Exploding bridgewire (EBW) detonators are used for a variety of purposes and subjected to a wide range of conditions during transport and use, however, few studies have been conducted on stability at high temperatures. Many EBW's contain pentaerythritol tetranitrate (PETN) as the initiating explosive, which has been shown to exhibit a relatively low melt at 141 degrees C, followed by an onset of decomposition of approximately 160 degrees C. In the present work, we have evaluated the behavior of PETN with X-ray radiography in commercial RP-80 EBW detonators at temperatures just above the melt. These experiments show that PETN remains stable in the solid-state, but after reaching the melt temperature is vulnerable to mixing, followed by rapid evolution of gases and decomposition. Our results indicate that the orientation of the thermally treated detonator with respect to gravity is important after the PETN reaches the melted state: this shows for the first time why thermal tests on PETN-based detonators often result in varied outcomes.