9.23. Three general mechanisms are distinguished whereby heat is trans­ferred from one point to another, namely: (1) conduction, (2) convection, and (3) radiation. These three methods for heat removal will be reviewed and their general characteristics outlined. The subsequent discussion, how­ever, will be devoted to the first two, since they are of major importance in existing reactor designs. The preliminary treatment will refer to heat transmission in systems without internal sources, and subsequently the problems associated with internal sources, such as exist in reactor fuel elements, will be taken up.