Just before the spaceship takes off for Fhloston Paradise, the audience gets to see the manual interface that the airport employees use to refuel the ship. On the tarmac beneath the spaceship, the ground crewman plugs in a portable control box to the underside of the plane, and presses a button to open a hatch in the ground, from which a new, glowing green radioactive fuel cell emerges.

One of the crewmen grabs it by its circular handles at the end, removes it from the hatch, and sets it on the ground.

He then uses the plugged-in control box to open a compartment on the underside of the spaceship, from which one of the ground crew removes the spent fuel cell by hand, and inserts it into the still-open hatch.

Finally they pick up the full fuel cell and insert it into the compartment on the plane.

This scene is there to set up how Cornelius stows away on the craft, but also serves as a cinematic pun when it crosscuts to a scene inside the ship (but which must be seen rather than read to appreciate.) For such a “throwaway” technology, it’s handled really well.

• The ground affords natural shielding from any collection of radioactive fuel cells.
• Being circular, the cells and the handles to manipulate the cells are orientation-less.
• There are familiar black-and-yellow-stripe warnings on the walls of the hatch and the revealed sides of the spaceship compartment. These warnings are only visible when it’s relevant.
• The radioactivity trefoil symbol has the same colors and appears on the fuel cell, the hatch, and the compartment.
• Having a portable and wired control box means that it’s not readily available for any passing hackers.
• The transparent container lets the material act as an additional warning to observers: There is danger here.
• The transparent container lets the fuel itself tell the ground crew which cell is spent and which one is full.

All told, short of making it automated, this is how it should work.