The life extension of the Phe´nix LMFR involved the inspection of reactor vessel internal structures : among other techniques, a visual inspection was performed of the above core structure, fuel assembly heads and upper components. To make this inspection possible, a partial draining of the main vessel from primary liquid sodium was carried out (sodium at 180°C and argon cover at 150°C). The test program aimed at obtaining further knowledge on the process of wetting of sodium — as pure metal — on Phe´nix Plant assembly heads — made of stainless steel —, as well as on the internal structure weldings, was carried out from November 1998 to January 1999. The main results were as follows: • the sodium meniscus measured during sodium lowering against the non-wet vertical structures reaches 10 mm in height. On wetted structures, it reaches only 5.3 mm. • when sodium level decreases, the process if very regular. However, re-flooding is carried out in stages. • a difference of 0.2 mm between two heads altitudes is enough to observe successively each of the heads. • the quality of sodium does not modify the wetting process (in the range of cold trap temperature : 110–140°C). • the influence of lighting is important. • the visibility limit of emerging electro-eroded cracks (from 0.17 to 1.0 mm) is at 0.20 mm. • the visibility of a horizontal welding, machined or not, is good when the lighting is sufficient. • the superficial flow of sodium only modifies the wetting process for the closest heads. A final test allowed to observe that the global inclination of the assembly head mock-up does not modify the wetting process. These experimental results were part of the feasibility demonstration of the visual inspection within the actual Phe´nix Plant that was undertaken in 2001.

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