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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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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