AbstractThe Helicoflex® metallic seal initially designed for static assemblies is presently used as a valve primary seal for specific applications where the elastomeric seal commonly used in such a configuration is totally unsuitable owing to low resistance to radiation and high temperature, and incompatibility with ultrahigh vacuum conditions. The hydrogen injection quick valves of the JET fusion reactor, as well as the aluminium ultrahigh vacuum valves of the TRISTAN accelerator already operate using this technology over a rather narrow range of working conditions. In order to widen the field of applications and establish the basic principles of the sealing function applied to a valve seat-seal assembly, tests were carried out in the Sealing Techniques Laboratory of the French Atomic Energy Commission (CEA). The seat configuration and seal holding device have been reconsidered. Seals have been tested using different grades of sealing lining material, i.e. successively aluminium, silver and copper. In this respect, such an assembly configuration was found to be in full compliance with the requirements of the new types of valve, those which are being considered for applications where ultrahigh vacuum is combined with a leak rate requirement of 10−10 Pa m3 s−1 and a high bake-out temperature.