4 January 1961: The 4241st Strategic Wing's Boeing B-52G-95-BW Stratofortress, serial number 58-0187, was on a 24-hour airborne alert mission off the United States' Atlantic Coast. Major Walter S. Tulloch, US Air Force, piloted the bomber with Captain Richard W. Hardin and First Lieutenant Adam C. Mattocks. Major Eugene Shelton, Radar Navigator; Captain Paul E. Brown, Navigator; First Lieutenant William H. Wilson, Electronics Warfare Officer; Major Eugene H Richards, Electronics Warfare Instructor; and Technical Sergeant Francis R. Barnish, Gunner were among the other members of the crew. It was equipped with two Mark 39 thermonuclear bombs, each with a 3–4 megaton explosive yield.
The B-52 was refueled by an air tanker while in flight. Major Tulloch was told by the tanker crew that the B-52's right wing was leaking fuel. More than 5,400 gallons (37,000 pounds/17,000 kilograms) of jet fuel were wasted in less than three minutes due to the massive leak. The B-52 was on its way to North Carolina's Seymour Johnson Air Force Base.
The bomber became increasingly difficult to control as they fell due to the unbalanced condition. Major Tulloch ordered the crew to abandon the fatal ship when the bomber lost control. Five crew members were evacuated, and one climbed through the top hatch. (Lieutenant Mattocks is thought to be the only member of the B-52 crew to have fled through the upper hatch.)
58-0187 shattered and exploded. Its debris reached an area of two square miles (5.2 square kilometers). Majors Shelton and Richards, as well as Sergeant Barnish, were killed.
The B-52's two Mark 39 bombs fell out of the bomb bay as the plane exploded. One buried itself to a depth of more than 55 meters (180 feet). The parachute retarding mechanism on the other plane worked perfectly, and it landed safely. An bomb disposal team promptly detonated it and carried it away.
The buried bomb was extremely difficult to recover. The ordnance crew retrieved the majority of the bomb after eight days, including the 92 detonators and conventional explosive "lenses" of the "primary," the first stage implosion portion. On January 29, the uranium-235/plutonium-239 "pit," the bomb's very core, was discovered. The "secondary," on the other hand, was never discovered.
The fusion fuel is held in the secondary, but it cannot detonate without the primary exploding first. There is no threat of an explosion because the secondary is buried.
“During a B-52 airborne alert mission structural failure of the right wing resulted in two weapons separating from the aircraft during aircraft breakup at 2,000 – 10,000 feet altitude. One bomb parachute deployed and the weapon received little impact damage. The other bomb fell free and broke apart upon impact. No explosion occurred. Five of the eight crew members survived. A portion of one weapon, containing uranium, could not be recovered despite excavation in the waterlogged farmland to a depth of 50 feet. The Air Force subsequently purchased an easement requiring permission for anyone to dig there. There is no detectable radiation and no hazard in the area.”
The military code term for an accident like this involving the loss of nuclear weapons is BROKEN ARROW.
Though official declarations claim that neither weapon was in danger of exploding, others claim that five of the six steps (or six of seven) required for a thermonuclear detonation were followed.
The aircraft commander's arming switch was the only one that had not been turned on.
The Mark 39 was a two-stage thermonuclear bomb with radiation-implosion. It was produced from 1957 to 1959, with over 700 units produced. It was fully fused, which meant it could be detonated by contact with the ground, as an air burst, or by "laid down," in which a sequence of parachutes would slow the bomb and allow it to land on its intended target before detonating. This gave the bomber enough time to clear the area.
With a weight of 6,500–6,750 pounds (2,950–3,060 kilograms), the Mark 39 was considered a light weapon. The bomb was roughly 11 feet, 8 inches (3.556 meters) in length and 2 feet, 11 inches in diameter (0.889 meters). The Mark 39 had a 3–4 megaton explosive output. (For comparison, the Redwing Cherokee nuclear weapons test at Bikini Atoll in 1956 had a yield of 3.8.)
In the mid-1960s, the Mark 39 was phased out of service and replaced by the more powerful Mk 41.