It's most likely going to work like other power buildings, only using fuel (nuclear rods) as there is an actual demand. If you use only half the capacity of the Nuclear Power Plant, a rod will "burn" twice as long, and thus produce half as much waste.
But it's not in the actual game yet, and game designers sometimes change things...
I find it a bit of a weird choice to have waste... It's not hard to have a bunch of chained containers for waste, and periodically go there to empty them out. (unless they make simple deletion impossible and require some form of waste treatment). Just storing them in containers doesn't seem like a really good approach either.
Then again, this may be a good use of the converter, convert 2 units of waste into some useful resource :)