A coupling (120; 220) for a radiation source assembly (100; 200) comprising an elongate radiation source (105; 205) and an elongate radiation transparent protective sleeve (110; 210) for receiving the elongate radiation source (105; 205), the coupling (120; 220) comprising: - a sleeve bolt element (125; 225) configured to secure the protective sleeve (110; 210) to a wall (115; 215) of a fluid treatment housing; f - a lamp plug element (150; 250) configured to: (i) be reversibly engageable with respect to the elongate radiation source (105; 205), and (ii) supply electrical power to the elongate radiation source (105; 205); - a lamp plug retaining element (145; 245a, 245b) engaged to the lamp plug element (150; 250); - a seal element (160; 260), wherein the lamp plug element (150; 250) comprises a recess portion for receiving the seal element (160;260), said seal element (160; 260) having an annular configuration; - wherein the sleeve bolt element (125; 225) comprises a first locking portion (130; 230) and the lamp plug retaining element (145; 245a, 245b) comprises also a locking portion configured to releasably engage the first locking portion so as to secure the lamp plug retaining element (145; 245a, 245b) in a first extended position corresponding to a first position of the seal (160; 260), in which the seal (160; 260) provides a fluid tight seal between the sleeve bolt element (125; 225) and the lamp plug element (150; 250); - wherein the lamp plug retaining element (145; 245a, 245b) is configured so that it can be moved to a second retracted position, the lamp plug element (150; 250) undergoing at the same time a non-rotational retraction in a direction substantially parallel to a longitudinal axis of the elongate radiation source (105; 205), said second retracted position of the lamp plug retaining element (145; 245a, 245b) corresponding to a second position of the seal (160; 260), in which the sleeve bolt element (125; 225) and the lamp plug element (150; 250) are unsealed; - wherein the sleeve bolt element (125; 225) further comprises a threaded portion (140; 240) separated from the first locking portion (130; 230) by a space portion (135; 235), and the lamp plug retaining element (145; 245a, 245b) comprises a threaded portion (147; 247), so that when the lamp plug retaining element (145; 245a, 245b) is in the second retracted position, the threaded portion (147; 247) of the lamp plug retaining element (145; 245a, 245b) is in the space portion (135; 235) of the sleeve bolt element (125; 225) and is prevented from further retraction by the threaded portion (140; 240) of the sleeve bolt element (125; 225); - wherein the lamp plug element (150; 250) may then continue to be disengaged from the sleeve bolt element (125; 225) by turning the lamp plug retaining element (145; 245a, 245b) so that its threaded portion (147; 247) engages and then disengages from the threaded portion (140; 240) of the sleeve bolt element. - a sleeve bolt element (125; 225) configured to secure the protective sleeve (110; 210) to a wall (115; 215) of a fluid treatment housing; f - a lamp plug element (150; 250) configured to: (i) be reversibly engageable with respect to the elongate radiation source (105; 205), and (ii) supply electrical power to the elongate radiation source (105; 205); - a lamp plug retaining element (145; 245a, 245b) engaged to the lamp plug element (150; 250); - a seal element (160; 260), wherein the lamp plug element (150; 250) comprises a recess portion for receiving the seal element (160;260), said seal element (160; 260) having an annular configuration; - wherein the sleeve bolt element (125; 225) comprises a first locking portion (130; 230) and the lamp plug retaining element (145; 245a, 245b) comprises also a locking portion configured to releasably engage the first locking portion so as to secure the lamp plug retaining element (145; 245a, 245b) in a first extended position corresponding to a first position of the seal (160; 260), in which the seal (160; 260) provides a fluid tight seal between the sleeve bolt element (125; 225) and the lamp plug element (150; 250); - wherein the lamp plug retaining element (145; 245a, 245b) is configured so that it can be moved to a second retracted position, the lamp plug element (150; 250) undergoing at the same time a non-rotational retraction in a direction substantially parallel to a longitudinal axis of the elongate radiation source (105; 205), said second retracted position of the lamp plug retaining element (145; 245a, 245b) corresponding to a second position of the seal (160; 260), in which the sleeve bolt element (125; 225) and the lamp plug element (150; 250) are unsealed; - wherein the sleeve bolt element (125; 225) further comprises a threaded portion (140; 240) separated from the first locking portion (130; 230) by a space portion (135; 235), and the lamp plug retaining element (145; 245a, 245b) comprises a threaded portion (147; 247), so that when the lamp plug retaining element (145; 245a, 245b) is in the second retracted position, the threaded portion (147; 247) of the lamp plug retaining element (145; 245a, 245b) is in the space portion (135; 235) of the sleeve bolt element (125; 225) and is prevented from further retraction by the threaded portion (140; 240) of the sleeve bolt element (125; 225); - wherein the lamp plug element (150; 250) may then continue to be disengaged from the sleeve bolt element (125; 225) by turning the lamp plug retaining element (145; 245a, 245b) so that its threaded portion (147; 247) engages and then disengages from the threaded portion (140; 240) of the sleeve bolt element.