Internal geometry and coolant choices for solid high power neutron spallation targets

The next generation of neutron spallation sources envisages high power proton beam interaction with a heavy metal target. Solid targets have potentially higher spallation efficiency due to the possibility to use metals with higher density than used in liquid metal targets, but to realize this potential the solid fraction must be high enough. As the power released in the form of heat can reach several MW in the target volume of typically 10 l, target cooling can be a serious challenge. Heat evacuation efficiency for different solid fraction geometries at high power is analyzed for different coolant options (helium, water and gallium) using empirical correlations for friction factors and Nusselt numbers. For estimation of the heat transfer efficiency a parameter γ is introduced characterizing how many watts can be transferred per temperature- and pressure-difference unit. It is demonstrated that water is preferable as a coolant in high convection cases whereas gallium – in medium Peclet number cases when heat conduction in the coolant is important. Strictly focusing on cooling, the results indicate that for a stationary target liquid metals are advantageous in particular conditions. Three options are compared featuring geometries with large internal surfaces and avoiding high pressures. The transition from a stationary target to a rotating one in the case of gallium as coolant improves the heat transfer conditions to a higher degree than for ordinary liquids or gases. An advantage of gallium can be derived from the fact that gallium also acts as a neutron generating medium allowing the target solid fraction to be reduced and a part of the deposited heat is localized in coolant directly.