Since any structural material in the path of the high intensity proton beam will suffer severe radiation damage, a “windowless target” design, i.e. without a physical separation between the accelerator beam line vacuum and the liquid target material is considered.
A nozzle shapes a liquid metal jet so that an optimal target free surface to accept the proton beam is created. A dedicated pump delivering the flow required for the jet formation, is located near the edge of the reactor vessel, where more space is available and radiation levels are low. The heat generated by the proton beam is transported towards the liquid metal of the primary system by means of a heat exchanger also located near the vessel edge. Nozzle, pump and heat exchanger are combined into a closed loop keeping most of the spallation products confined and separated from the primary system.
A windowless design means that the target zone and the beam line share a common vacuum. Sufficiently high vacuum conditions must be reached above the target surface to avoid plasma formation and to guarantee compatibility with the vacuum of the accelerator beam line. Adequate pumping capacity is foreseen while at the same time, outgassing of the LBE, liquid metal evaporation and emanation of volatile spallation products must be kept under control.
The absence of a window leads to several challenging issues in the spallation target design that require an extended research programme. Through the execution of a research programme of more than ten years on this topic, SCK•CEN has demonstrated the viability of this concept.