To add to what OP said, the deaths were not immediate. In the first incident, the scientist conducting the experiment died 25 days afterward from acute radiation poisoning. “Acute” doesn’t mean he suddenly had symptoms 25 days later. It just took that long for him to die after he got radiation poisoning.

The nearby security guard died 33 years after the incident, though his death (acute myeloid leukemia) can also be attributed to that incident.

The second incident with the demon core involved at least eight people. One of them died nine days after the incident. The other seven lived for several years. Of those seven, at least three of them died from complications related to the incident (such as leukemia or other causes). Of the remaining four, one of them refused to take part in any medical studies, and their medical records were withheld from them. Another died in the Korean War four years later. Two of them seem to have died of natural causes, or at least the cause was not specified.