Defense Secretary Donald Rumsfeld said on Tuesday low-yield nuclear weapons may be useful in destroying deadly chemical and biological weapons stocks as he pressed Congress to lift a 10-year ban on research and development of smaller nuclear arms.

The Senate was debating whether to allow research on low-yield weapons with about one-third the force of the bomb dropped on Hiroshima in World War II, which Democrats said would signal the United States was pursuing new battlefield weapons and would spur an arms race....

--Reuters

I've been upset about this for months -- last fall, I linked this story (from Popular Mechanics) about the "tiny nukes." It cites a skeptic, who seems thoroughly reasonable in his doubts:

Rob Nelson, a physicist with the Princeton University Program on Science and Global Security, and an expert on nuclear weapons design, has looked carefully at the relationship between the depth of a primary-powered explosion and geological damage. He argues that the sort of deep penetrator proposed by [Stephen] Younger [of the Defense Department] would, in fact, release rather than contain radioactive fallout. While it is true that most material would remain within the blast area, a radioactive cloud seeping from the crater would release a plume of gases that would irradiate anyone in its path.

He has calculated that a weapon with a yield of about 0.1 kiloton--about one two-hundredth the energy of the bomb dropped on Hiroshima--would have to penetrate to a depth of 230 ft. to fully contain the explosion in the manner that Younger has described. Nelson cautions that if it were used to root out terrorists near a major Third World city such as Baghdad, the casualties could be in the hundreds of thousands.

I'm grateful to Yahoo for pointing me to this Jane's report on the nuclear bunker-busters. It's actually rather sanguine about these weapons -- to the point of being Strangelovian:

How much radiation is acceptable to release into the environment is, of course, debatable. During the era of US nuclear testing, explosions got progressively deeper as the acceptable amount of released radiation was continually reduced. Eventually, nuclear devices were only exploded at depths greater than 1,000 feet because of concerns over low-level seepage. Comparable depths for nuclear bombs dropped from airplanes can never be achieved. Some analysts, however, might argue that such tight standards are not required in a wartime setting.

With that nod to the notion of an "acceptable" level of nuclear fallout in civilian areas, the report goes on to explain just what might happen if such a weapon were used:

In the example above of attacking Tarhunah, a half-kiloton bomb would spread highly radioactive debris over a circle of 300m diameter. The 5kT bomb would do the same over a 700m diameter circle. Both bombs would also release significant fallout that could travel tens of miles before falling to earth. Of course, the crater walls as well as the immediate debris field have also trapped a significant amount of radioactivity that would otherwise land as fallout far from the explosion. If the fallout landed uniformly over a one square kilometre area, the radiation from the half-kiloton explosion would produce three rems per hour, 24-hours after the detonation, while the 5kT bomb would produce 50 rems per hour under the same conditions. These are significant amounts and threaten the health and safety of the populations far from the target. An eight-hour exposure to the larger bomb's fallout would kill about half the people exposed. It is also likely that the exposure levels would be higher from breathing in or otherwise ingesting the fallout, causing even greater harm.

All these calculations assume that the nuclear weapon will survive being driven deep into the earth. In fact, a warhead will see, on average, forces well over 10,000 times the force of gravity as it ploughs through the earth.

Yahoo also links a Federation of American Scientists report. I won't pretend to understand the science in either the Jane's report or this one, but here's the FAS's conclusion:

... the use of any nuclear weapon capable of destroying a buried target that is otherwise immune to conventional attack will necessarily produce enormous numbers of civilian casualties....

it is simply not possible for a kinetic energy weapon to penetrate deeply enough into the earth to contain a nuclear explosion.

The FAS also makes a point that should be obvious:

...by seeking to produce usable low-yield nuclear weapons, we risk blurring the now sharp line separating nuclear and conventional warfare, and provide legitimacy for other nations to similarly consider using nuclear weapons in regional wars.