Low-background steel is steel produced prior to the end of World War II. Following Atomic bombing of Hiroshima and Nagasaki, and the nuclear weapons testing during the Cold War, world background radiation levels increased 7% across all radioactive elements. Steel produced during or after this testing became contaminated with certain radionuclides and consequently, is unsuitable for certain radiation-sensitive purposes. However, anthropogenic background radiation levels have fallen again since the Partial Nuclear Test Ban Treaty - The man-made radioactive elements in the air have decayed to a 0.5% increase on naturally occurring levels.
From 1856 until the mid-20th century, steel was produced using the Bessemer process, in which air is blown into a blast furnace in order to convert Pig iron into steel. Since the mid-20th century, the Bessemer process has been supplanted by the BOS process, which uses pure oxygen instead of air.
Both processes are susceptible to contamination from airborne dust. Present-day dust carries radionuclides, particularly cobalt 60, which are deposited in the steel, making it weakly radioactive. Cobalt-60 may also contaminate steel when used as a tracer or when radioactive sources are mixed into the scrap metal supply.
Certain techniques and devices require very low radiation materials. Geiger counters, medical applications (Whole body counting and Lung counters) and physics applications (photonics) frequently require an extremely low radiation environment, called a Low background counting chamber. A low background counting chamber is a room built with extremely heavy radiation shielding made from low-background steel.
Naval vessels constructed prior to the Cold War are a primary source of low-background steel. Chief among these are reserve fleets, the German fleet scuttled at Scapa Flow, and Nazi U-boats scuttled as part of Operation Deadlight.
Decreasing background radiation
World anthropogenic background radiation levels peaked at 0.15 mSv above natural levels in 1963, the year that the Partial Nuclear Test Ban Treaty was enacted. Since then, anthropogenic background radiation has decreased exponentially to 0.005 mSv per year above natural levels.
- Is steel from scuttled German warships valuable because it isn't contaminated with radioactivity? Straightdope. December 10, 2010.
- Reducing Risks in the Scrap Metal Industry - Sealed Radioactive Sources IAEA. Sept 2005. IAEA/PI/A.83 / 05-09511
- United Nations Scientific Committee on the Effects of Atomic Radiation