a safe geometry nuclear fuel powder blender of a pneumatic type having a plurality of narrow flat - walled blending chambers or &# 34 ; slab tanks &# 34 ; extending radially outward from a pneumatic spouting tube having an inlet and an outlet at bottom and top , respectively , open to each slab tank or blending chamber and contained within a cylindrical cone - bottomed shell filled with neutron - absorbing material between the blending chambers .