O.K., found the law, if anyone is interested.
§ 2332a. Use of weapons of mass destruction
(c) Definitions.— For purposes of this section—
...
(2) the term “weapon of mass destruction” means—
(A) any destructive device as defined in section 921 of this title;
(B) any weapon that is designed or intended to cause death or serious bodily injury through the release, dissemination, or impact of toxic or poisonous chemicals, or their precursors;
(C) any weapon involving a biological agent, toxin, or vector (as those terms are defined in section 178 of this title); or
(D) any weapon that is designed to release radiation or radioactivity at a level dangerous to human life”
The problem here is that ”(A) any destructive device as defined in section 921 of this title”:
§ 921. Definitions
(4) The term “destructive device” means—
(A) any explosive, incendiary, or poison gas—
(i) bomb,
(ii) grenade,
(iii) rocket having a propellant charge of more than four ounces,
(iv) missile having an explosive or incendiary charge of more than one-quarter ounce,
(v) mine, or
(vi) device similar to any of the devices described in the preceding clauses;
So while the military and most of the world means nuclear, chemical, radiological, or biological when they say WMD, the U.S. law says anything that goes boom. Anything at all.