I am of the understanding that part of this is because there are different profit margins in mind with a civilian reactor generating power to be sold and a military reactor powering a vessel.

When that is combined with deregulation (or an anti-regulatory mindset) where things like insulation on water intake is deferred or ignored because it impacts the economics of the power plant, then building one becomes difficult.

Agree... although I do believe part of the problem is that a lot of US Naval reactors run on weapons grade uranium. Someone here probably knows more about this.

The fuel in US Naval nuclear reactors is enriched to a much higher percentage than civilian reactors due to size and longevity considerations. It has to fit the ship and refuels take months/years. A ship undergoing a refuel isn't a ship you can use.

In a civilian plant, you can have multiple reactors and refuel them on a rotating schedule to avoid downtime, having a larger reactor vessel isn't a problem, and all of that is also going to be less expensive - which is a huge factor.

I think the difference i >90% for military use and 3-5% for civilian use.

Some new SMRs are planning on using >5%.

With that grade fuel, can a nuclear submarine denote like a bomb?!

Large nuclear reactors have better economics (per energy produced) than small reactors due to economy of scale benefits.

But SMRs can still be useful for small towns, remote communities, district heating, process heat for nearby industry and so on.