They were running tests, so some warnings were expected.
Accidents are kinda inevitable with new technologies, especially nuclear. The US actually had its main nuclear accident, Three Mile Island, in 1979, which was quite a bit earlier than the USSR's Chernobyl in 1986. The human errors that caused both accidents were quite similar, the US just had a slightly better reactor design that prevented the same steam blowout that Chernobyl suffered.
Because of the backdrop of the Cold War, the US didn't share anything it learned from its mistakes at Three Mile Island with the Soviet Union.
Notice that after their respective disasters, the USA and USSR/Russia have not had similar meltdowns again. Since the end of the Cold War, no accidents have been caused again by similar issues due to the sharing of reactor info.
The Fukushima meltdown was due to corporate incompetence and skimping on disaster preparedness by TEPCO, so isn't comparable.