A typical Candu reactor uses slow neutrons. The neutrons can be handled with water, and when a slow neutron settles in on a U235 atom, it decides to split.
A fast reactor uses high-speed, very energetic neutrons. These things blast through water, and would jellify you in an instant! They have to use super-hot liquid sodium to control them, and capture the heat energy. Liquid sodium makes a boiler explosion look like a firecracker!
But fast neutrons can blast out all that nuclear waste, and make the problem go away! That is why it has been dragged out of the dusty cupboard and dangled in front of the Obama blue ribbons. Really, if you eliminate all reasonable approaches, you have to go with this!
"The world has spent $100 billion trying to build fast reactors, but they were too costly and unreliable," says Frank von Hippel, a nuclear energy expert and a director of the Program on Science and Global Security at Princeton University. "I know there are enthusiasts, but history isn't encouraging," he says.
Yeah, well there's that tiny little problem....