Abstract: The present invention relates to a coupling, employing a thermal sleeve, for connecting a secondary conduit to a main conduit.The coupling according to the invention connects a main conduit with a secondary conduit which opens through a lateral opening into the said main conduit and includes a tubular sleeve which is located according to the axis of the secondary conduit, in which the entry mouth of the sleeve is connected in a sealed manner to the said secondary conduit, and its end engages in the said lateral opening thus constituting an orifice, and is so designed that the said sleeve is shaped so that it converges according to the axis of the secondary conduit, in the sense directed from the outside towards the axis of the main conduit.This coupling may be used in the piping system for a nuclear reactor.