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.

Well this is telling you to have any metals that have been passed though Thailand ran under a geiger counter. At least in the US any major recyclers perform this operation to prevent radioactive materials showing up in the product stream.