Military watchdog groups are worried about plans to detonate a gigantic conventional explosion at the Nevada Test Site on June 2, calling it a possible prelude to resumption of nuclear tests.

The experiment is called "Divine Strake," in which 700 tons of ammonium nitrate and fuel oil will be blown up. (A strake is a line of metal plating along a ship's hull.)

The explosive material, similar to that used by domestic terrorists to destroy the Murrah Federal Building in Oklahoma City in 1995, will explode with a force equivalent to 593 tons of TNT. It is expected to raise a mushroom cloud of dust, but officials say it won't be visible off the test site.

"There is no danger to the population of Las Vegas and the surrounding communities," says an agency release. "The test does not use a nuclear device, and it does not test a weapon."

An environmental assessment dated November 2005 is on file with the state of Nevada's clearing house. It says besides the explosives, two tracer compounds would be used: Glo Germ Powder and Fluorescein USP.

Glo Germ Powder would be placed on tarps surrounding the charge hole in order to see how material disperses during the test, says the statement. Glo Germ Powder is "considered to be hazardous if it is burned, and toxic gases can be formed," the environmental statement says.

"The powder would not be mixed in the . . . blasting agent so it would not be subjected to the oxidizing effects of the detonation."

The environmental assessment also says Dugway Proving Ground in western Utah was considered as a possible site for the blast. It and other alternative sites "were eliminated because of the need to conduct the detonation in a limestone bed with specific geological properties," says the statement.

The experiment is to assess the capability of computer modeling to predict ground shocks and the response of a tunnel to the blast. The tunnel involved has no radiation and has not been used in nuclear testing, according to the agency.

"Better predictive tools will reduce the uncertainties involved with defeating very hard targets, and therefore reduce the need for higher yield weapons to overcome those uncertainties," adds the release.

All indications are that this is part of the Robust Nuclear Earth Penetrator program, nicknamed the "bunker buster" program, said Steve Erickson, director of the Salt Lake City military watchdog group Citizens Education Project.

"We expected there'll be one further test" later, he added.

The purpose of the test is to "determine what it would take for a small penetrating nuclear warhead to collapse a hardened bunker," Erickson said.