Yes, both countries' nuclear weapons programs were extraordinarily expensive over time. The first underground reactor of the Mining and Chemical Combine was built in 1958, well after the USSR had started stockpiling fission and fusion weapons. The original Mayak production complex had ordinary above-ground reactors.

I believe that the the "AD", "ADE-1", and "ADE-2" reactors here are the only underground ones:

https://en.wikipedia.org/wiki/Soviet_atomic_bomb_project#Plu...