> old Chernobyl-like pressure water reactors in Tallin or St Petersburg

You must be confusing Tallinn with some other city - Estonia does not and has not had any nuclear power plants.

The map covers Sweden, Finland, Estonia, Latvia and Russia. Estonia and Latvia have no nuclear power or weapons programs, and the first four are EU countries where staying silent about an incident would be very unlikely. That leaves Russia.

Interestingly enough the possible source area includes the Arkhangelsk naval base, where the Russian nuclear submarine fleet is stationed. An incident there may explain the lack of information.