It’s obsolete, it’s defunct, and it hasn’t been touched in nine years. But it’s still worth looking at. It’s the Nuclear Weapons Archive, last updated in 2007 after a rocky ride around various sponsoring non-profits and hosting sites, and it’s full of interesting nuclear documents, like this short British run-down on what it will take to make His Majesty’s first nuke, as of 1947. (The link is to a .pdf).
Another, similarly defunct site that was a parallel and cooperative site with the Nuclear Weapons Archive was the Trinity Atomic Web Site, which appears to have assumed ambient temperature in 2005, but exists in a sort of undead (and un-updated) state.
But if you really want to understand the technical factors involved in the production of the first A-Bombs, factors that are often glossed over by highly verbal but innumerate and scientifically weak writers, you need to buy one book: Atom Bombs by John Coster-Mullen.
Coster-Mullen is not a professional historian or archivist, but you would never know that from his book. (He is actually — we are not making this up! — a truck driver). Through sheer determination and hard work, he mastered the subject and wrote the definitive work on it (with equally definitive documentation and illustrations). If you go to the Amazon link, and select all buying options, the seller coster60 is the author himself.