Performance of Metal and 

Polymeric O-Ring Seals in 

Beyond-Design-Basis 

Temperature Excursions

Office of Nuclear Regulatory Research

NUREG/CR-7115
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Performance of Metal and 

Polymeric O-Ring Seals in 

Beyond-Design-Basis 

Temperature Excursions

Manuscript Completed: November 2011

Date Published: April 2012

Prepared by

Jiann C. Yang

Edward J. Hnetkovsky

National Institute of Standards and Technology

Engineering Laboratory

Gaithersburg, Maryland 20899

Felix Gonzalez, NRC Project Manager

NRC Job Code N6550

Office of Nuclear Regulatory Research

NUREG/CR-7115

iii 

ABSTRACT 

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

of one type of metallic seal and two different polymeric compound (ethylene-propylene and 

polytetrafluoroethylene) seal designs typically used in spent fuel transportation packages. 

Fifteen tests were conducted using a small scale-model package 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 over the specified 

maximum operating temperatures of these seals for a pre-determined period (typically 9 h). The 

pressure drop technique was used to determine if leakage occurred during thermal exposure. A 

total of fifteen tests, twelve used metallic seals, two used ethylene-propylene compound, and one 

used polytetrafluoroethylene, were performed. Leakage was observed in some of the thermal 

exposure tests. The time when leakage occurred varied. The overall goal of the project is to 

provide insights to the performance of these seals when exposed to beyond-design-basis 

temperature conditions that could result due to a severe fire. 

v 

TABLE OF CONTENTS 

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

TABLE OF CONTENTS ................................................................................................................ v

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

LIST OF TABLES ......................................................................................................................... ix

EXECUTIVE SUMMARY ........................................................................................................... xi

ACKNOWLEDGMENTS ........................................................................................................... xiii

ABBREVIATIONS ...................................................................................................................... xv

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

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

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

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

2.2 Experimental Procedure ................................................................................................ 2-4

2.3 Test Matrix .................................................................................................................... 2-5

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

3.1 Test #1 (Vessel #1) ....................................................................................................... 3-1

3.2 Test #2 (Vessel #2) ....................................................................................................... 3-2

3.3 Test #3 (Vessel #3) ....................................................................................................... 3-5

3.4 Test #4 (Vessel #4) ....................................................................................................... 3-7

3.5 Test #5 (Vessel #5) ....................................................................................................... 3-9

3.6 Test #6 (Vessel #2 Refurbished) ................................................................................. 3-11

3.7 Test #7 (Vessel #1 Refurbished) ................................................................................. 3-12

3.8 Test #8 (Vessel #6) ..................................................................................................... 3-14

3.9 Test #9 (Vessel #1 Re-Refurbished) ........................................................................... 3-16

3.10 Test #10 (Vessel #7) ................................................................................................... 3-18

3.11 Test #11 (Vessel #3 Refurbished) ............................................................................... 3-20

3.12 Test #12 (Vessel #3 Refurbished) ............................................................................... 3-21

3.13 Test #13 (Vessel #8) ................................................................................................... 3-23

3.14 Test #14 (Vessel #9) ................................................................................................... 3-25

3.15 Test #15 (Vessel #3 Refurbished) ............................................................................... 3-26

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

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

APPENDIX A ............................................................................................................................. A-1

APPENDIX B ............................................................................................................................. B-1

APPENDIX C ............................................................................................................................. C-1

APPENDIX D ............................................................................................................................. D-1
 
vii 

LIST OF FIGURES 

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

Figure 2-2. Photographs of the experimental apparatus. ............................................................ 2-3

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

O-ring groove (above) and the O-ring bonded to the test vessel body (below). ...... 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 #2. ....................... 3-4

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

extended, including the complete cool-down phase). .............................................. 3-4

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

Figure 3-6. Temporal variations of vessel pressure and temperature in Test #3 (time scale 

extended, including the complete cool-down phase). .............................................. 3-6

Figure 3-7. Isothermal reference helium leakage rate during the 9 h heating in Test #3. .......... 3-6

Figure 3-8. Temporal variations of vessel pressure and temperature in Test #4. ....................... 3-7

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

extended, including the complete cool-down phase). .............................................. 3-8

Figure 3-10. Isothermal reference helium leakage rate during the 9 h heating in Test #4. ........ 3-8

Figure 3-11. Temporal variations of vessel pressure and temperature in Test #5. ..................... 3-9

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

extended, including the complete cool-down phase). ......................................... 3-10

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

Figure 3-14. Temporal variations of vessel pressure and temperature in Test #6. ................... 3-11

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

extended, including the complete cool-down phase). ......................................... 3-12

Figure 3-16. Temporal variations of vessel pressure and temperature in Test #7. ................... 3-13

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

extended, including the complete cool-down phase). ......................................... 3-13

Figure 3-18. Temporal variations of vessel pressure and temperature in Test #8. ................... 3-14

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

extended, including the complete cool-down phase). ......................................... 3-15

Figure 3-20. Isothermal reference helium leakage rate during the 9 h heating in Test #8. ...... 3-15

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

performance. ........................................................................................................ 3-16

Figure 3-22. Temporal variations of vessel pressure and temperature in Test #9. ................... 3-17

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

extended, including the complete cool-down phase). ......................................... 3-17

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

gauge. .................................................................................................................. 3-18

Figure 3-25. Temporal variations of vessel pressure and temperature in Test #10. ................. 3-19

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

extended, including the complete cool-down phase). ......................................... 3-19

Figure 3-27. Temporal variations of vessel pressure and temperature in Test #11. ................. 3-20

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

extended, including the complete cool-down phase). ......................................... 3-21
viii 

Figure 3-29. Temporal variations of vessel pressure and temperature in Test #12. ................. 3-22

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

extended, including the complete cool-down phase). ......................................... 3-22

Figure 3-31. Isothermal reference helium leakage rate during the 22 h heating in Test #12. .. 3-23

Figure 3-32. Temporal variations of vessel pressure and temperature in Test #13. ................. 3-24

Figure 3-33. Temporal variations of vessel pressure and temperature in Test #13 (time scale 

extended, including the complete cool-down phase). ......................................... 3-24

Figure 3-34. Temporal variations of vessel pressure and temperature in Test #14. ................. 3-25

Figure 3-35. Temporal variations of vessel pressure and temperature in Test #14 (time scale 

extended, including the complete cool-down phase). ......................................... 3-26

Figure 3-36. Temporal variations of vessel pressure and temperature in Test #15. ................. 3-27

Figure 3-37. Temporal variations of vessel pressure and temperature in Test #15 (time scale 

extended, including the complete cool-down phase). ......................................... 3-27

Figure 3-38. Isothermal reference helium leakage rate during the 25 h heating in Test #15. .. 3-28

Figure 3-39. Photograph of the ethylene-propylene compound o-ring after thermal exposure 3-29

Figure 3-40. Photograph showing the disintegration of the tested ethylene-propylene compound 

O-ring when an attempt was made to remove the O-ring from the groove. ........ 3-29

Figure A-1. Design drawing of test vessel body. ....................................................................... A-1

Figure A-2. Design drawing of removable flange for metallic seal (vessel cap). ..................... A-1

Figure A-3. Design drawing of removable flange for polymeric seal (vessel cap). .................. A-2

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

 
ix 

LIST OF TABLES 

Table 2-1. Nominal test conditions and parameters .................................................................... 2-5

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
 
xi 

EXECUTIVE SUMMARY 

The objective of this work is to provide experimental seal performance data for one type of 

metallic seal and two different polymeric compound seals typically used in spent fuel 

transportation packages for beyond-design-basis temperature excursions (beyond the seal 

manufacturer rated operating temperatures). 

Fifteen tests were conducted using a scale-model package (a pressure vessel) made of stainless 

steel SS 304 filled with helium initially at either 5 bar (for metallic seal) or 2 bar (for polymeric 

seals) 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 pre-determined duration, which varied from 

several hours to one or two days. The vessel pressure was monitored to determine if a leak 

occurred at the exposed temperature. 

Fifteen tests including two shakedowns were performed to establish seal performance at beyonddesign-basis thermal exposure conditions. Of the fifteen tests conducted, twelve tests used 

metallic seals, two used ethylene-propylene compound seals, and one used a 

polytetrafluoroethylene (PTFE) seal. 

Of the five repeat metallic-seal tests, leakage was observed in three of the tests 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 one test, measurable leakage occurred 

approximately 6.9 h after the test temperature of 800 °C (1472 °F) had been reached. In another 

test, measurable leakage occurred about 2.8 h into the 800 °C (1472 °F) exposure. Yet, in 

another test, leakage was observed roughly 3 h into the test. In the two shakedown tests (Tests 

#1 and 9) using metallic seals, leakage was not observed during the respective 30 min and 4 h 

exposure to 800 °C (1472 °F). No leakage was observed for the remaining tests using metallic 

seals exposed to temperatures just below 750 °C (1382 °F) for 9 h. 

No leakage was observed in one ethylene-propylene compound seal tested at 300 °C (572 °F) for 

more than 20 h. Leakage was observed immediately after the vessel had attained the nominal 

target temperature of 450 °C (842 °F) in another ethylene-propylene compound seal test. 

Leakage was also observed during the cooling phase in the test that used a PTFE seal after it had 

been subjected to 300 °C (572 °F) exposure for 22 h. 
 
xiii 

ACKNOWLEDGMENTS 

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

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

of NRC, Harold E. Adkins Jr. 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. 
 
xv 

ABBREVIATIONS 

ANSI American National Standards Institute 

ASME American Society of Mechanical Engineers 

CFR Codes of Federal Regulations 

CRUD Chalk River Unidentified Deposits 

DAQ Data Acquisition 

FDS Fire Dynamics Simulator 

HAC Hypothetical Accident Conditions 

ISO International Organization for Standardization 

NIST National Institute of Standards and Technology 

NRC United States Nuclear Regulatory Commission 

RES NRC Office of Nuclear Regulatory Research 

SNF Spent Nuclear Fuel 

PTFE Polytetrafluoroethylene 

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 spent nuclear fuel (SNF) transportation packages, during 

fire exposures that exceed the hypothetical accident conditions (HAC) fire described in 10 CFR 

Part 71 Section 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. 

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 spent fuel 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 finite 

element 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 release of radioactive 

material, such as Cobalt 60 (from CRUD) or Cesium 137 (from fission products), could not be 

ruled out with available information. 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 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). A similar 

experimental configuration was used to examine the performance of elastomeric seal at 

temperatures below 0 °C (32 °F) (Jaunich et al., 2011). Testing of package seals to determine 

their performance in beyond-design-basis 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 using a scale-model 

package (a pressure vessel) to house a test seal. Since it is difficult to control a real fire 

environment for testing, an electric furnace was used to provide various controlled thermal 

environments. 

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 exposure 

environment proved to be less demanding than other more sensitive test methods, as described in 
1-2 

ANSI N14.5-1997. Since the use of the temporal variation of vessel pressure as a means to 

detect potential leakage is best applied to isothermal conditions (ANSI N14.5-1997), all the 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). 
2-1 

2 TECHNICAL APPROACH 

2.1 Experimental Apparatus 

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, 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 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 Oring 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 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 ft·lb; 

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

uncertainty with a coverage factor of 2 [ISO, 1993]) was used for the metallic seals and 271 N·m 

(200 ft·lb) for the ethylene-propylene compound 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 ethylene-propylene compound 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 straighthole with a bevel-groove and was all-around 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 

 

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 xray 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 

fourth arm of the union cross via a port 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 Ktype 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) 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. 

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

 

6 See Appendix B for measurement uncertainty discussion. 

Electric

furnace

Pressure 

vessel

Pressure

transducer

Vacuum

Helium

Thermocouple

Valve

Thermocouple

Thermocouple

Thermocouple
2-3 

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

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

Figure 2-2. Photographs of the experimental apparatus. 

Flange thermocouples

Furnace interior

Test fixture

Stainless steel tubing

Valves

Flange 

thermocouples

Vessel interior 

thermocouple

Pressure transducer

Stainless steel tubing

Furnace exterior

Thermal insulation board
2-4 

2.2 Experimental Procedure 

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)7. 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 Table 2-1 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 ethylene-propylene compound 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 oil-less 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 closed 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 had reached the target 

value, the vessel was heated according to the pre-determined exposure duration (see Table 2-1 

below). After the thermal exposure, the furnace was then 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. 

 

7 Generally, in HAC analyses using computer codes, SNF Transportation package design seals do not reach the 

HAC temperature of 800 °C (1472 °F). 
2-5 

2.3 Test Matrix 

Table 2-1 summarizes the test conditions and parameters for this test series. 

Table 2-1. Nominal test conditions and parameters 

Test # Vessel # Nominal initial vessel conditions Exposure duration 

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

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

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

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

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

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

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

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

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

about 4 h + cool-down 

10 7 24 °C at 5 bar (Metallic seal) Incremental heating from 427 °C to 627 oC 

with 100 oC increment§

 + cool-down 

11 3** 24 °C at 2 bar 

(ethylene-propylene seal)

Incremental heating from 150 oC to 300 °C 

with 50 oC increment§§ + cool-down 

12 3** 24 °C at 2 bar (PTFE seal) Incremental heating from 150 oC to 300 °C 

with 50 oC increment§§ + cool-down

13 8 24 °C at 5 bar (Metallic seal) Incremental heating from 427 °C to 727 oC 

with 100 °C increment§

 + cool-down

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

15 3** 24 °C at 2 bar 

(ethylene-propylene seal) 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 for second time. 

§

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.

3-1 

3 RESULTS AND DISCUSSION 

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

method was applied under isothermal conditions. 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. If 

there was a very small leak, the reduction in pressure due to the reduction in helium in the vessel 

from the leak could easily be compensated by the increase in pressure due to increasing 

temperature. The net effect might still show an increase in pressure, thus masking the leak. 

Therefore, only the vessel pressure data at an elevated constant temperature were used to 

properly detect potential leak. Another indicator for leak was to determine whether the vessel 

pressure could be restored to the initial vessel pressure after the thermal exposure test. The 

restoration of initial vessel pressure indicated no leak had occurred during a test. 

3.1 Test #1 (Vessel #1) 

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

apparatus. The entire heating process took about 4.5 h 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 an 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, which indicated 

that no leakage had occurred. 

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. 
3-2 

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

O-ring groove (above) and the O-ring bonded to the test vessel body (below). 

3.2 Test #2 (Vessel #2) 

Before 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 temperature. Figure 3-2 is the test 

Vessel cap

O-ring groove coated with silver 

Metallic O-ring

Vessel body
3-3 

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. 

The vessel was disassembled and assembled again using a metallic seal to start Test #2. 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 #2. The rapid decrease in the 

furnace temperature signifies turning-off the furnace 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 cooldown 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 rest 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 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. 

Vessel #2 (no O-ring)

Time (s)

0 20x103 40x103 60x103 80x103 100x103

Pressure (bar)

0

1

2

3

4

5

6

Time (h)

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

LR,He (cm3/s)

-0.04

-0.02

0.00

0.02

0.04

Vessel pressure

Helium leakage rate
3-4 

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

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

extended, including the complete cool-down phase). 

Vessel #2 (Test #2)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

13.8

14.0

14.2

14.4

14.6

14.8

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #2 (Test #2)

Time (s)

0 100x103 200x103 300x103 400x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-5 

3.3 Test #3 (Vessel #3) 

Tests #3 is a repeat of 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 #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 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. 

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

Vessel #3 (Test #3)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-6 

Figure 3-6. Temporal variations of vessel pressure and temperature in Test #3 (time scale 

extended, including the complete cool-down phase). 

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

Vessel #3 (Test #3)

Time (s)

0 100x103 200x103 300x103 400x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Test #3

Time (s)

12000 17000 22000 27000 32000 37000 42000

Pressure (bar)

0

2

4

6

8

10

12

14

16

LR,He (ref cm3/s)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Vessel pressure

Helium leakage rate
3-7 

3.4 Test #4 (Vessel #4) 

Test #4 is another repeat of the thermal exposure condition used in Test #2. Figure 3-8 shows 

the test results from Test #4. In this test, the pressure remained relatively constant for about 

10 000 s (2.8 h) initially during the 9 h constant-temperature 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 #3, and #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. 

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

Vessel #4 (Test #4)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-8 

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

extended, including the complete cool-down phase). 

Figure 3-10. Isothermal reference helium leakage rate during the 9 h heating in Test #4. 

Vessel #4 (Test #4)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Test #4

Time (s)

12000 17000 22000 27000 32000 37000 42000

Pressure (bar)

0

2

4

6

8

10

12

14

16

LR,He (ref cm3/s)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Vessel pressure

Helium leakage rate
3-9 

3.5 Test #5 (Vessel #5) 

The results for 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 cooldown. 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. 

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

Vessel #5 (Test #5)

Time (s)

0 10000 20000 30000 40000 50000 60000

Pressure (bar)

4

5

6

7

8

9

10

11

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-10 

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

Vessel #5 (Test #5)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

11

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-11 

3.6 Test #6 (Vessel #2 Refurbished) 

The results for 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 #6. 

Vessel #2 refurbished (Test #6)

Time (s)

0 10000 20000 30000 40000 50000 60000

Pressure (bar)

9.4

9.6

9.8

10.0

10.2

10.4

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-12 

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

extended, including the complete cool-down phase). 

3.7 Test #7 (Vessel #1 Refurbished) 

Figure 3-16 shows the results for 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-hour constant-temperature heating period. No 

leakage was observed during Test #7. Figure 3-17 is the complete thermal exposure history of 

Test #7. 

Vessel #2 refurbished (Test #6)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

11

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-13 

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

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

extended, including the complete cool-down phase). 

Vessel #1 refurbished (Test #7)

Time (s)

0 10000 20000 30000 40000 50000 60000

Pressure (bar)

4

5

6

7

8

9

10

11

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #1 refurbished (Test #7)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

11

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-14 

3.8 Test #8 (Vessel #6) 

The test conditions were similar to Tests #2, 3, and 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 

gradual 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 timepressure and time-temperature histories of Test #8. Figure 3-20 shows the calculated isothermal 

reference helium leakage rates during the 9 h heating. 

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

Vessel #6 (Test #8)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure 

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-15 

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

extended, including the complete cool-down phase). 

Figure 3-20. Isothermal reference helium leakage rate during the 9 h heating in Test #8. 

Vessel #6 (Test #8)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure 

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Test #8

Time (s)

12000 17000 22000 27000 32000 37000 42000

Pressure (bar)

0

2

4

6

8

10

12

14

16

LR,He (ref cm3/s)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Vessel pressure

Helium leakage rate
3-16 

3.9 Test #9 (Vessel #1 Re-Refurbished) 

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 gauge8 to ensure that 

the pressure transducer functioned properly. Figure 3-21 shows the set-up 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 and was located outside the furnace at room temperature (see Appendix D for theoretical 

volume and pressure calculation) as shown in Figure 3-21. 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 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. 

 

8 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). 
3-17 

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

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

extended, including the complete cool-down phase). 

Vessel #1 re-refurbished (Test #9)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0

2

4

6

8

10

12

14

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #1 re-refurbished (Test #9)

Time (s)

0 100x103 200x103 300x103 400x103 500x103

Pressure (bar)

0

2

4

6

8

10

12

14

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-18 

Figure 3-24. Comparison of pressure measurements from pressure 

transducer and pressure gauge. 

3.10 Test #10 (Vessel #7) 

In 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. 

Pressure transducer readings (bar)

0 2 4 6 8 10 12 14

Pressure gauge readings (bar)

0

2

4

6

8

10

12

14

45o line
3-19 

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

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

scale extended, including the complete cool-down phase). 

Vessel #7 (Test #10)

Time (s)

0 50x103 100x103 150x103 200x103

Pressure (bar)

0

2

4

6

8

10

12

14

Temperature (oC)

0

100

200

300

400

500

600

700

Time (h)

0 5 10 15 20 25 30 35 40 45 50 55

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #7 (Test #10)

Time (s)

0 100x103 200x103 300x103 400x103 500x103

Pressure (bar)

0

2

4

6

8

10

12

14

Temperature (oC)

0

100

200

300

400

500

600

700

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-20 

3.11 Test #11 (Vessel #3 Refurbished) 

An ethylene-propylene compound 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 refacing the flange to remove the O-ring groove originally machined for the metallic seal and remachining a new O-ring groove for the ethylene-propylene compound O-ring which has different 

groove dimensions from its metallic counterpart. 

Figure 3-27 and Figure 3-28 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 3-28. Post-test inspection of the exposed vessel 

revealed that the O-ring was still pliable and could be easily removed from the O-ring groove. 

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

Vessel #3 refurbished (Test #11)

(ethylene propylene seal)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Temperature (oC)

0

50

100

150

200

250

300

350

400

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-21 

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

scale extended, including the complete cool-down phase). 

3.12 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 was used for 

this test. The refurbishment of the vessel simply consisted of cleaning of the O-ring groove 

previously used for the ethylene propylene seal test (Test #11). Figure 3-29 and Figure 3-30 

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 3-31, 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 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. Posttest inspection of the exposed vessel revealed that the compressed O-ring was still intact and 

could be easily removed from the O-ring groove. 

Vessel #3 refurbished (Test #11)

(ethylene propylene seal)

Time (s)

0 100x103 200x103 300x103 400x103 500x103

Pressure (bar)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Temperature (oC)

0

50

100

150

200

250

300

350

400

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-22 

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

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

scale extended, including the complete cool-down phase). 

Vessel #3 refurbished (Test #12)

(TFE seal)

Time (s)

0 20x103 40x103 60x103 80x103 100x103 120x103

Pressure (bar)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Temperature (oC)

0

50

100

150

200

250

300

350

400

Time (h)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #3 refurbished (Test #12)

 (TFE seal)

Time (s)

0 100x103 200x103 300x103 400x103 500x103

Pressure (bar)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Temperature (oC)

0

50

100

150

200

250

300

350

400

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-23 

Figure 3-31. Isothermal reference helium leakage rate during the 22 h heating in Test #12. 

3.13 Test #13 (Vessel #8) 

For 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 #1 to #12, two additional evacuating and filling 

cycles were applied in order to determine if the evacuation process had any effect on the results.9 

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 

727 °C for 9 h. Figure 3-32 and Figure 3-33 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 pressure reading. 

 

9 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. 

Test #12

Time (s)

34x103 44x103 54x103 64x103 74x103 84x103 94x103 104x103

Pressure (bar)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

LR,He (ref cm3/s)

-0.02

-0.01

0.00

0.01

0.02

Vessel pressure

Helium leakage rate
3-24 

Figure 3-32. Temporal variations of vessel pressure and temperature in Test #13. 

Figure 3-33. Temporal variations of vessel pressure and temperature in Test #13 (time 

scale extended, including the complete cool-down phase). 

Vessel #8 (Test #13)

Time (s)

0 50x103 100x103 150x103 200x103 250x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

Time (h)

0 10 20 30 40 50 60

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #8 (Test #13)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperatuer (oC)

0

100

200

300

400

500

600

700

800

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-25 

3.14 Test #14 (Vessel #9) 

In 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 #2, #3, and #4. 

Figure 3-34 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 3-35. 

However, similar to the observation in Test #13, 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. 

Figure 3-34. Temporal variations of vessel pressure and temperature in Test #14. 

Vessel #9 (Test #14)

Time (s)

0 10000 20000 30000 40000 50000

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-26 

Figure 3-35. Temporal variations of vessel pressure and temperature in Test #14 (time 

scale extended, including the complete cool-down phase). 

3.15 Test #15 (Vessel #3 Refurbished) 

In Test #15, an ethylene-propylene compound seal together with the refurbished Vessel #3 (used 

in Test #11) 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 #11 was the 

temperature used for the thermal exposure test. Instead of 300 °C (572 °F) as used in Test #11, a 

higher temperature, 450 °C (842 °F) was used. 

Figure 3-36 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 3-37. The pressure trace in Figure 3-36 clearly indicates a leak occurred soon 

after the vessel had attained the nominal target temperature of 450 °C (842 °F). Figure 3-38 

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. 

Vessel #9 (Test #14)

Time (s)

0 100x103 200x103 300x103 400x103 500x103

Pressure (bar)

0

2

4

6

8

10

12

14

16

Temperature (oC)

0

100

200

300

400

500

600

700

800

900

Time (h)

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Vessel pressure

Vessel internal temperature

Flange temperature 1

Flange temperature 2

Furnace temperature
3-27 

Figure 3-36. Temporal variations of vessel pressure and temperature in Test #15. 

Figure 3-37. Temporal variations of vessel pressure and temperature in Test #15 (time 

scale extended, including the complete cool-down phase). 

Vessel #3 refurbished (Test # 15)

Time (s)

0 20x103 40x103 60x103 80x103 100x103 120x103 140x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 5 10 15 20 25 30 35

Vessel pressure 

Vessel internal temperature 

Flange temperature 1

Flange temperature 2

Furnace temperature

Vessel #3 refurbished (Test #15)

Time (s)

0 100x103 200x103 300x103 400x103 500x103 600x103 700x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

Temperature (oC)

0

100

200

300

400

500

Time (h)

0 20 40 60 80 100 120 140 160 180

Vessel pressure 

Vessel internal temperature 

Flange temperature 1

Flange temperature 2

Furnace temperature
3-28 

Figure 3-38. Isothermal reference helium leakage rate during the 25 h heating in Test #15. 

From Figure 3-36 and Figure 3-37, 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 off-gas 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 

3-39), 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 3-40). Additional tests 

would be needed to further examine this finding. 

Test #15

Time (s)

26x103 46x103 66x103 86x103 106x103

Pressure (bar)

0

1

2

3

4

5

6

7

8

9

10

LR,He (ref cm3/s)

-0.02

-0.01

0.00

0.01

0.02

Vessel pressure 

Helium leakage rate
3-29 

Figure 3-39. Photograph of the ethylene-propylene compound O-ring after thermal 

exposure. 

Figure 3-40. Photograph showing the disintegration of the tested ethylenepropylene compound O-ring when an attempt was made to remove the 

O-ring from the groove. 

4-1 

4 SUMMARY 

Fifteen small-scale metallic and polymeric seal performance tests including two “shakedown” 

tests were performed to explore seal performance during beyond-design-basis thermal exposure 

conditions. Twelve tests used metallic seals, two used ethylene-propylene compound seals, and 

one used a PTFE seal. 

Of the five repeat metallic-seal tests (Tests #2, #3, #4, #8, and #9), leakage (decreasing vessel 

pressure) was observed in three of the tests (Tests #3, #4, and #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 #3, measurable leakage occurred 

approximately 6.9 h after the test temperature 800 °C (1472 °F) had been reached. In Test #4, 

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

leakage was observed roughly 3 h into the test. The two shakedown tests 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 ethylene-propylene compound seal tested at 300 °C (572 °F) for 

more than 20 h; however, leakage was observed immediately after the vessel had attained the 

nominal target temperature of 450 °C (842 °F) in another ethylene-propylene compound seal test 

(Test #15). Leakage was also observed in the test (Test #12) that used a PTFE seal after it had 

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

Since only a very limited number of tests were conducted using polymeric seals, further testing is 

recommended to verify the observations. In addition, the effect of scale on seal performance 

under beyond-design-basis thermal exposure should also be explored to ensure that the results 

are not dependent on the size of the test fixture. 

 
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/CR6894, 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. 

Jaunich, M., von der Ehe, K., Wolff, D., Voelzke, H., and Stark, W., “Understanding Low 

Temperature Properties of Elastomer Seals,” Packaging, Transport, Storage and Security of 

Radioactive Material 22 (2):83-88 (2011). 

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 

A.1 Design Drawings of the Test Vessel 

Figure A-1. Design drawing of test vessel body. 

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

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

 
B-1 

APPENDIX B 

B.1 Vessel Internal Volume 

Vessel internal volume (excluding the volume of the through hole for welding the tubing flush to 

the vessel interior, see Figure A-1) 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. 

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

 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 

*post-test measurements 

 

C-1 

APPENDIX C 

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) ݑ ௖ൌ ටݑ்஼

஼஽஺ݑ ൅ ଶ௃஼ݑ ൅ ଶ

ଶ ൌ 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) 

௉ݑට ൌ௖ ݑ௅ݑ ൅ ଶ

ଶ ൅ ݑு

௅ݑ ൅ ଶ்ݑ ൅ ଶ

஼஽஺ݑ ൅ ଶ

ଶ

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 

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 

ܰଵ ൅ ܰଶ ൌ ܰଵ௢ ൅ ܰଶ௢ ൌ ܰ

Using the ideal gas law, 

ܸܲଵ

ܴܶଵ

൅

ܸܲଶ

ܴܶଶ

ൌ 

ܲ௢ሺܸଵ௢ ൅ ܸଶ௢ሻ

ܴܶ௢

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 

ܶଶ ൌ ܶ௢ and ܸଶ ൌ ܸଶ௢ (no volumetric thermal expansion in System 2), then the above equation 

can be written as 

ܸܲଵ

ܶଵ

൅

ܸܲଶ௢

ܶ௢

ൌ 

ܲ௢ሺܸଵ௢ ൅ ܸଶ௢ሻ

ܶ௢

Note that due to volumetric thermal expansion in System 1 during the thermal exposure, ܸଵ ്

ܸଵ௢. The final vessel volume V1 can be expressed in terms of P as 

ܸଵ ൌ 

ܲ௢ܶଵሺܸଵ௢ ൅ ܸଶ௢ሻ

ܲܶ௢

െ ܶଵܸଶ௢

ܶ௢

Since leakage was not observed in Tests #2, #5, #6, #7, and #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 #2, the calculated percent 

volume expansion from 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 
D-2 

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. 

VARIABLES: 

N1 Amount of substance in system 1 To Initial Temperature of the system 

N2 Amount of substance in system 2 T1 Temperature of system 1 

N1o Initial amount of substance in system 1 V1 Volume of system 1 

N2o Initial amount of substance in system 2 V1o Initial Volume of system 1 

P Pressure of the system V2o Initial Volume of system 2 

Po Initial Pressure of the system 

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 #2 25 4.96 9.52 × 10-5 802 14.55 1.01 × 10-4 5.8 

Test #5 25 4.98 9.53 × 10-5 425 10.34 9.91 × 10-5 4.0 

Test #6 25 4.94 9.52 × 10-5 425 10.20 9.97 × 10-5 4.7 

Test #7 24 4.99 9.57 × 10-5 426 10.39 9.97 × 10-5 4.1 

Test #14 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 

െ ݀ܰ

݀ݐ ൌ െ ൬

݀ܰଵ

൅ ݐ݀

݀ܰଶ

݀ݐ ൰ ൌ െ

݀ܲ

݀ݐ ൬ ܸଵܴܶଵ

൅ 

ܸଶ௢

ܴܶ௢

൰

Since V1/RT1 is an order of magnitude larger than V2o/RTo, the above equation can be 

approximated by 

െ ݀ܰെ ؆ ݐ݀

݀ܲ

݀ݐ ൬ ܸଵ

ܴܶଵ

൰

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 9.1. 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. 

െ ݀ܰെ ؆ ݐ݀

݀ܲ

൬ ݐ݀

ܸଵ௢

ܴܶଵ

൰

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). 



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NUREG/CR-7115 Performance of Metal and Polymeric O-Ring Seals in 

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