China trades in nuclear power technology so it must be a signatory to some nuclear regulatory agency or they wouldn't be able to build foreign nuclear power plants which it has done. In order to be able to do that, one has to follow a lot of rules which includes allowing outside independent inspectors into the country and allow them to go anywhere they want. If China has a facility to produce fissile material, then it would have to be legal since no one but the Indians are complaining about it. The Indians are the only ones to discover it? The US stockpiles fissile material so they can't really complain if China does too.
Another issue with the assumption that they would know from the independent foreign investigators is that China's nuclear technology mastery is deep and long. The country is also vast.
Looking for nuclear facilities that are not declared is absolutely impossible particularly if those facilities are intended to be hidden. Every major military and tech power has a lot of hidden ability and facilities doing stuff that people even within its own government wouldn't know about if they don't hold those clearances.
For all we know (including those in the low clearance public office), nuclear weapons as some MAD guarantee is already long superceded by superior weapons in China. Nukes are kept around but are far from the be all end all WMD.
Hiding activity and purpose of facilities, even their size and locations is really all but too easy for China. There are ways around SAR satellites gaining valuable information or any sort of effort to measure details below dedicated material used for these purposes. You can't hide a carrier being constructed but you can hide nearly everything else well enough. Then there's jamming and blinding satellites which China's been able to do since the 2000s if not earlier... if American complaints on those abilities hold any truth.
If it were that easy to measure and expose strategically important things, everything flying in Area 51 and whatever other facilities used for testing aircraft and missiles in the US would be totally exposed to the Chinese and Russians since the 1980s and earlier for the Russians/Soviets. There are so many ways to counter satellites the US have not retired the U-2 and surely have long had an in service replacement for the SR-71. The US is truly the only other capable nation as it is the only other one that doesn't show and tell everything it has. The Russians are beginning to be like the Indians with the whole "I have this and that and I'm really strong so please respect my power!" lol.
