Suppose there is a isolated system in which there is only one stationary rocket.
As the rocket is in a stationary position the initial momentum will be 0, now the nuclear engine is started, as a result momentum increases but in nuclear engine, there is no exhaust gases or such things to counteract the increase in momentum of rocket. So the total momentum in the system increases. As a result law of conservation of momentum is violated.
Can you please say what is the mistake in my thought experiment.
Edit: The question was a result of my misunderstanding of how nuclear engine works.