WASHINGTON, Jan. 29 — A successful terrorist attack on a spent fuel storage pool at a large nuclear reactor could have consequences “significantly worse than Chernobyl,” according to a new scientific study. But it said the risk could be cut sharply by moving some of the spent fuel to dry casks near the reactors and making changes in how the rest is stored.

The report, which will be published this spring in a scientific journal of Princeton University, is one of the few broad analyses of the risk posed by spent fuel that is being made public. Because there is no long-term storage site for nuclear fuel, the risk it poses would persist for years even if the reactors where it is now stored are shut down, as some critics are seeking for the Indian Point plants in New York.

Many reactor operators have already moved some fuel to dry casks because their pools have filled up as the federal program to bury the fuel has slipped further into the future. Burial can be done only with fuel that is more than five years old, because newer fuel gives off so much heat that it must be kept in water. The older fuel can be kept dry because air will safely dissipate its heat. It would cost $3.5 billion to $7 billion to move old fuel to dry casks, the authors predicted.

But if the federal government opens a burial site at Yucca Mountain near Las Vegas, as it says it will in about a decade, some of that money will have to be spent anyway to put fuel in casks for shipment to a permanent burial site.

The paper will appear in the spring issue of Science and Global Security, a journal at Princeton. Some of its eight authors began briefing federal officials in Washington today. Their recommendations include reinforcing the casks to make them less vulnerable and designing them so that if a large airplane were crashed into them, the plane’s fuel could not pool around them and overheat them as it burned.

The nuclear industry has generally argued that a successful attack is highly unlikely. At the Nuclear Energy Institute, the industry’s trade association, Stephen D. Floyd, a senior director, said that an aircraft could not do the job, and that adversaries could not assemble a larger attack without attracting attention.

Security improvements, Mr. Floyd said, should be limited to those that address realistic threats. “Otherwise you’re going to chase your tail and spend this country out of existence on what-if scenarios,” he said.

Today, two of the authors briefed one of the five members of the Nuclear Regulatory Commission, Edward McGaffigan Jr., who said the study overstated the possible effects. At Indian Point, Mr. McGaffigan said, even if terrorists could puncture a spent fuel pool, it would be very difficult to drain the water because the fuel is almost entirely below ground.

“If you’re at all rational as an Al Qaeda planner, you don’t choose this target where you have a probability of failure,” he said. “You have other targets where you have a higher probability of success.”

But the authors, reviewing European tests and other studies, suggest that a plane moving fast enough could cause the building over a spent fuel pool to collapse, or could create an explosion under a pool.

The authors say that the industry has raised the risk by reconfiguring its pools over the years to squeeze in more and more fuel and that in their current configuration the pools are vulnerable to heating up and catching fire if they are breached.

Nuclear experts, including at the Nuclear Regulatory Commission, acknowledge that the vulnerability of the pools has increased because they are so full. With older fuel removed, the authors say, remaining fuel could be spread out, and reactor operators could install air-moving equipment that could help keep it cool even if the pool were drained.

The Nuclear Regulatory Commission ruled in December that it was impossible to determine the likelihood of a terrorist attack and thus has resisted using its standard method for deciding whether to make improvements, which is to multiply the probability of an event with its consequences, and using the product to rank the hazard and determine how much should be spent to reduce it. “This situation calls for more explicit guidance from Congress,” the paper said.

Nuclear plants create radioactive material as they operate, splitting atoms of uranium, which are only slightly radioactive, into a variety of products that are highly unstable and give off gamma rays or subatomic particles to achieve stability. Nearly all the radioactive material stays inside the fuel, which continues to generate heat over the years as it gives off radiation.

When the nuclear plants were designed, engineers believed that the fuel would be removed quickly from the pools, and were more concerned about releases from inside the containment, where water and steam at high temperatures and pressures seemed more prone to escape. Spent fuel pools are designed to withstand earthquakes, hurricanes, tornadoes and other natural hazards, but were not explicitly designed with terrorism in mind.

The authors plan to brief some members of Congress on Thursday. The authors include Frank N. von Hippel, a Princeton physicist; Gordon R. Thompson, director of the nonprofit Institute for Resource and Security Studies, in Cambridge, Mass.; Alison Macfarlane, of the Securities Studies Program and the Massachusetts Institute of Technology; Edwin Lyman, president of the Nuclear Control Institute; and Robert Alvarez, a former adviser to the energy secretary.

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