Good thing you've got fusion plants
Because you're going to need a lot of energy. Of critical importance is the ionization energy of all the elements. As plasma contains normal elements stripped of their electrons, enough energy will need to be pumped into each trash cube to liberate enough electrons to form a plasma.
The energy required to energize/plasma-ize 10m^3 of random garbage will be immense. At the low end, let's continue consider hydrogen. It takes 1312 kJ/Mol for complete ionization. But hydrogen is easy. It's already a gas.
Iron is trickier. It's a solid and does not yield energy through fusion. Iron's ionization energy is 762.5 kJ/Mol for the first electron. Ionization energies rapidly increase for each electron there after.
I don't know if iron has to be heated to boiling before it can turn into a plasma. If it does then the energy requirements for this reactor are staggering. Assuming linear specific heat values at all temperatures of 1kg of iron of 449 J/kg K; it takes 1407166 J to boil 1kg of iron. Add in the heat of vaporization and it gets even more expensive.
Reactor Design Considerations
There's a couple things you'll need to account for in this reactor.
* Don't let the plasma touch anything. I've seen temperatures of 30,000 Kelvin in the literature. With the energies under discussion, the plasma temperatures may be considerably higher. Normal matter does not perform well when hit with high energy atoms. 435
This is a long standing problem in fusion power plants under development now.
* This is not fuel. Anything iron and heavier does not yield energy when fused.
* Injecting cold matter into this reactor will require ridiculously high energy flows to sustain. Each 10m^3 will need to be heated to plasma temps, which will vary wildly from cube to cube. As mass leaves the plasma chamber, it will take thermal energy that must be restored in order to maintain the plasma.
* The reactor must contain the rapid expansion of gas phase products. Water expands 1700x when turned to steam. Other solids or liquids may expand even more. Not only must the reactor contain the expanding gasses but also the high speed chunks of arbitrary mass accelerated by those products. In other words, without preprocessing, each cube is a 10m^3 fragmentation bomb.
* Watch out for the really caustic elements such as fluorine and chlorine. These elements aren't bad when bound to other elements at room temperature, but this reactor is not normal. Special precautions will have to be taken or the reactor may eat itself.
Design trade-offs
Okay, so somehow the reactor works. It separates each element and doesn't destroy itself. You've expended an enormous amount of energy to get a very hot gas. Well done!
However, you now have a big hot cloud of random elements that need to be sorted, cooled and turned into products that can be sold. How will you sort these? How will you prevent unhelpful chemical reactions when the atoms cool down?
Having done a little research into hydrocarbon plasmas, I can say without a doubt that this area is crazy complex and very hard to manage, even with single element plasmas. This reactor must accept all elements in any ratio or quantity. This is the pdf manual for a real life plasma calculator that only does one element plasmas. All of those input variables will change between batches and perhaps within each batch. 435