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Old 23rd century type impulse packs (part of the impulse engines) were insulated with an element that could decay to lead. When a ship had been through trauma it was best to check the points of the packs, because if they had decayed, when activated they could take out the entire impulse deck.
Lead was mentioned in the final draft script of TOS: "The Corbomite Maneuver" (dated 3 May1966), as that script referred to the Fesarius as being "in the shape of a round cluster of [...] lead shot." This allusion was written out of the teleplay by the time a second revised final draft of the script (dated 20 May 1966) was issued.