Why does when the steel was produced affect the background radiation? A girder that existed in 1938 still existed in the 40’s and 50’s, why is it not contaminated but new steel was/is?
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Part of making steel is blowing air up through the boiling metal to have the free oxygen in the air remove some of the impurities of the metals, particularly carbon content which turns into CO2.
Unfortunately if your air contains radioactive cobalt-60 some will remain in your steel making your steel mildly radioactive
Nuclear tests starting in the mid 40s significantly increased the quantity of radioactive elements in the air so that any steel made with free air would be radioactive enough that it would be a constant false positive for something like a Geiger counter
Low background steel was mainly from old warships like the German fleet from WW1 that was scuttled in Scapa Flow Scottland. Since this steel was already turned from iron into steel it didn’t have to have the air blown through it again and didn’t pick up excess radioactive dust and could be used for sensitive equipment
>A girder that existed in 1938 still existed in the 40’s and 50’s, why is it not contaminated
It is, but only on the surface. You can grind off the surface and the inside metal from 1938 won’t have ever been contaminated by radiation.
Steel made after the 50s/60s is radioactive *all the way through*, because the radioactive contamination was *mixed into the molten metal itself* while the steel was being made.
Steel production uses oxygen from the atmosphere to drive out impurities from the iron ore. The air today still has a lot of radioactive elements left over from nuclear-related activity (mostly bombs) in the last century. As a result, new steel is contaminated with those elements. Yes, we are breathing these elements in all the time. But the amounts are low enough that it doesn’t cause (much) harm to humans, but it’s still high enough to cause issues for sensitive equipment made using steel.
We can make new low-background steel today but it’s expensive. You have to expend a lot of energy to filter/clean the air of radioactive contaminants. For now it’s still cheaper to reclaim it from old sunken warships.
Some answers focus on atmospheric contamination, but there are other very important factors.
A big factor in reduced radioactivity in steel is better control and monitoring of radioactive sources that used to end up getting into scrap- recycling
Nearly all the steel made in the last 80 years includes some recycled steel. “virgin” steel (using iron ore straight from the mine without recycled scrap added) isnt particularly necessary for any industrial reason so a mill trying to make virgin steel can’t compete for price.
Radioactive sources used to get into the scrap recycling market a lot. Nowadays, If you go to a scrap yard, you’ll see that at the entry weigh station there are a couple of very large boxes on either side of the scale. These contain giant radiation detectors. There are a lot of industrial and medical uses of very high radiation sources. For instance “moisture density gauges” (troxler gauges) which are used in road construction. Road crews aren’t always super good with their paperwork, especially in countries that are still developing. So these get lost and eventually wind up being sold as scrap metal. (*Everything* that’s made mostly of iron copper or aluminum eventually winds up in a scrap yard). From there it goes to a steel mill where it contaminates a hundred tons of steel. When that steel gets recycled it just ends up contaminating a thousand tons of steel, etc.
So lost sources that got scrapped in the 1950s ended up contaminating new steel for decades because the radioactive steel would just keep going back in via recycling. It’s getting better because of dilution with new iron ore and also because these sources use isotopes with half lives of 10s of years. And most countries now are doing a better job of keeping track of the sources and monitoring the scrap metal.
But it’s probably still the case that some insanely stupid Soviet cold-water relic in Siberia is occasionally getting chucked into a scrapper with broken radiation detectors in Russia and recontaminating the world steel supply. Look up “Soviet nuclear powered lighthouse” if you want a good scare.
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