It seems that they only discovered this tech in the late '90s, so it's bleeding edge stuff. Basicly, if you aim "soft" x-rays (like the kind the dentist uses) at a block of hafnium-178, you get a gamma ray output that's 60 times the energy of the x-ray input. No fission, no fusion, no neutrons -- just gamma rays. Gamma rays need a lot of mass to shield, but they don't have the contamination problem that neutrons and alpha particles do.
As the title suggests, they're talking about using this as an aircraft powerplant -- using the gamma output to heat up the air and run a jet turbine. Apparently, you could run the x-ray machine on a solar panel.
The most fun of all: they stumbled on this accidentally. It's all so wonderfully comic-book! Can't you just see some Pulp Era Inventor bombarding the Mysterious Hafnium Isomer with X-Rays and discovering an Astounding New Power Source?
The basic story is pretty straightforward, but the physics behind it is frankly beyond me. Blah blah blah handwave blah energy states blah quantum blah magic.
Some scientists question the technology, but their experimental technique seems fundamentally flawed. "They used weak, limp, girly x-rays, so we'll use bigger, badder, far more impressive x-rays from our Superatomic Hyperphallic Compensator Collider! Nope, we didn't get a proportionally larger output; they must be wrong!"
Did they try actually duplicating the experiment? I'm hardly a nuclear physicist, but even I can come up with three or four plausible reasons why low-energy x-rays would produce an effect where high-energy ones wouldn't.