By Jonathan Weisman, USA TODAY

WASHINGTON — Energy Department scientists will begin work next month on a new bunker-busting nuclear weapon that could mark the most significant advance in the U.S. nuclear arsenal in a decade.

Research into a weapon that could penetrate deeply buried structures, such as those designed to make nuclear, chemical or biological weapons, is a key part of President Bush's push to rejuvenate the U.S. nuclear weapons program.

The research project marks a shift from designing weapons of mass annihilation to smaller arms that the administration says would better deter "rogue" states but critics say could make nuclear war more plausible.

Documents from the Energy Department, which oversees nuclear weapons, say Bush also plans to:

Reassemble design teams at the nation's nuclear weapons labs, which disbanded the teams in 1992 after the first President Bush had agreed to a nuclear test moratorium.

Shorten from years to months the lead time it would take to resume nuclear testing.

Ramp up spending on manufacturing sites to build nuclear weapons and components.

"The need is clear for a revitalized nuclear weapons complex," the Pentagon's new review of nuclear strategy says. News organizations have obtained portions of the classified Nuclear Posture Review. Researchers at Lawrence Livermore National Laboratory in California and Los Alamos National Laboratory in New Mexico will determine whether an advanced earth-penetrating nuclear weapon can be built. It would be assembled from existing warheads and components and placed in a 5,000-pound shell.

Everet Beckner, the National Nuclear Security Administration's deputy administrator for defense programs, says the program starts small: There likely will be fewer than a dozen designers at each lab, the "bunker-buster" study will cost $40 million to $50 million over two to three years, and Energy officials will seek congressional approval before designing a weapon.

Bush's father canceled the last major weapons research program, a short-range attack missile warhead, in 1991. He halted all new weapons research in 1992.

President Clinton shifted the nuclear weapons program from research, testing and production to dismantling warheads and ensuring the safety and reliability of older weapons without testing.

The U.S. arsenal has had one type of nuclear "bunker buster" since 1997. Scientists took an existing bomb and refitted it with a hardened nose cone and new tail fins. The aim of the new weapon is to go deeper into the ground to cause less surface damage.