In a fluid treatment system includes a housing with fluid inlet and outlet for a process fluid, an irradiation zone disposed between the fluid inlet and fluid outlet, and at least one radiation source module with a radiation source having a discharge vessel with outer and inner wall. The inner wall encloses an internal volume with at least one opening and an internal electrode for igniting and maintaining a discharge. The radiation source module also includes a submersible frame with a guide to guide the process fluid into and out of the internal volume of the radiation source. The dissipation of the heat generated by the discharge in the discharge gap via a fluid flow in the internal volume of the lamp and in contact with the internal electrode is substantially more effective than dissipation via a cooling channel separated from the internal electrode. It is therefore substantially easier to maintain the discharge at an approximately optimal temperature.