Downward flow was chosen for the DFR, because the top shield and mechanisms are kept cool at the reactor inlet and down-flow avoids levita­tion of the fuel subassemblies.

Against these reasons must be set the disadvantages that the coolant flow is in the opposite direction to natural circulation for the thermal siphon

and is opposed to buoyancy forces. The cover gas must be pressurized and the instrumentation at the core outlet is very difficult because of its remote­ness from the operating floor.

It should be added that a considerable number of problems have arisen due to the choice of down-flow; it is unlikely that such a choice would be made in any present or future system.