Infinia Technology Corporation's (ITC) proposed Modular Stirling Power System (MSPS) is a free-piston Stirling system that addresses NASA needs in 12-kW increments. The MSPS utilizes a support structure that couples 1 heater head with 4 power modules and provides a high efficiency of 25% between very conservative acceptor and rejecter temperatures of 823 K and 475 K (30% @ 900/450 K). Proven ITC technology provides high intrinsic reliability and maintenance-free operation for >15 years. It directly leverages 3-kW power modules developed for Infinia's solar-Stirling PowerDish that have been deployed in over 400 engines to enable full-scale demonstration under a Phase II SBIR. The MSPS will employ innovative fabrication and/or laser welding processes for reliable liquid metal pumped loop compatibility with a next-generation heater head designed for nuclear system integration. Phase I will culminate in a concept definition and design as a foundation for analysis, detailed design, fabrication, and testing in Phase II. Development of the MSPS concept opens many avenues of application for commercial and government markets. Within NASA, the system will provide mission support for future space transportation and surface power with a very reliable, high-efficiency Stirling converter for the conversion of reactor heat into electricity.