Question

which best describes fusion nuclear reactors?
they use rods containing uranium for fuel.
they are currently used for electricity in several highly populated countries.
they are used in some naval vessels, such as submarines.
they have been successful, but they are very expensive to operate.

Explanation:

Fusion nuclear reactors are still in experimental stages and not yet widely - used for electricity generation or in naval vessels. They don't use uranium rods like fission reactors. While they show promise, they are expensive to operate due to the high energy requirements to initiate and sustain fusion reactions.

Answer:

They have been successful, but they are very expensive to operate.