This technology was so highly classified that any mention of it by those developing it would lead to their lifetime incarceration, stated clearly in the non-disclosure agreements they had to sign They could not even mention the theory and general technology behind it. The background tech only came to the public attention when Russia and China started commercializing it, and this forced the Americans to acknowledge it. It was a Russian ice breaker that was the first commercial vessel to use nuclear power, and even at that it was wrapped in military secrecy. But America refused to allow any development on a Western equivalent for 'military security' reasons.

https://interestingengineering.com/energy/commercial-nuclear-adoption-ship

But the most effective way for America to completely prevent any development of this nuclear technology was to make it essentially impossible for any commercial outfit to get insurance on these propulsion systems, making it impossible for them to enter any port.