Salted versions of both fission and fusion weapons can be made by surrounding the core of the explosive device with a material containing an element that can be converted to a highly radioactive isotope by neutron bombardment.
The radioactive isotope used for the fallout material would be a high-intensity gamma ray emitter, with a half-life long enough that it remains lethal for an extended period.
[5][6] Physicist W. H. Clark looked at the potential of such devices and estimated that a 20 megaton bomb salted with sodium would generate sufficient radiation to contaminate 200,000 square miles (520,000 km2) (an area that is slightly larger than Spain or Thailand, though smaller than France).
He publicly sounded the alarm against the possible development of salted thermonuclear bombs capable of annihilating mankind on a University of Chicago Round Table radio program.
[10][11] His comments, as well as those of Hans Bethe, Harrison Brown, and Frederick Seitz (the three other scientists who participated in the program), were attacked by the Atomic Energy Commission's former Chairman David Lilienthal, and the criticisms plus a response from Szilard were published.
[12] In his 1961 essay, Clark suggested that a 50 megaton cobalt bomb did have the potential to produce sufficient long-lasting radiation to be a doomsday weapon, in theory, but was of the view that, even then, "enough people might find refuge to wait out the radioactivity and emerge to begin again.