In order to provide fresh fuel at the distributor, free from overheating/soak, unused fuel was returned to the tank where it entered very close to the high pressure fuel pump.

Pressure regulation is achieved by returning unused fuel to the tank.

A pile of unused fuel lay ready.

According to one paper the following difference between the cladding failure mode of unused and used fuel was seen.

The unused fuel is not very radioactive, but when the reactor runs, it splits the atoms of uranium into pieces, and the "daughter" atoms, some solid and some gaseous, are highly radioactive.

For example, the catalytic converter uses a catalyst and oxygen to burn off any unused fuel and certain other chemicals in the exhaust.

A mechanism for dissolving unused nuclear fuel malfunctioned Saturday, spraying radioactive acid over three workers at a chemical plant, the Energy Department said.

It should be noted that spent nuclear fuel storage basins pose an enormous risk and hazards, and therefore all unused fuel should be transferred to dry cask storage as soon as possible.

The silica gel was used to keep the unused fuel dry during storage and transport.

Or perhaps it only becomes non-homogenous when the user makes a wrong mixture, or leaves unused fuel in the model, occasionally getting cold, or allows the hydrocarbons to evaporate away faster than the nitromethane.