A 238Pu-Powered heat source for use in an experimental model of a cardiac pacemaker was developed. The encapsulated source has a thermal power of 0.134 ± 0.013 W and is fueled with three 238Pu metal pins. Each fueled pin was TIC welded into a tantalum inner capsule. The inner capsules were electron beam welded into Hastelloy-C middle capsules. These middle capsules were platinum plated to prevent a reaction between the Hastelloy-C and the titanium outer capsules. Three middle capsules were electron beam welded into each outer capsule.