The main obstacle is cultural. Even contemplating the building of nuclear weapons is considered a deep disrespect to the memory of the people who died in the nu

The main obstacle is cultural. Even contemplating the building of nuclear weapons is considered a deep disrespect to the memory of the people who died in the nuclear bombings of Hiroshima and Nagasaki. There would be a lot of domestic opposition to any such project, and it might even be difficult to recruit people and companies to contribute to it, because involvement in that work could harm their reputation. But they definitely aren't lacking the technological, material, or financial ability. (Source: lived in Japan and discussed this with various engineers there)