A report on NPR this morning spoke of removing “unexploded ordnances” in Vietnam. This immediately sent me to dictionaries to check my understanding of “ordnance.” The word refers to canons, artillery, or munitions generally, and it may, according to some sources, refer as well to related matériel.
In any case, “ordnance” is a collective noun. We normally don’t speak of one ordnance or many ordnances. An artillery shell is ordnance. A pile of artillery shell is also ordnance.
The NPR reporter Michele Kelemen was clearly wrong in her use of the phrase “unexploded ordnances.” In fact, it is difficult to imagine when “ordnances” might be properly used, though one can imagine comparing the military resources of two countries and remarking on the differences in their “ordnances.” Even that might be a stretch.
As an aside, I should note that “ordnance” should not be confused with “ordinance” (a decree, regulation, etc.), a word that is often used in the plural. It is amusing, however, to think about what “unexploded ordinances” might be.