A chemical reaction heat source for use in heaters for downhole applications is provided. The heat source has a solid fuel composition that comprises thermite and a binding agent. The binding agent serves to maintain the solid form of the solid fuel composition during burning and ensure a predetermined uniform heating pattern can be provided for longer. The solid fuel composition can be provided in the form of blocks. The solid fuel composition can also be provided in the form of a plurality of fragments that, during burning, behave more like powdered thermite and can flow.