On this day the USA conducted the most powerful nuclear test in its history. It was conducted on the famous Bikini Atoll in Micronesia (the bikini swimwear was named after it).
The nuclear test performed on this day was codenamed Castle Bravo, and produced an explosion of around 15 megatons. Since the Hiroshima bomb had only 16 kilotons, this explosion was 938 more powerful than it!
The force of the explosion surprised even the experts, who had expected it to measure only 6 megatons. Therefore, the actual explosion was around 250% more powerful than expected. It was this that caused the radiation incident – not even the Americans were prepared for an explosion of that magnitude. Sadly, as many as 239 locals on the neighboring islands were exposed to a significant amount of radiation, as were 28 Americans on a nearby atoll.
The Japanese tuna fishing boat Daigo Fukuryū Maru was also located near the Bikini Atoll at the time of the explosion. Some of the 23 fishermen aboard the boat suffered radiation sickness, and one even died of it.
Why was the Castle Bravo explosion so powerful? It was because the bomb was thermonuclear (i.e. a fusion bomb). Lithium (Li) was used to produce the thermonuclear reaction, specifically a combination of 40% Li-6 and 60% Li-7 isotope.
This proved an unusually powerful combination due to the amount of tritium (a radioactive isotope of hydrogen) which was created during the reaction. The fireball produced by the explosion had a diameter of around 7,000 meters. The resulting crater had a diameter of around two kilometers and was some 76 meters deep.