system
Given the following post, respond as if you're commenting on Reddit:
human
Why cant we use nuclear fuel until it's spent?

We use plutonium and uranium to fuel our nuclear plants. In hospitals, they're used in xray machines. At a certain point, they are considered "spent", but still contain enough energy to be considered a problem. Why can't we use them until they're gone, just like a log would be just carbon and water vapor when it's burnt out?
gpt
Spent is an interesting word in nuclear fuel.  Normally the fuel is considered spent if there is enough ingrowth of neutron absorbers to render the control rods useless in the all the way out position.  Then it needs to be reprocessed and the bad stuff removed.  And to do that, requires that the fuel rods be removed, the zirconium outer liner removed, and the metals dissolved.  Then it goes into a chemical separation, and new fuel can be made from the remaining uranium and for mixed oxide fuel, the plutonium.  This ALL has to be done, once the rods are removed, in a hot cell.  The rods have to sit for months to years to let the short lived isotopes decay away.