Because plutonium production requires differend kinda ractors what have unique heat signature, they release all kinda things into athmosphere where such things can be sniffed by sensors, infastructure isn't easy to hide and so on. All compact weapons China tested in 90s used plutonium or composite pits. HEU manufacturing is much easier to hide as centrifuges or other enchrichment ways don't have those problems plutonium production.
There are many possibilities.
- China's older plutonium based warheads what are/were used on DF-3, DF-4, DF-5 had lot of plutonium so they can get quite a lot by recycling.
- It's also possible that they use composite pits on their 90s warheads aka making more by using both HEU + plutonium so each new warhead takes a smaller nible of fissile material stock.
- Another possibility is that Chinese scientists have developed new HEU based weapon and trust that it works as well on real world as it does on computer simulations, etc.
-it require approx. 3TWyear to make one tonnes of Pu-239. This is a lot of heat , and as easy to hide as an elephant . The North38 periodically checking the NK pu reactor with satellite thermal images from commercial sources, I presume there are many hobbyist NGO who can afford the thermal images of 32°29'36.92" N 105°35'39.14" E. Up to date no one started to cry loudly due to heat emission from there.
-The U235 primary require way more uranium, that reduce the load of ICBMs, and most likely require new tests to prove the design. It should be audible from many seismic observatory .
But, interestingly, at the Chinese Oak Ridge they made a new building on the reactor site, they finished a new access bridge to the isotope separation site, and demolished the old bridge to the reactor site and there is building activity on a new bridge.
So , there has been lot of investment activity into the site.
But still no news about extreme level of continuous heat emission