power source is electricity, that's all the engine needs, how it gets it doesn't matter (while it's still in testing) as long as power generation can keep up with demand. so concerns about how it will perform with different power generation schemes only matters if they can produce the power it needs. once outfitted or planned to be then lightweight power sources become more relevant.
a nuclear plant would not be used in any case...as they are rather large and not suited for space...much less launch a plant is a rather large complex of reactors and support equipment that covers a rather large area and going into the thousands, at a conservative guess tons.
yes i know, the power required is electric but for the ship to travel to lets say Mars in 2 or less months, it will require Megawatts + of power which today only nuclear reactors can, even the CEO of Ad astra behind the company behind the Vasimr engine says so, unless the e-cat fusion/fission reactor story is not fake then that will change things.
Also the Russians have already made a space nuclear reactor decades ago so it is possible. NASA has made one and tested it on an ion engine years ago too.