Abstract: One fast cool-down furnace embodiment discloses a double wall in the interspace of which a cooling air stream is flowable to provide fast cool-down of a load of wafers. The double wall embodiment is retrofittable on existing diffusion furnace, and includes a cylindrical member aligned in concentric relation with the diffusion tube by combination plenum defining and supporting manifolds. In a second embodiment, the fast cool-down furnace includes a reaction vessel and a heating core relatively movable relative to each other in such a way that in a fast cool-down condition the core and reaction vessel are spaced apart. Cooling tubes are embedded in the movable core. Fans or other aids to cooling are provided. Operation is either in a controlled cool-down mode or in a quick quench mode.