In episode 3, the Belarusian physicist Ulyana Khomyuk is detained by the KGB. Valery Legasov, who led the commission that investigated the Chernobyl catastrophe, meets her in the prison cell. These scenes were filmed in a real former KGB prison in Vilnius, in cell nr 14.  This  building now houses the Museum of Occupations and Freedom Fights.

Screenshot: HBO Chernobyl (2019)

More about the museum: http://www.genocid.lt/muziejus/en/ziejus/en/