[en] This paper describes the application of novel feedback control techniques to the welding of the thermal sleeve joint for fast reactor heat exchangers. It is required to produce a full penetration Tee-butt weld between a tube and flange, without fusion into the tube bore. Two complementary feedback control systems are therefore employed: (a) the penetration of the Tee-butt weld is ensured by viewing the back of the weld with photodiodes, the signals from which control the current and travel speed; and (b) protrusion into the bore is prevented by measuring the radiation from the bore surface by means of photodiodes: the signal from these is applied to a magnetic arc deflector, which controls the position of the arc with respect to the tube to maintain the required bore temperature. The system has been demonstrated experimentally, and it is shown that the feedback control system enables consistently good quality welds to be made in a situation where quality control is notoriously difficult. (author)