It is actually very difficult to push the nuclear weapon much further back in time than IOTL without a massive POD that greatly alters society. The keys would be to marginalize the Church's influence in the West and/or alter the basics of Chinese political and social life. There was a signficant period in both the East and West where science for science's sake effectively stopped (much more obviously in the case of Christian Europe) due to sociatal conditions and the demans of the rulers of the region.
It is important to remember that Nuclear Weapons were not the result of a single practical application of a well know, long held, principal, but the result of a headlong rush to knowledge that didn't even begin until the 1890's which required a series of breakthroughs, each building on the previous, that resulted in the Bomb. The implosion weapons required breakthroughs in chemical and electrical engineering as well as the creation of the nuclear reactor to even become possible, while the "gun" style Uranium weapon also required breakthroughs in chemistry as well as physics.
Without all the pieces you get no result.