What conditions are necessary for heat to flow?

if it is heat flow through a metal, it would be necessary to have a conductor so that the electrons can flow through it (delocalised electrons).

In terms of radiation, it would require a good absorber of heat (e.g. a black surface) so that the thermal energy flows through on the surface area.

uranium fissions/breaks up giving out energy and neutrons

d. air pressure decreases as altitude increases.