A lingering aversion to all things nuclear since the end of the Cold War has tamped most official talk about new research and development.

Defense Secretary Donald Rumsfeld (search) has gotten testy in the past when pressed on the issue. He has repeatedly insisted actual development of the weapons is nowhere on the administration's agenda.

"It is a study, it is nothing more and nothing less and it is not pursuing and it is not developing, it is not building, it is not manufacturing, it's not deploying and it's not using," Rumsfeld said in May 2003 of the Robust Nuclear Earth Penetrator (search), or "bunker buster" bomb and the low-yield "mini-nuke."

The RNEP is a B61 or B83 nuclear bomb re-fitted with an extra-hard nose that would allow the weapon to pierce the earth before detonating. It is hardly a new concept: government scientists have been studying them since the end of the Cold War, and under former President Bill Clinton the B61 was altered to penetrate up to 10 feet of frozen soil.

Because RNEPs are modified versions of existing weapons, building them does not violate any of Washington's nuclear treaties. Supporters of RNEPs are quick to point out that they are not new weapons, just "new capabilities." But critics fear that the research will lead to new testing, which may present Washington, D.C. with a host of diplomatic problems.

Mini-nukes are lower-yield weapons (5 kilotons maximum) that are designed for use with precision-guidance technology to allow the military to take out specific, known targets while containing casualties.

"In terms of anthrax, it's said that gamma rays can, you know, destroy the anthrax spores, which is something we need to look at, and in chemical weapons, of course, the heat can destroy the chemical compounds and make them not develop that plume that conventional weapons might do that would then drift and perhaps bring others in harm's way," Gen. Richard Myers, U.S. Joint Chiefs of Staff chairman, said in 2003.

Mini-nukes don't just release radiation, however. A 5-kiloton bomb would cause about a third of the destruction experienced in Hiroshima (search) in 1945.

According to scientists, the biggest obstacles to developing a bomb that could take out a deeply-buried facility are speed — a velocity faster than one kilometer per second would likely destroy the vessel before impact — and the casing, which must be strong enough to keep the bomb intact after hitting the ground.

Dr. Robert Nelson (search), a Princeton University astrophysicist and expert on nuclear weapons policy, told FOXNews.com it is highly unlikely a strong enough material could ever be made to protect the bomb from exploding until well after impact.

In addition, the current bunker-busters in America's arsenal can reach only 10 or 20 feet below ground before exploding — a fact even the weapon's advocates acknowledge.

"You're not getting much better than 30 feet into concrete," Nelson said. "The reason is the materials that can survive the impact stresses don't exist, [the existing materials] just crumple."

"The study is to see if that outer shell can be hardened, if the bomb could go through rock," NNSA spokesman Bryan Wilkes told FOXNews.com. "The answer might be no, but the study — a feasibility study — was stopped, so we might never know."

The weapons aren't supposed to actually reach the bunkers; the shockwaves from the impact should be enough to destroy them. But if the target is more than 1,000 feet underground, a 100-kiloton-yield bomb — 10 times the size of the bomb dropped on Hiroshima — would be required.

Government officials contend that the damage from an RNEP sturdy enough to reach such a deep destination would be largely restricted to below the earth's surface. But opponents say such a feat is impossible.

"You might be able to drill down with a low-yield weapon, but [not far] enough to contain the blast," Nelson said. "There's no such thing as a clean surgical strike — it's dirty."

Joseph Cirincione (search), director of the Non-Proliferation Project at the Carnegie Endowment for International Peace, said that while the RNEP would cause less ground damage than the bombs America dropped on Japan, the radiation fallout would be much greater.

"Nuclear fallout increases the deeper you go with a nuclear weapon, because it throws up hundreds of tons of debris," Cirincione said. "Radiation increases with increased ground penetration because all that earth is then irradiated, thrown up and scattered downwind."

An Energy official who asked not to be named acknowledged "some fallout" was inevitable, but told FOXNews.com that the consequences of deploying a nuclear weapon are the reasons the threshold for using them is so high.

The RNEP's critics believe that conventional weapons are better suited to the task of disabling underground bunkers.

"In order for a small nuclear weapon to work on an underground bunker, you need to have excellent intelligence on where it is," Cirincione said. "Without it, you wouldn't risk using the nuclear weapon, and if your intelligence is that good you wouldn't need a nuclear weapon to begin with."

Both Cirincione and Nelson cited studies showing that conventional weapons would be as effective in disabling underground facilities and would cause fewer casualties. Non-nuclear munitions and materials could be used to seal entrances and exits, block ventilation and cut off power and water supplies. Thermobaric weapons can also be used to destroy biological and chemical weapons with minimal explosive possibility, Nelson said.

But advocates for the weapons research say those points may be irrelevant in the new War on Terror.

"Those who have chosen to de-fund these programs are putting this country at serious risk," argues Jack Spencer of the Heritage Foundation.

"The critics worry about proliferation, but what they don't understand is ... these programs allow us to build down our nuclear weapons force," Spencer said. "Can we credibly convince [North Korean dictator] Kim Jong Il that his behavior could result in us annihilating 20 million of his people? We have to convince him that his behavior will result in the complete destruction of his territory."

Rumsfeld in 2003 told the Senate Appropriations Defense Subcommittee that pursuing the RNEP and mini-nuke would have no effect on anti-proliferation efforts.

"The idea that our studying a ... nuclear deep-earth penetrator is going to contribute to proliferation, I think, ignores the fact that the world is proliferating ... It is happening without any studies by us," he said.