I created several concentric circles and I want to cut them in pieces where the radial lines pass as if was a pizza (leaving an empty space with the width of the line). Each of those resulting pieces should be independent of the rest so I can assign different colors to the pieces, move them, etc. I selected all objects and tried all the options under Path but I'm missing something cause I don't get the desired result. How can I achieve it?
The first thing to note is that boolean operations in Inskcape require two combined/united objects to work properly.
Assuming the thin lines are strokes, select them all and convert them to paths using Path > Stroke to path. If they're already paths with fills, ignore this step, but make sure they are all selected.
Do Path > Union
Assuming the circles are made of strokes, then select them all and convert them to paths using Path > Stroke to Path. If they're already paths with fills, ignore this step but make sure they are all selected.
Do Path > Union
The steps above converted all the objects to just two separate objects, so now the boolean operations can work.
Select both objects and do Path > Difference
Do Path > Break apart
The circles will be cut into separate moveable pieces, leaving a gap which is the thickness of the lines, and you can now assign different fill colours to each object.
An example
