immobilization of this pin and consequently the block 62 in normal operation of the generator on the other hand during the maintenance phase the closure member 76 is raised by sliding against the wall of the casing 26 advantageously in lateral guides 80 fig5 b fixed on the latter such that after retraction of the pin and lifting of the closure member the block 62 can be pivoting without risk of encountering any obstacle fig6 and 7 illustrate an alternative embodiment in which the block 62 for occupying the tube lane is arranged so as to have a substantially larger longitudinal dimension consisting for example of three juxtaposed elements 82 84 and 86 each provided with at least one element for passage of the secondary fluid these elements being solidly attached to each other and arranged in the extension of each other the first element 82 situated furthest to the left in fig6 includes an extension for the articulation of the block 62 about the spindle 61 on the clevis 60 carried by the plate 40 while the opposite element 86 has a larger height dimension and is optionally provided with a protective sleeve 88 the blocking pin 68 integral.