The NASA Glenn Research Center is developing the next generation of Stirling Radioisotope Generators (SRGs) to power deep space science missions. One potential technology gap is the waste heat rejection approach for higher power Stirling convertors. The previous 140W Advanced Stirling Radioisotope Generator (ASRG) used a copper alloy conduction flange to transfer heat from the convertor to the generator housing radiator surface. The conduction flange would incur a substantial mass and thermal performance penalty for larger Stirling systems. The Radial Core Heat Spreader (RCHS) is a passive two-phase thermal management device developed to solve this issue by using water vapor instead of copper as the heat transport media.