Industrial smelters are big expensive bits of kit.

Radiation monitors are pretty cheap - at least ones sufficient to identify a Cesium 137 source in a pile of scrap steel.

Radiation in your steel is a hugely costly mistake - at a minimum, you're gonna have to give up all your costly smelting equipment and stock.

Radiation isn't super rare either - major events may be rare, but every few weeks a big smelter will detect small things that ought not be smelted.

So I don't see why any smelter wouldn't have a radiation detector on the input hooked up to warning sirens, for purely financial self interest.