Abstract: So as to be able to carry out various operations, especially for cleaning observation and simple interventions, on an installation placed in an active cell, the present invention provides a mobile intervention chamber including a caisson (40) adapted to be docked on an access orifice formed in an upper horizontal partition of the cell. The caisson (40) contains the modular elements (88) of an intervention pole (94), these elements being placed on a tool holder barrel (78). With the aid of auxiliary handling arms and a retractable support (110), a pole (94) of a suitable type and length is long-distance constructed on a mechanism (128) mounted on a rotary plate (120) integrated in the ceiling of the caisson (40). Shielding windows (232, 236) and lighting and display systems facilitate the carrying out of intervention operations.