The overall program objective is to develop a high-temperature passive thermal management system for the Radioisotope Power Conversion system that energizes the refrigeration system applicable to Venus missions. The innovation consists of a high temperature alkali metal variable conductance heat pipe (VCHP) integrated with a two-phase heat collection / transport package from the General Purpose Heat Source (GPHS) stack to the Stirling convertor heater head. The thermal management system collects the heat from the GPHS modules, and delivers heat as required to the Stirling system. Any excess heat is removed by the VCHP. Excess heat must be removed when the Stirling system is shut down, or in the early stages of a mission powered by a short-life radioisotope. In Phase I, it was demonstrated experimentally and theoretically that the VCHP allows the Stirling convertor to: stop during transit to Venus, pre-cool the system before re-entry, work on Venus and execute brief stoppages on Venus. The reservoir is exposed to the environment temperature during the mission and this is a key for the HTTMS to work passively. The other component of the system, the two-phase heat transport package (HTP), minimizes the temperature drop between the multi-GPHS stack and the heater head. In Phase II, a full scale HTTMS will be designed and a representative multi-segment of the full scale HTTMS will be build and tested in relevant environment. This multi-segment contains two or three parallel/redundant heat paths from the simulated GPHS stack to the heater head simulator, in addition to the backup cooling system (VCHP). The full-scale multi-segment HTTMS will be integrated and tested with the corresponding full scale multi-segment of the Intermediate Temperature Thermal Management System (ITTMS) of the Venus Lander.