I do not know as I don't have full access to INEL (the nuclear reactor development site in Idaho) records, perhaps a good idea for a FOIA request, the first meltdown would almost certainly have occurred in one of their test reactors long before any commercial reactor.
I do know that the core of EBR-I (the first US breeder/power reactor prototype) was damaged in a meltdown on November 29, 1955; it was repaired and finally deactivated in 1964. This reactor has been decontaminated and is open for public tours (docent guided and self guided). I have been there 3 times (the first before decontamination of the floors were complete).
I very much suspect they had at least one test reactor deliberately designed to thermally damage or even melt its core to study the effects.
Unclassified Controlled Nuclear Information
The only country to have used nuclear warfare in history is the USA
Russia currently owns the biggest stockpile of nuclear weapons in the world. Also Russia made and detonated the biggest nuclear bomb in history called the Tsar bomb.
The test was codenamed Trinity, the bomb was codenamed Gadget.
The nuclear weapons of the cold war still exist. They were in responsible controlled hands during the cold war (which is why there wasn't a war). The problem: Keeping them out of un-controlled hands.
The first controlled nuclear reaction took place in 1942, and the first nuclear meltdown in US history occurred in 1979, marking a span of 37 years between these two events.
The first controlled nuclear reaction took place in 1942 at the University of Chicago. The first nuclear meltdown occurred at the Chernobyl Nuclear Power Plant in 1986, which is 44 years later.
The first controlled nuclear reaction occurred in 1942, while the first nuclear meltdown happened in 1952, so 10 years passed between the two events.
coz if nuclear meltdown it can explode
Very good question. It probably happened in the 1950s on an experimental reactor on the INEL site in Idaho. I know that their EBR-1 reactor was damaged by a partial meltdown in that time period, but I don't know if it was the first. I have considered making a FOIA request to them on that subject.
a nuclear reactor had a nuclear meltdown
Under the worst possible conditions, a meltdown can occur in a nuclear submarine. It is an event of low probability, however.
The nuclear meltdown at Three Mile Island occurred in Middletown, Pennsylvania, United States on March 28, 1979. It was the most serious accident in the history of the American commercial nuclear power generating industry.
Russia
Chernobyl, Russia
Human stupidity
yes