At the end of "The Doomsday Machine" Kirk says, "Ironic, isn't it? Way back in the 20th century, the H-Bomb was the ultimate weapon, their doomsday machine, and we used something like it to destroy another doomsday machine. Probably the first time such a weapon has ever been used for constructive purposes."
In our world, at least (we know TOS 20th century history is different in a number of ways), US Operation Plowshare and a similar Soviet program attempted to apply nuclear explosives for peacetime, constructive purposes. The first shots were in 1961, so TOS writers could have known about it. Propaganda films were made, so the various projects in Plowshare were not secret.
By the way, William Shatner narrated three documentaries in The Atomic Bomb Collection, the last which addressed the Plowshare projects.
In our world, at least (we know TOS 20th century history is different in a number of ways), US Operation Plowshare and a similar Soviet program attempted to apply nuclear explosives for peacetime, constructive purposes. The first shots were in 1961, so TOS writers could have known about it. Propaganda films were made, so the various projects in Plowshare were not secret.
By the way, William Shatner narrated three documentaries in The Atomic Bomb Collection, the last which addressed the Plowshare projects.