http://dsc.discovery.com/news/2009/03/11/plutonium-bombs.html Basically, adding enough Am to the reactor to bias the FP production to non-weapons-grade Pu-238 rather than some of the fissionable Pu isotopes. Still can be engineered into the weapons-grade material, but adds some extra hardships, and may decrease the risk of helping out some countries with their power needs.
The Israeli's developed it? That's ironic. After all, they were the ones who stole/we're allowed to steal fissionable material from us in the first place.
