Helium Leakage Testing of SAFSHIELD® 2773A with Silicone O-rings

The Croft Associates SAFSHIELD® 2773A is designed as a Type B(U) Package to the IAEA Regulations for the Safe Transport of Radioactive Materials [1].

The SAFSHIELD® 2773A packaging comprises of a high integrity lead shielded stainless flask (Design No 2774) which provides containment and shielding of the contents. The flask is carried within a double skinned carbon steel outer casket (Design No 2773) providing regulatory impact and thermal protection.
The 2774 flask has a conventional double O-ring closure design incorporating silicone O-ring seals. It is a regulatory requirement to verify the containment system i.e., the O-rings and flask closure at manufacture and periodically during annual maintenance. The containment verification, depending on product form and State Variations, requires helium leakage testing to ANSI N14.5 [2] or ISO 12807 [3] against a leakage rate acceptance criteria of: 1.0 x 10-7 ref-cc.s-1 or 1.0 x 10-8 Pa.m3.s-1 SLR respectively.

The operational temperature range of silicone O-ring seals is very desirable for transportation packages, however, the permeability coefficient of helium through silicone is up to 10 times greater than that of ethylene propylene and fluoroelastomer (Viton) O-rings. This high permeation creates challenges for the helium leakage testing and consequently, despite their performance characteristics, silicone O-ring seals are seldom utilised in high integrity containment applications where helium leakage testing is required.

In response to this issue Croft, with the support of Leak Testing Specialists Inc, have developed a test methodology which provides for accurate timed release of helium into the containment system and a test configuration that significantly reduces the System Response Time (SRT), i.e., the time taken for the Mass Spectrometer Leakage Detector (MSLD) to reach a steady state reading from release of helium, before the onset of permeation. The methodology utilises a fine capillary from the helium quartz reference leak to the test port and then a high conductance, multi-port arrangement to maximise the helium conductance to the MSLD.

This paper explains the problems associated with helium leakage of high integrity enclosures utilising silicon O-rings and how these problems were resolved, validated, and verified through a dedicated test programme.