Free surface flow (video) |
In the windowless spallation target, the surface is formed by the free flow of the PbBi through a properly designed nozzle. In order to avoid excessive evaporation at the target surface, surface temperatures should be kept low (< 450 °C for PbBi). Consequently, the target nozzle should be designed in such a way that the convective removal of the deposited heat is sufficient to prevent overheating. The flow is chosen to run away vertically from the beam heating to warrant as low surface temperatures as possible. |
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The PbBi flows down through an annular gap surrounding the beam transport line. The free surface is formed at the confluence point of the spallation nozzle. From here the flow returns through the central pipe in the middle of the subcritical core. At the confluence point, a recirculation zone is created which size can be varied by varying the equilibrium between inlet and outlet flow. Heat deposition in recirculation or stagnant zone should be minimised, as it would lead to the formation of hot spots. Along the same line, emanation of liquid-metal droplets from the surface should be prevented by design. For a vertical target flow as in MYRRHA - which is the most sensitive for the creation of recirculation zones and droplets - a possible mitigation tool is adding a few percent of swirl to the flow for stabilisation. To gain insight in the characteristics and expertise in the creation of an adequate free surface flow, SCK•CEN is performing an R&D program for free surface flow. |