The Soviet Nuclear Disaster the West Covered Up.
sobify.com
sobify.com
"Kyshtym – The Nuclear Disaster Both the Soviets & the U.S tried to hide"
https://en.wikipedia.org/wiki/Kyshtym_disaster
"Kept it secret" on Wikipedia and "cover-up" in the article title both imply malice, but it's not very clear to me that the CIA had an obvious obligation to report everything bad that happened in the Soviet Union which doesn't impact non-Soviet citizens. When they do publish something, it's for propaganda reasons, and, as stated, this incident wasn't really suitable for that.
I'd add that this disaster doesn't seem to implicate the US nuclear industry at all, sounds like it was part of their plutonium breeding effort, which is for weapons. Back then and for a very long time Oak Ridge would enrich natural uranium to, what, 5% U-235, and then it goes into a separate cycle which could and no doubt someday will include reprocessing (although not for weapons grade plutonium, too much Pu-238 and Pu-240 if you let it brew long enough), but didn't then nor does it today.