[MereInterest]

That’s true, but it’s more the timescale that helps. There’s a decent amount of radioactive background produced by cosmic rays hitting the upper atmosphere, much of it as gaseous elements that are easily incorporated into steel while smelting. It isn’t harmful at that level, but you do need to wait a few decades for those to decay away.

World War Two is rather convenient in that respect, as there are large quantities of steel that were left to sit around for several decades after those ships sank.

It’s been a while since I’ve done low-background gamma-ray spectroscopy, but I believe there were some setups that went even further, using lead that had been smelted by the Romans. That way, any contamination present at the time of smelting would have a few thousand years to decay away.

[HyperSane]

Wikipedia says that for the lowest radiation levels high-purity copper is used.

[MereInterest]

That would make sense, since the longest lifetime of an unstable copper isotope is ~13 hours, and so chemical separation plus a week of waiting would give you a low background. By comparison, iron has Fe-60, a naturally occurring gamma-ray emitter with a 2.6M year half-life, and 10 million years is too long to wait. For iron shielding, you'd need isotopic separation to remove the Fe-60, which is wicked expensive.