It is a NUCLEAR POWERED submarine ; an SSN , as opposed to a SSBN , or "Bomber" as they call them. So to call it a "Nuclear Sub" in everyday parlance is fine. USS Nimitz cannot launch nuclear missiles,but she is still a "Nuclear Aircraft Carrier", again in everyday parlance.You can always tell a "nuclear powered " submarine by the escaping steam from the connong- tower vent ,as per the recent shots of "Astute"; you wouldn't get that from any "conventional" submarine !!