US, 1952, near meltdown

history: the time Jimmy Carter stopped the first nuclear meltdown, long
before he was president. On December 12, 1952, the NRX research reactor at
Chalk River Laboratories, Ontario, Canada, went through a partial meltdown
due to mechanical failure and human error. The lab had failed to provide
adequate cooling for several experimental fuel rods, resulting in them
rupturing and melting, sending 10,000 Curies of fission products flooding
into the basement of the reactor building, riding on about a million
gallons of cooling water.

 IFL Science 17th Dec 2021

 https://www.iflscience.com/technology/how-jimmy-carter-stopped-the-first-nuclear-meltdown-long-before-he-was-president/