Cecil Kelly Criticality Accident
A criticality accident occurred on December 30, 1958, at the Los Alamos National Laboratory in Los Alamos, New Mexico, in the United States. It is one of 60 known criticality events that have occurred globally outside the controlled conditions of a nuclear reactor or test, though it was the third such event that took place in 1958 after events on June 16 at the Y-12 Plant in Oak Ridge, Tennessee, and on October 15 at the Vinča Nuclear Institute in Vinča, Yugoslavia. The accident involved plutonium compounds dissolved in liquid chemical reagents; within 35 hours, it killed chemical operator Cecil Kelley by severe radiation poisoning.
https://en.wikipedia.org/wiki/Cecil_Kelley_criticality_accident

Goiânia Accident
The Goiânia accident [ɡojˈjɐniɐ] was a radioactive contamination accident that occurred on September 13, 1987, in Goiânia, Goiás, Brazil, after an unsecured radiotherapy source was stolen from an abandoned hospital site in the city. It was subsequently handled by many people, resulting in four deaths. About 112,000 people were examined for radioactive contamination and 249 of them were found to have been contaminated.
In the consequent cleanup operation, topsoil had to be removed from several sites, and several houses were demolished. All the objects from within those houses, including personal possessions, were seized and incinerated. Time magazine has identified the accident as one of the world's "worst nuclear disasters" and the International Atomic Energy Agency (IAEA) called it "one of the world's worst radiological incidents".
https://en.wikipedia.org/wiki/Goi%C3%A2nia_accident

SL-1 Nuclear Meltdown
Stationary Low-Power Reactor Number One, also known as SL-1 or the Argonne Low Power Reactor (ALPR), was a United States Army experimental nuclear reactor in the western United States at the National Reactor Testing Station (NRTS) in Idaho about forty miles (65 km) west of Idaho Falls, now the Idaho National Laboratory. On January 3, 1961, at 9:01 pm MST, an operator fully pulled out the reactor’s central control rod, causing the reactor to go prompt critical. The intense heat generated an extreme water hammer which propelled the reactor vessel to the roof of the reactor building. The operator standing on top of the reactor lid was pinned to the roof of the reactor building with expelled control rod shield plugs, while the reactor expelled water, debris, and fuel. The release of materials hit two other operators, killing them. The reactor vessel then fell down to its original position. Press reported a steam explosion killed all three of its young military operators, pinning one of them to the ceiling with a reactor vessel plug. It remains the only U.S. reactor accident to cause immediate deaths.
Part of the Army Nuclear Power Program, SL-1 was a prototype for reactors intended to provide electrical power and heat for small, remote military facilities, such as radar sites near the Arctic Circle, and those in the DEW Line. The design power was 3 MW (thermal), but some 4.7 MW tests were performed in the months before the accident. Operating power was 200 kW electrical and 400 kW thermal for space heating.
During the accident, the core power level reached nearly 20 GW in just four milliseconds, causing the explosion. The direct cause was the over-withdrawal of the central control rod that absorbed neutrons in the reactor's core. The accident released about 80 curies (3.0 TBq) of iodine-131, which was not considered significant, due to its location in the remote high desert of Eastern Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.
https://en.wikipedia.org/wiki/SL-1

Demon Core
The demon core was a sphere of plutonium that was involved in two fatal radiation accidents when scientists tested it as a fissile core of an early atomic bomb. It was manufactured by the Manhattan Project, the U.S. nuclear weapon development effort during World War II. It was a subcritical mass that weighed 6.2 kilograms (14 lb) and was 8.9 centimeters (3.5 in) in diameter.
The core was prepared for shipment to the Pacific Theater as part of the third nuclear weapon to be dropped on Japan, but when Japan surrendered, the core was retained for testing and potential later use in the case of another conflict.
The two criticality accidents occurred at the Los Alamos Laboratory in New Mexico on August 21, 1945, and May 21, 1946. In both cases, an experiment was intended to demonstrate how close the core was to criticality with a tamper (layer of dense material surrounding the fissile material), but the core was accidentally put into a critical configuration. Physicists Harry Daghlian, in the first accident, and Louis Slotin, in the second, suffered acute radiation syndrome (ARS) and died soon afterward, while others present in the laboratory were also exposed. The core was melted down during the summer of 1946 and the material recycled for use in other cores.
https://en.wikipedia.org/wiki/Demon_core

More:
https://en.wikipedia.org/wiki/Therac-25
https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents
https://en.wikipedia.org/wiki/Anatoli_Bugorski
https://en.wikipedia.org/wiki/Operation_Crossroads
https://en.wikipedia.org/wiki/Castle_Bravo

https://en.wikipedia.org/wiki/Lia_radiological_accident
https://www-pub.iaea.org/MTCD/publications/PDF/Pub1660web-81061875.pdf (Lia, Georgia Incident, I would not recommend looking through the entire report, there are some very bad pictures of radiation burns in there.)

I think Kyle Hill made videos on all of these topics I might be wrong though