The discovery of isotope-239 and possible military applications the radioactive igniting agent entailed was one of the key reasons for RobCo's hostile takeover of REPCONN Aerospace right before the Great War.
Isotope-239 igniting agent is a fuel used for the rockets at the REPCONN test site in the quest Come Fly With Me. The item is marked as a quest item, and cannot be dropped. The rocket souvenirs sold at the Dino Bite gift shop in Novac contain the same agent inside them.
Behind the scenes
The only nuclide with an atomic mass of 239 and a half life capable of lasting 200 years would be plutonium-239, a form of fissile material that can be used as fuel for both nuclear reactors and nuclear weapons. It is produced in uranium-fueled nuclear reactors.
The isotope-239 located next to the dead pack brahmin, along with the other items near it, may fall through the map if touched and therefore become unobtainable. [verified]
- Fallout: New Vegas Official Game Guide Collector's Edition p. 320: "[2.35] REPCONN Headquarters
REPCONN Corporation was an up-and-coming regional aerospace firm, and keen contributor to Las Vegas political campaigns. They operated a test facility in the Black Mountains to the south (much to the chagrin of the local populations, who long complained of the "REPCONN bombs" landing in their fields and destroying property). The company was purchased by the giant RobCo Company just before the Great War, in a hostile takeover. New security countermeasures were installed, and more militaristic plans were undertaken, especially after the discovery of a special radioactive igniting agent that interested certain senior RobCo members. These countermeasures are still in effect, so investigate this building with caution."
(Fallout: New Vegas Official Game Guide Collector's Edition Tour of the Mojave Wasteland)