Investigation Summary
The incident occurred during a period of high Cold War tensions when the U.S. was secretly rotating nuclear assets. The B-50 was carrying a Mark 4 nuclear bomb, a design based on the 'Fat Man' implosion weapon. Crucially, the plutonium pit (the core required for a nuclear chain reaction) was not installed in the weapon at the time of the flight, a standard safety practice to prevent accidental nuclear detonation. When the aircraft suffered engine failure, the crew faced a dilemma: attempting a landing with the heavy, volatile weapon or jettisoning it. Military doctrine dictated jettisoning to prevent the weapon from falling into the wrong hands or causing a disaster at a friendly airfield. The crew set the bomb's conventional explosives to trigger, which resulted in a non-nuclear blast that was felt by residents of Rivière-du-Loup. The U.S. military initially covered up the event, claiming the explosion was caused by 500-pound practice bombs.
Final Conclusions
The incident was classified as a 'Broken Arrow' (an accidental event involving a nuclear weapon that does not create a risk of nuclear war). The Air Force confirmed the nuclear nature of the incident in the 1980s. The primary cause was mechanical engine failure, which necessitated the jettisoning of the payload according to established safety protocols of the era.
Video Analysis
No video analysis linked for this case file yet.
Systems & Failure Modes
Mark 4 Nuclear Bomb
An improved version of the Fat Man bomb design, utilizing a conventional high-explosive assembly to compress a fissile core.
Uranium Tamper
A layer of U-238 surrounding the core designed to reflect neutrons back into the reaction; this material was scattered by the conventional detonation.
Interesting Facts
- 01The plutonium core (pit) was not present in the weapon during the flight.
- 02The explosion scattered approximately 100 pounds (45kg) of radioactive uranium-238.
- 03The incident was kept secret from the public for over 30 years.
- 04Standard protocol required jettisoning nuclear weapons if the transport aircraft had engine trouble.
- 05The explosion was powerful enough to shake area residents in Rivière-du-Loup.
Safety Actions & Advisories
Separation of Components
The practice of shipping plutonium pits separately from the high-explosive bomb assemblies was a key safety measure that prevented a nuclear yield during this accident.
Known Controversies & Unanswered Questions
Decades-Long Cover-Up
Reference ↗For over 30 years, the U.S. military maintained a cover story that the explosion felt by residents of Rivière-du-Loup was caused by the detonation of 500-pound practice bombs. It was not until the 1980s that the Air Force officially confirmed the incident involved a Mark 4 nuclear weapon and the dispersal of radioactive uranium-238.