I think Arkhangelsk being involved seems likely. The Russian ship "Akademik Aleksandrov" aka Project 20183 was apparently in that area recently[1], and seems obviously related to Poseidon[2]. If the source of the radiation were a Poseidon, that would certainly explain the secrecy.

As for claims of the fission products coming from a "civil source", I don't think enough is known about Poseidon's reactors to rule that out. I could be wrong, but as far as I've read the details of that reactor are up in the air (besides being small, obviously.)

[1] https://twitter.com/FrankBottema/status/1276930424170844162

[2] http://www.hisutton.com/Akademik-Aleksandrov.html