Apollo's heat shield worked because of the aerodynamic properties of the CM.
Something besides the ratio of surface area relative to mass?
You cannot put nuclear fuel in a reentry capable aerodynamic body.
Clearly you can, because you could easily add 1 kg to the internal mass of the Apollo capsule and it would still be able to safely re-enter the atmosphere.
Everything you have just described has effectively rendered the fuel as unusable. We're not talking about an RTG. This stuff needs to function as reactor fuel.
Why? I mean yes, eventually, but you can put the reactor up there inert, put the fuel up there inert, and have a manned mission to assemble the thing. Nothing inherently requires that the reactor be active or in a ready-to-activate state during launch.
Besides, you need to be able to do a safety inspection with a CT scanner or similar to verify that there are no weld failures or other damage caused by the launch process or the landing process, or else you risk the thing immediately spewing radioactive steam as soon as you turn it on, and contaminating the reactor vessel in such a way that renders it irreparable, all because you cut corners, so you're probably going to want to have a manned mission to activate it anyway, or else some very high-end robot tech. Either way, you should be able to come up with a way to then unwrap the fuel and install it into the reactor after you've safely landed the whole thing on the moon, because that's relatively simple compared with all the other stuff that needs to be done before you can safely start up the reactor.
Your armchair physics expert take on this is absurd.
You're making a huge number of very questionable assumptions about how this should be done, and dismissing my comments based on those flawed assumptions. I'm not the one being absurd here. There are ways to do this that are very, very low risk. Whether they choose to do it that way or not is a different question.
Launching fissile material into space is dangerous. Period.
Not particularly. U-235 has a half-life of 704 million years. This is not the stuff that makes reactors scary. It's the short-half-life byproducts that are super dangerous to be around.
The NIOSH workplace exposure limits for Uranium are 0.05 mg uranium per cubic meter. That means as long as the explosion evaporates the material over at least 20 million cubic meters, even if it evaporates into the air, you're not likely to cause too much harm. This is only about twenty empire state buildings worth of air, by the time you're flying at an altitude where fuel could realistically evaporate, it should evaporate into many orders of magnitude more air than that.
And realistically, AFAIK, no failed spacecraft has ever completely evaporated during reentry other than tiny satellites that are designed to do so, so that isn't a realistic concern anyway, IMO, unless you're planning to ship fissile material inside tiny satellites, and realistically, probably not even then, given the quantities involved.
Or to put this another way, if the entire 1 kg chunk of U-235 got somehow flattened out into a sheet (so that the uranium wouldn't shield you from most of its own radiation) and you were to lie down next to it, you'd still probably get less than the equivalent of one chest x-ray per hour of radiation. Mind you, I wouldn't want to leave a kilogram of uranium lying around on a children's playground, but realistically, the swingset is probably more likely to kill someone. The risk is nonzero, but not so nonzero that it's worth worrying about, IMO.
That doesn't mean it doesn't need to be done, but you acting like it's no risk, waving your hands to make the risk disappear, isn't helping a fucking thing.
From a safety point of view, the highest risk would be it landing on the ground somewhere, and some terrorist finding it and stealing the nuclear material before the government does. And given that there's only a 29% chance of it hitting land, and maybe a 0.1% chance of any given land region being within a short distance of a terrorist cell, I'm not sure that's worth thinking about too hard, either.
The risk is nonzero, but it is so laughably small that I'd be more worried about the spacecraft physically hitting someone and killing them on impact than the tiny amount of U-235 killing someone.
I guess if the spacecraft missed you by a few meters and you somehow didn't die from the dust cloud, the criticality event from the impact might give you cancer someday, but...