For solid-fueled designs like the Integral Fast Reactor, the fuel is metallic uranium in steel-encased rods. For fast molten salt reactors, it's pretty much just pure fuel, since it gets melted into the liquid reactor fuel.
After startup, the fuel can be unenriched uranium, so there's no concern about an explosion, or any significant security concern. The only part that requires care and high security is the startup fuel, which has to be enriched to about 20% U235. (Bomb-grade is over 90%.)
Even so, the fuel isn't shipped in paper bags. It moves inside containers, insider other containers. If you saw it heading down the road on a truck it would be a much larger/heavier object.
I would still not recommend assembling a beachball-sized mass of any sort of uranium. It may not be critical, but you are heading in that direction. The local criticallity officer will not be happy. Even depleted uranium, the stuff once used in bullets, probably shouldn't be so assembled.
I read that an awful lot of it was fired into Iraq by the US in the last 'war' there, and will be causing birth defects there for a long time. I don't think someone reading the calm sentence Depleted uranium is still used in military large-caliber bullets would have any idea of the horrifying reality.
(Depleted) Uranium is a pyrophoric material - it spontaneously ignites in the right conditions (e.g. when it's shot at a tank and penetrates it), making a better job of killing said tank's occupants. This causes it to vaporize, and can now be breathed in - the exact set of circumstances where its nature of alpha emitter is dangerous for health.
While this is horrific, and I hope it stops, please keep in mind that any other use case which doesn't involve burning it or aerosolizing it creates no health hazard - you can build glassware with a high U content and drink from it.
any other use case which doesn't involve burning it or aerosolizing it creates no health hazard
That doesn't sound quite right:
"Normal functioning of the kidney, brain, liver, heart, and other systems can be affected by uranium exposure, because, besides being weakly radioactive, uranium is a toxic metal. Uranium is also a reproductive toxicant. ...Uranium metal is commonly handled with gloves..."
After startup, the fuel can be unenriched uranium, so there's no concern about an explosion, or any significant security concern. The only part that requires care and high security is the startup fuel, which has to be enriched to about 20% U235. (Bomb-grade is over 90%.)