[Red Badger]

http://en.wikipedia.org/wiki/Pebble_bed_reactor

[xzins]

It is not impossible to get weapons grade nuclear material from pebble bed reactors. It might not be easy, but it is not impossible. Additionally, there are dirty bombs.

[From One - Many]

From the article:
Since the fuel is contained in graphite pebbles, the volume of radioactive waste is much greater, but contains about the same radioactivity when measured in becquerels per kilowatt-hour. The waste tends to be less hazardous and simpler to handle.

Thus safe and less hazardous but still able to be manufactured into A-bombs.

[From One - Many]

Also from the article: Proponents point out that this is a plus, as it is difficult to re-use pebble bed reactor waste for nuclear weapons.

Thus difficult but not impossible.