“The ancillary systems needed for a reactor have similar breakdown rates as diesel except the consequences of a breakdown are catastrophic.”
Those are ancillary systems as you admit - ancillary, and thus their breakdown rate it not likely to be higher. And our military navies have already years of experience on how such ancillary system breakdowns are handled, safely, on our nuclear powered fleets - for which quite obviously there is not a history of “catastrophic” disasters.
The systems are basic mechanics and break down all the time. Motors, pumps, valves, pipes, etc. They are present and similar in every type of ship engine. Piston, turbine, nuclear.
The difference with nuclear is you can’t have any of the components fail for even a few minutes without absolutely having a redundant backup in place to go online immediately. Redundancy and mission-critical systems multiply the cost of everything in a plant.
There may be a point where is becomes profitable if fuel is extremely expensive but the safety concerns are such that it will likely never be allowed. The government allows private stationary plants but with extreme oversight and multiple extensive security perimeters in reasonably remote areas. Such a thing is not possible on a freighter that travels to major ports around the globe. Only on military vessels with capability to fight back against a military attack is it safe.
There are some inherently safe reactor designs that I am hopeful for but they are still prohibitively expensive and inneficient for private industry.