We're talking about TIG welding here, in submarines and civilian nuclear reactors.
Call it 60 hours of actual practice to gain "proficiency". A lesiurely formal course lasts 6+ months.
Hence I'm happy to go with a 6month figure.
Note that China has an extensive civilian nuclear industry (with associated TIG welders) who I also expect to be security-cleared.
Another flaw of your thinking which is becoming clear — you treat security clearance and welding qualification as some sort of master key, where you just need one to do everything. Reality works more like a hotel with hundreds of rooms, each key only allows access to one room. You need a new key even if you just want to access the adjacent room — new qualification.
No, we are not talking about just TIG welding here, we are using welding to illlustrate one of many bottlenecks. The weldings on nuclear reactors are different to those on submarines and those on aircraft carriers because of difference in composition in the steel; they are not just "TIG welding".
You also can't just stuff people working on one project to another project and expect them to excel. Beside, they are already occupied, they are not free to work on another project. Doing what you proposed and everything is going to suffer.
If things were so simple, it wouldn't have taken China decades to reach today's industrial might, and more countries would have been involved in the heavy industry.
But at the same time, security clearances cannot become a major impediment to increased wartime production...
I agree, but I don't see much else that China can be doing.
The path has already been set to:
1. Secure the 1IC ~2030
2. Secure the 2IC ~2035
The next step (2040s onwards) would be a pre-eminent blue-water Navy, which becomes a nice-to-have, if the 1IC and 2IC can be secured.
Wartime production for naval vessels is a thing of the past.
You can't just ramp up production at war time because the size of the skilled labour is inelastic. If you talk about needing 10+ years to ramp up production, then you aren't talking about war time, you are talking about peace time.