Rumsfeld defended the proposed 8.5 million-dollar study of a "robust nuclear earth penetrator" at a Senate hearing after it came under fire from Senator Diane Feinstein, a California Democrat.

Feinstein noted that Energy Secretary Samuel Bodman has acknowledged in previous hearings that no missile could bore deep enough into the earth to trap all fallout from a nuclear explosion.

"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," Feinstein said.

Rumsfeld said more than 70 countries have programs to build facilities underground, and have available to them equipment that can dig chambers the size of a basketball court from rock in a single day.

"We can't go in there and get at things in solid rock underground," Rumsfeld said.

"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.

He said the administration wanted see if it is feasible to develop weapons casings hard enough to penetrate "not with a large nuclear weapon but with either a conventional capability or a very small nuclear capability in the event that the United States of America at some point down the road decided they wanted to undertake that type of project."

"It seems to me studying it makes all the sense in the world," he said.

The proposal has been attacked by arms control advocates as a step toward developing a weapon that would lower the threshhold for the use of nuclear weapons.

Congress last year killed funding for the study, 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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