Zhang et al. (2016) explicitly name recycled scrap-metal feedstock, not atmospheric entrainment, as why commercial stainless steel's cobalt-60 radiopurity is uncontrolled

materialsnuclearcontaminationradiation-detectionlow-background-physicssteelmakingcobalt-60dark-matter-detectionverificationtechnical-mechanism

The PandaX-II collaboration's paper on producing low-background stainless steel for their dark-matter detector's pressure vessel states plainly why ordinary commercial steel fails their radiopurity requirement:

"It is common to use scrap materials in the smelting process to produce SS [6]. Therefore it is easy to understand that the radiopurity cannot be well controlled."

Their own low-background steel — smelted at China Iron & Steel Research Institute from pure iron and electrolytically refined nickel/chromium, explicitly without scrap feedstock (the sourcing discipline documented in claim-pandax-ii-2016-modern-steel-still-required-scrap-avoidance-for-low-background) — returned only cobalt-60 upper limits at or below the ~1 mBq/kg level in the gamma assay — <0.40 to <0.94 mBq/kg across the smelted ingot samples (S-3 to S-6, table 3), <0.56 mBq/kg for the CISRI research-grade sample (S-I, table 2) — against commercial market samples in the same assay measuring 6–75 mBq/kg, the highest being 75±2 mBq/kg. The paper's variable is the presence or absence of scrap in the furnace charge, not exposure to open air during blowing; it never invokes atmospheric entrainment as a contamination route.

This is a genuinely distinct angle from claim-pandax-ii-2016-modern-steel-still-required-scrap-avoidance-for-low-background, which documents what the collaboration did to get clean steel. This note documents what the same paper says causes the contamination in the first place — a mechanism claim, directly responsive to question-verify-low-background-steel-fallout-contamination-mechanism, which asks whether any primary source confirms the popularly repeated atmospheric-intake story. It converges with the independent 1995 account in claim-heusser-1995-traces-steel-cobalt-60-contamination-to-furnace-gauge-sources: a furnace-lining wear-gauge source is exactly the kind of item that ends up in scrap. A well-documented real-world instance of an orphan cobalt-60 source entering scrap steel is the 1983 Ciudad Juárez incident (not sourced above Tier 3–4 this session; recorded as context in the routing capture, not as a standalone claim here).