On This Day: Atomic bomb jettisoned over St. Lawrence, 1950

On Nov. 10, 1950, a United States Air Force bomber with engine trouble jettisoned a Mark 4 atomic bomb, without its fissile core, over the St. Lawrence River near Rivière-du-Loup, Que., and the weapon’s conventional explosive detonated.

The Canadian Encyclopedia said a B-50 Superfortress suffered severe engine malfunctions and its crew dropped the bomb, which self-destructed over the water. The encyclopedia said the explosion spread about 45 kilograms of uranium into the air.

A Mark 4 without a plutonium core cannot produce a nuclear blast. Wikipedia’s account of the incident said the core was not installed and that the trigger was always removed before flight and shipped separately. It said the core’s absence was probably due to its high cost and relative scarcity.

Wikipedia said the aircraft was flying from Goose Bay, Labrador, toward Davis-Monthan Air Force Base in Arizona and later diverted to Loring Air Force Base in Maine. It said the bomb was released at 10,500 feet (3,200 metres) and that American protocol called for jettisoning a nuclear device after engine trouble rather than landing with it. Wikipedia’s list of military nuclear accidents said the crew set the bomb to self-destruct at 2,500 feet (760 metres).

The Canadian Encyclopedia said the first Mark 4 weapons deployed in Canada were 11 to 15 bombs, sent with 43 American long-range bombers to Goose Air Base in the summer of 1950. It said the deployment was kept from the Canadian public.

Wikipedia said the explosion shook area residents. It also said the incident was immediately covered up and explained away as the detonation of 500-pound military practice bombs. It said the Air Force did not confirm a nuclear incident until the 1980s. Its account cites books including Farley Mowat’s Eastern Passage (2010) and Dirk Septer’s Lost Nuke (2012).

It was the second such accident in Canada that year. History.com reported that on Feb. 13, 1950, a B-36 bomber in trouble over British Columbia jettisoned a Mark 4 over the Pacific, also without its plutonium core, before the crew bailed out over Princess Royal Island. History.com said five of the 17 crew members were never found. The Canadian Encyclopedia said both 1950 incidents were covered up and became widely known only after the Cold War.

The sources reviewed disagree on one point. Wikipedia’s article says the bomb was destroyed before it hit the ground, and also that it was detonated presumably on impact. They do not give the time of the explosion, the size of the conventional charge or how and when Canadian officials were told, and Canada News Media has not found those details in a primary document.

Other entries in our On this day series include the 1965 Ontario blackout and the 1942 Oran harbour raid.

File photo of a ferry crossing on the St. Lawrence River at L’Isle-aux-Coudres, Que. It does not show the 1950 incident.

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