A joint report of the U.S. Departments of Defense and Energy estimates that more than 10,000 potential hardened and deeply buried targets worldwide contain crucial infrastructure and possibly chemical or biological weapons. Although many of these targets are vulnerable to conventional weapons, hundreds are fortified below 25 to 100 meters of concrete. Nuclear weapons are the only sure means to defeat these strongholds, some defense analysts say, calling for a new generation of weapon: a low-yield, earth-penetrating warhead that would deliver a knockout blast without releasing plumes of deadly radioactivity. But such weapons, various physicists argue, are not technically feasible.

"Earth-penetrating weapons cannot penetrate deeply enough to contain the nuclear explosion and will necessarily produce an especially intense and deadly radioactive fallout," concludes Robert W. Nelson of the Program on Science and Global Security at Princeton University. In a paper to appear this summer in the journal Science and Global Security, Nelson calculates that a one-kiloton, earth-penetrating "mini nuke" used in an urban environment such as Baghdad would spread a lethal dose of radioactive fallout over several square kilometers and result in tens of thousands of civilian fatalities. Regardless of its impact velocity or its construction material, no missile can penetrate reinforced concrete more than about four times its length, Nelson calculates, a number supported by data he received from Sandia National Laboratories via the Freedom of Information Act.

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