I read from here that

Japan does not possess any programs for the development of weapons of mass destruction (WMDs), but it is the only non-nuclear weapon state in possession of a full nuclear fuel cycle and has advanced WMD-relevant industries.

This seems to give me a naive impression that
Japan has the capability of making nuclear weapons in a short period of time (say in weeks, if they want to). Can anyone clarify this?

For example, I am not sure if the making of atomic bombs need significantly more complicated industrial system than building nuclear plants, as Japan currently have.