Uranium.

This is coolbert:

Merely that ISIL has gotten their hands on uranium enough to cause consternation in some quarters.

"ISIS Has Seized 88 Pounds Of Uranium In Northern Iraq"

The Islamic State (the terrorists formerly known as ISIS) have stepped up their game and come out swinging... by seizing 88 pounds of nuclear material (uranium compounds) from Mosul University. As Iraq's UN Ambassador warned, 'these nuclear materials, despite the limited amounts mentioned, "can be used in manufacturing weapons of mass destruction... or in combination with other materials in its terrorist acts.'"
. . . .

"Nearly 40 kilograms (88 pounds) of uranium compounds were kept at Mosul University, Iraq's U.N. Ambassador Mohamed Ali Alhakim told U.N. Secretary-General Ban Ki-moon in the July 8 letter"

That weapon of mass destruction presumably the "dirty bomb". A radiological device, a conventional explosion creating a spew of radioactive elements spread over a wide area, contamination requiring a massive and unprecedented clean-up of questionable effectiveness.

Uranium in the raw form of itself having a background radiation level but enrichment through processing as needed for an effective "dirty bomb" beyond the capability of ISIL?

coolbert.