New York ? The “dirty bomb” allegedly planned by terror suspect Jose Padilla would have been a dud, not the radiological threat portrayed last week by federal authorities, scientists say.

At a June 1 news conference, the Justice Department said the alleged al-Qaida associate hoped to attack Americans by detonating “uranium wrapped with explosives” in order to spread radioactivity.

But uranium’s low radioactivity is harmless compared with high-radiation materials — such as cesium and cobalt isotopes used in medicine and industry that experts see as potential dirty bomb fuels.

“I used a 20-pound brick of uranium as a doorstop in my office,” American nuclear physicist Peter D. Zimmerman, of King’s College in London, said to illustrate the point.

Zimmerman, co-author of an expert analysis of dirty bombs for the U.S. National Defense University, said last week’s government announcement was “extremely disturbing — because you cannot make a radiological dispersal device with uranium. There is just no significant radiation hazard.”

Other specialists agreed. “It’s the equivalent of blowing up lead,” said physicist Ivan Oelrich of the Federation of American Scientists.

When Padilla was arrested in June 2002, after returning to Chicago from Afghanistan and Pakistan, Attorney General John Ashcroft said the ex-Chicago gang member and Muslim convert had planned a dirty bomb that could “cause mass death and injury.” Washington, D.C., was the likely target, his department said.

But it wasn’t until Deputy Atty. Gen. James Comey’s briefing for reporters last week that authorities said Padilla had uranium in mind for his radiological dispersal device, or RDD, the technical term for such a weapon. Comey said the detainee disclosed he’d also been sent to set off natural gas explosions in U.S. apartment buildings.

“Just saying the word ‘uranium,’ the public automatically assumes, ‘Oh, it sounds bad,”‘ said physicist Charles Ferguson of the Washington office of California’s Monterey Institute of International Studies. He co-authored one of the most detailed reports on the dirty-bomb threat.

Those studying the RDD potential envision a combination of explosives with a lethal radioisotope, such as cesium-137, diverted from use in cancer radiotherapy, for example, or from machines that irradiate food. Particularly if in powder form, it could spew intense radioactivity over a section of a city, making it uninhabitable.

Radiation from uranium, on the other hand, is billions of times less intense than that of cesium-137, cobalt-60 and other radioisotopes. It’s not radioactivity but another property of uranium — its ability in some forms to sustain atomic chain reactions — that makes it a fuel for nuclear power and bombs.

The Justice Department didn’t respond directly when asked this week whether it had consulted with experts and knew that uranium wouldn’t make a dirty bomb.

Instead, spokesman Mark Corallo said Padilla’s statements, in view of his al-Qaida links, made clear that he was “willing to cause devastating harm to innocent Americans.”