Abstract: The RHU(Radiosotope Heater Unit), RTG (Radioisotope Thermoelectric Generator) and space nuclear reactors have wide applications in deep space exploration missions. The safety principles are investigated based on their technology characteristics and the applications of nuclear sources in outer space are introduced, such as the RHU/RTG of ALRH (Apollo Lunar Radiosotope Heater), GPHS(General Purpose Heater Source) and LWRHU (Lighted Weighted Radiosotope Heater Unit), MMRTG (Multi-Mission Radioisotope Thermoelectric Generator), and space nuclear reactor of TOPAZ-II.The safety tests and analysis done by the United States and Russia are studied, and the safety validation and assess approach are reviewed, The work focuses on the technology details of the tests and analysis of RHU/RTG for different missions. The work would serve as reference for the application of nuclear source in outer space.