Traveller: Why Are Radioactives Expensive?

Reflecting on Traveller and gross planetary product, I got to thinking about radioactives.  They're worth a million credits a ton, and it's a little puzzling.  What are radioactives used for in Traveller?  Nuclear weapons are purely the Imperium's prerogative, so that's pretty much out (but radioactives aren't illegal on their own, so it's also not like a black market price inflation sort of thing).  They're not being used for starship armor, probably (almost certainly not for crystal-iron or titanium steel.  Maybe for "bonded superdense"?  But that sounds more like neutronium than depleted uranium).  Is DU being used in infantry weapons and armor?  But you don't really need radioactives for DU, it's depleted.  It's probably not being used for medical imaging with all the higher-TL stuff available.  And it's not being used for power because fusion technology exists.

Fusion, incidentally, sounds like a decent way to synthesize radioactives, since that's how stars do it.  Horribly energy intensive, yes - but you have fusion power.  So fuse hydrogen for energy, and then use that energy to fuse lead or whatever to get your radioactives.

I've got half a mind to remove fusion power and make starships fission powered, as a means of explaining why anybody in Traveller gives a hoot about radioactives.  Doesn't matter how far in the future you go, fusion is still 10 years away.

But I don't think I'd want to use Mongoose's rules for fission power, which are pretty punitive.  I'm fine with the gameplay of the fusion reactor, just not the in-world implications.  Keep the same reactor volume and cost, keep the same fuel volume, but you only have to replace it once every, oh, ten years or so, and have the total cost of the replacement fuel rods add up to what you would've spent on refined hydrogen fuel for a typical fusion reactor over that time.  And then lift HOSTILE's hyperdrive and maneuver drive rules, you're heating hydrogen on the reactor and using that for thrust.