There are various isotopes that cannot themselves initiate chain reactions, but that can be used to boost or moderate an ongoing explosion. Tritium and reactor-grade enriched uranium, for example, both have weapon uses, but neither is useful by itself for creating a nuclear weapon.
Also, please note that just because a material is "weapons grade," that does not mean terrorists or rogue states can actually make a weapon out of it. Plutonium is relatively easy to make—there's no doubt that Iraq and North Korea both have pounds and pounds of the stuff. But it's extremely difficult to build an actual bomb out of plutonium—making a big mess is easy, but getting a true critical mass takes precision mathematics and engineering that only a handful of countries possess.
On the flip side, any college physics student could make a uranium bomb, but weapons-grade enriched uranium is extremely difficult to make. So the only "weapons-grade" material we really need to worry about is enriched uranium. But since weapons-grade uranium doesn't have very many uses, and is so difficult to make, very little of it is produced, which means there isn't much lying around to steal and it's easier to keep an eye on. Aside from the Chelyabinsk incident (the details of which are too sketchy to evaluate), no one has indicated that Russia is missing the 10-15kg of HEU required to build a nuclear device. The actual risk of terrorists setting off a true nuclear explosion is very small. The "Axis of Evil" states will be able to manage it on their own if we leave them alone for another few years, but that's about it.
That's not to say somebody equipped with a pound of plutonium couldn't make a hell of a mess, just that it wouldn't be any more devastating than, say, a few hundred tons of jet fuel or a pound of weaponized anthrax.