Widespread use of nuclear power for civilian merchant shipping is a non-starter. It's simply too expensive to train and pay enough qualified nuclear engineers. The US Navy barely manages this for a few dozen nuclear powered vessels with a huge budget and limited concern for cost effectiveness.

Plus there are the security concerns. Every civilian nuclear power plant has armed guards on duty around the clock. Those guards have to also be trained and paid. Many foreign countries have laws against bringing weapons into their ports.

If we want to reduce CO2 emissions from merchant ships then use nuclear or renewable power on shore to manufacture carbon neutral synthetic liquid fuel. Use that to run the ship engines.