> The Nuclear Non-proliferation Treaty (NPT) forbids signatories who don’t already have a bomb from making one. It also says they must put sensitive nuclear material, like enriched uranium, under international safeguards, monitored by the International Atomic Energy Agency (IAEA), a watchdog. But the rules have a submarine-shaped loophole. States are allowed to remove nuclear material from safeguards if they are for “a non-proscribed military activity”, such as submarine propulsion. No non-nuclear-armed state has ever tested that loophole—until now.

Source: https://www.economist.com/international/2021/09/17/what-does...

So it seems that it is technically not violating NPT.