While demonstrating his technique to visiting scientists at Los Alamos, Canadian physicist Louis Slotin manually assembled a critical mass of plutonium. A momentary slip of a screwdriver caused a prompt critical reaction. Slotin died on May 30 from massive radiation poisoning, with an estimated dose of 1,000 rads (rad), or 10 grays (Gy). Seven observers, who received doses as high as 166 rads, survived, yet three died within a few decades from conditions believed to be radiation-related.^[4]

Slotin worked with the same bomb core as Daghlian which became known as the "demon core." It was later melted down and combined with existing weapons-grade material.

A simulated nuclear bomb containing TNT and uranium, but without the fissile core needed to create a nuclear explosion, was proactively dumped in the Pacific Ocean after a Convair B-36 bomber's engines caught fire during a test of its ability to carry nuclear payloads. The crew reported releasing the weapon out of concern for the amount of TNT inside, alone, before they bailed out of the aircraft. The bomber eventually crashed at an unknown location in Canada. Four years later the wreckage was found and searched, but no bomb was found. The weapon was briefly thought to have been located by a civilian diver in 2016 near Pitt Island but this was subsequently found not to be the case.^[5]

A USAF B-36 bomber, AF Ser. No. 44-92075, was flying a simulated combat mission from Eielson Air Force Base, near Fairbanks, Alaska, to Carswell Air Force Base in Fort Worth, Texas, carrying one weapon containing a dummy warhead. The warhead contained conventional explosives and natural uranium but lacked the fissile core of an actual weapon. After six hours of flight, the bomber experienced mechanical problems and was forced to shut down three of its six engines at an altitude of 12,000 feet (3,700 m). Fearing that severe weather and icing would jeopardize a safe emergency landing, the weapon was jettisoned over the Pacific Ocean from a height of 8,000 ft (2,400 m). The weapon's high explosives detonated upon impact with a bright flash visible. All of the sixteen crew members and one passenger were able to parachute from the plane and twelve were subsequently rescued from Princess Royal Island.^[6] The accident was categorized as a Broken Arrow, that is an accident involving a nuclear weapon but which does not present a risk of war.^[7]

A USAF B-29 bomber AF Ser. No. 44-87651 with a Mark 4 nuclear bomb (without a fissile core) on board, flying to Guam experienced malfunctions with two propellers and with landing gear retraction during take-off and crashed while attempting an emergency landing at Fairfield Suisun-AFB. In the resulting fire, the bomb's high-explosive material exploded, killing nineteen people from the crew and rescue personnel. Brigadier General Robert F. Travis, command pilot of the bomber, was among the dead.^[9]

Returning one of several U.S. Mark 4 nuclear bombs secretly deployed in Canada, a USAF B-50 had engine trouble and jettisoned the weapon at 10,500 feet (3,200 m). The crew set the bomb to self-destruct at 2,500 ft (760 m) and dropped over the St. Lawrence River. The explosion shook area residents and scattered nearly 100 pounds (45 kg) of uranium (U-238) used in the weapon's tamper. No fissile core was installed in the bomb.^[10]

A USAF B-47 Stratojet, AF Ser. No. 52-534, on a non-stop mission from MacDill Air Force Base, Florida, to an overseas base descended into a cloud formation at 14,000 feet over the Mediterranean in preparation for an in-air refueling and vanished while transporting two capsules of nuclear weapons material in carrying cases; a nuclear detonation was not possible.^[14] A French news agency reported that the plane had exploded in the air Northeast of Saïdia, in French Morocco in the same general location of its last known position. The plane was lost while flying through dense clouds, and the cores and other wreckage were never located.^[15][16][17]

A USAF B-47 crashed into a storage igloo, spreading burning fuel over three Mark 6 nuclear bombs at RAF Lakenheath. A bomb disposal expert stated it was a miracle exposed detonators on one bomb did not fire, which presumably would have released nuclear material into the environment.^[18]

A fire began in a theoretically fireproof area inside the plutonium processing building, in a glovebox used to handle radioactive materials, igniting the combustible rubber gloves and plexiglas windows of the box. The fire quickly spread to the plutonium as various safety features failed. The fire spread through the ventilation system as the containment ability of the facility became compromised, with plumes of radioactive smoke sent high into the outside air. The fire raged inside the building for 13 hours over the night of the 11th & 12th before firefighters could finally extinguish it. In the aftermath, Department of Energy officials, and the Dow Chemical officials who ran the facility, did not admit the extent of the catastrophe, or the radiation danger, to local officials or the media. Knowledge of the extent of the damage and contamination was kept from the public for years. After the fire, plutonium was detected near a school 12 miles (19 km) away and around Denver 17 miles (27 km) away. An independent group of scientists conducting off-site testing 13 years later found plutonium contamination in areas in nearby Rocky Flats to be 400 to 1,500 times higher than normal, higher than any ever recorded near any urban area, including Nagasaki. The Atomic Energy Commission then conducted its own off-site study, and that study confirmed plutonium contamination as far as 30 miles (48 km) from the plant.^{[20][21][22][23]}

A cooling system failure at the Mayak nuclear processing plant resulted in a major explosion and release of radioactive materials. A large area was subjected to radioactive contamination and thousands of local inhabitants were evacuated.^[24]

Technicians mistakenly overheated Windscale Pile No. 1 during an annealing process to release Wigner energy from graphite portions of the reactor. Poorly placed temperature sensors indicated the reactor was cooling rather than heating. The excess heat led to the failure of a nuclear cartridge, which in turn allowed uranium and irradiated graphite to react with air. The resulting fire burned for days, damaging a significant portion of the reactor core. About 150 burning fuel cells could not be removed from the core, but operators succeeded in creating a firebreak by removing nearby fuel cells. An effort to cool the graphite core with water and the switching off of the air cooling system eventually quenched the fire. The reactor had released radioactive gases into the surrounding countryside, primarily in the form of iodine-131 (¹³¹I). Milk distribution was banned in a 200-square-mile (520 km²) area around the reactor for several weeks. A 1987 report by the National Radiological Protection Board predicted the accident would cause as many as 100 long-term cancer deaths, although the Medical Research Council Committee concluded that "it is in the highest degree unlikely that any harm has been done to the health of anybody, whether a worker in the Windscale plant or a member of the general public." The reactor that burned was one of two air-cooled, graphite-moderated natural uranium reactors at the site used for production of plutonium.^[25][26][27] A 2007 study concluded that because the actual amount of radiation released in the fire could be double the previous estimates, and that the radioactive plume actually travelled further east, there were 100 to 240 cancer fatalities in the long term as a result of the fire.^[28][29][30]

A USAF B-47 bomber jettisoned a Mark 15 Mod 0 nuclear bomb (the nuclear capsule was not aboard the aircraft; the bomb contained a simulated 150-pound (68 kg) cap made of lead)over the Atlantic Ocean after a midair collision with a USAF F-86 Sabre during a simulated combat mission from Homestead Air Force Base, Florida. The F-86's pilot ejected and parachuted to safety.^[34] The USAF claimed the B-47 tried landing at Hunter Air Force Base, Georgia three times before the bomb was jettisoned at 7,200 ft (2,200 m) near Tybee Island, Georgia. The B-47 pilot successfully landed in one attempt only after he first jettisoned the bomb. A 3-square-mile (7.8 km²) area near Wassaw Sound was searched for nine weeks before the search was called off.

A USAF B-47E bomber, number 53-1876A, was flying from Hunter Air Force Base in Savannah, Georgia, to England in a formation of four B-47s on a top-secret mission called Operation Snow Flurry to perform a mock bombing exercise. The flight navigator/bombardier was checking the locking harness on the massive (7,600 pounds (3,447 kg)) Mark 6 nuclear bomb when he accidentally pushed the emergency release lever. The bomb fell on the bomb-bay doors, smashing them open and going into a 15,000 feet (4,572 m) free fall. The high-explosive detonator went off after it hit the ground 6.5 miles east of Florence, South Carolina, in Mars Bluff, creating a 70 feet (21 m) wide crater, 30 feet (9 m) deep.^[35] A nearby house was destroyed and several people were injured.^{[34]: 136–137 [36]} A nuclear detonation was not possible because, while on board, the weapon's core was not in the weapon for safety reasons.

A USAF B-52 bomber caught fire and exploded in midair due to a major leak in a wing fuel cell 12 miles (19 km) north of Seymour Johnson Air Force Base, North Carolina. Five crewmen parachuted to safety, but three others died—two in the aircraft and one on landing. The incident released the bomber's two Mark 39 nuclear bombs. Three of the four arming devices on one of the bombs activated, causing it to carry out many of the steps needed to arm itself, such as the charging of the firing capacitors and, critically, the deployment of a 100-foot (30 m) diameter retardation parachute. The parachute allowed the bomb to hit the ground with little damage. The fourth arming device—the pilot's safe/arm switch—was not activated, preventing detonation. The second bomb plunged into a muddy field at around 700 mph (300 m/s) and disintegrated. Its tail was discovered about 20 feet (6 m) down and much of the bomb recovered, including the tritium bottle and the plutonium. However, excavation was abandoned due to uncontrollable ground water flooding. Most of the thermonuclear stage, containing uranium, was left on site. It is estimated to lie around 55 feet (17 m) below ground. The Air Force purchased the land and fenced it off to prevent its disturbance, and it is tested regularly for contamination, although none has so far been found.^[48]

USAF B-52 bomber departed Mather Air Force Base, California and experienced a decompression event that required it to fly below 10,000 feet. Resulting increased fuel consumption led to fuel exhaustion; the aircraft crashed near Yuba City, California with two nuclear bombs, which did not trigger a nuclear explosion.

USAF B-52 on airborne alert duty encountered a severe winter storm and extreme turbulence, ultimately disintegrating in midair over South Central Pennsylvania.^[50] Only the two pilots survived. One crew member failed to bail out and the rest succumbed to injuries or exposure to the harsh winter weather. A search for the missing weapons was initiated, and recovery was effected from portions of the wreckage at a farm northwest of Frostburg, MD.

A U.S. Navy A-4E Skyhawk aircraft with one B43 nuclear bomb on board fell off the aircraft carrier USS Ticonderoga into 16,200 feet (4,900 m) of water while the ship was underway from Vietnam to Yokosuka, Japan. The plane, pilot and weapon were never recovered. There is dispute over exactly where the incident took place—the U.S. Defense Department originally stated it took place 500 miles (800 km) off the coast of Japan; The accident was said to occur 68 miles (59 nmi; 109 km) from Kikai Island, Kagoshima Prefecture, Japan.^[54] U.S. Navy documents later show it happened about 80 miles (130 km) from the Ryukyu Islands and 200 miles (320 km) from Okinawa.^[55]

A USAF B-52 carrying four hydrogen bombs collided with a USAF KC-135 jet tanker during over-ocean in-flight refueling. Four of the B-52's seven crew members parachuted to safety while the remaining three were killed along with all four of the KC-135's crew. The conventional explosives in two of the bombs detonated upon impact with the ground, dispersing plutonium over nearby farms. A third bomb landed intact near Palomares, Almería (Spain) while the fourth fell 12 miles (19 km) off the coast into the Mediterranean sea. The U.S. Navy conducted a three-month search involving 12,000 men and successfully recovered the fourth bomb. The U.S. Navy employed the use of the deep-diving research submarine DSV Alvin to aid in the recovery efforts. During the ensuing cleanup, 1,500 tonnes (1,700 short tons) of radioactive soil and tomato plants were shipped to a nuclear dump in Aiken, South Carolina. The U.S. settled claims by 522 Palomares residents for $600,000. The town also received a $200,000 desalinization plant. The motion picture Men of Honor (2000), starring Cuba Gooding, Jr., as USN Diver, Master Chief Petty Officer Carl Brashear, and Robert De Niro as USN Diver, Chief Petty Officer Billy Sunday, contained an account of the fourth bomb's recovery.^[56]

A fire broke out in the navigator's compartment of a USAF B-52 near Thule Air Base, Greenland. The bomber crashed 7 miles (11 km) from the airbase, rupturing the bomber's bomb bay and causing the conventional explosives in the four B28FI thermonuclear bombs to detonate, fragmenting and spreading the radioactive primary and secondary components across a large area. The burning bomber and its fuel load melted through the ice, dropping wreckage to the seafloor underneath. The recovery and decontamination effort was complicated by Greenland's harsh weather. Contaminated ice and debris were returned and buried in the United States. Recovered bomb fragments were recycled by Pantex, in Amarillo, Texas. The incident caused outrage and protests in Denmark, as Greenland is a Danish possession, and Denmark forbade nuclear weapons on its territory.

At about 6:30 p.m., an airman conducting maintenance on a USAF Titan-II missile at Little Rock Air Force Base's Launch Complex 374-7 in Southside (Van Buren County), just north of Damascus, Arkansas, dropped a nine-pound (4 kg) socket from a socket wrench, which fell about 80 feet (24 m) before hitting and piercing the skin on the rocket's first-stage fuel tank, causing it to leak. The area was evacuated. Overnight, at about 3:00 a.m., the hypergolic fuel exploded. The W53 warhead landed about 100 feet (30 m) from the launch complex's entry gate; its safety features operated correctly and prevented any explosion, chemical or nuclear. An Air Force airman, David Livingston, was killed and the launch complex was destroyed.^[74]

During the final testing of a new saltless uranium processing method, there was a small explosion followed by a fire. The explosion occurred in an unvented vessel containing unreacted calcium, water and depleted uranium. An exothermic reaction in the vessel generated enough steam to burst the container. This small explosion breached its glovebox, allowing air to enter and ignite some loose uranium powder. Three employees were contaminated. BWXT Y-12 (now B&W Y-12), a partnership of Babcock & Wilcox and Bechtel, was fined $82,500 for the accident.^[81]

According to the version presented by Russian officials, it was a result of a failed test of an "isotope power source for a liquid-fuelled rocket engine".^[82][83][84] Nonproliferation expert Jeffrey Lewis and Federation of American Scientists fellow Ankit Panda suspect the incident resulted from a test of the Burevestnik cruise missile.^[85] However, other arms control experts disputed the assertions; Ian Williams of the Center for Strategic and International Studies and James Acton of the Carnegie Endowment for International Peace expressed skepticism over Moscow's financial and technical capabilities to field the weapon,^[86] while Michael Kofman of the Wilson Center concluded that the explosion was probably not related to Burevestnik but instead to the testing of another military platform.^[87] According to CNBC, the Russians were trying to recover a missile from the seabed which was lost during a previously failed test.^[88]