One estimate I read was that it would be comparable to a fast breeder reactor for the same energy output. The neutrons have to go somewhere, and they will irradiate the materials the reactor is made of to make them intensely radioactive. You cannot construct a real piece of engineering (magnet cores, laser optics, field coils, vacuum pumps, boiler tubing, pressure vessels, shielding) entirely out of isotopes that won't turn into a waste problem.