There are 3 good reasons it has not happened, yet.

The ‘PWR’ is the wrong technology

Almost all the 600 conventional ‘nuclear reactors’ that have been built since 1954, are Pressurised Water Reactors (PWR).

Nuclear = PWR.

The PWR is ‘the’ nuclear reactor we know today.

Although PWRs have become much safer and much more efficient over time, the physics of the core-process and the engineering to make them so, are both hallmarks of an age now passing.

We all know about nuclear accidents. The public mistrust of the PWR is too widespread now for this to change substantially. A PWR on a ship is a very unpopular idea with port authorities, charterers or flag states.

Proliferation of spent PWR fuels aka ‘ nuclear waste’.

A civilian PWR ‘runs out of fuel’ before 1% of the energy in its fuel has been consumed. Long lived ‘nuclear waste’ is produced from 99% of the remaining spent fuel. This ‘waste’ must be either reprocessed (which is expensive) or stored for up to 250,000 years (which is unpopular).

In contrast, PWRs that power naval submarines and aircraft carriers can run for a lifetime of close to 50 years without refuelling. That’s because they run on ‘military-grade’ High Enriched Uranium (HEU). HEU is a tightly controlled substance and commercial ships cannot use HEU because it poses an intolerable proliferation risk.

If the merchant fleet were to decarbonize using PWRs, we would have to run entirely on civilian grade Low Enriched Uranium (LEU), like power stations do.

A PWR running on LEU needs to be refuelled every 1-2 years which would mean potentially disposing of spent nuclear fuel in ports.

That’s not going to happen.

Costs are way too high

The enormous cost of a mini-PWR does not help either. Each naval reactor can cost as much as 10 new supertankers.

That will not work for any type of merchant ship, ever.

As a result of these three factors PWR powered nuclear propulsion of commercial ships has been, and always will be, a non-starter for commercial shipping.

However, things are about to change with the introduction of the marine molten salt reactor (m-MSR).

With an m-MSR we get the benefits, but not the problems of nuclear.

An m-MSR is radically different from a PWR and we still get the power with zero-emissions.