>It's heavy, and any accident would render a chunk of Mars radioactive for quite some time.

Do you think that nuclear fuel is some kind of green glowing stuff like in Simpsons? New nuclear pellets is just a bunch of uranium dioxide, which is mildly alpha radioactive. In other words, as long as you are not inhaling its dust, it's more or less safe to handle even with bare hands.

Even the worst possible kind of accident is likely to be absolutely irrelevant from the radiation safety point of view considering high levels of natural solar radiation on the surface of Mars.