Investigation on Release Rates of Vacuum Tight Threaded Junction (VTTJ) Samples Exposed to Ultrapure Water

The Vacuum Tight Threaded Junction (VTTJ) is an innovative vacuum-compatible non-welded junction developed and patented by Consorzio RFX. This technique was used to join non easily weldable materials, such as copper and stainless steel, to manufacture several in vacuum components for the two experiments of the Neutral Beam Test Facilities, i.e. SPIDER and MITICA. SPIDER and MITICA experiments are actively cooled by Ultrapure Water (UPW) to electrically insulate in-vessel components that are polarised to high voltage levels. The most heated components, made of copper or copper alloy, are connected to stainless steel piping by means of VTTJ technique and cooled by UPW. VTTJ has been tested up to 500 bar internal pressure and up to 700°C showing excellent leak tightness in vacuum conditions and high mechanical strength. However, no investigations on the corrosion performance of these coupled metals in UPW exposed to high voltages have been carried out. VTTJ prototypes were sectioned and it was observed that not only this junction is exposed to a high water velocity (up to 7 m/s), it is also subjected to stagnant water regions. This paper presents first experimental results of stainless steel and copper samples exposed to stagnant Ultrapure water at ambient temperature to investigate their corrosion behaviour in the VTTJ. Trace metal analysis using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to quantify the release of metals when stainless steel and copper are either exposed separately to UPW or when they are coupled together in stagnant conditions. Separate experiments on VTTJ joined pipes inserted in a dedicated cooling loop at ambient temperature and velocity up to 10 m/s were performed to investigate the phenomenon of erosion-corrosion at SPIDER operating conditions. This is to inform on the mechanism of corrosion of the couple copper and stainless steel in the junction.