This is pretty simple, when you add a fuel with burnt result on an object (such as (portable) nuclear reactor, nuclear locomotive), it will consume it and stock the energy into a slider. If you somehow delete that object, by mining it or picking it up from the grid, it won't return the used product for that stocked fuel.
Just to be more precise, here are the steps:
1. Place an equipment/entity that uses fuel with a result (nuclear fuel, or other moded ones).
2. Place the fuel with a result on the fuel slot.
3. Right after, mine the entity/remove the equipment.
Expected: Get back the fuel on its used version.
What I get: Nothing.
Voiding fuel result when picking up entity/equipment
Re: Voiding fuel result when picking up entity/equipment
See viewtopic.php?f=28&t=91653, which started as an identical bug report and within a merged topic you will find https://mods.factorio.com/mod/sw-recover-burnt-result.
