According to a report by the National Research Council in Washington, the plan to develop a nuclear weapon that could penetrate the earth and destroy underground bunkers is seriously flawed.

The report says the weapon could not go deep enough to eliminate fallout, as some advocates have asserted, and estimates that -- depending on weather conditions -- as many as one million people could be killed or affected by radiation.

John F. Ahearne, an expert on nuclear arms who headed the 15-member committee that wrote the report, said an earth-penetrating weapon "could kill a devastatingly large number of people." The report also says that trying to reduce fallout and civilian damage by making a smaller weapon was impractical because its destructive force would be insufficient to destroy military targets.

The report's conclusions agree with criticisms made by experts outside the government, but it draws upon secret federal studies and carries the political weight of the National Research Council of the National Academies -- the leading US scientific advisory group.

In Washington, debate over the Robust Nuclear Earth Penetrator, known as the bunker buster, has been underway for more than three years. Critics contend that aside from the danger of radioactive fallout, military intelligence can never ensure that the weapons hit the right targets and that they might fail to work. They also say that the weapon could create an illusion of limited consequences that could lead to a wider nuclear conflict.

The new study was mandated by the US Congress in 2003. The report claims that if a warhead penetrates ten feet into the earth before detonating, much of its energy will go into the ground, forming a shock wave that can destroy underground structures.

But it also found that attacking bunkers at depths of 650 feet would require a blast of 300 kilotons, or 20 times larger than the bomb that destroyed Hiroshima. A target 1,000 feet deep would require a weapon 67 times as large.

The report concludes that the size of the weapons means they would produce heavy radioactive fallout and cause deadly damage.