Both of those are pre-Challenger and the first paragraph of Kosmos article reads: "Kosmos 954 (Russian: Космос 954) was a reconnaissance satellite launched by

Both of those are pre-Challenger and the first paragraph of Kosmos article reads: "Kosmos 954 (Russian: Космос 954) was a reconnaissance satellite launched by the Soviet Union in 1977. A malfunction prevented safe separation of its onboard nuclear reactor; when the satellite reentered the Earth's atmosphere the following year it scattered radioactive debris over northern Canada, prompting an extensive cleanup operation." so caution seems reasonable.