I had to cut a large number of branches from a walnut (its crown started mixing with electric wires). If those branches are chopped (by a machine), can resulting peaces be used as mulch (given juglone content of any walnut)?
It depends what you want to grow. Not all plants are sensitive to juglone. According to this from a US university, beans, corn, beets, onions, and raspberries are tolerant, for example. Plants that are intolerant include the Solanaceae family, (potatoes, tomatoes, etc), blackberries, cabbage, and rhubarb.
If your branches are big enough, walnut wood is valuable for woodworking. Somebody might pay you to take them away, if you can advertise them locally.