system
Given the following post, respond as if you're commenting on Reddit:
human
How do world powers develop new nuclear weapons/warheads/systems if testing has been banned since 1996?
gpt
Countries like the US and Russia have enough knowledge and data that they can be pretty confident a design will work just from models. Testing isn't just to make sure a bomb works. They gather scientific information from it to improve future designs. 

They can also do sub-critical tests, which can test part of a warhead without actually causing a nuclear explosion.