This topic has also been covered by Rachel Maddow and Randi Rhodes on Air America. The Bush Administration wants to test their underground bunker busting nuclear bomb for use in Iran. However, they are forbidden by treaties from testing nuclear weapons. Therefore, they are going to set off the largest conventional bomb ever to simulate the effects of the new nuclear weapon. The concern is that the blast will create a tall mushroom cloud that may spread radioactive dust from the nearby test sites used in past decades for nuclear bomb tests. The test has been temporarily delayed by a lawsuit. Here's an article:
This topic was also covered in Time Magazine in April.
http://www.gallupindependent.com/2006/apr/041006fallout.html"Concerns over blast mushroom
Utah lawmaker is worried about fallout from explosion in Nevada
U.S. Rep. Jim Matheson ...wants assurance that a 700-ton blast planned for June 2 at Nevada Test Site will not disperse radioactive remnants from previous nuclear tests into the atmosphere.
Matheson said though he understands that the June 2 "Divine Strake" test is not a nuclear test, "I am greatly concerned that you have not provided the public with adequate assurances that the test is not being conducted in order to further misguided attempts to build new low-yield nuclear devices.
"You are well aware that at 700 tons (595-ton equivalent yield) this demonstration will not simulate an actual conventional bomb because no bomber in the U.S. fleet has the capacity to carry a weapon of this size," he said... Congressmen were concerned in 2003 when Congress "mistakenly repealed" the Spratt-Furse ban on research and development of low-yield nuclear weapons, Matheson said, because they felt that advocates for repealing the ban were yielding to those who actively support the development of new nuclear weapons. At the time, the Department of Defense and the National Nuclear Security Administration assured Congress that the ban should be repealed because it hindered research efforts and that no actual weapon was being stealthily developed.
...Matheson said another cause for concern is that DTRA recently confirmed that Divine Strake is the Tunnel Target Defeat Advanced Concept and Technology Demonstration specified in the agency's FY2007 budget. He said the budget document states that the demonstration"will develop a planning tool that will improve the warfighter's confidence in selecting the smaller proper nuclear yield necessary to destroy underground facilities while minimizing collateral damage." Matheson said, "That sounds like preparation for a low-yield nuclear weapon to me."