To satisfy the safety requirements of a terrestrial 60‐Watt Radioisotope Heat Source, it was necessary to determine whether the internal pressure buildup from the helium generated from Pu‐238 decay would pose problems in a fire environment (time/temperature/internal pressure). Nine T‐111 strength members were pressure burst tested; three were of the flat end cap weld design and six of the girth weld design. The flat end cap weld design was not successful, with failure times of less than 180 seconds. The girth weld design gave favorable results, with lifetimes ranging from 8 to 240 hours. All of the girth weld capsules failed in the center of the weld. The stress rupture model equation for determining test life using base metal properties does not predict the time to failure for the girth weld design under the accident scenario environment.