> Low-background steel is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s. With the Trinity test and the nuclear bombings of Hiroshima and Nagasaki in 1945, and then subsequent nuclear weapons testing during the early years of the Cold War, background radiation levels increased across the world. Modern steel is contaminated with radionuclides because its production uses atmospheric air. Low-background steel is so-called because it does not suffer from such nuclear contamination. This steel is used in devices that require the highest sensitivity for detecting radionuclides.
Apparently one source of low-background steel are ships built before 1945, especially military ships as they are large and can be in use for decades. The US navy has scraped a few old ships for this use and there are theories that some WW2 wrecks are being salvaged from the ocean bed for this purpose, but there is no conclusive evidence.
And not just before 1945: lead is used for radiation shielding, but freshly-mined lead is slightly radioactive itself... shipments of lead that were mined 2000 years ago and sitting at the bottom of the Mediterranean since then, much less so: https://www.nature.com/news/2010/100415/full/news.2010.186.h...
Not a lot globally and at this timescale. Radionuclides is about it.
Nuclear bombs put energy and radioactive particulates into the atmosphere. The effects of the radioactive particulates are fairly well understood. The amount of energy from even the largest bombs is negligible at those scales.
