Performance of Metal and Polymeric O-Ring Seals in Beyond-Design-Basis Temperature Excursions (NUREG/CR-7115, Revision 1)
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Manuscript Completed: September 2015
Date Published: November 2015
Jiann C. Yang
Edward J. Hnetkovsky
National Institute of Standards and Technology
Gaithersburg, Maryland 20899
Felix Gonzalez, NRC Project Manager
NRC Job Code N6550
Division of Risk Analysis
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
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.