The Bush administration this week is expected to announce a major step in the building of the country's first new nuclear warhead in nearly...

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WASHINGTON — The Bush administration this week is expected to announce a major step in the building of the country’s first new nuclear warhead in nearly 20 years.

It will propose combining elements of competing designs from two weapons laboratories in an approach some experts argue is untested and risky.

The new weapon would replace the nation’s existing arsenal of aging warheads with a new generation meant to be sturdier, more reliable, safer from accidental detonation and more secure from theft by terrorists.

The announcement, to be made by the interagency Nuclear Weapons Council, avoids making a choice between the two designs for a new weapon, the Reliable Replacement Warhead, which at first would be mounted on submarine-launched missiles.

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The effort, if approved by President Bush and financed by Congress, would require a huge refurbishment of the nation’s complex for nuclear design and manufacturing, with the bill estimated at more than $100 billion.

But the council’s decision to seek a hybrid design could delay production. It also raises the question of whether the U.S. will be forced to end its moratorium on underground nuclear testing to make sure the new design works.

On Friday, Bryan Wilkes, a spokesman for the National Nuclear Security Administration of the Energy Department, said the government would not proceed with the Reliable Replacement Warhead “if it is determined that testing is needed.”

But other officials, including Robert Joseph, undersecretary of state for arms control and international security, have said the White House should make no commitment on testing.

Congress authorized exploratory research for the weapon three years ago and has financed it at relatively low levels since. But now the costs will begin to increase.

The two teams competing to design the weapon, one at Los Alamos in New Mexico and the other at the Livermore National Laboratory in California, approached the problem with different philosophies, nuclear officials and experts said.

Livermore drew on a single design that was detonated in the 1980s. The weapon, however, never entered the nuclear stockpile.

The Los Alamos team drew on aspects of many weapons from the stockpile and pulled them together in a novel design that has never undergone testing.

A winner of the competition was to have been announced in November. But federal officials said they had a hard time choosing between the two designs, calling both excellent.

The question now, arms experts said, is whether an approach that combines the two will produce a clever hybrid or an unworkable dud.