The congressionally-mandated study comes as US Defense Secretary Donald Rumsfeld is pressing Congress for 8.5 million dollars to study the feasibility of designing nuclear weapons casings hard enough to bore through rocket and concrete before detonating.

Conducted by the National Research Council of the National Academy of Sciences, the study found that earth-penetrating nuclear weapons could effectively destroy deeply buried bunkers with a smaller bang than nuclear weapons detonated on the surface.

But, said John Ahearne, head of the committee that produced the report, "Using an earth-penetrating weapon to destroy a target 250 meters deep -- the typical depth for most underground facilities -- potentially could kill a devastatingly large number of people."

Many of the more important strategic hard and deeply buried targets are beyond the reach of conventional weapons and can only be held at risk with nuclear weapons, the report said.

Earth-penetrating weapons are being considered because they can destroy facilities deep underground using up to 25 times less energy than a nuclear weapon detonated on the surface, according to the report.

But they cannot be detonated deeply enough to avoid mass casualties on the surface, according to the study.

"For attacks near or in densely populated urban areas using nuclear earth-penetrator weapons on hard and deeply buried targets (HDBTs), the number of casualties can range from thousands to more than a million, depending primarily on weapon yield," the report said.

"For attacks on HDBTs in remote, lightly populated areas, casualties can range from as few as hundreds at low weapon yields to hundreds of thousands at high yields and with unfavorable winds," it said.

Destroying a bunker buried 200 feet (roughly 70 m) underground would require a 300-kiloton earth-penetrating weapon. But a one-megaton weapon would be needed to destroy a target 300 feet (100 m) down, the study said.

"Current experience and empirical predictions indicate that the earth-penetrator weapons cannot penetrate to depths required for total containment of the effects of a nuclear explosion," the study said.

Nevertheless, the administration says it wants to study the technical feasibility of making nuclear weapons cases that can bore through rock before detonating.

"It is beyond me as to why you are proceeding with this program when the laws of physics won't allow a missile to be driven deeply enough to retain the fallout which will spew in hundreds of millions of cubic feet if it is a hundred kilotons," said Senator Diane Feinstein, a Democrat, told Rumsfeld Wednesday.

Rumsfeld replied: "It seems to me studying it makes all the sense in the world."

He argued that more than 70 countries now have programs to build facilities deep underground.

"The only thing we have is very large, very dirty nuclear weapons. So the choice is: do we want to have nothing and only a large, dirty nuclear weapon, or would we rather have something in between. That is the issue," he said.

Congress last year killed funding for a study on the technical feasibility of making nuclear weapons cases that are hard enough to bore through rock.

But the administration has requested resumed funding in its 2006 budget proposal. Besides 8.5 million dollars in 2006, the proposal calls for another 14 million dollars to complete the study in 2007.

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