Build Thermonuclear Device is a project in Civilization VI. It becomes available after researching Nuclear Fusion and completing Operation Ivy.
This project requires Uranium (20 Uranium in Gathering Storm) and can be undertaken multiple times. Each time it is completed, it adds one Thermonuclear Device - a one-use weapon with a blast radius of 2 and a maintenance cost of 16 Gold per turn - to the player's arsenal.