According to the scientists, their reactors fuse nickel with hydrogen, which produces copper and a lot of heat as a byproduct. Very little hydrogen is consumed in the process, and minimal radiation is contained by lead shielding. What radiation is produced has a very short half-life and leaves none in the materials when the reactor is shut down. They estimate the generation cost to be 1 cent per kilowatt hour.
Other scientists are of course very skeptical, especially given the criminal background of Rossi. The Bologna scientists have answered questions about their device very vaguely, and declined to allow closed-loop testing. Peer-review journals have not published their research, and they have been rejected a patent because they don't provide scientific evidence of how the reactor works, or how it doesn't defy the accepted laws of physics. Although the reactor appeared to perform as promised in the press conference demonstration, it looked more like a movie prop than a recognizable apparatus.
Arguments in the scientists' favor are that there have been several scientific discoveries that weren't understood in the beginning, and there has been interest from several companies that have seen the reactors in operation. The reactors have been examined and verified by Giuseppe Levi, a nuclear physicist from INFN (Italian National Institute of Nuclear Physics) who helped set up the press conference and demonstration.
We shall see whether this is a bona fide achievement, or the snake oil it appears to be.