tests, the maintenance of unchanged vessel pressure over the 9-h thermal exposure 

period and the recovery of the initial vessel pressure at room temperature after the cool-down 

phase indicate the absence of a leak18. It should be noted that in Phase I, a leak was evident 

after a few hours into the thermal exposure period in three of the five tests using a single 

metallic O-ring configuration, and a leak was also observed in a single exploratory test using a 

single EPDM O-ring immediately after the vessel had attained the test temperature of 450 °C 

(842 °F).

 16 The DAQ was inadvertently turned off during the heat-up period, and the data was not logged. The time 

zero in the figure was the time when the DAQ was initiated at the beginning of the 9-h thermal exposure 

period. The data collected over the 9-h test period were never affected or compromised.

17 The numbering sequence for the vessel designation continues from Phase I.

18 The data logging was terminated during the cooling phase of Test # EPDM-Metallic-3 due to site poweroutage; however, the recovery of the initial vessel pressure was manually noted from the computer display 

when the vessel was cooled to room temperature after the power outage was over.
3-68

Figure 3-68. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-1 (Vessel #10).
3-69

Figure 3-69. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-2 (Vessel #11).
3-70

Figure 3-70. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-Metallic-3 using the cap of Vessel #12 and the body (with boss 

re-surfaced) of Vessel #10.

Figure 3-71 shows the photographs taken after the vessels were disassembled for post-test 

inspection for the three tests. Unlike the observations made in the single EPDM O-ring tests 

(see Section 3.2.1), a globule of O-ring material was not observed to seep out at the interface 

between the two flanges of the vessel when the test vessel was first removed from the furnace 

after the test. However, upon disconnecting the two flanges, char residues left by the EPDM Oring materials were apparent on both bosses at the vicinity of the EPDM O-ring groove, 

indicating that oozing of the EPDM O-ring material had occurred during the thermal exposure 

and the materials that had seeped out from the interface were completely charred. The tested 

EPDM O-ring turned into a thick layer of highly compacted powdery char in the O-ring groove 

and could not be removed from the groove without destroying the structural integrity of the char 

layer. It should be noted that the condition of the EPDM O-ring depicted in Figure 3-71 does not 

truly represent the actual in situ condition of the tested O-ring because the damage to the O-ring 

might have been further exacerbated when the two flanges were forcibly removed and 

separated for post-test inspection. Similar to the observations made in Phase I, the metallic Oring was partially bonded to the boss of the vessel body flange when it was disconnected from 

the flange of the vessel cap. The O-ring groove was coated with silver from the melted metallic 

O-ring.
3-71

Figure 3-71. Photographs showing the thermally degraded EPDM-Metallic O-rings after 

9-h thermal exposure at 800 °C (1472 °F) for the two EPDM-Metallic O-ring 

tests.

3.3.2 Blank-Metallic O-Ring

In this test, the cap of Vessel #13 (with double O-ring configuration) and the body (with boss resurfaced) of Vessel #11 was used; however, the outer O-ring groove was left blank and a 

metallic O-ring was placed in the inner groove. Figure 3-72 shows the temporal variations of the 

vessel internal pressure and temperature, two flange temperatures and furnace temperature for 

Test # Blank-Metallic-1. As indicated in the figure, the vessel pressure remained unchanged 

during the 9 h constant-temperature heating period, and no leakage was observed during this 

test. Figure 3-73 shows the photographs taken after the vessel were dismantled for post-test 

inspection. The thermally-exposed O-ring was observed to suffer damages similar to those 

sustained in the double metallic O-rings tested at 900 °C (1652 °F).
3-72

Figure 3-72. Temporal variations of vessel pressure and temperatures in 

Test # Blank-Metallic-1 using the cap of Vessel #13 and the body (with boss 

re-surfaced) of Vessel #11.

Figure 3-73. Photographs showing the thermally degraded Metallic O-ring after 9-h 

thermal exposure at 900 °C (1652 °F) for the Blank-Metallic O-ring test.

Test # Blank-Metallic-1 Test # Blank-Metallic-1
3-73

3.3.3 EPDM-EPDM O-Rings

Two tests (Test # EDPM-EDPM-1 and Test # EPDM-EPDM-2) were conducted at 900 °C 

(1652 °F) using the EPDM-EPDM double O-ring configuration. Figure 3-74 and Figure 3-75

show the temporal variations of vessel pressure and temperatures for these two tests. Leaks

occurred before the test vessel reached the target temperature of 900 °C (1652 °F)19. Note that 

in the two figures, the four temperatures were off-scale for less than 1 h but returned to normal

due to the problems associated with the thermocouples and the DAQ system. The DAQ 

recorded unrealistically high temperatures from the four thermocouples. The photographs in 

Figure 3-76 illustrate the extent of damage to the O-rings after the thermal exposure tests. The 

black powdery materials (most likely from metal dusting) on of the flange came from the vessel 

wall and was the result of striking the vessel with a mallet in order to loosen the bolts and other 

mechanical actions taken to dismantle the vessel for post-test inspection. In addition, the vessel 

interior wall was found to be coated with a very thin layer of what appeared to be metallic

material. The isothermal leakage rates could not be estimated because the pressure drop 

occurred during the heat-up period. Tests # EPDM-5 and # EPDM-6 also revealed a smaller 

layer of material under the same thermal exposure condition (see Section 3.2.1).

Figure 3-74. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-EPDM-1 using Vessel #16.

 19 The method for leak rate estimation described in Appendix D could not be used to estimate the leak rate 

because the heat-up period was not isothermal.
3-74

Figure 3-75. Temporal variations of vessel pressure and temperatures in 

Test # EPDM-EPDM-2 using Vessel #17.
3-75

Figure 3-76. Photographs showing the thermally degraded EPDM O-rings after 9-h 

thermal exposure at 900 °C (1652 °F) for the EPDM-EPDM O-ring test.

3.3.4 Butyl-Butyl O-Rings

One exploratory test was performed using a double butyl O-ring configuration. Vessel #16 was 

cleaned and re-used for this test. Figure 3-77 shows the test results. Similar to the EPDMEPDM tests, a leak occurred before the target temperature of 900 °C (1652 °F) was attained, 

and a layer of material of metallic nature was also formed on the interior wall of the vessel (see 

Figure 3-78)

19. Incidentally, all four thermocouples also failed to function for about one hour.

Test # EPDM-EPDM-2

Layer of 

material

Test # EPDM-EPDM-2

Test # EPDM-EPDM-1

Layer of 

material

Test # EPDM-EPDM-1
3-76

Figure 3-77. Temporal variations of vessel pressure and temperatures in 

Test # Butyl-Butyl-1 using Vessel #16.

Figure 3-78. Photograph showing the thermally degraded butyl O-rings after 9-h thermal 

exposure at 900 °C (1652 °F) for the butyl-butyl O-ring test.

Test # Butyl-Butyl-1 Test # Butyl-Butyl-1

Layer of material
3-77

3.3.5 Metallic-Metallic O-Rings

Four tests (Test # Metallic-Metallic-1, Test # Metallic-Metallic-2, Test # Metallic-Metallic-3, and 

Test # Metallic-Metallic-4) were conducted. For Test # Metallic-Metallic-1, the cap of Vessel 

#14 and the body of Vessel #12 was used. For Test # Metallic-Metallic-2, the cap of Vessel #15 

and the body of Vessel #13 was used. Test # Metallic-Metallic-3 and Test # Metallic-Metallic-4 

used Vessel # 18 and Vessel # 19 respectively. As described in Section 2.1.3, Test # MetallicMetallic-1 and Test # Metallic-Metallic-2 used 416 N m (307 lb ft) torque, whereas Test # 

Metallic-Metallic-3, and Test # Metallic-Metallic-4 used 976 N m (720 lb ft) torque.

Figure 3-79 shows the temporal variations of the vessel internal pressure and temperature, two 

flange temperatures and furnace temperature20 for Test # Metallic-Metallic-1. The vessel 

pressure started to decrease after more than 4 h at 900 °C (1652 °F) and continued to decrease 

over the remaining test period. The gradual decrease in vessel pressure during the thermal 

exposure and the failure to recover the original vessel pressure (≈ 5 bar) at room temperature 

after the cool-down phase are the two indicators of a leak. However, after the vessel cooled to 

room temperature, the vessel pressure did not ever equilibrate with atmospheric pressure, but 

remained at elevated pressure (≈ 2 bar), implying that the O-rings still retained some residual 

sealing capability after the thermal exposure. Figure 3-80 shows the calculated reference 

helium leakage rates during the 9 h isothermal heating period at 900 °C (1652 °F).

Figure 3-81 shows the temporal variations of the vessel internal pressure and temperature, two 

flange temperatures and furnace temperature for Test # Metallic-Metallic-2. This test is subtly 

different from Test # Metallic-Metallic-1 in that the vessel pressure abruptly (within minutes) 

dropped from ≈ 14 bar to ≈ 11 bar after more than 4 h at 900 °C (1652 °F), and then continued

to decrease gradually over the remaining test period. Other observations remain similar to Test 

# Metallic-Metallic-1. Figure 3-82 shows the calculated reference helium leakage rates over the 

9 h isothermal heating period at 900 °C (1652 °F). The sudden drop in vessel pressure is 

clearly marked as a spike in the leakage rate plot.

Figure 3-83 shows the temporal variations of the vessel internal pressure and temperature, two 

flange temperatures and furnace temperature for Test # Metallic-Metallic-3. The vessel held 

pressure for about 8 h at 900 °C before a leak occurred. It is interesting to note that when the 

vessel cooled to room temperature after 9-h thermal exposure, the vessel pressure still 

maintained a pressure of about 1.6 bar before the vessel was dismantled for post-test 

inspection more than a week later. Figure 3-84 shows the calculated reference helium leakage 

rates over the isothermal heating period at 900 °C (1652 °F) where the leak occurred. The 

sudden drop in vessel pressure is again clearly marked as a spike in the leakage rate plot. In 

this test, the vessel pressure was stabilized and maintained eventually at about 1.7 bar as the 

vessel was cooled to room temperature.

 20 The furnace temperature readings in this figure are somewhat (about 40 °C) lower than those reported 

in other figures due to the slight misplacement of the furnace thermocouple, but the furnace own 

temperature read-out indicted that the furnace was at the set-point temperature.
3-78

Figure 3-79. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-1 using the cap of Vessel #14 and the body of 

Vessel #12.

Figure 3-85 shows the temporal variations of the vessel internal pressure and temperature, two 

flange temperatures and furnace temperature for Test # Metallic-Metallic-4. The vessel was 

maintaining pressure for about 2 h before the leak occurred, as indicted by the sudden drop of 

vessel pressure. Contrary to the observations made in Test # Metallic-Metallic-3, the leak 

occurred much sooner in this test although both tests were prepared and conducted at the same 

conditions. In addition, the vessel pressure never stabilized and continued to drop to 

atmospheric value as the vessel cooled to room temperature. Figure 3-86 shows the calculated 

reference helium leakage rates over the isothermal heating period at 900 °C (1652 °F) where 

the leak occurred.

Figure 3-87 shows the photographs taken after the vessels were disassembled for post-test 

inspection for the four tests. The thermally-exposed O-rings were observed to suffer damages 

similar to those sustained in the metallic O-rings tested at 800 °C (1472 °F).
3-79

Figure 3-80. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-1 using the cap of Vessel #14 and the body of 

Vessel #12.
3-80

Figure 3-81. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-2 using the cap of Vessel #15 and the body of 

Vessel #13.
3-81

Figure 3-82. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-2 using the cap of Vessel #15 and the body of 

Vessel #13.
3-82

Figure 3-83. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-3 using Vessel #18.
3-83

Figure 3-84. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-3 using the cap of Vessel #18.
3-84

Figure 3-85. Temporal variations of vessel pressure and temperatures in 

Test # Metallic-Metallic-4 using Vessel #19.
3-85

Figure 3-86. Isothermal reference helium leakage rate during the 9 h heating in 

Test # Metallic-Metallic-4 using Vessel #19.

Figure 3-87. Photographs showing the thermally degraded Metallic-Metallic O-rings after 

9-h thermal exposure at 900 °C (1652 °F) for the two Metallic-Metallic O-ring 

tests.

Test # Metallic-Metallic-1

Test # Metallic-Metallic-1

Test # Metallic-Metallic-2

Test # Metallic-Metallic-2

Test # Metallic-Metallic-3

Test # Metallic-Metallic-3

Test # Metallic-Metallic-4

Test # Metallic-Metallic-4

4-1

4 SUMMARY

This study provides experimental seal performance data for metallic and polymeric seals 

typically used in radioactive material transportation packages for beyond-design-basis 

temperature excursions. Tests were conducted using a small-scale test package. The study 

was divided into three phases. Phases I and II used a single O-ring configuration, whereas 

Phase III used a double O-ring configuration.

In Phase I, fifteen tests including two shakedowns were performed including some at beyonddesign-basis thermal exposure conditions. Of the tests conducted, 12 tests used metallic seals, 

two used EPDM seals, and one used a PTFE seal.

Of the five repeat metallic-seal tests (Tests # Metallic-2, # Metallic-3, # Metallic-4, # Metallic-8, 

and # Metallic-12), leakage (decreasing vessel pressure) was observed in three of the tests 

(Tests # Metallic-3, # Metallic-4, and # Metallic-8) during the 9 h 800 °C (1472 °F) exposure. 

The times when the leakage occurred (the vessel pressure started to decrease) varied in the 

three tests performed. In Test # Metallic-3, measurable leakage occurred approximately 6.9 h 

after the test temperature 800 °C (1472 °F) had been reached. In Test # Metallic-4, measurable 

leakage occurred about 2.8 h into the 800 °C (1472 °F) exposure. In Test # Metallic-8, leakage 

was observed roughly 3 h into the test. The two shakedown tests (Tests # Metallic-1 and # 

Metallic-9) were conducted using a 30 min and 4 h exposure to 800 °C (1472 °F), respectively, 

and the seal appeared to hold vessel pressure. No leakage (unchanged vessel pressure within 

the pressure measurement uncertainty) was also observed in the two metallic seal tests that 

used 100 °C (212 °F) incremental heating from 427 °C (800 °F) to 627 °C (1161 °F) and from 

427 °C (800 °F) to 727 °C (1341 °F), respectively, with at least 9 h exposure at each 

temperature increment. Three repeat metallic seal tests were also conducted at the seal 

maximum operating temperature of 427 °C (800 °F) for 9 h. The seal maintained vessel 

pressure within the measurement uncertainty of the pressure transducer in all three tests.

No leakage was observed in one EPDM seal tested at 300 °C (572 °F) for more than 20 h (Test 

# EPDM-a); however, leakage was observed immediately after the vessel had attained the 

nominal target temperature of 450 °C (842 °F) in another EPDM seal test (Test # EPDM-b). 

Leakage was also observed in the test (Test # PTFE-a) that used a PTFE seal after it had been 

subjected to 300 °C (572 °F) exposure for 22 h during the cooling phase.

In Phase II, additional experimental seal performance data for the two polymeric (EPDM and 

PTFE) O-ring seals were obtained. For exploratory purposes, three additional O-ring seals 

made of other polymeric materials (butyl, Viton, and silicone) were also studied. Thermal 

exposure tests were conducted at 316 °C (600 °F) for 8 h.

A total of eighteen tests were performed: four tests used EPDM O-rings, four used Viton, four 

used silicone, three used butyl, and three used PTFE. For the EPDM, butyl, PTFE, and silicone 

O-rings, the results, except one silicone O-ring test (Test # Silicone-1), showed that no leak was 

observed within measurement uncertainty. For all the Viton O-ring tests, the vessel pressure 

peculiarly increased during the thermal exposure, and the reason for this increase remains 

unresolved.

In Phase III, leak tests were performed using a double O-ring configuration. The outer-inner Oring combinations were metallic-metallic, EPDM-metallic, blank-metallic, EPDM-EPDM, and 

butyl-butyl. Thermal exposure tests were conducted at 800 °C (1472 °F) or 900 °C (1652 °F) for 

9 h. A total of eleven tests were carried out using a double O-ring configuration: four tests used 
4-2

metallic-metallic, three used EPDM-metallic, one used blank (no O-ring)-metallic, two used 

EPDM-EPDM, and one used butyl-butyl. No leaks were observed with the EPDM-metallic 

double O-ring configuration. For the four metallic-metallic O-ring tests, leak was observed in the 

two tests when the same torque for single metallic O-ring was used. Even when the appropriate 

torque value was applied for the double O-ring configuration, the other two metallic-metallic 

tests leaked, but only after being at a 900°C environment between 2 h and 8 h; there was no 

leakage prior to that period. No leak was observed in the blank-metallic test. Leaks were

detected in all the EPDM-EPDM and butyl-butyl tests even before the test temperature was 

reached. Table 4-1 provides a detailed summary of all the tests where constant vessel pressure 

was not observed during thermal exposure, and Table 4-2 provides a concise summary of all 

the tests.
4-3

Table 4-1. Summary of tests where constant vessel pressure was not observed during 

thermal exposure

Test 

Phase

Test # 

and seal 

material

Comments Exposure 

conditions

Nominal 

initial 

vessel 

condition

Seal 

configuration

I

Metallic-3

Measurable leakage occurred at 

approximately 6.9 h after the 

test temperature of 800 °C 

(1472 °F) had been reached.

Heat-up + 9 h at 

800 °C (1475 °F) + 

cool-down

24 °C at 

5 bar

Single 

Metallic-4

Measurable leakage occurred 

about 2.8 h into the 800 °C 

(1472 °F) exposure.

Metallic-8

Leakage was observed roughly 

3 h after reaching the target 

temperature.

PTFE-a

Leakage was observed in the 

test after it had been subjected 

to 300 °C (572 °F) exposure for 

22 h during the cooling phase.

8 h of incremental 

heating from 150 oC 

to 300 oC (572°F) + 

more than 20 h at 

300 oC (572 °F) + 

cool-down

24 °C at 

2 bar

EPDM-b

Leakage was observed 

immediately after the vessel 

had attained the nominal target 

temperature of 450 °C (842 °F).

Heat-up + more than 

24 h at 450 oC 

(842 °F)

II

Silicone-1

Leakage was observed after 

about 2 h at the target 

temperature.

Heat-up + 8 h at 

316°C + cool-down

Viton-1 For all the Viton O-ring tests, 

the vessel pressure peculiarly 

increased during the thermal 

exposure, and the reason for 

the pressure increase remains 

unresolved.

Viton-2

Viton-3

Viton-4

III

EPDMEPDM-1

Leakage occurred before test 

temperature was reached.

Heat-up + 9 h at 

900 °C + cool-down

24 °C at 

5 bar Double

EPDMEPDM-2

ButylButyl-1

MetallicMetallic-1 Leakage occurred when the 

torque for a single metallic O- Metallic ring was used. -

Metallic-2

MetallicMetallic-3 Leakage occurred even when 

the appropriate torque was Metallic used. -

Metallic-4
4-4

Table 4-2. Summary of all the test results

Phase Test # Nominal initial vessel 

conditions Exposure conditions Leakage

I

Metallic-1 24 °C at 5 bar 30 min at 800 °C

Metallic-2 24 °C at 5 bar 9 h at 800 °C

Metallic-3 24 °C at 5 bar 9 h at 800 °C Yes

Metallic-4 24 °C at 5 bar 9 h at 800 °C Yes

Metallic-5 24 °C at 5 bar 9 h at 427 °C

Metallic-6 24 °C at 5 bar 9 h at 427 °C

Metallic-7 24 °C at 5 bar 9 h at 427 °C

Metallic-8 24 °C at 5 bar 9 h at 800 °C Yes

Metallic-9 24 °C at 5 bar 427 °C and then to 800 °C for about 4 h

Metallic-10 24 °C at 5 bar 427 °C to 627 oC with 100 oC increment

Metallic-11 24 °C at 5 bar 427 °C to 727 oC with 100 °C increment

Metallic-12 24 °C at 5 bar 9 h at 800 °C

EPDM-a 24 °C at 2 bar 150 oC to 300 °C with 50 oC increment

EPDM-b 24 °C at 2 bar > 24 h at 450 oC Yes

PTFE-a 24 °C at 2 bar 150 oC to 300 °C with 50 oC increment Yes

II

EDPM-1 24 °C at 2 bar 8 h at 316 °C

EPDM-2 24 °C at 2 bar 8 h at 316 °C

EPDM-3 24 °C at 2 bar 8 h at 316 °C

EPDM-4 24 °C at 2 bar 8 h at 316 °C

EPDM-5 24 °C at 5 bar 9 h at 900 °C

EPDM-6 24 °C at 5 bar 9 h at 900 °C

Silicone-1 24 °C at 2 bar 8 h at 316 °C Yes

Silicone-2 24 °C at 2 bar 8 h at 316 °C

Silicone-3 24 °C at 2 bar 8 h at 316 °C

Silicone-4 24 °C at 2 bar 8 h at 316 °C

Butyl-1 24 °C at 2 bar 8 h at 316 °C

Butyl-2 24 °C at 2 bar 8 h at 316 °C

Butyl-3 24 °C at 2 bar 8 h at 316 °C

Viton-1 24 °C at 2 bar 8 h at 316 °C

Viton-2 24 °C at 2 bar 8 h at 316 °C

Viton-3 24 °C at 2 bar 8 h at 316 °C

Viton-4 24 °C at 2 bar 8 h at 316 °C

PTFE-1 24 °C at 2 bar 8 h at 316 °C

PTFE-2 24 °C at 2 bar 8 h at 316 °C

PTFE-3 24 °C at 2 bar 8 h at 316 °C

III

EPDM-Metallic-1 24 °C at 5 bar 9 h at 800 °C

EPDM-Metallic-2 24 °C at 5 bar 9 h at 800 °C

EPDM-Metallic-3 24 °C at 5 bar 9 h at 900 °C

Metallic-Metallic-1 24 °C at 5 bar 9 h at 900 °C Yes

Metallic-Metallic-2 24 °C at 5 bar 9 h at 900 °C Yes

Metallic-Metallic-3 24 °C at 5 bar 9 h at 900 °C Yes

Metallic-Metallic-4 24 °C at 5 bar 9 h at 900 °C Yes

Blank-Metallic-1 24 °C at 5 bar 9 h at 900 °C

EPDM-EPDM-1 24 °C at 5 bar 9 h at 900 °C Yes

EPDM-EPDM-2 24 °C at 5 bar 9 h at 900 °C Yes

Butyl-Butyl-1 24 °C at 5 bar 9 h at 900 °C Yes
5-1

5 REFERENCES

Adkins, H.E., Jr., Koeppel, B.J., Cuta, J.M., Guzman, A.D., and Bajwa, C.S, “Spent Fuel 

Transportation Package Response to the Caldecott Tunnel Fire Scenario,” US NRC 

NUREG/CR-6894, Rev. 1, PNNL-15346, 2007.

Adkins, H.E., Jr., Cuta, J.M., Koeppel, B.J., Guzman, A.D., and Bajwa, C.S., “Spent Fuel 

Transportation Package Response to the Baltimore Tunnel Fire Scenario,” US NRC 

NUREG/CR-6886, Rev. 2, PNNL-15313, 2007.

American National Standard for Radioactive Materials − Leakage Tests on Packages for 

Shipment, ANSI N14.5-1997.

ASME B16.5-2009, Pipe Flanges and Flanged Fittings NPS 1⁄2 through NPS 24 Metric/Inch 

Standard.

ASME Boiler and Pressure Vessel Code Section VIII − Rules for Construction of Pressure 

Vessels Division 1 (BPVC-VIII-1-2007).

Bronowski, D.R., “Performance Testing of Elastomeric Seal Materials under Low- and HighTemperature Conditions: Final Report,” Sandia Report SAND94-2207, Sandia National 

Laboratories, June 2000.

Dunn, D.S., Shewmaker, R.E., and Chowdhury, A.H., “Analysis of Structural Materials Exposed 

to a Severe Fire Environment,” US NRC NUREG/CR-6987, 2009.

ISO Guide to the Expression of Uncertainty in Measurement, first edition, 1993, International 

Organization for Standardization, Geneva, Switzerland.

ISO 12807 (1996) Safe Transport of Radioactive Materials − Leakage Testing on Packages. 

ISO, International Organization for Standardization.

Marlier, R., “First tests results for determination of seal life of EPDM O-rings at high temperature 

(determined by unique method),” Packaging, Transport, Storage and Security of Radioactive

Material 21 (1):37-40 (2010).

McGrattan, K.B. and Hamins, A. “Numerical Simulation of the Howard Street Tunnel Fire, 

Baltimore, Maryland, July 2001,” U.S. NRC NUREG/CR-6793, 2003.

Title 10, Code of Federal Regulations (CFR), Part 71, Packaging and Transportation of 

Radioactive Material, January 1, 2010, U.S. Government Printing Office, Washington, DC.

A-1

APPENDIX A DESIGN DRAWINGS OF TEST VESSEL

Figure A-1. Design drawing of single O-ring test vessel body.
A-2

Figure A-2. Design drawing of removable flange for single metallic seal (vessel cap).
A-3

Figure A-3. Design drawing of removable flange for single polymeric seal (vessel cap).
A-4

Figure A-4. Design drawing of double O-ring test vessel body. Ø1.25 in Ø4.20 in

1.75 in

0.375 in

Ø1.25 in

Ø5.75 in

Ø8.00 in

32

Ø2.75 in

Ø1.25 in

Ø0.75 in

Ø1.25 in

Ø0.375 in

Ø1.25 in

3.625 in

1.00 in

45 degree

R0.25 in
A-5

Figure A-5. Design drawing of removable flange for double O-ring (outer 

EPDM-inner metallic) seal (vessel cap). Ø1.25 in Ø4.20 in

1.75 in

0.375 in

O-ring groove

I.D. 2.178 - 0.004

O.D. 2.524 + 0.004

Depth 0.101 +/- 0.003

Ø1.25 in

Ø5.75 in

Ø8.00 in

Ø1.25 in

O-ring groove

I.D. 3.065

O.D. 3.387

Depth 0.104 +/- 0.003

Ø1.25 in

6332

16
A-6

Figure A-6. Design drawing of removable flange for double O-ring (outer polymeric-inner 

polymeric) seal (vessel cap).Ø1.25 in Ø4.20 in

1.75 in

0.375 in

Ø1.25 in

Ø5.75 in

Ø8.00 in

Ø1.25 in

O-ring groove

I.D. 3.065 in

O.D. 3.387 in

Depth 0.104 in +/- 0.003 in

Ø1.25 in

16

O-ring groove

I.D. 2.213 in

O.D. 2.533 in

Depth 0.104 in +/- 0.003 in

16
A-7

Figure A-7. Design drawing of removable flange for double O-ring (outer 

metallic-inner metallic) seal (vessel cap). Ø1.25 in Ø4.20 in

1.75 in

0.375 in

O-ring groove

I.D. 2.178 - 0.004

O.D. 2.524 + 0.004

Depth 0.101 +/- 0.003

Ø1.25 in

Ø5.75 in

Ø8.00 in

Ø1.25 in

O-ring groove

I.D. 3.051

O.D. 3.399 + 0.004

Depth 0.101 +/- 0.003

Ø1.25 in

6332

6332

B-1

APPENDIX B VESSEL INTERNAL VOLUMES

Vessel internal volume (excluding the volume of the through hole for welding the tubing flush to 

the vessel interior) was measured at room temperature using an internal micrometer with a 

resolution of 0.005 mm (0.0002 in.). The inside diameter of the vessel was the averaged value 

from measurements made at several (at least five) depth locations of the vessel cavity using an 

electronic digital bore gauge with a resolution of 0.002 mm (0.0001 in.).

Table B-1. Measured internal volumes of pressure vessels at room temperature

Vessel # Internal volume (m3)

Vessel #1 9.56 × 10-5

Vessel #2 9.52 × 10-5

Vessel #3 9.56 × 10-5

Vessel #3 (refurbished) 9.53 × 10-5

Vessel #4 9.55 × 10-5

Vessel #5 9.53 × 10-5

Vessel #6* 9.46 × 10-5

Vessel #7 9.57 × 10-5

Vessel #8* 9.55 × 10-5

Vessel #9 9.53 × 10-5

Vessel #10 9.54 × 10-5

Vessel #11 9.54 × 10-5

Vessel #12 9.57 × 10-5

Vessel #13 9.66 × 10-5

Vessel #14 No vessel body; only cap

Vessel #15 No vessel body; only cap

Vessel #16 9.72 × 10-5

Vessel #17 9.60 × 10-5

Vessel #18 9.75 × 10-5

Vessel #19 9.63 × 10-5

*post-test measurements

C-1

APPENDIX C UNCERTAINTY ESTIMATE OF 

MEASUREMENTS

C.1 Uncertainty Estimate in Thermocouple Measurements

The uncertainty budget associated with the thermocouple measurements is summarized in

Table C-1.

Table C-1. Summary of standard uncertainty components in thermocouple 

measurements

Standard 

uncertainty 

component

Source of 

uncertainty

Value of 

standard 

uncertainty

Source and comments

uTC Type-K thermocouple 0.64 °C

The thermocouples have a limit of 

error of 2.2 °C (from manufacturer 

specifications). An assumed 

rectangular probability distribution 

(ISO, 1993) results in 2.2 °C × 

0.29 = 0.64 °C.

uADC

Analog to digital 

conversion 0.04 °C

The K-type thermocouples have 

an output range of ± 50 mV; 100 

mV/216 (16-bit) = 0.00153 mV, 

which corresponds to 0.04 °C 

using an approximate sensitivity of 

40 μV/°C.

uCJ

Cold-junction 

compensation 1 °C

Software cold-junction 

compensation using 25 °C as 

reference temperature.

Combined standard uncertainty (uc) uucc = �uuTTTT

2 + uuAAAA2 + uu CC2 = 1.2 °C

Expanded uncertainty U (with coverage factor k = 2) U = k uC = 2.4 °C
C-2

C.2 Uncertainty Estimate in Pressure Transducer Measurements

The measurement uncertainty of the pressure transducer was estimated based on the 

specifications of the transducer provided by the manufacturer (see Table C-2). The uncertainty 

budget is listed in Table C-3.

Table C-2. Manufacturer’s specifications of the pressure transducer

Excitation 24 VDC to 32 VDC

Input range (0 to 500) psia ; (0 to 34.02) bar

Output range (0 to 5) VDC ± 0.03 VDC

Linearity 0.05 % FSO (full scale output)

Hysteresis 0.05 % FSO

Repeatability ± 0.05 % FSO

Thermal zero drift 0.001 % FSO/°F

Operating temperature range − 46 °C to 121 °C

Table C-3. Summary of standard uncertainty components in pressure measurements

Standard 

uncertainty 

component

Source of 

uncertainty

Value of standard 

uncertainty

Source and 

comments

uP Transducer output 0.0087 V

A rectangular probability 

distribution was assumed

(ISO, 1993), resulting 0.29 

× 0.03 V = 0.0087 V.

uL Linearity 0.0025 V 0.05 % × 5 V

uH Hysteresis 0.0025 V 0.05 % × 5 V

uR Repeatability 0.0025 V 0.05 % × 5 V

uT Thermal zero drift 9 × 10-4 V 18 °F × 0.001 % × 5 V/°F§

uADC

Analog to digital 

conversion 7.63 × 10-5 V 5 V/216 (16-bit)

§ Since the pressure transducer was mounted outside the furnace, the temperature of the transducer was close to room 

temperature, and a 10 °C (18 °F) rise from room temperature was assumed to be the worst case.

The combined standard uncertainty (uc)

uucc = �uuPP

2 + uuLL2 + uuHH2 + uuLL2 + uuTT2 + uuAAAA2

Then uc = 0.00976 V. Based on the calibration data of the pressure transducer provided by the 

manufacturer, P (bar) = −0.0147 + 6.8835 × output (V), the combined standard uncertainty, uc = 

0.00976 V, corresponds to uc = 0.0525 bar. The expanded uncertainty U of the pressure 

transducer with a coverage factor k of 2 is U = k × uc = 0.11 bar.
D-1

APPENDIX D VESSEL VOLUME EXPANSION AND LEAK 

RATE ESTIMATION

D.1 Vessel Volume Estimation due to Thermal Expansion

The total measurement system is idealized and treated as two systems, the pressure vessel 

(inside the electric furnace) and the rest (outside the furnace) which contains tubing, 

connections, the pressure transducer, and two valves. The pressure vessel is designated as 

System 1 using subscript 1, and the rest System 2 using subscript 2.

Figure D-1 shows the initial state of the two systems. They are at the same initial temperature

To and pressure Po. The initial amount of substance in System 1 is N1o, and System 2, N2o. The 

initial volume of the pressure vessel is V1o, and the rest is V2o. N is the total amount of 

substance in the two systems. Figure D-1 also shows the final state of the two systems. If 

there is no leak in the total system, then

NN1 + NN2 = NN1oo + NN2oo = NN

Using the ideal gas law,

PPVV1

RRTT1

+

PPVV2

RRTT2

= PPoo(VV1oo + VV2oo)

RRTToo

It is assumed that during the thermal exposure the heat loss to the surrounding from System 2 

is sufficient enough to keep System 2 at its initial temperature To. This assumption is 

reasonable since the tubing system remains cold to touch during the thermal exposure 

experiments. With TT2 = TToo and VV2 = VV2oo (no volumetric thermal expansion in System 2), then 

the above equation can be written as

PPVV1

TT1

+

PPVV2oo

TToo

= PPoo(VV1oo + VV2oo)

TToo

Note that due to volumetric thermal expansion in System 1 during the thermal exposure, VV1 ≠

VV1oo. The final vessel volume V1 can be expressed in terms of P as

VV1 = PPooTT1(VV1oo + VV2oo)

PPTToo

− TT1VV2oo

TToo

Since leakage was not observed in Tests # Metallic-2 (Test #2), # Metallic-5 (Test #5), #

Metallic-6 (Test #6), # Metallic-7 (Test #7), and # Metallic-12 (Test #14), the test parameters 

and the experimental results from these five thorough tests were used in the calculations of V1. 

The time-averaged values of T1 (average of the four thermocouple readings) and P over the 

constant heating period were used. A V2o value of 7 mL was estimated based on the internal 

dimensions of all the piping and fittings. Table D-1 tabulates the calculated V1 and the percent 

volume expansion, (V1-V1o) × 100/V1o. The volume expansion is less than 6 % at the maximum 

temperature (800 °C) used in this test series. Compared to Test # Metallic-2 (Test #2), the 
D-2

calculated percent volume expansion from Test # Metallic-12 (Test #14) is a bit low. This could 

be due to the assumptions used in analysis and the difficulty in delineating exactly Systems 1 

and 2 from test to test since the test fixture was placed inside the furnace at a slightly different 

location in each test.

Figure D-1. Schematics showing the systems used in the thermodynamic analysis.

To

V1o

Po To V2o Po

N1o

N2o

System 1

System 2

Pressure vessel volume

Plumbing + pressure transducer volume

Initial state

T1

V1

P To V2o P

N1

N2

System 1

System 2

Pressure vessel volume

Final state

Plumbing + pressure transducer volume
D-3

Table D-1. Calculations of vessel volume expansion at high temperatures

To (°C) Po (bar) V1o (m3) T1 (°C) P (bar) V1 (m3)

Volume 

expansion

(%)

Test #

Metallic-2 25 4.96 9.52 × 10-5 802 14.55 1.01 × 10-4 5.8

Test #

Metallic-5 25 4.98 9.53 × 10-5 425 10.34 9.91 × 10-5 4.0

Test #

Metallic-6 25 4.94 9.52 × 10-5 425 10.20 9.97 × 10-5 4.7

Test #

Metallic-7 24 4.99 9.57 × 10-5 426 10.39 9.97 × 10-5 4.1

Test #

Metallic-12 24 4.98 9.53 × 10-5 801 14.79 9.92 × 10-5 4.1
D-4

D.2 Isothermal Leakage Rate Estimation

If there is a leak after the final state is attained, then the leakage rate at the final-state 

temperature T1 can be estimated by

− dd

dddd = − �

ddNN1

dddd +

ddNN2

dddd � = − dd

dddd � VV1

RRTT1

+

VV2oo

RRTToo

�

Since V1/RT1 is an order of magnitude larger than V2o/RTo, the above equation can be 

approximated by

− dd

dddd ≅ − dddddd � VV1RRTT1

�

Knowing the pressure-time history at the final-state temperature T1 and the final-state volume V1

due to thermal expansion, the leakage rate can be estimated using the above equation. In the 

formula described in ANSI N14.5-1997 and ISO 12807 for leakage rate estimation using 

pressure drop technique, no correction is made for the thermal expansion of vessel volume at 

high temperatures. For the conditions used in this test series, the thermal expansion of the 

vessel volume at the highest T1 (800 °C) was estimated to be less than ≈ 6 % of the initial 

volume at room temperature, as described in Section D.1. In conforming to the ANSI and ISO 

practices and without the introduction of additional uncertainty associated with the estimation of 

V1 at different values of T1, V1 is approximated in the above equation using V1o with less than

6 % reduction in the leakage rate estimation.

− dd

dddd ≅ − dd

dddd �

VV1oo

RRTT1

�

The time derivatives of vessel pressure (dP/dt) can be computed numerically from the pressuretime history curve using the scientific graphing package SigmaPlot® macro with a running 

average length of 1. Following ANSI N14.5-1997, the reference helium leakage rates 

LR,He (ref·cm3

/s) can be obtained by dividing – dN/dt by 4.09 × 10-5 mol/cm3, which is the amount 

of substance (ideal gas) contained in 1 × 10-6 m3 (1 cm3

) at 1.01 × 105 Pa (1 atm) and 298 K 

(25 °C).
E-1

APPENDIX E O-RING MASS LOSS MEASUREMENTS

As an aside, mass loss measurements were also performed on the five polymeric O-rings used 

in Phase II to examine the extent of pyrolysis of the O-ring materials during thermal exposure. 

Table E-1 summarizes the test matrix.

To perform a mass loss measurement, an O-ring, used as-received, was placed in an acetonecleaned square pan (10 cm × 10 cm × 3 cm) made of stainless steel (SS 304) sheet (with a 

thickness of 0.05 mm). The pan with the O-ring was weighed using a PioneerTM Ohaus PA214 

electronic balance with a resolution of 0.0001 g and a maximum capacity of 210 g. The pan 

was then covered with a 0.05 mm stainless steel (SS 304) sheet, and placed in the preheated 

electric furnace, the same one used in Phase I, set at 316 °C (600 °F) for 8 h. This test 

temperature exceeds the maximum operating temperatures of all the O-ring materials. An 

exploratory test series was also performed using EPDM O-rings at a higher temperature of 450 

°C (842 °F) for 8 h. After the thermal exposure, the covered pan with the O-ring was cooled to 

room temperature in the furnace overnight before being weighed without the cover. The mass 

loss was determined from the difference between the masses of the pan with the O-ring before 

and after the thermal exposure. A minimum of five repeated measurements for each polymeric 

material were performed. Digital photographs of the tested O-rings were also taken for 

examinations.

Table E-1. Test matrix for O-ring mass loss measurements

O-ring material Number of tests Test conditions

EPDM 7 316 °C (600 °F) for 8 h

Silicone 5 316 °C (600 °F) for 8 h

Butyl 5 316 °C (600 °F) for 8 h

Viton 5 316 °C (600 °F) for 8 h

PTFE 7 316 °C (600 °F) for 8 h

EPDM 4 450 °C (842 °F) for 8 h

Table E-2 summarizes the O-ring mass loss measurements at 316 °C (600 °F) for 8 h. The last 

column entries in the table were obtained by dividing the O-ring mass loss (column 6 in the 

table) by the original O-ring mass (column 4 in the table), expressed in percent. The entries 

with negative percent losses indicate the measured mass losses were negligible and that the 

differences in O-ring masses before and after thermal exposure could not be resolved within the 

uncertainty of the electronic balance used. Table E-3 summarizes the examinations of the Orings after the thermal exposure. Figure E-1 shows sample photographs of the O-rings after 

thermal exposure. Note that only the fragmented EPDM and butyl O-rings are shown in the 

figure, and not every post-tested EPDM or butyl O-ring was found to be broken into pieces. Orings made of materials other than EPDM and butyl remained intact after thermal exposure.

Table E-4 summarizes the EPDM O-ring mass loss measurements at 450 °C (842 °F) for 8 h. 
E-2

After thermal exposure, the EPDM O-ring turned into powdery pieces (see Figure E-2). More 

than 60 % of the original O-ring mass was lost after thermal exposure at 450 °C (842 °F) for 8 h.

Figure E-1. Photographs showing EPDM and butyl O-rings after thermal exposure at 

316 °C (600 °F) for 8 h.

Figure E-2. Photograph showing an EPDM O-ring after thermal exposure at 

450 °C (842 °F) for 8 h.

EPDM Butyl
E-3

Table E-2. O-ring mass loss measurements at 316 °C (600 °F) for 8 h

O-ring 

material

Pan

(g)

O-ring + Pan

(g)

O-ring

(g)

O-ring + Pan 

(after 

thermal 

exposure)

(g)

O-ring

mass loss

(g)

% loss

EPDM 9.2259 11.3318 2.1059 11.2508 0.081 3.8

EPDM 9.4152 11.5303 2.1151 11.0647 0.4656 22.0

EPDM 9.5216 11.6260 2.1044 11.5542 0.0718 3.4

EPDM 9.2336 11.3193 2.0857 10.7297 0.5896 28.3

EPDM 9.5225 11.6188 2.0963 11.5499 0.0689 3.3

EPDM 9.4152 11.5213 2.1061 11.3265 0.1948 9.2

EPDM 9.4720 11.5744 2.1024 11.5164 0.058 2.8

Silicone 9.4559 11.8344 2.3785 11.7802 0.0542 2.3

Silicone 9.2265 11.6080 2.3815 11.5631 0.0449 1.9

Silicone 9.4213 11.8182 2.3969 11.7522 0.0660 2.8

Silicone 9.5315 11.9012 2.3697 11.8453 0.0559 2.4

Silicone 9.4739 11.8730 2.3991 11.8175 0.0555 2.3

Butyl 9.2263 11.5613 2.3350 10.9928 0.5685 24.3

Butyl 9.5208 11.8608 2.3400 11.6351 0.2257 9.6

Butyl 9.4155 11.7732 2.3577 11.6068 0.1664 7.1

Butyl 9.4727 11.7954 2.3227 11.5301 0.2653 11.4

Butyl 9.5218 11.8744 2.3526 11.6390 0.2354 10.0

Viton 9.4674 12.9175 3.4501 12.894 0.0235 0.7

Viton 9.4716 12.8799 3.4083 12.828 0.0519 1.5

Viton 9.2329 12.6557 3.4228 12.5874 0.0683 2.0

Viton 9.5219 12.9306 3.4087 12.902 0.0286 0.8

Viton 9.4552 12.8948 3.4396 12.8666 0.0282 0.8

PTFE 9.4153 13.4239 4.0086 13.4223 0.0016 0.04

PTFE 9.5215 13.5299 4.0084 13.5289 0.0010 0.02

PTFE 9.4553 13.4582 4.0029 13.4590 -0.0008 -0.020

PTFE 9.2253 13.2429 4.0176 13.2430 -1.00E-04 -0.002

PTFE 9.4731 13.4598 3.9867 13.4580 0.0018 0.045

PTFE 9.4514 13.3707 3.9193 13.3683 0.0024 0.061

PTFE 9.5221 13.5216 3.9995 13.5200 0.0016 0.040
E-4

Table E-3. O-ring examinations after thermal exposure at 316 °C (600 °F) for 8 h

O-ring material Observations

EPDM

After thermal exposure, the color of the O-ring did not change; 

however, it became quite rigid and lost its elasticity. If compressed, 

the O-ring would crumble. In some tests, the O-rings were broken 

apart (in the pan) into chunks after thermal exposure*. When this 

happened, abnormally high mass losses were measured.

Silicone After thermal exposure, the color of the O-ring did not change; the 

O-ring remained very elastic and intact.

Butyl

After thermal exposure, the color of the O-ring did not change; 

however, it became quite rigid and lost its elasticity. If compressed, 

the O-ring would crumble. In some tests, the O-rings were broken 

apart (in the pan) into chunks after thermal exposure*. When this 

occurred, abnormally high mass losses were measured.

Viton After thermal exposure, the color of the O-ring did not change; the 

O-ring remained very elastic and intact.

PTFE After thermal exposure, the color of the O-ring did not change; the 

O-ring retained the same hardness and remained intact.

*Since the pan was covered when it was being heated inside the electrical furnace, it was unclear 

whether the fragmentation of the O-ring occurred.

Table E-4. O-ring mass loss measurements at 450 °C (842 °F) for 8 h

O-ring 

material

Pan

(g)

O-ring + Pan

(g)

O-ring

(g)

O-ring + Pan 

(after 

thermal 

exposure)

(g)

O-ring

mass loss

(g)

% loss

EPDM 8.9926 11.0971 2.1045 9.7920 1.3051 62.01

EPDM 8.9948 11.1007 2.1059 9.7887 1.3120 62.30

EPDM 8.9952 11.1018 2.1066 9.7887 1.3131 62.33

EPDM 8.9944 11.0838 2.0894 9.7880 1.2958 62.02
E-5

Within the resolution of the electronic balance, PTFE O-rings were found to have negligible

mass losses. Silicone O-rings were found to have mass losses ranging from 1.9 % to 2.8 % 

with an arithmetic mean of 2.3 % and a standard deviation of 0.3 %. For Viton O-rings, the 

lowest and highest measured mass losses were 0.7 % and 2 % respectively, with an arithmetic 

mean of 1.2 % and a standard deviation of 0.6 %. The mass loss results from EPDM and butyl 

O-rings were scattered due to three outliers in the EPDM measurements and one in the butyl’s. 

For EPDM O-rings, the lowest and highest measured mass losses were 2.8 % and 28.3 % 

respectively, with an arithmetic mean of 10.4 % and a standard deviation of 10.5 %. For butyl 

O-rings, the lowest and highest measured mass losses were 7.1 % and 24.3 % respectively, 

with an arithmetic mean of 12.5 % and a standard deviation of 6.8 %. The examination of the 

EPDM and butyl O-rings after thermal exposure revealed that some of the O-rings were broken 

into several segments in the covered pan during thermal exposure. When the test O-ring 

fragmented during thermal exposure, abnormally high mass losses (outliers) were recorded. 

Since all the O-rings used in the experiments came from the same batches (lots), the reason 

why few EPDM and butyl O-rings fragmented and the majority did not during the thermal 

exposure remains unclear.

F-1

APPENDIX F LEAK RATE MEASUREMENTS WITHOUT 

O-RING

As an aside in Phase II, several helium leak rate measurements were conducted at room 

temperature using Vessel #3 without an O-ring and different torques to tighten the bolts that 

hold the vessel flanges together. The torques21 used were 414 N m (305 lb ft, recommended 

for the metallic O-rings), 203 N m (150 lb ft), and 136 N m (100 lb ft, the lowest setting for the 

torque wrench used). Figure F-1 shows the measurement results. The vessel pressure started 

to decrease and leakage occurred immediately. The instantaneous leakage rate, which is 

proportional to the instantaneous slope of the pressure-time curve, is inversely proportional to 

the torque used; the higher the torque, the slower the leak.

Figure F-1. Temporal variation of normalized vessel pressure at 24 °C without O-ring at 

different torques used to tighten the vessel flanges.

 21 The vessel could not be pressurized if the bolts were tightened by hand.





NUREG/CR-7115, Rev. 1 Performance of Metal and Polymeric O-Ring Seals in Beyond-Design-Basis 

Temperature Excursions