[en] The Strontium Heat Source Development Program began at the Pacific Northwest Laboratory (PNL) in 1972 and is scheduled to be completed by the end of FY-1981. The program is currently funded by the US Department of Energy (DOE) By-Product Utilization Program. The primary objective of the program has been to develop the data and technology required to permit the licensing of power systems for terrestrial applications that utilize ⁹⁰SrF₂-fueled radioisotope heat sources. A secondary objective of the program has been to design and qualification-test a general purpose ⁹⁰SrF₂-fueled heat source. The effort expended in the design and testing of the heat source is described. Detailed information is included on: heat source design, licensing requirements, and qualification test requirements; the qualification test procedures; and the fabrication and testing of capsules of various materials. The results obtained in the qualification tests show that the outer capsule design proposed for the ⁹⁰SrF₂ heat source is capable of meeting current licensing requirements when Hastelloy S is used as the outer capsule material. The data also indicate that an outer capsule of Hastelloy C-4 would probably also meet licensing requirements, although Hastelloy S is the preferred material. Therefore, based on the results of this study, the general purpose ⁹⁰SrF₂ heat source will consist of a standard WESF Hastelloy C-276 inner capsule filled with ⁹⁰SrF₂ and a Hastelloy S outer capsule having a 2.375-in. inner diameter and 0.500-in. wall thickness. The end closures for this study, the general purpose ⁹⁰SrF₂ heat a Hastelloy S outer capsule having a 2.375-in. inner diameter and 0.500-in. wall thickness. The end closures for the outer capsule will utilize an interlocking joint design requiring a 0.1-in. penetration closure weld