I usually don't go into issues surrounding nuclear weapons because I don't particularly care to defend them. But there is a rather fun myth going around, usually told as a practical joke. It was tried on me today.
I was observing a technician at one of BP's contractors and he tried to sell me on the notion that the Hiroshima bomb magnetised all the metals on the surface of the planet. As proof of this, he used his screwdriver to lift a screw. Hazaaa!! Both politeness and not thinking about it too much when he said it meant I just nodded.
Apparently, because of this, a tool made from metal recovered from beneath the sea, shielded from this terrible effect somehow, is worth three times as much as a tool made from the horribly deformed magnetised metal from the surface.
The notion, of course, that a nuclear explosion can magnetise anything is bunk. There is such as thing as an electromagnetic pulse, which is a burst of electromagnetic radiation. Metals and electrical equipment can act as antennas for this energy, producing very high voltage power surges, which is damaging to electronics, hence the value of the gamma ray bursts as tactical weapons.
What a bomb can also do is irradiate either through the blast of neutrons or through the dispersal of fallout. Strictly from the untrustworthy grapewine, this irradiation has caused problems for nuclear power stations, whose materials are strictly regulated. The radioactivity in regular metals is too much for it. Any waste from this metal, such as segments of piping, that has not come into contact with any core materials, must be treated a low level waste with all the extra expense that entails.
But magnetising the metals on the surface? No.