Such a weapon has long been sought by nuclear weapons scientists and some military strategists, including key members of the Bush administration, as a way of reaching targets that are hidden deep underground without incurring huge collateral damage. Advocates also say that by developing such smaller nuclear weapons, the United States could safely reduce its current stockpile of 6,000 much more powerful warheads.

Interest in low-yield weapons has been rising with concern that Iraq's Saddam Hussein could hide his biological and chemical arsenals in underground bunkers. Another hardened target that has drawn attention is Russia's long-term construction of a nuclear war command center under Yamental mountain.

One senior adviser to Defense Secretary Donald H. Rumsfeld said that the Iraqi leader would not be deterred by current U.S. nuclear weapons "because he knows a U.S. president would not drop a 100-kiloton bomb on Baghdad" and destroy the entire city and its population to reach his weapons of mass destruction.

The prospect that the Pentagon would recommend that the Bush administration develop a new, low-yield nuclear weapon has become the focus of attention for groups committed to traditional arms control. The Federation of American Scientists (FAS) plans to release a report this week that argues "that adding low-yield warheads to the world's nuclear inventory simply makes their eventual use more likely."

A report on the Pentagon study is to be sent to Congress in July. Seven years ago, Congress barred research and development of a low-yield precision-guided nuclear weapon, out of concern that it would blur the line between conventional and nuclear weapons.

But an amendment last year to the defense authorization bill by Sens. John W. Warner (R-Va.) and Wayne Allard (R-Colo.) required the Pentagon to study how to defeat hardened and deeply buried targets. The Defense Department was specifically asked to determine what weapons might be needed, including low-yield nuclear devices. The Energy Department, which controls the nuclear labs, is assisting the Pentagon.

The July report is due at the same time a review of U.S. strategic nuclear deterrence policy, ordered by Rumsfeld, could be completed. That study deals with offensive and defensive systems, nuclear as well as conventional, administration sources said.

In a paper presented last month, Paul Robinson, head of Sandia Nuclear Laboratories, said he believed "low-yield weapons with highly accurate delivery systems" would be desirable "for deterrence in the non-Russian world." Robinson, however, said the devices could help decision-makers "contemplate the destruction of some buried or hidden targets while being mindful of the need to minimize collateral damage."

Stephen M. Younger, chief of nuclear weapons research at Los Alamos National Laboratory, suggested in a paper last summer that accurate, low-yield nuclear weapons could be better suited to attacking buried, concrete bunkers and mobile missiles than today's U.S. arsenal of silo-busting weapons that each have the explosive power of 30 Hiroshima bombs.

To destroy moderately hard targets, such as missile silos, Younger urged the development of low-yield weapons to be placed on highly accurate missiles. A new, five-kiloton warhead -- with less than half the destructive power of the Hiroshima bomb -- would vaporize a 30-foot-thick silo door if it were delivered by a precise missile, he wrote.

A new nuclear bomb has not been developed in the United States since the 1980s, and nuclear testing was halted in 1992. Each year the Energy Department spends about $4.5 billion in its stockpile stewardship program that keeps warheads safe and secure. Tiny elements of nuclear materials are exploded in "sub-critical" tests, which are allowed under the testing moratorium because they do not create a nuclear chain reaction.

Because many tested U.S. weapon designs exist from the period before the moratorium, one senior U.S. weapons scientist said recently that a low-yield weapon could be developed without testing. He added that with information developed on earth penetration for the Pershing II intermediate-range missile in the 1980s and the B-61 more recently, "we could build [a low-yield earth penetrator] tomorrow; it is not hard to do."

Critics say such a weapon would not be able to penetrate deep enough to keep radioactive debris from getting into the atmosphere. The FAS study, by Princeton University theoretical physicist Robert W. Nelson, argues that "in order to be fully contained, nuclear explosions at the Nevada Test Site must be buried at a depth of 650 feet for a five-kiloton explosive."

Based on that analysis, Nelson concludes: "This mission does not appear possible, without causing massive radioactive contamination. No American president would elect to use nuclear weapons in this situation -- unless another country had already used nuclear weapons against us."

The government nuclear weapons scientist said a one-kiloton warhead would have to dig down only 175 feet for its radioactive material to remain contained.

© 2001 The Washington Post Company

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