Completely off the cuff speculation here, but I think this is a plutonium MLIS (molecular laser isotope separation) facility:CNN piece on recent nuclear infrastructure expansion in Sichuan.
1) All other processes in the weapons production chain are accounted for elsewhere - pit production at Pingtong, explosive lenses at other facilities, etc.
2) This is exactly what an MLIS plant would look like - heavy shielding for radioactivity, blast doors for criticality accidents, massive air filtration to handle plutonium in the gas phase.
3) China has a vast latent stockpile of reactor grade plutonium and very little weapons grade. This is exactly the facility needed to clean up the reactor junk.
4) China recently said that Japan has enough material for "5,500" nuclear weapons. Japan has RGPu which is unsuitable for bombs, so such a statement wouldn't be made unless China thought it was technically feasible to upgrade it. I've seen claims you could guerilla up some bombs out of RGPu, so this is my weakest argument.
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