110 to provide a substantially fluid tight seal to prevent leakage of fluid from fluid treatment zone 117 e g water from a pressurized water treatment zone sleeve bolt element 125 comprises threaded portions 130 140 which are separated by a space portion 135 lamp plug retaining element 145 comprises a threaded portion 147 and a proximal portion 149 which is received in a recess 155 in lamp plug element 150 an o ring 160 is provided in a recess 157 in lamp plug element 150 in fig1 it can be seen that radiation source assembly 100 is shown u201c in use u201d thus o ring 160 forms a substantially fluid tight seal with an interior surface of sleeve bolt element 125 threaded portion 130 of sleeve bolt element 125 is engaged with threaded portion 147 of lamp plug retaining element 145 this is the so called first position of o ring 160 when it is desired to access radiation source 105 for servicing or any other purpose lamp plug retaining element 145 is rotated to disengage threaded portion 130 of sleeve bolt element 125 from threaded portion 147 of lamp plug retaining element 145 this results in threaded portion 147 of lamp plug retaining element 145 moving into space portion 135 of sleeve.